document 001 rev 0 010905 - corby cycle...phase 1 - outline strategy corby water cycle...

Halcrow Group Limited Corby Water Cycle Strategy Phase 1 - Outline Strategy Technical Report September 2005

Environment Agency

Halcrow Group Limited Endeavour House Cygnet Park Hampton Peterborough Cambridgeshire PE7 8FD England Tel +44 (0) 1733 560033 Fax +44 (0)1733 427988 www.halcrow.com Halcrow Group Limited has prepared this report in accordance with the instructions of their client, Environment Agency, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk. © Halcrow Group Limited 2005

Halcrow Group Limited Corby Water Cycle Strategy Phase 1 - Outline Strategy Technical Report September 2005

Environment Agency

Halcrow Group Limited Endeavour House Cygnet Park Hampton Peterborough Cambridgeshire PE7 8FD England Tel +44 (0) 1733 560033 Fax +44 (0)1733 427988 www.halcrow.com

Environment Agency Corby Water Cycle Strategy Phase 1 - Outline Strategy Technical Report

Contents Amendment Record This report has been issued and amended as follows: Issue Revision Description Date Approved by

1 0 Final Report 1/9/05 AJM/GJH

Phase 1 - Outline Strategy

Corby Water Cycle Strategy/Report/001 Rev 0

Contents 1 Executive Summary 1

2 Introduction 2 2.1 Background 2 2.2 The Parties 3 2.3 Overview 4 2.4 Objectives and Scope of Phase 1 Strategy 5

3 Approach to the Water Cycle Strategy 7 3.1 Overview 7 3.2 Funding 7 3.3 Legal Agreement 8 3.4 Cost Apportionment Mechanism and Developer Checklist 10 3.5 The Planning Process 11

4 Data Gathering 14 4.1 Overview 14 4.2 Flood Risk Assessment 15 4.3 Water Quality, Foul Flooding and Sewage Treatment 16 4.4 Water Supply 17 4.5 Sustainable Development 17

5 Catchment Characteristics and Existing Situation 19 5.1 Introduction 19 5.2 Data 19

6 Proposed Developments 25 6.1 Introduction 25 6.2 Existing Allocations 25 6.3 Proposed Developments 25

7 Ownership of Water Infrastructure 29 7.1 Introduction 29 7.2 Rivers 30 7.3 Flood Storage Reservoirs and Balancing Ponds 30 7.4 Maintenance Responsibilities 31 7.5 Water and Sewage Treatment 31 7.6 Other Water Infrastructure 31

8 Flood Risk Strategic Options 32 8.1 Selection of Generic Options 32 8.2 Application of Generic Options and Outline Strategy 34 8.3 Flood Risk Management (Rivers and Reservoirs) 34 8.4 Surface Water 35 8.5 Groundwater and SUDs 35 8.6 Low Flows and Water Quality 37



8.7 Costs 37

9 Foul Flooding, Water Quality and Sewage Treatment Analysis 38 9.1 Foul Flooding Analysis 38 9.2 Works Required by 2021 41 9.3 Works Required by 2031 47 9.4 Long term outline solutions to foul flooding 54 9.5 Water Quality Analysis 55 9.6 Sewage Treatment 56 9.7 Option 1 – Existing Corby STW plus new STW at Priors

Hall/Weldon serving new properties 58 9.8 Alternative Option 2 – Existing Corby STW plus new STW

at Priors Hall/Weldon and new STW at Stanion 61 9.9 Results and Option Selection 62

10 Water Supply 65 10.1 Introduction and existing headroom 65 10.2 Infrastructure Requirements 65

11 Phasing of Developments and Infrastructure 67 11.1 Introduction 67 11.2 Flood Risk Management 67 11.3 Groundwater 69 11.4 Water Supply 69 11.5 Foul Flooding 70 11.6 Sewage Treatment 70 11.7 Summary 71

12 Sustainable Development 72 12.1 Phase 1 Strategy 72 12.2 Additional Proposals 73 12.3 Operation and Maintenance 74

13 Compliance with Strategy 75 13.1 Introduction 75 13.2 Developer Checklist 76

14 Conclusions and Recommendations 77 14.1 Conclusions 77 14.2 Recommendations 79



Appendices

Appendix A Protocol

Appendix B Cost Apportionment Mechanism

Appendix C Data Review

Appendix D Dwelling Completion Spreadsheet

Appendix E Flood Defence Options Matrix

Appendix F Technical Notes


Doc No WNCSDS/DOC/001 Rev: 0 Date: September 2005

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1 Executive Summary

The Government’s Sustainable Communities Plan has identified the need for significant increases in housing and employment across the south and south east of England; over the past 5 years 30% fewer houses were built than were needed. It aims to provide affordable housing to match increasing demand, address infrastructure issues and promote urban regeneration. Four potential major growth areas, or Sub-Regions, have been identified, Corby lies within the Milton Keynes and South Midlands Sub-Region.

Catalyst Corby (the Urban Regeneration Company for Corby) has set out a Regeneration Framework which will oversee the provision of the 28,000 new homes and 30,000 new jobs needed by 2031 to accommodate the growth needed within Corby.

This Water Cycle Strategy represents the Environment Agency, Catalyst Corby and Corby Borough Council’s desire to look at all elements of the Water Cycle (principally flooding, water supply and sewage treatment) strategically to ensure that development needs do not overwhelm existing infrastructure. The Strategy also identifies those issues which would compromise development and defines high level targets for best practice with regard to receiving, using and dealing with water. Anglian Water Services Ltd (AWS) and Bee Bee Developments have been key consultees throughout this process.

The output of this Strategy is an outline timeline that shows when developments can take place, what infrastructure will be required and how much it is likely to cost. Developer contributions to these works have been calculated and a Protocol has been drafted which commits the Environment Agency, Catalyst Corby and Corby Borough Council to act as one towards the approval of planning applications from a water cycle perspective. The Protocol also guides Developers as to what they need do to comply with the overall Strategy and so gain planning approval.

This Strategy proposes various flood mitigation measures, highlights the need for their integrated operation and maintenance and defines what needs to be done to manage water demand and meet with sustainability criteria. It identifies the key constraints of water supply and sewage treatment, and concludes that water companies are hindered from putting strategic infrastructure in place owing to their funding constraints. This issue could compromise development plans for Corby and potentially developments in all the other Sub-Regions. It needs to be resolved at high level with the Office of the Deputy Prime Minister (ODPM) and the Water Regulator, Ofwat in discussion with the water companies and other key stakeholders.

In order to meet the proposed levels of growth Corby will require approximately £72 million of additional infrastructure to be in place. This infrastructure will comprise additional flood storage, water recycling and sustainable urban drainage systems, additional water supply and treatment facilities and a combination of upgraded existing and new Sewage Treatment Works.



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2 Introduction

2.1 Background

As a result of the Government’s Sustainable Communities Plan four potential growth areas (or Sub-Regions) have been identified in the south and south east of England. They are: 1) Milton Keynes and South Midlands, 2) London-Stansted-Cambridge-Peterborough, 3) Ashford and 4) the Thames Gateway.

The Milton Keynes and South Midlands Sub-Region is shown in Figure 2.1 below and comprises:

• Aylesbury

• Bedford, Kempston and northern Marston Vale

• Corby, Kettering and Wellingborough

• Luton, Dunstable and Houghton Regis

• Milton Keynes

• Northampton

In April / May 2004 Halcrow Group Limited completed the “Milton Keynes and South Midlands Development Scoping Report” which identified Aylesbury and Corby as priority sites where growth issues needed to be urgently addressed. A Business Case to the ODPM entitled “Integrated Water Cycle Studies for Aylesbury and Corby” presented the case for funding of strategic assessments of the Water Cycle in both towns. Despite an initial delay this funding has since been granted for Corby allowing the Environment Agency to appoint Halcrow Group Ltd to undertake Phase 1 of this Water Cycle Strategy. This is therefore the first Strategy to be undertaken within the Milton Keynes and South Midlands Sub-Region and it is expected that other areas will follow suit.

Catalyst Corby’s Regeneration Framework details the growth that will be required to sustain the regeneration of Corby (28,000 new homes and 30,000 new jobs by 2031). This growth will be achieved by a combination of infill developments and developments on brownfield and greenfield sites. Expansion has already started with several sites currently under construction. Applications have been received for further developments and the Environment Agency is objecting to them on the grounds that a strategy for providing infrastructure is not in place. There are, however, constraints that will limit the sustainability and phasing of such growth if not addressed strategically. The purpose of this Strategy is to identify constraints that will limit the sustainability and phasing of such developments and what needs to be done to overcome those constraints.



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Figure 2.1 Milton Keynes and South Midlands Sub-Region

2.2 The Parties

The Environment Agency has taken the lead on the strategic assessment of the Water Cycle in Corby. Given their overall flood defence and main river maintenance responsibilities, in addition to their regulatory role in consenting discharges to watercourses, they are best placed to assume this role. The Environment Agency are working in partnership with Catalyst Corby (as the Urban Regeneration Company) and Corby Borough Council (as the Planning Authority and Drainage Authority for the non-Agency watercourses) to deliver the overall Water Cycle Strategy. The North Northamptonshire Joint Planning Unit (JPU) are working with Corby, Kettering, Wellingborough, East Northamptonshire and North Northamptonshire Councils to “create an overall town planning strategy for the area”.



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At high level a Project Board has been formed whose members comprise senior representatives of all the above organisations.

A Project Group has also been formed to produce this Strategy and that comprises members from all the above groups plus Halcrow Group Ltd, Anglian Water Services Ltd (as water supply and sewerage will be key components of any growth) and Bee Bee Developments (to provide a Developer’s perspective). Bee Bee Developments are promoting the Priors Hall development which is one of the largest development sites within Corby. The organogram below summarises the situation.

Figure 2.2 Water Cycle Strategy Organogram

2.3 Overview

This Outline Strategy Report represents the completion of Phase 1 of the Corby Water Cycle Strategy which has been divided into three phases.

Phases 1 and 2 are linked and relate to the setting and refining of the Strategy. Phase 3 is a separate undertaking and will relate to the implementation of the Strategy.

Phase 2 will entail development of the high level Strategy given in this Report. It will refine the modelling undertaken to date (via an integrated flood risk model), complete outstanding survey works for currently missing data, test the assumptions made and verify the Developer contributions that have been estimated as part of the Phase 1 works.

Phase 3 will oversee the implementation of the Strategy with the emphasis on vetting Developers’ proposals to ensure that they comply with strategic requirements.

Project Group

Catalyst Corby Environment Agency Corby Borough Council North Northants JPU Anglian Water Services Halcrow Group Limited Bee Bee Developments

Strategy Approval

Strategy Production

Project Board

Catalyst Corby Environment Agency Corby Borough Council North Northants JPU



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2.4 Objectives and Scope of Phase 1 Strategy

2.4.1 Objectives

A “Proposal for Consultancy Services” setting out the objectives and scope of the work was agreed between Halcrow Group Ltd and the Environment Agency in March 2005.

The overall objective was to produce an integrated, sustainable Water Cycle Strategy for Corby which facilitated the planned development of the town to the satisfaction of the Environment Agency and other key stakeholders. Figure 2.3 below summarises all the components of the water cycle that have been investigated as part of this work.

Figure 2.3 The Water Cycle



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Phase 1 would entail data collection, collation and outline Strategy preparation. It would include an outline timeline to demonstrate how development should be phased and would give consideration to how urgent and imminent developments should be progressed to ensure they would not compromise the final Phase 2 Strategy, due to be published during summer 2006. Strategic options would need to be reviewed and a preferred solution identified and costed. A key aspect of the Phase 1 study would be the preparation of a legal agreement (or protocol) to allow for the continued review and approval of detailed development proposals; and a cost apportionment mechanism to determine what charges Developers should be levied in order to fund strategic infrastructure investment.

2.4.2 Scope

The overall scope of the consultancy services for Phase 1 was defined as:

• project management

• desk top data collection and collation

• consultation with key consultees

• development of the legal agreement framework (or protocol)

• strategic overview and development of preliminary models

• testing the sequencing of the planned development sites

• identification of an outline Strategy together with requirements for each of the development areas before development could proceed

• review of urgent and imminent Developer proposals for compliance with the Strategy

• identification of high level cost estimates and design of a cost apportionment mechanism

• outline scoping of the Phase 2 works that would be required

• the preparation of a detailed report and a high level summary publication addressing the above points.



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3 Approach to the Water Cycle Strategy

3.1 Overview

The Corby Water Cycle Strategy has been split into the three phases previously discussed.

The approach with Phase 1 has been to base assumptions principally on existing data (e.g. Strategic Flood Risk Assessments and Environment Agency Flood Maps) and use models (of rivers or pipe networks) only where available (e.g. AWS’ sewer network model). It was not the intention to construct new models as part of the Phase 1 works, owing to limited timescales, and hence this will be one of the key outputs of Phase 2. Phase 1 was therefore designed to allow developments such as Priors Hall and West of Stanion to progress during the interim period which will exist until the publication of the final Phase 2 Strategy next year. However, decisions made now must not compromise the final Strategy.

No survey works, other than brief visual inspections, have been undertaken during the Phase 1 works. Where data is insufficient to develop a completed final Strategy, a scope for additional survey works has been put together for completion during Phase 2. These survey works have been costed so that appropriate allowances are made for additional data gathering in the next phase.

The output of this phase is two reports: this detailed Technical Report summarising all the work that had been undertaken and showing how the Strategy has been derived in detail; and the Summary Report which is intended for a much wider audience. It details they key messages of the Strategy and will be used to facilitate discussions at high level where they need to take place to address specific issues.

In addition to the Summary and Technical Reports that summarise the outputs of this phase, a GIS (Geographical Information System) database has been established to store drawings, reports, maps, photographs and other key data in an easily accessible and interlinked format. This database will form a working platform that will ensure all available information is readily available for developing the Phase 2 Strategy; and for assessing the detailed design proposals for specific developments under Phase 3. Ultimately this will provide an important management tool for the Environment Agency and other key stakeholders.

3.2 Funding

The Environment Agency is funding the Phase 1 works using monies made available by the ODPM. The costs of the forthcoming Phase 2 will be shared between the Environment Agency (using ODPM funding), Catalyst Corby, Corby Borough Council and Bee Bee Developments.

The Environment Agency has set up a fund that it will initially administer to retain contributions from all of the parties. Costs for future strategic works will be met from this fund. Ultimately it will be used to retain Developer contributions for strategic works, levied by Corby Borough Council via



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Section 106 agreements, and to pay the contractors undertaking those works.

3.3 Legal Agreement

The original intention of Phase 1 was to establish a legal agreement mechanism that the key parties would sign up to. It was envisaged that the key parties would be the Environment Agency, Catalyst Corby, Corby Borough Council and Anglian Water Services Ltd (AWS). The purpose of this agreement would be to bind the parties together to consider developments against the Strategy and not to grant planning permission to developments that were not considered to meet with its requirements. This agreement would be heavily based upon an existing agreement for Peterborough’s Padholme Catchment which binds the Environment Agency, Peterborough City Council and English Partnerships together under common agreement.

During detailed discussions it became apparent that this system could not be replicated for Corby. Given their heavily regulated situation AWS could not be a party to this agreement. This is because their charging mechanism for recouping monies from Developers, certainly for domestic developments, is rigidly set down by Ofwat. They are not therefore able to recoup monies via a strategic fund operated by individual Section 106 agreements with Developers, as this would not be permitted under the Water Act. In effect this means that AWS, and all other water companies, can only recoup money from Developers via their standard charge on each house that they propose to build. This money is payable when the Developer requisitions the water company for water and sewerage for the new development. It means that water companies can only react to demand on a piecemeal basis as required by Developers. There is no method of funding strategic development which would see infrastructure in place before it was required.

This means the agreement which has been prepared can only operate between the Environment Agency, Catalyst Corby and Corby Borough Council and it can only apply to flood defence/land drainage works and not to water supply or sewage treatment. This is precisely the basis on which the document has been prepared and a copy is included within Appendix A. For reasons of expediency the agreement is currently a Protocol: its function is the same as that of an agreement but given the above compromises it was felt that a Protocol was more appropriate at this stage. It is still the intention to convert this to a legal agreement at a later stage (and the wording is heavily influenced by that of the Padholme Catchment to facilitate this process). There are also concerns over the status of a legal agreement given that it would be based upon works required by the Regeneration Framework (which has no legal standing) rather than the Corby Local Plan which is currently being converted to a Local Development Framework. It appears sensible to convert the Protocol to a legal agreement once the Local Development Framework is finalised in early 2007.

The Protocol document defines how the Environment Agency, Catalyst Corby and Corby Borough Council will act together to assess Developer proposals, obtain contributions from them and procure and manage the construction of the necessary flood defence works. The Protocol contains a summary flowchart which defines what needs to be done at each stage, by whom and at what point in the process Developer contributions are



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required. It is principally intended as an aid to Developers to guide them through the process and is shown in Figure 3.1 below.

Figure 3.1 Protocol Flowchart

In addition, the Protocol stipulates that Developers’ detailed proposals are to be reviewed by Halcrow Group Ltd to ensure that they meet with the requirements of the Strategy. This review (Phase 3) is to be funded by the Developers themselves as part of the overall planning approval process. The Protocol provides Developers with the assurance that their proposals will not be objected to on water cycle grounds provided that they have met with the requirements of the Strategy.

The Protocol also contains a Cost Apportionment Mechanism and a Developer Checklist which are discussed below.



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3.4 Cost Apportionment Mechanism and Developer Checklist

3.4.1 Cost Apportionment Mechanism

Until a more integrated method of funding strategic works can be devised, flood defence and water supply/sewerage will remain as separate issues. The latter will be levied directly on Developers by water companies using well established mechanisms. The cost of the former will be split between Developers according to the cost apportionment mechanism contained within Appendix B. Phase 1 has established high level costs for the works that will be required. Given the uncertainties over their precise nature at this stage all parties must accept that these costs are necessarily robust. Developers need to know what to allocate now and it will be impossible to go back for further funding if current estimates prove to be inadequate. The fund, set up by the Environment Agency to retain these contributions, has been established on the understanding that any unused funds will be returned to Developers in the same proportion as they were originally levied. Contributions will be required to fund the construction of strategic works which must be in place before development begins. However, this is not to say that all contributions are required on day one and in reality contributions will be staggered in accordance with the overall development programme.

During the current interim period, before the Phase 2 Strategy is finalised, cost apportionment will follow a simple formula which will charge Developers a set amount per house. All Developers will be charged the same rate. The number of houses has been calculated with reference to Corby Borough Council’s current Local Plan (10 June 1997) and the update provided by their “Urban Housing Capacity Study” dated June 2005. Only those houses above the current allocation will be subject to this charge. There is no corresponding flat rate for industrial development and each site will be looked at on a case by case basis, again provided that it has not been identified in the current Local Plan. Where industrial developments have not been identified, Developers will be given the choice of contributing to the Strategy or attenuating on site to a 1 in 100 year standard such that proposals do not compromise the Strategy. The bulk of the industry proposed for Corby has already been identified therefore the above mechanism will apply almost exclusively to houses at this point. The Strategy has estimated that within the growth plans some 23,600 houses are over and above the current allocation.

It is the intention of the Strategy to refine this cost apportionment mechanism during the Phase 2 development. This will allow good practice to be incentivised from the “do minimum” position. It would not be equitable to charge two different Developers the same rate if one were adopting more sustainable practices than the other. This approach cannot be adopted yet as it requires more work and an understanding of all the developments which are not yet known. If this was applied in haste there would be the risk of under recovery of funds for strategic costs and hence a compromise to the Strategy. With the current situation any over recovery of costs will be reimbursed to Developers but this mechanism cannot operate in reverse.

Further details are given in Appendix B but the Strategy proposes that developments are ultimately considered and ranked against the following criteria:



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• the watercourse into which they discharge – developments which discharge to watercourses which have more capacity will be favoured

• their location with regard to the town and hence their effect on the flood hydrograph – developments downstream will have less of an impact for Corby than those upstream and this should be reflected in the eventual contributions.

• a sustainability factor - this will take into account best practice and reward those Developers who employ the most sustainable solutions (this will be determined with reference to the Developer Checklist below).

3.4.2 Developer Checklist

The final aspect of the Protocol is a Developer Checklist. This sets out the high level measures that Developers are expected to meet in a simple tick box format. It is not restricted to flood defence so includes water supply and sewage treatment targets to ensure a fully strategic approach. An example of the Checklist is included within the Protocol in Appendix A and further details are given in Section 13.

3.5 The Planning Process

This first phase of the Strategy has been designed to get the planning process moving without compromising the ultimate Strategy next year. Figure 3.2 below shows the total development planned for Corby between now and 2031.

Where it is feasible to do so urgent developments, such as Priors Hall, have been considered now with regard to what the existing infrastructure can accommodate and what the ultimate Strategy will involve. Provided the Developer Checklist is adhered to flood risk will be mitigated; and if sewage treatment and water supply capacities as they stand are not exceeded, there is no water cycle related reason why developments cannot go ahead provided an adequate contribution to strategic works is made.

Taking the example of Priors Hall: an application has already been received for the Stage 1 works, which will involve building 650 houses and the Academy college. Given its downstream location flood risk is already less problematic for receiving watercourses in Corby than, say, that of the Western Extension whose run-off will discharge upstream of the existing town. Following the Developer Checklist and limiting run-off to greenfield rates by a combination of attenuation, recycling and groundwater recharge means that that development does not compromise other developments and does not make the existing situation (whether in Corby or downstream) any worse. AWS have confirmed that water supply and sewerage needs can be met for this first phase. Provided the Developer contributions are received including sufficient funding for future maintenance there is no reason to maintain an objection on water cycle grounds. This methodology will allow urgent developments already in the planning process to be considered on a first come first served basis.



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Figure 3.2 Development Areas



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This is not to say that all Development will be approved or that the Environment Agency will not retain a standing objection to new developments, as there will come a point at which further development is unacceptable. In these cases the blocker is likely to be the capacity of Corby Sewage Treatment Works to accept increased flows. AWS maintain their regulatory position in that they are obliged to provide infrastructure within 6 months of receipt of a requisition from a Developer (for domestic development). Whilst it can be argued that temporary measures could be put in place this approach is not strategic or sustainable and would not satisfy the Environment Agency who would maintain an objection on the grounds that there is not a Strategy in place.

The situation is similar but not as immediately urgent for water supply. AWS’ intention is to have additional treatment capacity by the end of the AMP4 period (2010) although this is subject to planning approval risks.

Therefore there is limited existing capacity to allow planning approvals to be given for some but not all developments. Unless sewage treatment is given urgent attention, and the situation with regard to water supply is carefully monitored, they will prove to be the limiting factors that dictate the rate of future development.

During Phase 2 urgent and imminent developments will continue to be reviewed against the emerging detailed strategy. The principal tool will be the Developer Checklist but where possible the preliminary results of the modelling that will be undertaken will be used in any assessment. However, the majority of developments (i.e. those which are not yet urgent) will need to await the outcome of the Phase 2 Strategy next year. As stated in 3.4.1 during this interim period a flat rate will be levied on each house that a Developer proposes to build. Once the Phase 2 Strategy has been finalised the weighted cost apportionment mechanism will apply and any previous overcharging will be refunded.



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4 Data Gathering

4.1 Overview

The first step in the development of the Phase 1 Strategy involved information gathering and literature review. The aim was to collate the information available and compare it with the Phase 1 scope to ascertain how it would be met. Missing data was identified and either requested subsequently or where unavailable was identified as part of the Phase 2 scope. A list of documents and data referred to in the production of this Strategy is included in Appendix C.

Key sources of data were the Environment Agency, Catalyst Corby, Corby Borough Council and AWS.

Key development data was taken from Catalyst Corby’s “Regeneration Framework” document and in particular their “Dwelling Completion” spreadsheet which sets out the incremental and total growth against the various sites on a yearly basis. A copy of this spreadsheet can be found in Appendix D. Planning data was obtained initially from Environment Agency records and latterly from Corby Borough Council. Two visits were made to the Council Offices in order to obtain comprehensive planning details for all the applications received, and then to update that information for more recently received data. A copy of Corby Borough Council’s Local Plan was then requested so that new developments could be compared with current allowances. Although this document is out of date, and is being superseded by the preparation of the Local Development Framework, the number of allocations has been updated in Corby Borough Council’s “Urban Housing Capacity Study”. It was this document which was ultimately used to confirm the number of current allocations.

Key documents previously prepared by Halcrow were also consulted, notably the “Milton Keynes and South Midlands Development Scoping Report”, the “Integrated Water Cycle Studies – Aylesbury and Corby” Business case to the ODPM and the “Scope of Works for Aylesbury and Corby – Appendix to the Business Case” which estimated budgets and the scope of the works that would be required to undertake the Strategy.

Information from the North Northamptonshire Joint Planning Unit in the form of “Options for North Northamptonshire - Towards a joint Core Spatial Strategy”, and the Joint Planning Committee via the “North Northamptonshire Local Development Framework, draft Local Development Scheme” were also reviewed as they added a wider development perspective.

All of this was considered against the “Milton Keynes and South Midlands Sub-Regional Strategy” by the Government Offices for the South East, East Midlands and East of England which underpins all of the above and sets out Strategic Policies to guide implementation.

Where possible existing models were used and modified although no new models were constructed for this phase of the Strategy. AWS’ Infoworks network model was obtained and modified to reflect the current state of



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development. Existing Halcrow models for the upper Nene catchment (sections of the Willow and Harpers Brook) where used where possible to verify otherwise qualitative assessments.

As previously stated the legal agreement/protocol was based upon the existing “Padholme Catchment Land Drainage Agreement” provided by the Environment Agency and the “Flood Defence Strategy for the Padholme Catchment” was also reviewed to derive any lessons learned.

In considering sustainable development the recently published “Planning Sustainable Communities – A Green Infrastructure Guide for Milton Keynes and the South Midlands” by the MKSM Environment & Quality of Life Sub-Group was consulted.

The final key general document was the WSP “Corby Utilities and Strategy Study” Report commissioned by Corby Borough Council. Although this considered the capacity to accommodate development by all the major utilities it contributed useful water supply and sewerage data as well as recommendations for energy usage.

Data gathered against specific technical areas is considered below.

4.2 Flood Risk Assessment

4.2.1 Surface Water

There is limited data for the surface water system in Corby. The primary source of data has been AWS surface water network maps. Although the location of pipes appears in sufficient detail, there is little information about pipe invert levels, pipe diameters and urban catchments covered by each drainage network discharging into the various watercourses.

AWS’ foul network model contains some surface water network data but it is not comprehensive, although details have been used where it was practicable to do so.

Anecdotal data is provided in Bullen Consultants’ Stage 1 and 2 Strategic Flood Risk Assessments which give historic details of when and where surface water flooding has occurred. These details have been cross referenced in the production of this Strategy.

4.2.2 Groundwater

A comprehensive Phase 1 data collection exercise was undertaken to identify the location of major and minor aquifers, groundwater catchments, springs, Source Protection Zones (SPZs) and Nitrate Vulnerable Zones (NVZs). Groundwater models (Sub-Regional and Local) and groundwater quality data have been consulted and groundwater resource estimates have been made.

A number of data sources were consulted (refer to Appendix C for further details) and key data was obtained from the Geological Survey of Great Britain (the identification of major and minor aquifers and geological classification) and the Environment Agency in the form of a groundwater vulnerability map, the Nene Catchment Abstraction Management Strategy (CAMS) for assessing groundwater resources in development areas, and Source Protection Zone information.



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Where relevant existing reports for local developments were reviewed, in particular: Taylor Woodrow Developments Ltd’s data for the land west of Stanion and Wardell Armstrong’s data for Priors Hall.

4.2.3 Rivers and Reservoirs

During the early stages of the project a site visit was made, with the Environment Agency, to understand flood risk issues. This was supplemented by Environment Agency floodmaps (from their website) and the data contained within the Strategic Flood Risk Assessments (Stages 1 and 2) by Bullen Consultants: in particular the modified floodmaps produced for their Stage 2 document. Flooding history was also taken into account by reviewing Bullen’s data and the Environment Agency’s records. Local knowledge has also been sought in discussion with Environment Agency staff. AWS’ surface water plans were also reviewed and their impact on the river system qualitatively assessed.

Use was made of existing modelling data where possible, such as Halcrow’s ISIS models for the Willow and Harpers Brooks (although they are as yet unchecked and do not cover the entire system).

Discharge hydrographs for a range of return periods from Corby STW were abstracted from AWS’ Infoworks model to enable the impacts of storm tank spills on flooding to be assessed. During the site visit to Corby STW the culvert immediately downstream of the works was reviewed as this presents a major existing constraint.

Where possible, condition survey data of watercourses were gathered although this was largely restricted to an Environment Agency condition assessment of the Willow Brook. Other data has been gathered from local knowledge and is recorded on the Constraints Map which is part of the GIS database.

Details of the balancing ponds and reservoirs within Corby are very limited and this will need to be rectified during the Phase 2 works. High level data was again obtained from the Bullen Strategic Flood Risk Assessments although comprehensive details for Gretton Brook and Great Oakley Reservoirs were obtained from Environment Agency Condition Survey Reports. Local knowledge accounts for the rest of the information considered.

4.3 Water Quality, Foul Flooding and Sewage Treatment

4.3.1 Water Quality and Foul Flooding

AWS’ network model was issued to Halcrow for information/modification and this has been the primary assessment tool for water quality and foul flooding. River quality data has been supplemented by information obtained from the Environment Agency. Water Framework Directive requirements have been assumed based upon our current understanding of what river basin management plans are likely to contain. The Environment Agency have also provided flow data for the Willow Brook northern, central and southern arms, Harpers Brook, Gretton Brook and the River Welland for input into the model.

Water quality results have been compared with data from Imperial College’s “A Water Quality Investigation of the Willow Brook, a River Contaminated



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with Domestic and Industrial Waste Water” which was copied from Environment Agency files.

4.3.2 Sewage Treatment

A thorough understanding of sewage treatment issues has been possible owing to the data that AWS have made available and local knowledge concerning the works. A site visit was made to Corby STW in order to understand existing constraints and assess process performance and overall condition. AWS have also provided two process audit reports. The first is a general process audit of the works and the second looks specifically at the Kaldnes Reactors in order to assess their future capacity.

The “Corby Utilities and Strategy Study” by WSP has also been used to verify our findings as it first pointed out the limitations on sewage treatment capacity.

AWS’ Infoworks model and SAR ground profile data obtained from the Environment Agency have enabled us to model the effects of different locations of sewage treatment works within Corby.

4.4 Water Supply

Water supply was not included within Halcrow’s original brief. However, at the Project Group’s Inception Meeting on 6 April 2005 it was agreed that the Strategy should be extended to cover water supply issues albeit at high level. The only source of information at that time was WSP’s “Corby Utilities and Strategy Study” Report.

Since then additional data has been obtained from AWS allowing a more complete, although still very high level assessment to be made.

Through Halcrow’s involvement with planning and environmental mitigation works at Rutland Water there is a good background understanding of AWS’ position and the risks associated with it. This experience has been drawn upon whilst compiling this Strategy.

4.5 Sustainable Development

4.5.1 Sustainable Urban Drainage (SUDs)

A comprehensive trawl of internet and hard copy data was made whilst investigating SUDs, in addition to drawing on Halcrow’s own experience of sustainable drainage systems in new developments. Appendix C gives further details of the websites and publications consulted. The key sources of data were the Environment Agency and CIRIA (for rainwater systems) and Coventry University (for a comparison of various SUDs devices’ effectiveness).

Examples of best practice both in this country and in Australia, where techniques are rather better established, were gathered and considered as part of the Strategy development.

Details were combined with geological knowledge of the area to ascertain what techniques would be suitable in which locations.

4.5.2 Other

Water efficiency was considered an integral part of the Strategy and so research into water saving devices was conducted. The focus of this was



18

rainwater harvesting techniques, in order to reduce potable water demand in the first place but this was supplemented by investigation of water efficient appliances. AWS provided additional information on water efficient appliances and this has been incorporated within the Report.

“Planning Sustainable Communities – A Green Infrastructure Guide for Milton Keynes and the South Midlands” was also consulted with regard to current best practice that could be applied to the Corby Water Cycle Strategy.

WSP’s Utilities report considered power and energy and made recommendations with regard to combined heat and power and waste heat recycling from Corby STW. Halcrow has assessed these proposals and with the benefit of more detailed information on the STW and have made recommendations of our own (refer to Section 12). These concern alternative routes for sludge disposal which are more environmentally friendly than current practices.

Finally, this opportunity has been taken to make recommendations for other sustainable measures that should be considered. Although some are not directly related to the water cycle they nevertheless form the basis of good design practice that supports a strategic approach. Further details are given in Section 12.



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5 Catchment Characteristics and Existing Situation

5.1 Introduction

This section describes the catchment characteristics (e.g. geology and groundwater resources) and also the existing situation within Corby in terms of flooding (whether surface water or fluvial), current water quality issues and performance of the Sewage Treatment Works.

Corby is a comparatively upstream catchment containing the Willow Brook, Harpers Brook and the Gretton Brook. Given its upstream location these watercourses are small in comparison to the size of the town (approximately 60,000 inhabitants at present). Catchments comprising Corby are relatively small with shallow gradients and there have been very few severe flooding problems as a result. The geology for most of Corby is impermeable and when flooding has occurred in the past it has been largely caused by short, intense storms rather than longer, less severe ones.

It is important to assess current performance as a part of the strategic approach to water cycle to provide the best opportunity to resolve current shortcomings, as opposed to only seeking to mitigate the effects of future development. Existing and new developments must be able to exist side by side with similar standards of protection and service. This reflects the Environment Agency’s recent change from provision of flood defences to flood risk management.

5.2 Data

5.2.1 Surface Water

Surface water flooding of roads and open spaces is likely to occur in general once every 10 years (the current required standard is 30 years for flooding of roads and 100 years for flooding of properties). There are however localised areas where flooding occurs more regularly (approximately once every two years) along Gainsborough Road, between Jubilee Avenue and School Place (based on data contained with Bullen Consultants Phase 1 and 2 Strategic Flood Risk Assessment Reports).

5.2.2 Groundwater

The British Geological Survey Drift Sheet 171 (Kettering), indicates that the geology of Corby comprises Pleistocene and Recent Boulder Clay overlying Jurassic Inferior Oolite. To the north, east, and south of Corby, Gretton Brook, Willow Brook and Harpers Brook have eroded the Boulder Clay to expose the underlying Inferior Oolite, which comprises Northampton Sand Ironstone; Upper Lincolnshire Limestone; Limestone and Marl. Springs are located at the boundary between the Boulder Clay and Inferior Oolite.

Groundwater Vulnerability Map Sheet 24 (North Northamptonshire and West Fens) shows that the west of Corby is defined as a major aquifer (Lower Lincolnshire Limestone) with soils of low leaching potential. The central section of Corby is defined as a non-aquifer and to the east it is



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defined as a minor aquifer with soils of high leaching potential. Further east, the Weldon area is underlain by a major aquifer.

The Nene CAMS does not define any aquifers in the Corby area, reflecting the absence of abstractions of groundwater notably for public water supply purposes. Corby lies outside the area of any Source Protection Zones (SPZs), based on data from the Environment Agency website.

There are a number of recognised minor aquifers in the catchment that contribute to river flow and are used for small scale water supply. These are the Marlstone Rock, Northampton Sand, Lincolnshire Limestone, Cornbrash and recent sands and gravels.

A previous study by Taylor Woodrow Developments Limited (Land West of Stanion R8) demonstrates the typical situation with regard to groundwater. It was found that there are no indications, or history, of the site being at risk of groundwater flooding. Landmark Envirocheck identified 8 boreholes on this site and 108 within a 1km radius of the centre of the site. However, the borehole reports showed that no groundwater was found on site. Groundwater was found 0.5km south of site at 4m below ground level. The site is however located within a Nitrate Vulnerable Zone.

5.2.3 Fluvial Flows (Main River Tributaries)

The analysis of all the information that has been gathered on flood risk management has identified key flood risk areas and other minor areas at risk. The key flood risk areas may be summarised as follows (refer also to Figure 5.1 opposite):

• Cottingham Road

• Tunwell Lane

• The Grove

• Corby STW (area around storm tanks and inlet works)

• Weldon

• Gainsborough Road

• Kingsthorpe Avenue

• Newark Drive

• Great Oakley

The standard of protection is thought to vary between 2 years and 30 years and this is below even the minimum indicative standard of 50 years for urban areas.

The assessment of the key flood risk areas and their current standard of protection is based on engineering judgment as a result of site visits and information from various sources.

A detailed analysis of the flood risk areas confirms that there are strong links between flood areas 1, 2, 3, 4 and 7 and between 5 and 6 as shown on Figure 5.1. This linkage indicates that works carried out in one of these areas would have a significant effect on the other(s). There are also links between other flood risk areas but these are likely to be less significant.

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There are locations within Corby where flooding results from a combination of surface water, fluvial and foul sewer flooding. The key area where this is likely to be the case is Cottingham Road.

5.2.4 Water Quality

Corby STW discharges to the Southern Arm of the Willow Brook. The River Quality Objective (RQO) both upstream and downstream of the works is RE4, and the river currently meets its RQO for both upstream and downstream locations. In fact, the river significantly outperforms its target, with both upstream and downstream recording a General Quality Assessment (GQA – the Environment Agency’s routine monitoring program) of GQA B, which is equivalent to an RQO of RE2.

The Northern Arm of the Willow Brook currently fails it RE4 standard, whereas the Central Arm complies with its RE5 standard and would comply with an RE4 standard.

From the confluence of the Southern and Central Arms the Willow Brook complies with its RE3 standard to the confluence with the Northern Arm just downstream of Deene Lake. The Willow Brook then complies with its RE3 target downstream of this point.

Harpers Brook currently exceeds its RE1 classification.

As the Gretton Brook is not a classified GQA watercourse the Environment Agency are not obliged to sample it hence there is no water quality data available. It is unlikely that this will change in the future with proposed development.

5.2.5 Foul Flooding and Sewage Treatment

Corby is a separately sewered catchment, with the majority of flows draining by gravity to Corby STW. Flows from the Great Oakley catchment in the south west are pumped by Great Oakley Terminal Pumping Station to Corby STW, and flows from the Weldon area east of the STW are pumped to the works by several pumping stations, refer to Figure 5.2.



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Figure 5.2 Location of Corby STW and pumping stations

Whilst the system is a separately sewered catchment, there is a significant storm response in the network: providing evidence of either misconnections of roof drainage and/or highway drainage to the foul network, or sewer dereliction causing rainfall related infiltration. Therefore the sewerage system shows some of the characteristics of a combined system, namely the risk of foul flooding and wet weather storm sewage discharges.

The Sewage Treatment Works experiences a number of problems (based on a site visit to the works, discussions with operational staff and process audit reports supplied by AWS), namely:

• the inlet works is under capacity and frequently floods, in addition screenings and grit removal are ineffective with screenings carry over into settlement tanks and process units and grit deposition in the inlet channels upstream of the detritor

• flooding of the storm tank area when river levels are high. There have also been commissioning issues with the new high level storm tank. The floodwall constructed to protect this area is inadequate and is circumvented on both ends from loss of ground that has occurred during previous floods

• sludge storage facilities are extremely limited with little capacity to absorb any delays in tankering (all sludge is currently tankered to Great Billing STW)

• leakage of glass coated steel thickener tanks although this has been attended to with local repairs

• there is limited process headroom in the Kaldnes process units although considering limitations their performance is good



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• hydraulic limitations of settlement tanks: the intermediate settlement tanks reportedly drown out at 650l/s (the current flow to full treatment) and the pipework supplying the final settlement tanks will not cope with this flow.

That said the works discharges a very good effluent quality owing to the Kaldnes units and the large tertiary sand filter which polishes effluent prior to discharge.

There is however a problem with flow non-compliance which is currently being resolved between the Environment Agency and AWS. The consented DWF is 10,760m3/d whereas the works discharges approximately 15,000m3/d.

5.2.6 Water Supply

WSP’s Report gives outline details of Corby’s trunk main system. It is expected that further details will be available from AWS during the Phase 2 Strategy production to confirm distribution requirements. The trunk main runs along the western fringe of Corby from Wing WTW to Milton Keynes; a spur runs through the area that will be the Western Extension and on towards the Oakley developments. Beanfield Water Tower and a covered reservoir are located to the west of Corby.

5.2.7 Reservoirs

Data for this section has largely been obtained from Bullen’s Stages 1 and 2 Strategic Flood Risk Assessments. Figure 5.3 opposite shows the location of Reservoirs and Balancing Ponds.

Gretton Brook

Gretton Reservoir is the only defence against upstream development and there has been previous evidence of flooding on the road next to the Reservoir.

Willow Brook

Reservoirs on Willow Brook north are: Stanier Road Flood Storage Reservoir, Pen Green Flood Storage Reservoir, Pen Green Balancing Pond and Phoenix Parkway Flood Storage Reservoir. The pumps within the Pen Green Lane Balancing Pond facility do not work and this compromises its current capacity. It would benefit from cleaning out and attention to the pumps in order to utilise its full capacity.

Willow Brook central flood storage comprises: upstream and downstream Willow Brook Balancing Reservoirs, the Balancing Reservoir upstream of The Jamb and the Crucible Road Reservoir. The bulk of the flow is then diverted to the Willow Brook south. Willow Brook central then continues via a culvert to Weldon Lagoon which attenuates surface water discharges before their eventual outfall to the Brook. In 2001 Weldon Lagoon (owned by AWS) was reduced in capacity to meet safety requirements and to bring it below the volume required by the Reservoirs Act. At that time an overflow spillway and clay cut-off trench to the embankment crest were installed. However, no compensation for loss of storage has been provided to date.

Willow Brook south contains Snatchill Reservoir, the Clay Ponds Reservoir and Soot Banks Reservoir. The Eurohub Balancing Reservoir attenuates local industry only. Further downstream Weldon Reservoir is the main

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facility but Quarry Road Reservoir provides additional downstream protection.

Harpers Brook has Great Oakley Balancing Reservoir, Oakley developments Balancing Reservoirs and Longcroft Road Reservoir which is presumed to balance highway drainage. The full capacity of the latter will never be exploited as the drainage into it is insufficient.



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6 Proposed Developments

6.1 Introduction

The following section summarises the development in Corby upon which this Strategy has been based. Overall a total of approximately 28,000 houses and associated industry to provide 30,000 new jobs are required by 2031 to support the regeneration programme. Figure 3.2 in Section 3 shows the development areas. The key sources of data have been Catalyst Corby’s Dwellings Completion Spreadsheet (refer to Appendix D for a copy) and their Regeneration Framework document. Planning application data from the Environment Agency and Corby Borough Council was reviewed against existing provisions in the Local Plan and the Urban Housing Capacity study.

The above considers new housing and industry. An A43 link road is planned for construction between 2007 and 2011 (according to the MKSM Sub-Regional Strategy) and the proposal for the most likely route has been considered as part of this Strategy.

6.2 Existing Allocations

Corby Borough Council’s Local Plan dated 10 June 1997 gives details of existing allocations for employment (Table 1 Employment Proposals) and housing (Table 2 Housing Land Proposals Corby Urban Area). Although the Local Plan is effectively out of date (and will be superseded by the submission of the Local Development Framework in March 2007) it is the most up to date version. It has been supplemented by the Council’s “Urban Housing Capacity Study” dated June 2005 which identifies additional allocations not detailed within the Local Plan.

Employment proposals give a total area of 253ha for industrial development although it does not state which Developers are promoting individual sites. It therefore appears that the bulk of the industry planned for Corby falls within the existing provisions of the Local Plan.

Housing proposals within the Local Plan accounted for a total of 3,359 additional houses in 1997. This figure comprised 229 houses for urban infill areas, 2,550 for the Oakley Vale development and 580 houses west of Stanion/off Stanion Lane. The Urban Housing Capacity Study updated this figure to 5,072 comprising: 682 completions to March 2004, 3,221 unimplemented planning permissions and 1,169 outstanding 1997 Local Plan allocations. Based on this information the majority of the housing envisaged by the Regeneration Framework falls outside the existing allocation.

6.3 Proposed Developments

6.3.1 Introduction

New housing and industry will double the population of Corby by 2031. It’s therefore not surprising that the town will grow substantially on its western, southern and eastern fringes with more modest development being planned for the northern extension.



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Given the numbers of houses required development will take place on a combination of existing sites, brownfield sites and new greenfield sites.

The table below shows how the total housing numbers have been derived. Note that this shows the total number of houses listed on Catalyst Corby’s Dwellings Completion spreadsheet which commences from 2002. Some 1,250 of the 28,699 were programmed for completion during the 2002 to 2004 period hence the net development total is 27,449 houses from 2005 to 2031.

Table 6.1 Housing Proposals from 2002 to 2031

Development Numbers of houses

Priors Hall 5,100

Weldon 500

Oakley Vale 2,900

West of Stanion 1,000

Northern Extension 688

Western Extension 6,548

Southern Extension 1,395

Oakley/Stanion 4,500

Urban infill 1,426

Town Centre 800

Additional Priors Hall/Weldon 715

Others 3,127

Total 28,699

6.3.2 Major Developments

The major housing developments are:

• Priors Hall and Weldon to the north east. Priors Hall will account for 5,100 houses under current regeneration proposals and Weldon for a further 500.

• Oakley Vale and Oakley/Stanion Extension on the southern boundary of Corby. Developments in Oakley Vale are already underway and this development will account for 2,900 houses. The adjacent Oakley/Stanion Extension will accommodate 4,500 houses.

• The Southern Extension which links Oakley Vale to the east with the Western Extension along the most southerly fringes of Corby will account for some 1,395 houses.

• The Western Extension will increase Corby’s boundary significantly to offer an additional 6,548 houses.



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• The Northern Extension above Rockingham Road is earmarked for a further 688 houses.

Remaining development will take place via urban infill (approx 1,426 houses), town centre redevelopment (800 houses) and other additional capacity which has not yet been allocated for any particular area (3,127 houses). Additional Priors Hall/Weldon capacity for 715 houses also exists.

The Major industrial developments are:

• Eurohub, which is an existing freight interchange centre to the south east of the town; it is planned to expand by a further 33 hectares on a brown field site.

• Max Park III which is a proposed brownfield development of two warehouse and office units each of 20,000m2, off the A43 east of Corby. The development follows the recent construction of Max Park II on a neighbouring site.

• Wm Morrison Supermarkets proposals for a 36 ha warehouse, distribution and Employment Park at Birchington Road, on the northern edge of Corby. The status of this proposal is currently uncertain however.

• Proposals for a new waste water treatment works for the existing Roquette processing factory (adjacent to the Morrison’s site). If successful the works will discharge 8,000m3 a day into the Willow Brook. Overall the site is less than 2ha in area. The Environment Agency are currently considering implications of the discharge requirements that will be necessary.

• A proposed mixed use development site of 11ha including a public house, hotel and health centre at Oakley Hay on the southern edge of Corby.

• The Phoenix Parkway development which comprises 8 plots of new light industrial units and offices on a 16 ha site on the eastern side of town.

• A 10 ha site of mixed use development on Cockerell Road which will include a car showroom, leisure and industrial units.

• A 7 ha business extension for Rockingham Road, north of Corby.

• The St James Industrial Estate proposals for 3 ha of commercial development on a brown field site in the eastern part of the town.

Of the above it appears that only Eurohub is not accounted for in the general areas identified in the existing Local Plan. Although other sites are not specifically identified their general areas are allocated for development.

There are several other smaller applications including DIY stores and an aggregate recycling centre to fill in brown field sites in areas in the eastern industrial part of Corby.

The town centre itself will be regenerated by renovation of existing retail units and the development of new shopping, leisure and residential areas. It will be supported by transport developments which will include a new train station.



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Some of the areas proposed for urban extension e.g. Priors Hall, have areas nominated for employment etc. The impact of these will be considered in Phase 2 of the Strategy development.

Finally, the A43 Link Road will be constructed as a dual carriageway to connect the A43 Stamford Road on the east to the A6003 on the south of Corby.



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7 Ownership of Water Infrastructure

7.1 Introduction

The situation with regard to ownership of Corby’s water infrastructure is complicated by the relatively high number of reservoirs and balancing ponds which exist. The principal organisations involved are the Environment Agency, Corby Borough Council and AWS although other parties are involved on a piecemeal basis. Key sources of information to identify ownership or areas of responsibility were the Environment Agency’s “Flood Defence and Land Drainage Operational and Emergency Contact Arrangements…” and data contained within Bullen Consultants Stage 1 Flood Risk Assessment. The extent of individual responsibilities is shown on Figure 7.1 below.

Figure 7.1 Watercourse Responsibilities



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The Environment Agency exercise an overall supervisory duty on all flood defence matters and Corby Borough Council exercise supervisory and enforcement powers over watercourses that the Environment Agency are not responsible for.

7.2 Rivers

The Environment Agency is responsible for maintenance and improvements required to main rivers. The boundaries have recently changed as some previously critical ordinary watercourses have now been en-mained and so fall under the Environment Agency’s remit. The full extent of the Environment Agency’s responsibility is shown on above Figure. Corby Borough Council are responsible for the remaining watercourses. There are no Internal Drainage Boards within Corby.

7.3 Flood Storage Reservoirs and Balancing Ponds

Current ownership is split between the Environment Agency, Corby Borough Council, AWS and other parties identified in the table below.

Table 7.1 Reservoir Ownership

Facility

Ownership Volume (m3)

Pen Green Lane Balancing Pond

English Partnerships 4,000

Soot Banks Flood Storage Reservoir

Prologis Kingspark Developments Ltd

2,200

Eurohub Balancing Pond

Eurohub ? but on site balancing anyway

Longcroft Road Balancing Pond

Atkins/Northants County Council

?

Gretton Brook Flood Storage Reservoir

Environment Agency 25,230

Weldon Flood Storage Reservoir


Great Oakley Flood Storage Reservoir


Stanier Road Flood Storage Reservoir

Corby Borough Council 645 est

Phoenix Parkway Flood Storage Reservoir

Corby Borough Council 10,000

Crucible Road Flood Storage Reservoir

Corby Borough Council 2,500

Quarry Road Flood Storage Reservoir

Corby Borough Council ?

Snatchill Flood Storage Reservoir

Corby Borough Council ?

Pen Green Flood Storage Reservoir

AWS 15,500

Weldon Lagoon AWS 17,650 when full but 8,600 currently



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As part of the recent en-maining exercise the Environment Agency will ultimately assume ownership of Crucible Road Flood Storage Reservoir and Phoenix Parkway Flood Storage Reservoir. The Environment Agency will adopt strategic facilities on main rivers installed to mitigate flood risk although they will not adopt non strategic facilities which provide attenuation for specific developments only, or those facilities which are contaminated irrespective of their purpose.

The Environment Agency anticipates assuming operating rights for Crucible Road Flood Storage Reservoir during autumn 2005 with full ownership by the end of the year. The timescale for the Phoenix Park Flood Storage Reservoir facility is not yet clear.

Finally, the Environment Agency is also considering its position with regard to Pen Green Flood Storage Reservoir which is currently owned by AWS. Improvements are required to return this facility to its full capacity and the Environment Agency may be best placed to adopt and manage it.

7.4 Maintenance Responsibilities

The Environment Agency is responsible for maintaining main rivers. For newly en-mained rivers the Environment Agency are obliged to follow Corby Borough Council’s maintenance regime until March 2007, after which time they will apply their own standards. Corby Borough Council has elected to transfer this responsibility to the Environment Agency rather than to undertake the works on their behalf during this interim period.

7.5 Water and Sewage Treatment

AWS own, operate and maintain Wing Water Treatment Works, Beanfield Reservoir/Water Tower, Corby Sewage Treatment Works and all associated pumping stations.

They also own, operate and maintain water distribution networks and, where adopted, foul and combined sewerage systems and any storm overflows.

7.6 Other Water Infrastructure

Atkins and Northamptonshire County Council have operational responsibility for highway culverts, bridges, underpasses, and highway drainage.



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8 Flood Risk Strategic Options

8.1 Selection of Generic Options

In order to develop an outline Strategy to mitigate flood risk it was necessary to look at both structural options (i.e. flood defence schemes) and non-structural (e.g. policy and guidance) options. This work was undertaken following reviews of existing studies, brainstorming sessions and consultations. Results have largely been qualitative and based upon engineering judgement given the comparative lack of data (i.e. modelling tools) when compared with other aspects of this Strategy.

The brainstorming session produced 46 options which were combined into the following nine broad groups:

• Flood Storage options

• Changes to watercourses

• Transfer of flood waters

• Legal proceedings

• Maintenance/Repairs/Replacements

• Sustainable Urban Drainage Systems

• Flood Warning/temporary flood defences

• Reduced dumping of trash on/near the watercourse

• Environmental Enhancements Appendix E contains a matrix which lists, describes and ranks each of the options. Each option was appraised using criteria that assessed:

• Economic/financial performance;

• Technical performance and flexibility to change;

• Environmental performance (including maximisation of benefit);

• Stakeholder/consultee acceptability and preference;

• Risk and uncertainty. A “traffic light” ranking system was used to determine the viability of each option. Considering the original forty six options, 14 were considered viable with a further 20 being potentially viable (‘green light’ and ‘amber light’ options respectively). The remaining options were disregarded as impracticable but will be reviewed during the Phase 2 Strategy.

All the ‘green light’ options were then used to develop the outline proposals for this Strategy. The ‘green light’ options are:



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Structural Options

• Creation of storage area(s)

• Increased storage in existing balancing ponds/storage reservoirs

• Raised banks (walls and earth banks)

• Redirection of flows by controlled flooding of car parks/roads

• Replacement of culverts to increase conveyance, reduce risk of blockage and minimise need for trash screens

• Redesign and replacement of trash screens to minimise maintenance where culverts cannot be designed out

• Permeable paving for new developments and/or existing hard standing/tarmac areas

• Restriction of post-development flows to pre-development figures with preference for larger attenuation features rather than a number of smaller ones

• Rainfall harvesting to reduce demand for potable water and provide limited flood storage

Non-Structural Options

• Increase public awareness about importance of watercourses e.g. as habitats, impact of trash on flooding, blockages and cost of maintaining watercourse

• Reduce road access to watercourses to prevent dumping of trash from vehicles into watercourses

• Inform all riparian owners of their legal duty to maintain watercourses and the requirement for consent for works affecting watercourses and flood attenuation measures

• Policy of preventing culverts from being reduced in size below 1.5m width/height/diameter when they are re-lined or replaced (use other means for upstream attenuation if required e.g. vertical gates)

• Prompt repairs of cracks and regular monitoring (CCTV surveys) and frequent clearance of debris at key locations

• Active regulation and enforcement where required to ensure safe access along watercourses for maintenance and inspection

The structural options will be the responsibility of the Developers as part of both their own site development and their contribution towards the overall Strategy.

The non-structural options are the principal responsibility of the Environment Agency and Corby Borough Council although other owners (refer to Section 7 for details) will be responsible for their individual facilities.

Owing to the uncertainties at this stage the effects of climate change were not considered in detail: this will be undertaken as part of the Phase 2



34

works. Instead robust assumptions were made to counter the relative lack of detail in base data.

8.2 Application of Generic Options and Outline Strategy

The matrix in Appendix E shows how the above options have been considered to derive an outline Strategy for Corby. In addition to these measures it is also apparent that “stand-alone” measures are needed as part of the overall Strategy to address specific problem areas.

Three key flood risk areas have been identified as independent and will therefore require “stand-alone” solutions and they are:

• Willow Brook South at Gainsborough Road (mainly due to surface water flooding). This is considered further in Section 9.

• Willow Brook North at Stephenson Way and at the railway crossing

• Willow Brook Central at Weldon

The outline Strategy is summarised in Figure 8.1 opposite.

8.3 Flood Risk Management (Rivers and Reservoirs)

Restricting post-development flows and volumes to pre-development levels is recommended as an adequate measure for the scale of developments when compared to the size and number of receiving watercourses.

New developments must not be to the detriment of existing ones and the flood risk situation emerging from this Strategy must be one of improvement to a consistent standard rather than maintenance of the existing disparate ones. The required standard for Corby is no flooding for a 1 in 100 year event and this will be achieved by the combination of the strategic works above and restrictions to development that this Strategy will recommend. Developers will be required to restrict run-off from new developments to onerous greenfield run-off rates of 2l/s/ha or better. Although this restricts the ultimate flowrate it does not necessarily return the volumes of flow to those prior to developments (refer to Section 8.6.3).

Therefore additional measures are required and the Strategy proposes these in the guise of rainwater harvesting and impermeable pavings for driveways. Without the benefits of detailed models this is as close as the Strategy can get to returning the post development volumes to roughly pre-development levels. Where practicable additional SUDs features will be recommended although it must be borne in mind that much of Corby lays on impermeable geology and so the effects of sustainable urban drainage systems is naturally limited.

In addition awareness of riparian responsibilities and protecting areas alongside watercourses should be promoted.

8.3.1 Urban Hydrology

Initial flow estimates have been obtained by using the modified rational method for urban catchments (based on key information from the AWS surface drainage network maps). These results have been compared against FEH hydrology estimates at key locations.

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It has been found that the FEH rainfall runoff hydrology estimates provide unrealistic smaller peak flows for short storm durations (between 1 to 4 hours), when compared to the modified rational method. The FEH hydrology is at the boundary of its applicability, as in many cases the catchments are very heavily urbanised (URBEXT parameter between 0.25 and 0.5). For this reason the modified rational method was chosen to model the urban catchment.

In order to model the critical section of the Willow Brook upstream of the STW hydrological estimates were obtained based on the modified rainfall run-off method for the urban areas of the catchment. The estimates were conservative because they did not take account of existing upstream storage areas. The estimates were however important for the identification of critical constraints and possible flood alleviation solutions at the STW.

8.4 Surface Water

The surface water system in Corby discharges into the watercourses both at their culverted and open channel sections. Flooding of the surface water drains could occur from badly designed outfalls or from high water levels in the watercourses which restrict flows from the drains. Although there are obvious locations where surface water flooding is a significant problem, it is not known if the system has been designed against flooding of roads and properties for 1 in 30 and 1 in 100 year events respectively. This will be investigated during Phase 2 of the study.

8.5 Groundwater and SUDs

8.5.1 Groundwater Resources

A review of Source Protection Zone data and consultation with AWS indicates that there are no public water supply groundwater abstractions in the Corby area.

The Nene Catchment Abstraction Management Strategy (CAMS) has not defined or assessed any CAMS aquifers. AWS and the Environment Agency have not identified the Corby area for the development of groundwater resources. Furthermore, AWS do not consider groundwater as part of a future Strategy for potable water supply to Corby.

However, minor aquifers at Corby do provide valuable local supply to small enterprises and this cannot be overlooked. Phase 2 will address this issue in more detail.

8.5.2 Reduced Groundwater Recharge

The construction of impermeable surfaces on large new development sites reduces the capacity of the ground to allow rainfall to permeate. This is compounded in those areas in Corby overlying boulder clay, where the standard percentage runoff is 60% when compared to 80% for tarmac and concrete surfaces. Therefore the ability of the developed Corby to naturally absorb rainfall is limited.

A groundwater recharge assessment has been completed to identify any potential reduction in recharge to the Minor Aquifers beneath Corby arising from the proposed developments.

Groundwater recharge will be reduced by the development in areas where the development sites lie over the exposed Minor Aquifers. The



36

development areas that were not overlying Boulder Clay (i.e. where they were overlying the aquifer) were estimated from Bullen’s Figure 4.0 Catalyst Corby Proposed Development Areas, and the Geological Survey of Great Britain Drift Sheet 171. The recharge lost was calculated by multiplying this area by the annual effective rainfall of 41mm taken from the Environment Agency website for the Anglian region for 2003. This leads to a loss of 234,008m3 of recharge per year. A summary of recharge loss for each development site has been summarised in Table 8.2 below.

Table 8.2 Reduction in Groundwater Recharge

Development Site Area (km2) Recharge Lost (m3/year)

Western Extension 0.13 5,125

Oakley Hay 0.06 2,563

Northern Extension 0.13 5,125

Earlstrees Industrial Estate 0.12 5,126

Willowbrook Industrial Estate 0.06 2,563

Weldon North Industrial Estate 0.15 6,150

Priors Hall and Weldon Extension - north

0.50 20,500

Priors Hall and Weldon Extension – north east

0.56 23,063

Priors Hall and Weldon Extension – south east

0.60 24,600

Priors Hall and Weldon Extension – south of the A43

0.60 24,600

Weldon South Industrial Estate 0.34 13,940

Corby East 0.76 30,750

St James Industrial Estate 0.08 3,280

Stanion 1.63 66,625

Total 5.72 234,010

8.5.3 Mitigation for Reduced Groundwater Recharge

The total volume of water that will no longer permeate into the ground could be significant over a period of one year (see above) unless extensive SUDs infiltration systems are built within the large proposed developments. However, the “full infiltration” option of keeping all rainwater at source would be very expensive in terms of the size of systems required and their maintenance owing to the presence of boulder clay at many locations.

A better approach would be to accept that there will be loss of water infiltrating into the ground but with the acknowledgment that this is



37

mitigated by a combination of SUDs options and other benefits. These will be a combination of rainfall harvesting (for watering gardens, washing vehicles etc), permeable driveways and sympathetic design of SUDs features where possible to allow some infiltration to occur.

8.6 Low Flows and Water Quality

All watercourses through Corby have relatively small upstream rural catchments. Most of the contribution during storm events originates from the impermeable areas in the town. All watercourses through Corby have very low flows during dry conditions which exposes them to poor water quality under the influence of various forms of urban pollution.

The soils of central Corby are classified as high leaching potential but this is a worst case vulnerability classification assumed for urban areas and restored mineral workings since it is based on fewer observations.

8.7 Costs

The total estimated cost for the strategic flood risk infrastructure that would need to be in place is approximately £10.3 million and the costs are summarised in Table 8.1 below. This figure does not include any contingency allowance but does represent a reasonable estimate of the costs envisaged at this stage. This is the cost that will be borne by Developers through Section 106 agreements and does not include for general maintenance and improvement works that would be undertaken by the Environment Agency or Corby Borough Council.

Table 8.1 Costs

Works required Estimated Cost (£k)

1 Survey Works 61

2 Additional flood storage 6,487

3 Channel/culvert improvements 2,580

4 Trash screens etc 700

5 Miscellaneous 500

TOTAL 10,328



38

9 Foul Flooding, Water Quality and Sewage Treatment Analysis

9.1 Foul Flooding Analysis

9.1.1 Overview

Although the new development areas will be separately sewered, it is important to accept that over time the system will start to deteriorate, and misconnections are inevitable. Some allowance must therefore be made for rainfall related runoff from these areas entering the sewerage system.

In order to ascertain what impact the population expansion will have on foul flooding and storm discharges, it is necessary to characterise its current performance and identify any constraints. It will then be possible to assess how to improve the network to maintain the same level of service, or even improve the level of service to higher design standards. The Strategy has looked at the constraints of the existing sewerage system and what should be done to provide an appropriate future standard.

The analysis has used AWS’ Infoworks network model for Corby which was then modified by Halcrow to account for existing (since the model was built) and proposed future development. A number of simulations were carried out to ascertain the worst case scenario and to model the impacts of development over time. In order to simplify final analyses the model was run using three basic scenarios: the current case as the baseline, the effects of partial development to 2021 and the effects of full development to 2031.

AWS have not had detailed input into the above work and it is anticipated that the Phase 2 work will involve close liaison with AWS to develop the detailed Strategy.

9.1.2 Existing Sewerage Constraints

It is expected that a sewerage system be able to convey a minimum of 3 x dry weather flow (3DWF) to the STW with minimal surcharge. This should ensure that dry weather flow during peak times can be carried satisfactorily by the system. During a storm the system will need to convey significantly more than this to ensure that there is no risk of foul flooding or of pollution of watercourses from storm discharges.

The verified model of Corby has been assessed against both peak dry weather flow and storm conditions to identify ‘at risk’ areas.

9.1.3 Peak Dry Weather Flow conditions

The model was run with the design baseflow increased threefold for twenty four hours. Several small sections of sewer were found to be surcharged highlighting potential risk areas (refer to the Figure 2 in the Technical Note in Appendix F for further details).



39

9.1.4 Storm conditions

From Sewers for Adoption the minimum level of service acceptable in the future is no foul sewer flooding during a 30year return period storm. This is standard industry practice as foul sewers are not sized for 100 year return periods given the costs versus the benefits of doing so. In addition AWS’ model would not have accurately predicted sewer sizes for return periods greater than 30 years.

This suggests a mismatch of standards between those for foul drainage and those for flood defence. This need not be so: although sewers are “only” designed for a 30 year return period, the 100 year standard for flooding of houses can still be met by sensitive design with increased ground conveyance. Therefore any flooding between the two return periods should not flood houses themselves although localised ground flooding would occur.

A series of Flood Estimation Handbook (FEH) design rainfall events were simulated using the Infoworks rainfall generator to determine the critical duration rainfall event for the Corby catchment. A 480 minute event produced the greatest extent and volume of foul flooding and was selected as the minimum design event.

Figure 9.1 shows the locations where significant foul flooding is predicted for a 30yr return period event.

Figure 9.1 Locations of foul flooding for 30 year event



40

The model demonstrated the following:

• the Pen Green area will not be affected by any predicted increase in growth. As the model is not accurately verified in this area it has been ignored for the remainder of the study.

• any development in the north west of Corby will cause an increase in the risk of foul flooding for a 30 year return period, in an area where foul flooding is already predicted to occur.

• the area immediately upstream of the STW is also at risk of foul flooding in a 30 year return period event.

9.1.5 Storm discharge

There are three overflows in the catchment that operate at a return period of less than 30 years. A 118 event time series rainfall dataset was provided by AWS. It was not possible to determine what period of time this dataset was representative of. However, the greatest rainfall event in the series has a total depth of 30.64mm. This compares to a 30.25mm FEH design rainfall for a 10 year 240 minute event. A storm spill analysis was carried on the model to determine the spill frequency and volume at each of the overflows. The results are shown in Table 9.1 below.

Table 9.1 Spill analysis for storm overflows in Corby

Overflow Location Spill Frequency (no)

Spill Volume (m3)

Great Oakley Pumping Station 0 0

Pen Green Pumping Station 5 2,459

Weldon Pumping Station (east) 1 35

STW storm tank (with no increase in volume)

5 2,464

Note that the Corby Technical College (CTC) pumping station does not have an overflow and so it not considered above. The Weldon North pumping station has not been studied in detail as it has been assumed that it will not be affected by growth given its location in the industrial area around the Steelworks. This will be reviewed as part of Phase 2 of the Strategy.

These results represent the baseline case against which the current population can be compared to the future population. They have also been used to ensure that there is no deterioration over the current standard when considering subsequent modelling. Currently the significance of these spills cannot be gauged in isolation to river needs and the integrated model that will be constructed during Phase 2 will be used to assess impacts more fully. Furthermore, little information was available regarding the suitability of the rainfall series supplied. Phase 2 will use a more contemporary and more local observed 10 year rainfall series to generate a 1,000 event series which is more suitable for a full water quality Urban Pollution Management (UPM) analysis. In lieu of this during Phase 1, a basic storm spill assessment was undertaken.



41

9.2 Works Required by 2021

9.2.1 Overview

The population forecasts for 2021 have been added to AWS’ model with an allowance for sewer dilapidation and surface water misconnections. Figure 4 in the Technical Note in Appendix F details how the population has been applied to the new catchment areas based on the population forecast.

The new population areas were added as large sub-catchments with the boundary and total areas covered taken from GIS data. The natural drainage connection direction was determined using a 3D visualisation of the existing Infoworks network with a ground model (based on the Environment Agency’s SAR data) superimposed. This allowed the most likely gravity drainage direction to be determined and hence the route of connections to the existing system.

Large new development areas were subdivided based on the direction of natural drainage, and the population apportioned to each of the sub-catchments was determined by the surface area of that catchment, as no more detailed information was available.

Figure 9.2 below shows the 3D visualisation including the new catchments.

Figure 9.2 Topography of Corby catchment

An allowance had to be made for an impermeable area in each of the new catchments. Even though the catchments will be separately sewered, over



42

time there will be an inevitable creep in the impermeable area connected as misconnections and sewer dilapidation begin to occur. A conservative modelling estimate for separate sewers is that 2% of the total catchment area will ultimately contribute runoff. It has been assumed that by 2031, 2% of the total new catchment area will contribute runoff to the sewerage system. This is almost certainly a conservative assumption, as the level of misconnection or dilapidation required to produce this amount of run-off would be unlikely to occur in this time frame. This figure was then adjusted pro-rata with development predicted to 2021 to give an adjusted runoff factor.


There are no new areas at risk of flooding in a 30 year design event created by the predicted developments to 2021. However, the areas already affected will be subject to a slightly higher risk of flooding although the model does not predict any increase in the extent of foul flooding risk. However, if population growth expands as predicted with no improvements in infrastructure, there will be an increase in the depth and volume of flooding.

Figure 9.3 below shows the likely extent of flooding in a 30yr design rainfall event for the forecast population growth to 2021. Figure 9.4 shows in more detail those areas most likely to be affected around Cottingham Road.

Figure 9.3 Likely locations of foul flooding in a 30 year design event

Pen Green Area PS

Cottingham Road

Medina Road

Beanfield Ave/

Newark Drive



43

Figure 9.4 Cottingham Road area flooding detail

9.2.3 Storm spill analysis

The Urban Pollution Management (UPM) assessment below uses the same 100 storm events as used for the baseline spill analysis to determine the effects on the watercourse. The assessment shows that the increase in foul flow has little impact on the number of spills from the intermittent discharges in the catchment. The spill regime is compared with the baseline in Table 9.2.

Table 9.2 Impact of 2021 population on storm discharges

Location Current Spill

Frequency

(no)

Current Spill

Volume (m3)

2021 if PS not

upgraded

2021 Spill if PS not upgraded

2021 if PS upgraded by 3DWF

2021 Spill if PS

upgraded by 3DWF

Pen Green Pumping Station

5 2,460 5 2,460 5 2,460

Weldon South Pumping Station

1 35 1 35 1 31

Great Oakley Pumping Station

0 0 0 0 0 0

165_002

400_160

400_170

400_200

400_310 40

430_080

430_100

430_110

430_130

440_110

440_120

440_130

487_000

492_000

495_025

495_050

495_060

495_070

497_020

SP87887903

SP87899001

SP87899002

SP88880801

SP88882801

SP88882802

SP88882803

SP88882901SP88883903

165_002

400_160

400_170

400_200

400_310 40

430_080

430_100

430_110

430_130

440_110

440_120

440_130

487_000

492_000

495_025

495_050

495_060

495_070

497_020

SP87887903

SP87899001

SP87899002

SP88880801

SP88882801

SP88882802

SP88882803

SP88882901SP88883903



44

However, the capacity of the system to accept storm sewage is inevitably decreased by the addition of foul flow without upgrading Great Oakley, Weldon and the CTC pumping stations. Without upgrading there would be an increased risk of either storm sewage discharging to local watercourses or an increase in the risk of foul flooding. The overflows from pumping stations within Corby are really emergency overflows and are not designed to provide hydraulic relief for smaller storm events. For example, Great Oakley Pumping Station does not show any storm spills for the ten year rainfall series simulated. However, a 15 year 480 minute design event whilst not causing a spill with the present population, does cause a spill when the additional population to 2021 is added: assuming the pumping station is not uprated. If the pumping station is upgraded from 64l/s to 94l/s (which is equivalent to the peak flows from the new population), there is no spill during the same design rainfall event. Therefore, it is important in the short term (say within the next 5 years) that all pumping stations are upgraded by a rate at least equivalent to 3DWF for the new population.

9.2.4 Pumping station capacity

The new pumping stations that will need to be provided to serve the West of Stanion and Priors Hall developments will only convey foul flows for those developments in the immediate future. There is therefore the need to size both pumping stations for a minimum of 3DWF. However, considering the strategic implications, provision must be made for the ultimate Formula A flow and further consideration given to the level of storage at the pumping stations. Whilst this will not be needed by 2021, a Formula A equivalent pump rate is the greatest that could be required long term to ensure that the pumping stations can cope with future rainfall runoff expected from older sewerage networks. Table 9.3 below details pumping station requirements to 2021.

Table 9.3 Pumping Station parameters for 2021 population

Pumping Station

Population DWF (l/s)

3DWF (l/s)

Formula A (l/s)

Storage (m3)

West of Stanion

4,998 12.2 33.0 97.8 265

Priors Hall 13,338 26.7 70.9 244.4 577

Gt Oakley (increase)

4,220 17.6 29.5 84.2 233

CTC (increase) 3,451 9.5 26.0 70.7 200

As no changes are proposed to either the existing Pen Green (which is not affected by growth) or Weldon (as Weldon growth will be served by Priors Hall pumping station) pumping stations they are not considered in Table 9.3.

9.2.5 West of Stanion/Oakley Stanion extension

No development can take place in this catchment without a new foul sewage pumping station and rising main being constructed.



45

The most direct route to the existing network would be a direct connection to Corby STW. This would entail a rising main running north (approximately 1km length) followed by a new gravity trunk sewer (also 1km long) to connect the rising main to the STW as shown in Figure 9.5 below.



46

Figure 9.5 New West Of Stanion Pumping Station

9.2.6 Priors Hall/Weldon

A new pumping station would be required to serve the development at Priors Hall and Weldon. Whilst some of the development may be accommodated by the existing Weldon north and south pumping stations, not all of it can gravitate to either pumping station. Therefore, a rising main approximately 3.5km long would be required to connect the new pumping station to Corby STW.

9.2.7 Southern Extension and south of Western Extension

Both of these areas drain to the existing Great Oakley Pumping Station which would need upgrading by a value equivalent to 3DWF.

9.2.8 Oakley Vale North

This area drains to the CTC pumping station which would need to be upgraded by 3DWF for the new population (i.e. from 30l/s to 45l/s).

9.2.9 Remaining gravity drainage catchments

Development in any of the gravity drained sub-catchments would cause an increase in the theoretical risk of foul flooding in the areas already identified to be at risk. However, the ‘trigger’ normally used to identify when a section of the sewerage network needs to be reassessed is when there is a

New West of Stanion PS

New rising main (1km length, 43m lift)

New gravity sewer (approx 1km length, 400mm diameter)



47

risk of internal foul flooding from a 1 in 5 year design rainfall event. There are no areas that are pushed through this trigger level by the new developments (the Pen Green area is predicted to flood during a 5 year return period rainfall event but is not affected by new development).

The Cottingham Road area, which has a history of combined surface water and foul flooding, is not predicted to flood during a 5 year rainfall event, although the extent and duration of flooding for a 10year return period event is increased by the new properties. The hydrographs shown as Figures 11 and 12 in the Technical Note in Appendix F show how the new population increases the depth, volume and duration of flooding at this location for a 10 year 480 minute rainfall event.

9.3 Works Required by 2031

9.3.1 Overview

The final projected population for 2031 was added to AWS’ model, using the same apportionment method as before where a catchment had to be sub-divided. The population figures by catchment are shown in Figure 14 in the Technical Note in Appendix F.

The impermeable area for each of the new sub-catchments, with the exception of the town centre and urban capacity sub-catchments, was increased to 2% of the total catchment area. The verified model already allows for impermeable area connections to the network in the town centre area, and it is likely that redevelopment will reduce this impermeable area, not increase it.

9.3.2 Peak Dry Weather Flow conditions

The surcharge which occurs during peak dry weather flow conditions across the whole network for the 2031 population was modelled. There are no issues with peak DWF being conveyed by the existing drainage network which is adequate for this task.


The additional population from the Western Extension creates a new foul flooding issue at the junction of Gainsborough Road and Jubilee Road adjacent to the school as shown in Figure 9.6.



48

Figure 9.6 Flooding at Gainsborough Road

0_150300_170

300_190

300_210

300_230

300_240300_250 300_255

300_260300_270

340_000

340_010

350_000

350_010

350_020

360_0

440_070

50_265new

50_270

50_280

50_30050_310

50_320

70_000

75_000

75_010

0_150300_170

300_190

300_210

300_230

300_240300_250 300_255

300_260300_270

340_000

340_010

350_000

350_010

350_020

360_0

440_070

50_265new

50_270

50_280

50_30050_310

50_320

70_000

75_000

75_010

Flooding in this area only occurs at the 1 in 30 year event, and not during any of the smaller return period events. Therefore the population increase does not cause foul flooding at this location for the ‘trigger’ level of 1 in 5 year return period event.

The areas already at risk of flooding (at baseline population and in 2021) will be subject to slightly higher risk of flooding, and the predicted severity of flooding for a 30year event as measured by flood depth and volume increases.



49

Figure 9.7 Location of flooding at Medina Road

100_030

100_050100_080

100_090

100_100

110_000

110_020

110_030

120_000

120_010

12

435_000

435_010

435_020

100_030

100_050100_080

100_090

100_100

110_000

110_020

110_030

120_000

120_010

12

435_000

435_010

435_020

For example, at Medina Road, the flood hydrographs for 2006, 2021 and 2031 are shown in the hydrographs (Figures 18 and 19) in the Technical Note in Appendix F. Figure 9.7 above shows the location of the flooding. Total flood volumes and depths for the three scenarios are given in Table 9.4 below.

Table 9.4 Total Flood volume and depth at Medina Road

Flood Volume (m3) Flood Depth (m)

Current 76 0.26

2021 95 0.19

2031 146 0.45



50

Figure 9.8 Predicted flooding for an M30-480 rainfall event for 2031 forecast population at Cottingham Road

165_002

400_160

400_170

400_200

430_080

430_100

430_110

430_130

440_110

440_120

440_130

487_000

492_000

495_025

495_050

495_060

495_070

497_020

SP87887903

SP87899001

SP87899002

SP88880801 SP88881802

SP88882801

SP88882803

SP88882901SP88883903

SP88884801

165_002

400_160

400_170

400_200

430_080

430_100

430_110

430_130

440_110

440_120

440_130

487_000

492_000

495_025

495_050

495_060

495_070

497_020

SP87887903

SP87899001

SP87899002

SP88880801 SP88881802

SP88882801

SP88882803

SP88882901SP88883903

SP88884801

The results of similar modelling showing flooding at Cottingham Road and the Newark Road area are shown in Figures 9.8 and 9.9.



51

Figure 9.9 Predicted flooding for a M30-480 rainfall event for Newark Road area for 2031 population

10_020 420_000

420_010

425_000

425_010

440_000

440_040

440_050

440_070

440_090

44450_000

450_020

450_040

460_000

460_010

470_000

470_010

10_020 420_000

420_010

425_000

425_010

440_000

440_040

440_050

440_070

440_090

44450_000

450_020

450_040

460_000

460_010

470_000

470_010

The increase in foul flows and runoff from the catchment will have an impact on the number of storm spills from the intermittent discharges if there is no improvement in the infrastructure.

The spill regime is compared with the baseline in Table 9.5 below.



52

Table 9.5 Spill analysis for 2031

Overflow Location

Current Spill

Frequency (no.)

Current Spill

Volume

Scenario 1 Spill

Frequency

Scenario 1 Spill

Volume

Scenario 2 Spill

Frequency

Scenario 2 Spill

Volume

Pen Green Pumping Station

5 2,459 5 2,459 5 2,486

Weldon South Pumping Station

1 35 1 35 1 31

Gt Oakley Pumping Station

0 0 0 0 0 0

CTC Pumping Station

0 0 0 0 0 0

STW storm tank

5 2,464 12 13,504 0 0

With Scenario 1 the pumping stations have not been upgraded to account for extra baseflow, and the flow to full treatment and storm tank size at Corby STW are kept at their current values.

With Scenario 2 the pumping stations have been upgraded by a rate equivalent to 3DWF, and the flow to full treatment and storm tank size at the works have been increased to the 2031 figure. These results show that there has been no change in the volume or frequency of spills for the 10 year rainfall series. However, there is reduced risk of storm overflows occurring at periods greater than 10 years and reduced risk of foul flooding caused by pumping station incapacity at return periods greater than 30 years.

9.3.4 Pumping Station Capacity

The sections below consider works that will be required to the pumping stations in order to meet the requirements of flows to 2031.

9.3.5 West of Stanion/Oakley Stanion extension

The capacity at the new West of Stanion Pumping Station needed for the 2031 population needs to be increased to allow for the increased population at the Oakley Stanion Extension as shown in Table 9.6.



53

Table 9.6 Pumping Station requirements for 2031

Location Population DWF (l/s)

3DWF (l/s)

Formula A (l/s)

Rising Main

Storage (m3)

West of Stanion

4,998 12.2 33 97.8 1km, 43m static head

265

Priors Hall 13,338 26.7 71 244.4 3.5km, 10m static head

577

Great Oakley

4,220 Increase from 64 to 94

Increase from 94 to 178

750m, 32m static head


CTC 3,451 9.5 Increase from 30 to 56


1.5km, 23m static head


9.3.6 Priors Hall/Weldon

The capacity at the new Priors Hall Pumping Station needed for the 2031 population needs to be increased as shown in Table 9.6.

9.3.7 Southern Extension and south of Western Extension

Both of these areas drain to the existing Great Oakley Pumping Station which would need to be uprated.

9.3.8 Corby Technical College (CTC)

The capacity of the CTC pumping station needed for the 2031 population will need to be increased as shown in Table 9.6.

9.3.9 Remaining drainage catchments

Development in any of the gravity drained sub-catchments will cause an increase in the theoretical risk of foul flooding in the areas already identified to be at risk. There are no areas that are pushed through the 1in 5 year trigger level by the new developments.

Note that the Pen Green area is predicted to flood during a 5year return period rainfall event but is not affected by new development. This will need to be considered further during Phase 2 but cannot be modelled in any more accuracy during this phase owing to the limitations of the model in this area.

The Cottingham Road area, which has an anecdotal history of flooding, does not flood during a 5 year rainfall event, although the extent and duration of flooding for a 10year return period event is increased by the new properties.



54

9.4 Long term outline solutions to foul flooding

9.4.1 Overview

As above, foul flooding from the sewerage system in Corby is not a critical issue when using the standard levels of service. None of the new developments would cause the risk of foul flooding to trigger investment when using the standard water company levels of service. However, to ensure that Corby is protected to a 30 year design level of service, the following improvements will be needed. Although there are no absolutes on when these improvements are needed the Strategy assumes that works will be carried out within the next 5 years as the existing system will not meet the 30 year standard at the moment.

The cost estimates are average estimates extrapolated from the cost estimate mechanism in sewers for adoption (attenuation tanks), and from a water company spreadsheet used to determine the cost of new pipework.

9.4.2 Cottingham Road area

In order to achieve a 30year return period storm design for the Corby sewerage system the conveyance at and around Cottingham Road where the foul flooding is most extreme would need to be increased, by removing the two parallel sewers and flow control device and replacing with a larger diameter pipe. This would require extensive civil works under a section of main road. The length of road affected would be approximately 1.5km and is shown below in Figure 9.10. The estimated cost for this work is £3.0 million.

9.4.3 Newark Road area

Initial indications are that the flooding in the Newark Road area would be relieved by the above solution. However, if the detailed design modelling did not alleviate the risk of flooding, an attenuation tank in this area of approximately 1,500m3 would be required, at an estimated cost of £1.5 million



55

Figure 9.10 Cottingham Road area – potential infrastructure upgrade

9.4.4 Medina Road area

30 year foul flood protection for the Medina Road area could be achieved with an attenuation tank of approximately 1,000m3 volume. The estimated cost for this work is £1 million

9.4.5 Gainsborough Road area

The above level of protection could be achieved in the Gainsborough Road area with an attenuation tank of approximately 2,000m3 in volume. The estimated cost for this work is £2 million

9.5 Water Quality Analysis

Corby STW discharges to the Southern Arm of the Willow Brook. The River Quality Objective (RQO) both upstream and downstream of the works is RE4, and the river currently meets its RQO for both upstream and downstream locations. In fact, the river significantly outperforms its target, with both upstream and downstream recording a General Quality Assessment (GQA – the Environment Agency’s routine monitoring program) of GQA B, which is equivalent to an RQO of RE2. Overall there are no current causes for concern although these standards must be at least maintained in all future proposals.

The implementation of the Water Framework Directive may require higher standards of water quality and this should be considered as part of the Phase 2 work.



56

9.6 Sewage Treatment

9.6.1 Introduction and existing headroom

AWS’ model has been updated and adapted to consider a wide range of scenarios and timescales. In order to focus the modelling exercise initial options were considered and ruled out where it was apparent that they would be less feasible than others. Therefore three options were considered in detail to meet the future requirements and the analysis was adjusted to consider an intermediate situation in 2021 and the final situation in 2031, with the appropriate populations being factored in to suit.

A site visit was made to Corby STW to verify assumptions being made about hydraulic and process capacity and to gauge the overall condition of the works. Reference was also made, as previously mentioned, to AWS’ overall and Kaldnes unit process audit reports.

The existing headroom has been estimated at the equivalent of 2,250 new homes henceforth. This is to say that no development above this level can take place without new works being required. This figure has been based on our understanding of the consent issues surrounding the works and that the Environment Agency is in the process of granting a higher consent (it is understood AWS are applying for 18,454 m3/d although this has not been confirmed by the Environment Agency) to mitigate against the existing flow non-compliance. This was then checked against hydraulic and process capacity. AWS has already confirmed that Priors Hall Phase 1 development (650 houses) can take place and this is not compromised by the above assessment. Our estimated figure is essentially a compromise as it’s apparent that some elements of the works are at capacity now whereas there is headroom available within others. In particular the inlet works is already poor and cannot handle current peak flows. In process terms there is headroom within the Kaldnes units (although AWS’ own process audit states that this is limited and there are operational problems to be overcome), the intermediate settlement and the activated sludge plant (again limited), although the Final Settlement Tanks are at full load. It is understood that the inlet works floods, as do the Intermediate Settlement Tanks at full flow and that pipework supplying the Final Settlement Tanks will not accommodate sustained flows at this level. The Kaldnes units will accommodate a hydraulic load of 550l/s which is less than the pumped forward rate of 650l/s.

What is clear is that however headroom for the entire works is derived it is limited by a number of factors and that there will need to be close liaison with AWS during Phase 2 of this Strategy to understand more fully what the constraints are.

Additional headroom could be liberated if industrial producers were encouraged to treat on site or discharge elsewhere. One example is the Roquette plant, who have applied to build their own on site treatment facility and this is currently being considered by the Environment Agency. Theoretically an additional 5,800 houses could be released if this were to be the case. This is based on the limiting hydraulic capacity as process capacity is notionally far greater than this. However this is not necessarily helpful as the existing Kaldnes units need strong industrial effluent to function as designed. Removing this load could jeopardise that and render them redundant. In addition, if there were to be future problems, Roquette could ask AWS to resume treating their flows. AWS could in theory refuse but



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the fact that they are already treating Roquette’s effluent means that they may not be able to refuse in practice. Therefore headroom liberated by industrial processes simply cannot be relied upon so has not been considered in further detail. Finally, the requirements of future industry in terms of process treatment are unknown at this stage and impacts will need to be assessed as part of the next phase.

9.6.2 Corby STW to 2021

The DWF for the new population to 2021 is 19,112m3/d. The flow to full treatment (at 3DWF) would be 531 l/s. Although this is higher than the current consented flow it is less than the actual flow to full treatment at present.

This appears anomalous as the modelled flow to full treatment, provided by AWS, is 650l/s. This was confirmed by the Works Manager during a site visit to Corby STW. It is based on the pump forward rate achieved by the inter-stage pumping station although this is significantly higher than the consented flow to full treatment of 380l/s. In reality the attenuation within the treatment works will smooth out this flow although the higher future flows will cause the pumps to run for longer and hence the attenuation will be reduced. This will highlight the existing hydraulic and process deficiencies if nothing is done to up-rate capacity.

The storm tank size based on the 68 l/head as required by Environment Agency policy is 6,600m3. Current storm tank capacity is approximately 3,000m3. It has not been possible to determine the exact storm capacity as another tank has recently been added. However, the previous capacity was 2,125m3 and according to the Works Manager this has been increased by 50%, which gives a new capacity of approximately 3,000m3.

If the population expansion continues unchecked until 2021 without any improvements at the STW, there is little impact on the predicted spill regime from the STW storm tanks, primarily because the flow to full treatment is substantially higher than 3DWF (see Table 9.7 below).

If storage is increased to 6,600m3 and the flow to full treatment (effectively the pump forward flow rate) is kept at the current rate, there are no storm discharges from the treatment works in the 10 year rainfall series.

Table 9.7 Storm tank spill analysis

Baseline population

Spill Frequency

Baseline population

Spill Volume

(m3)

2021 population

Spill Frequency

2021 population

Spill Volume

(m3)

2021 population with new

storm tank Spill

Frequency

2021 population with new

storm tank Spill Volume

(m3)

5 2,464 7 4,031 0 0

9.6.3 Corby STW to 2031

The DWF for the new population is 29,104m3/d and the flow to full treatment (at 3DWF) would be 783 l/s.



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The storm tank size based on 68 l/head would need to be 8,193m3 which prevents any modelled storm discharge to the river in the top 100 events in 10 years. In order to prevent foul flooding in the immediate upstream catchment, the inlet works would have to be hydraulically capable of handling peak flows of 2.4 m3/s.

The current downstream RQO is RE5, although this is unlikely to be a satisfactory target with all RE5 stretches soon to be re-designated as RE4, therefore effluent quality to meet RE4, RE3 and RE2 criteria have been provided in Table 9.6 below.

Table 9.6 STW requirements to 2031

P PE (load)

DWF (l/s)

3DWF (l/s)

Peak flow (l/s)

Storm Tank size

(m3)

Consent BOD/

ammonia (mg/l)

120,484 221,383 336 783 2.4 8,193 RE4 12/4 RE3 9/2 RE2 6/1

In order to prevent any deterioration in the current river quality, the STW would need to treat to an effluent quality of 6mg/l BOD and 1mg/l Ammonia as a 95 percentile standard. Whilst no guarantee can be given of the likely future standards under the Water Framework Directive, and whilst standards cannot be assumed at this stage because of the difficulty equating a biochemical water quality target with an ecological target, it is likely that RE2 will be the minimum standard acceptable, and the 6mg/l BOD and 1mg/l Ammonia effluent quality would meet an RE2 objective.

However, it must be noted that any increase in flow at the STW may need to be mitigated with an increase in flood protection on the river.

9.7 Option 1 – Existing Corby STW plus new STW at Priors Hall/Weldon serving new properties

An alternative option to pumping the new foul flow from the Priors Hall and Weldon areas back to the existing treatment works would be to provide a new local treatment plant serving this area. A likely discharge point, identified from the natural drainage of the existing catchment, is shown in Figure 9.11 below.



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Figure 9.11 New STW at Priors Hall/Weldon serving new properties

The benefit of this solution would be that it reduces the pressure on capacity (both hydraulic and process) at Corby STW. In addition, more ‘sustainable’ drainage removes the need for “abortive” pumping from the Priors Hall/Weldon area back upstream to Corby STW.

The disadvantages are that there would be more treatment works to operate and maintain and associated problems of new STWs such as the long lead in time needed for planning reasons. There is also the issue of under-load to the plant in the early stages before design capacity is reached.

Priors Hall/ Weldon STW



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Design parameters for each STW are summarised in Table 9.8 below:

Table 9.8 STW Design Parameters

P PE (load)

G DWF (l/s)

3DWF (l/s)

Peak flow (l/s)

Storm Tank size

(m3)

Consent

Corby STW

104,527 205,426 310 713 2.1 7,107 RE3 10/2.5 RE2 6/1

Priors Hall/Weldon STW

15,957 15,957 120/145 26 67 0.4 1,085 RE3 14/3.5 RE2 8.5/1.5

Peak flows during a 30 year FEH rainfall event at Priors Hall/Weldon, if it is assumed that 2% of the drainage catchment will contribute runoff to the system in future years, would be in the order of 400l/s. Therefore the inlet works would need to be able to cope with flows up to this figure to prevent any risk of flooding.

Assuming a minimum storage of 68 l/head the new STW would require a storm tank of 1,085 m3. This will not be required initially as the system will be fully separate but will be needed at a later stage once cross connections start to occur.

This option does not reduce the risk of flooding (for a 30year design event) at any locations other than at the STW inlet, therefore the suggested improvements to secure 30 year protection against foul flooding on the network are still needed.

If Corby STW is treated to meet an RE4 objective in the Willow Brook at the point of discharge, the Willow Brook downstream of Priors Hall/Weldon STW may not be able to meet its RQO of RE3 because of the combined impact of Corby STW and Priors Hall/Weldon STW. Therefore Corby STW needs to be treated to a consent standard of 10mg/l BOD and 2mg/l Ammonia as a 95 percentile standard to guarantee RQO compliance in the Willow Brook both downstream of Corby STW and downstream of Priors Hall/Weldon STW.

To ensure that there is no deterioration in current quality, both treatment works would have to treat to the effluent standards to meet RE2. This is the minimum standard that is likely to be acceptable by 2031 because of the stringent demands of the Water Framework Directive.



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Table 9.9 Pumping Station requirements

Pumping Station

Population DWF (l/s)

3DWF (l/s)

Formula A (l/s)

Storage (m3)

West of Stanion

12,461 21.6 56.2 228 465

Gt Oakley (increase)

5,780 10.0 26.0 111 216

CTC (increase) 3,451 9.5 26.0 70.7 200

The pumping station upgrades (for pumping stations serving the existing Corby STW) at CTC and Great Oakley will still be required, and the new pumping station serving the West of Stanion Oakley/Stanion extension will still be required, as detailed in Table 9.9 above.

9.8 Alternative Option 2 – Existing Corby STW plus new STW at Priors Hall/Weldon and new STW at Stanion

The development in the Southern Extension, Oakley Vale South, and Oakley/Stanion extension falls hydrologically within the Harpers Brook catchment. The natural drainage therefore will be towards Harpers Brook. There is a very small existing works at Stanion serving the village of Stanion.

A new STW at this point would remove the need for pumping stations to serve these areas. However, Harpers Brook is currently a high quality watercourse meeting the RE1 objective. A new treatment works would need to treat to a very high standard just to meet the RE1 objective. However, because the Harpers Brook is currently significantly better than RE1, it is not feasibly possible to reliably treat to a high enough quality to prevent any deterioration from the actual current quality (and not just deterioration to the RE1 objective) without reverse osmosis.


The benefit of this solution is that it reduces the pressure on capacity (both hydraulic and process) at Corby STW and more ‘sustainable’ drainage removes the need for abortive pumping from Priors Hall/Weldon area back upstream to Corby STW, as before.

It would also reduce the upgrade work needed for existing pumping stations as the CTC Pumping Station would not need to be upgraded, and Great Oakley Pumping Station would only need to be upgraded by 10l/s rather than 24l/s.

The disadvantages are that there would be additional treatment works to operate and maintain and associated problems of new STWs such as the long lead in time required for planning reasons and under-load in the early stages before capacity is approached. In this case the issue of negative PR is likely to cause a significant problem with a new STW discharging into a very high quality watercourse.



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Design parameters for each STW are summarised below:

Table 9.10 Parameters table

P PE (load)

G PG DWF (l/s)

3DWF (l/s)

Peak Storm Tank size (m3)

Consent

BOD/ ammonia

(mg/l)

Corby STW

85,377 186,276 276 630 2 5,800 RE4 13/4 RE3 10/2 RE2 6/1

Priors Hall/Weldon STW

Note 1 – Corby STW must treat to a minimum standard of RE3 Note 2 – Corby STW must treat to a minimum standard of RE2 15,957 15,957 120/145 26 67 400 1,085 14/4 Note 1

8.5/1.5 Note 2

Stanion STW

19,150 19,150 120 2,873 34 87 370 1,300 RE1 target 8/1 For no deterioration 3/0.17

9.9 Results and Option Selection

There are no immediate problems in the catchment with regard to foul flooding, and the current level of service provided by AWS to its customers will not be affected by the new developments. However, the new developments will increase the risk of foul flooding and to protect the population of Corby to a 30yr design standard, approximately £7.5M will need to be spent in the long term. This figure is not affected by the various options assessed for the location of the STWs.

Storm discharges are not currently a problem within the catchment. Whilst a full UPM assessment will not be carried out until Phase 2, the costs estimated for each option include an element of storage. Estimates have been conservative and the number of spills from the pumping stations and the storm tank will be reduced when compared to the current situation. It is highly unlikely that the UPM assessment will support the need for further storage.

The water quality downstream of Corby STW is currently very high and it outperforms its RQO. Assuming that the current quality and not the RQO target quality will determine the effluent quality consent, it is immediately apparent that a very high quality effluent will be needed from any of the options for STWs. This is especially true at Stanion STW where the current quality is RE1. From a river quality and cost viewpoint, the option for a new STW at Stanion to the Harpers Brook cannot be supported, unless there are large environmental or cost savings at Corby STW and on the Willow Brook.



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The capital costs are very much dependent on whether the existing Corby STW can accept any additional load or flow, and at what level a new STW is triggered.

It may be possible to provide additional capacity at the existing works to include the new population up to and including 2031. If the existing STW can be squeezed, there are significant cost savings as shown below. However, there are great difficulties and risks in doing this at Corby, as the present works employs a two stage process in view of the predominantly strong industrial effluent. As the proportion of domestic flow increases, the incoming wastewater will become weaker, and the flow will therefore increase to a greater extent than the load. This will cause hydraulic overload with the need to upgrade pumps, pipelines etc as well as the treatment units. This could be very difficult on congested areas of the site. Furthermore, the installed process may then be inappropriate for this weaker effluent. It may be simpler to provide a complete new treatment stream (including inlet works) for the new development on the treatment site, leaving the present treatment works to serve the existing catchment.

Therefore, the most cost effective option in this case is to uprate Corby STW to a load population equivalent of 205,426 and to provide a new local STW at Priors Hall/Weldon.

A new STW at the existing Corby site is the most likely long term option given the lack of spare capacity at the STW, the fact that the inlet works are overloaded, and the strict consent that is likely to be needed to meet the WFD requirements by 2031. However, these costs favour the local treatment options, assuming that the existing site capacity can be squeezed. A cost summary is given in Table 9.11 below.



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Table 9.11 STW Options Cost Summary

Location Notes Rebuild Corby

STW (£M)

Alternative Option 1

(£M)

Alternative Option 2

(£M)

STW Improvements to 2031

Corby STW - 34.0 12.2 9.1

Priors Hall/ Weldon STW

- 0.0 7.4 7.4

Stanion STW - 0.0 0.0 11.0

New Pumping Stations etc

PS & rising main 1.54 0.0 0.0 Priors Hall

Storage 0.45 0.0 0.0

PS & rising main 0.46 0.46 0.0 West of Stanion

Storage 0.26 0.26 0.0

Pumping Station Improvements

PS & rising main 0.47 0.47 0.47 Great Oakley

Storage 0.05 0.05 0.05

PS & rising main 0.61 0.61 0.0 CTC

Storage 0.2 0.2 0.0

Sewerage Improvements

Foul flooding protection

- 7.5 7.5 7.5

Total 45.54 29.15 35.52



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10 Water Supply

10.1 Introduction and existing headroom

There has been no detailed analysis of water supply infrastructure or water resources. Instead, the development required has been discussed with AWS who have supplied high level details to show how the extra demand will be met. In preparing details AWS have assumed that an additional 28,000 homes will require an extra 12.5Ml/d and that associated industry will need 2.5Ml/d.

Headroom calculations have been undertaken based on the predicted increase in capacity at Wing WTW assuming that it is used to serve all of the growth predictions for the Government’s Sustainable Communities Plan. In other words the additional 170,000 new houses will absorb the growth and that Corby accounts for some 28,000 of these houses i.e. Corby does not receive preferential treatment over other developments. Conservative water supply (i.e. accounting for leakage) figures of 200l/h/d have been assumed to derive a headroom equivalent of 5,250 houses henceforth. In practice figures should be more favourable than this but given the assumptions that needed to be made a conservative approach was adopted. Clearly these headroom figures are very high level and need to be discussed in detail with AWS during Phase 2 of this Strategy.

10.2 Infrastructure Requirements

The infrastructure improvements required and their estimated cost are given in Table 10.1 below. Whilst the provision of infrastructure (e.g. mains and booster pumping) for individual developments can be tailored to suit their individual timescales the whole Strategy is underpinned by the assumption that the additional volume of water can be provided by Rutland Reservoir and that Wing WTW can be upgraded to treat the increased flows.

AWS already have an AMP4 scheme to address both these issues and the supply and treatment of additional water hinges upon its successful outcome.



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Table 10.1 Water Infrastructure requirements and costs

Proposed Scheme Estimated Cost (£k)

1 Increase output at Wing WTW from 255Ml/d to 270Ml/d peak

2,000

2 Northern Extension – trunk main connection and short length of main

100

3 Priors Hall – upgrade booster feed pumps to Beanfield Tower and install 3km of 400 dia. main

850

4 Oakley – trunk main connection, new booster station and install 1km of 355 dia. main

530

5 Southern Extension – trunk main connection and install 1km of 355 dia. main

280

6 Western Extension – new 15Ml storage reservoir and booster station at Beanfield, trunk main connection and install 4km of 450 dia. main

5,750

7 Remaining sites – allow 10km of 400 dia. main plus connections, metering, system controls, telemetry etc

3,000

TOTAL 12,510



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11 Phasing of Developments and Infrastructure

11.1 Introduction

Developments have been considered against flood risk management, groundwater, water supply, foul flooding and sewage treatment. The most obvious constraints which will limit the pace of development will be the capacity of Corby STW and to a lesser extent the water supply which will depend on Wing WTW extensions being built in time. A critical factor will be liaison with AWS over all developments to ensure that water supply and sewage treatment headroom in particular are not exceeded.

Development will therefore proceed on a first come first served basis, to a point, as those Developers who are able to move more quickly will be in a position ask the water and sewerage undertaker to guarantee that capacity is available. Later developments may have to wait until suitable facilities are provided.

Another key question is which developments can take place from a flood risk management point of view without compromising the Strategy, at least in the short term. In particular, the Priors Hall Phase 1 and the land west of Stanion proposals are becoming urgent. Outline conclusions have been drawn from this Phase 1 Strategy where it has been possible to do so. The intention is to avoid compromising development which can be agreed to now, provided of course that the requirements of the Strategy (Developer Checklist and S106 contributions) are met. Where the position is less certain developments will need to await the outcome of the Phase 2 Strategy in summer 2006 before the programming can be finalised. It is intended that urgent developments continue to be considered in detail throughout the coming year so as to minimise all delays where possible.

The Phase 2 Strategy will consider a detailed timeline to show what developments can take place and by when. The output of this phase has been the draft timeline given below which has been assembled with particular reference to Catalyst Corby’s Dwellings Completion Spreadsheet (see Appendix D).

11.2 Flood Risk Management

The preferred approach to the sequencing of planned developments has been to compare the Dwellings Completion Spreadsheet with the timing of the Phase 2 Strategy submission and the results of the outline flood risk assessments undertaken to date. Developments may be considered as those which, by inspection or preliminary modelling, can be granted now, those which must be addressed as a matter of urgency during Phase 2 and those which must await the completion of the Phase 2 Strategy one year hence. In all cases it is assumed that developments will adhere to the Developer Checklist such that impacts on flood risk are mitigated as far as possible.



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As would be expected the larger upstream developments will have the greatest impact and should therefore be completed only when the strategic solutions are in place to mitigate those impacts.

Oakley Vale – developments here will ultimately utilise a number of the proposed flood mitigation measures. The build programme is ongoing and is scheduled for completion in 2009. This means that urgent consideration will need to be given during the Phase 2 development to proposals and a better understanding obtained of what is programmed and by when. It is conceivable that there could be some delays pending the outcome of the final Strategy. Preliminary work to date suggests that Oakley Vale neighbourhood centre and housing development could proceed with minimal impact to the Strategy (it is recommended that small flows are diverted to the Willow Brook South Arm at Oakley Road); developments above that would need to be analysed in detail.

West of Stanion – this development is favoured by its downstream location within Corby. It is one of the developments that require urgent attention during the early parts of the Phase 2 works. It is currently programmed to start during 2006 which is now unlikely although the Developer has been asked to confirm the current programme. Given the minimal works that will need to be in place before this development can proceed, delays should not be significant from a flood risk perspective. However, this will be confirmed in Phase 2 and it is recommended that hydraulic modelling of the Harpers Brook through Stanion is prioritised to assess the impacts on the Strategy.

Other allocations and permissions – the build rate shown at 25 houses per year is minimal and in practical terms so is the impact on flood risk. Therefore the Strategy need not delay imminent applications and will consider the bulk of the remainder in due course on publication of the Phase 2 works.

Urban Capacity – the situation with urban capacity is very similar to that for “Other allocations and permissions” above. Given the low initial build out rate it is unlikely that the Strategy will be affected significantly by any applications granted during the Phase 2 works. It is recommended that developments proceed as programmed until the publication of the Phase 2 Strategy at which point this will be reviewed. This approach is supported by the fact that development is likely to take place on brownfield sites thus the net impacts will be minimal.

Town Centre Living – comments are as those above for “Urban Capacity”. Given the low build out rates and the fact that commencement is not programmed until 2007 the impacts on immediate flood risk issues are minimal. The Phase 2 Strategy will be published before any houses are due to be built so there is minimal risk to this approach.

Priors Hall and Weldon – although these developments are separate they are considered together because of their locations. Priors Hall accounts for the largest share at 5,100 houses versus 1,215 for Weldon. Given the urgent nature of Priors Hall Phase 1, its impacts were modelled as part of the outline Strategy (Phase 1) works. Impacts on flood risk are minimal which means that the first development (650 houses plus the Academy) need not be held up on flood risk grounds. It is recommended that SUDs are incorporated at permeable locations across the site (i.e. where it’s practicable to do so) to minimise ground water losses. It is proposed that the



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remaining development be addressed after production of the Phase 2 Strategy in summer 2006.

Oakley/Stanion Extension – this is not programmed for commencement until 2016 at which point the requirements of the Strategy should be in place, it need not be considered further at this stage.

Western Extension – this requires many of the identified flood mitigation measures to be in place owing to its location within Corby. That said it is not programmed to start until 2010 and even then with a phased rate of build. This should give sufficient time for the appropriate flood mitigation measures to be in place, especially considering that not all of them will be required immediately. The programming of the Western Extension should therefore be achievable within the requirements of the Strategy.

Northern Extension – a 2020 start date is shown for the phased building of some 688 houses. Given that the Strategy will be well established by this time the mitigation measures required should long since have been in place therefore there should be no delay to the programme as shown.

Southern Extension – similar comments to those for the Western Extension can be made although the build programme is shown commencing in 2005. However, the programme starts in earnest in 2010 which again should give sufficient time to allow phasing of flood mitigation works to be put in place. Given its location many of the flood mitigation measures identified will be applicable to this development.

Additional Capacity – housing under this Phase will not be commenced until 2026 hence no further consideration has been given, as the Strategy should be well established by that time.

The above makes no reference to prioritisation for industrial developments. This needs to be better understood during the Phase 2 works but as previously discussed the Strategy will give Developers the option of attenuating to the 1 in 100 year standard on site or contributing to the Strategy itself. In the former case there will be no adverse effects on flood risk and in the latter the Strategy will need to consider each on a case by case basis.

11.3 Groundwater

There are no issues with the sequencing of developments provided that: a) groundwater is not used for water supply and b) groundwater recharge is mitigated wherever possible through the provision of permeable driveways, rainwater harvesting and SUDs where the geology allows.

11.4 Water Supply

As stated previously, the estimated headroom for water supply issues is some 5,250 houses before AWS’ scheme for Rutland Water and Wing WTW extension comes to fruition. It is ongoing and is due for completion at the end of the AMP4 period (2010). Between now and the end of 2010 the Dwellings Completion spreadsheet shows developments totalling 4,575 houses. Assuming the works proceed as planned there is just sufficient capacity. If works are delayed by 1 year the additional 1,050 houses shown (giving a new total of 5,625) would exceed the theoretical capacity. It is therefore important for the Regeneration Framework that AWS’ programme for water supply works is not delayed. There is however a



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planning risk associated with this as there are likely to be objections to AWS’ current proposals. The situation will need to be monitored during the Phase 2 works.

11.5 Foul Flooding

Foul flooding works to bring Corby’s infrastructure to an acceptable standard (from a strategic point of view) comprise 1.5km of new pipework to the Cottingham Road area and plus additional foul storage tanks to Newark Road, Medina Road and Gainsborough Road areas. Although it has been assumed that these will be put in place over the next 5 years they are not necessarily dependent upon any one development and the current situation is not sufficiently poor to cause internal flooding (DG5) issues to property.

Therefore from a strategic point of view they are required now but this need not hold up any developments in the meantime. This is an area that will be given particular attention during Phase 2 to optimise the Strategy requirements.

11.6 Sewage Treatment

The Strategy recommends that Corby STW is up-rated and that a new STW is provided to serve Priors Hall and Weldon. Until this happens it has been conservatively estimated that Corby STW has a residual capacity for another 2,250 homes which means that headroom will be exhausted in 2007 at the predicted rate of development. This means, for instance, that Priors Hall Phase 1 (650 houses) and land west of Stanion development (970 houses) could be accommodated with headroom for an additional 630 houses before any works are undertaken (note that ultimately Priors Hall would be served by the new STW). After that point it does not matter where developments are as they all depend equally on strategic improvements being made.

It has been assumed that a new STW at Priors Hall/Weldon would be operational during 2010 and that Priors Hall effluent until that point is discharged to the existing Corby STW. Referring to the Dwellings Completion Spreadsheet this would compromise development principally for the Oakley Vale site. Summarising housing completions (excepting Oakley Vale, the Western Extension and the subsequent Phases of Priors Hall and Weldon) between 2005 and 2010 gives:

West of Stanion 650 Other allocations 150 Urban capacity 250 Town centre living 250 Priors Hall Phase 1 650 Oakley/Stanion 100 Southern Extension 250 Total 2,300

This corresponds roughly with the calculations for available headroom and shows that at least the early parts of most developments could theoretically be accommodated. Assuming that by 2010 there is a new STW at Priors Hall/Weldon and that suitable upgrades have been made to Corby STW the development can take place as shown on the Dwellings Completion spreadsheet for future years. However, the housing that will have been delayed amounts to 1,800 houses (Oakley Vale), 100 houses (Western



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Extension) and 475 houses (Priors Hall and Weldon) to give a total of 2,375 houses i.e. 50% of the Regeneration Framework’s programme.

If developments were delayed beyond 2010 the impacts would be more serious as development is scheduled to proceed at roughly 1,000 houses per year thereafter. The above list is not definitive as proportions can be varied to suit between developments but it does demonstrate the impacts of the limited STW capacity. It is not inconceivable that temporary arrangements are put in place over the next 5 years to mitigate the shortfall however this does represent a compromise to the Strategy. Any proposals for temporary works would need to be addressed in detail against strategic requirements.

11.7 Summary

In general terms groundwater, foul flooding and water supply issues should not hold up the development envisaged by the Regeneration Framework. Water supply will need to be carefully monitored to ensure that AWS’ plans can be fulfilled in the time envisaged. Foul flooding issues will raise difficult funding questions but should not compromise overall development.

From a flood risk management point of view many of the developments which require the majority of the strategic works to be in place are those which are already towards the end of the programme. Where they are not the phased approach and brownfield infill mean that sufficient time should be available to implement works before development is compromised. Key pinch points are currently Oakley Vale which is ongoing and Priors Hall/Weldon after Priors Hall Phase 1 development Phase 2 will need to give urgent consideration to these and the land west of Stanion proposals. However Priors Hall Phase 1 need not be delayed on flood risk grounds.

The major constraint is the capacity of Corby STW which has the potential to delay half of the Regeneration Framework until strategic solutions can be put in place between now and, say, 2010.



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12 Sustainable Development

12.1 Phase 1 Strategy

Sustainable development has been taken into account as part of the development of this Strategy. The key elements of sustainability in this Strategy are:

• Rainwater recycling (to provide limited flood storage)

• Sustainable urban drainage systems where possible e.g. permeable driveways

• Consideration of sewage treatment assets to make most use of existing plant and to site new works to avoid pumping as far as possible (note that Phase 2 will need to consider the sustainability impacts of recommended performance versus the energy and CO2 etc implications of achieving it)

• Minimisation for potable water demand by a combination of rainwater harvesting and specification of water efficient appliances via the Developer Checklist.

It should be noted that rainwater recycling provides a dual role: limited flood storage and reduction of potable water demand. These two roles need to be balanced during detailed design as a full storage tank will not attenuate any run-off and an empty one during “drought” conditions will not supply any water for garden irrigation/vehicle washing/other external uses. Nevertheless it still forms a valuable part of an integrated strategy on water cycle management.

Water efficient appliances should comprise: low flush and dual flush toilets, self closing and spray taps, efficient low flow shower heads (rather than power showers), low usage white goods and waterless urinals for public facilities. In addition pipe design should include lagging to minimise wastage associated with waiting for taps to run hot or cold. Finally, gardens and community spaces should be designed with limited water needs in mind.

All of the above are in keeping with the green infrastructure endorsed by the MKSM Sub-Regional Strategy and the need to “take account of and integrate with natural processes and systems” as recognised by the “Planning for Sustainable Communities – A Green Infrastructure Guide for Milton Keynes and the South Midlands” document co-authored by the Environment Agency. This document recommends that all developments should be built to the highest environmental standards which include:

• water efficiency measures

• sustainable drainage systems

• water recycling

Water efficiency and water recycling are addressed by the above measures and SUDs have been incorporated where practicable but proposals are naturally limited by Corby’s geology which is largely impermeable.

This document also proposes a strategic approach to Section 106 agreements and this is effectively part of this Strategy via the work that is



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being done developing the Protocol and Cost Apportionment Mechanism so that Developers can see what their liabilities are.

A pilot study for green infrastructure in Corby is currently nearing completion and the results will be incorporated into the Phase 2 Strategy.

Finally, English Nature has adopted Accessible Greenspace Standards and the strategic proposals for numerous flood storage reservoirs can be used to contribute towards this goal. There are definite opportunities for using storage reservoirs for such multifunctional purposes. One possibility is to use a third of the higher surface area for amenity and leisure activities although details will need to be finalised as part of a more detailed design. Developers for larger sites (> 100 ha) will be required by the Developer Checklist to ensure that 25% of balancing facilities are designed for multi-functional use to meet this guidance.

12.2 Additional Proposals

12.2.1 Introduction

The strategic approach towards the development in Corby can be used to benefit other areas in addition to water infrastructure. The measures given below are suggestions for what might be done on a larger scale and may or may not relate directly to the water cycle but are nevertheless worthy of further consideration.

12.2.2 Waste management database

Given the amount of construction activity that there will be within Corby over the next 25 years there may be benefit to the establishment of a waste management database to which all the principal contractors contribute. This would not be designed to log every waste product arising but could be used to log “useful products” or suppliers for the benefit of others. For instance it offers the opportunity to balance cut from one site with fill requirements on another and so avoid double handling.

12.2.3 Services database

The Institution of Civil Engineers (ICE) are currently conducting research into an integrated services database that will combine records of assets from utilities companies and highway authorities etc in conjunction with the Ordnance Survey who will agree standard recording techniques (source: New Civil Engineer, 9 December 2004). This pre-empts the requirements of the Traffic Management Act which stipulates that utilities/highways must store data digitally by 2008. It is recommended that an update is obtained from the ICE and that a system be adopted for Corby which records all new and existing infrastructure data as it becomes available.

12.2.4 Composting sludge (to grow energy crops)

WSP’s “Corby Utilities Capacity and Strategy Study” Report makes reference to the potential use of biomass to grow energy crops. In the context of the Corby STW this would entail treated sludge (but not enhanced treated sludge) being used on land to grow non-food crops which are then used as a fuel. There are odour issues to address and EC guidelines to adhere to but it represents a potentially sustainable strategy for a low capital investment. The land take is significant and it has been estimated that some 500ha would be required. This can be assessed in further detail during the Phase 2 works if required.



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12.2.5 Discussion of other suggestions in WSP Report

In addition to comments above, WSP’s “Corby Utilities Capacity and Strategy Study” Report made various other recommendations for green infrastructure and alternative energy sources.

Grey water recycling has not been integrated into this Strategy owing to residual hygiene concerns (e.g. fungal growth) for both industrial and domestic use. In addition many Councils are opposed to grey water recycling.

Combined heat and power is a common consideration for a large works although its application at Corby would be on a small scale owing to the size of the catchment and would not be economically viable (AWS have discontinued CHP facilities at sites larger than Corby). Currently AWS transport sludge from Corby STW to other sites for treatment so there would be limited biogas to harvest on site and in addition the sludge produced at Corby STW is not understood to be amenable to digestion (through which biogas is generated in the first place). Given this and high maintenance costs associated with CHP it would not be feasible for Corby STW.

Similarly, the potential to extract waste heat from sewage would need careful consideration as reducing the temperature of sewage reduces treatment rates (particularly with regard to nitrification) and this could have a detrimental effect, especially in winter months. It’s unlikely that the value of heat extracted would outweigh the additional costs.

12.2.6 Recycled aggregates

Recycled aggregates, as well as being more sustainable, can often be cheaper for low grade applications such as road sub-bases. It is recommended that recycled or secondary aggregates are considered where practicable. Alternatively demolition arisings could be recycled on site for the same purpose (or be input into the waste management database above) and a website has been set up to give online guidance to specifiers and Developers on the use of such materials (www.aggregain.org.uk).

12.3 Operation and Maintenance

Development must be sustainable in the first instance but operation and maintenance should also be sustainable so as not to compromise the long-term implementation of the Strategy. Detailed requirements will be produced in response to detailed designs but in principal it is recommended that a maintenance company is formed which has responsibility for operation and maintenance of all the strategic flood mitigation assets (except those that the Environment Agency already have responsibility for). This should also be extended to cover Developer installed facilities to give a holistic approach throughout Corby.

An application has been made by others for funding to investigate this issue and it is recommended that progress be reviewed as part of the Phase 2 Strategy works.



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13 Compliance with Strategy

13.1 Introduction

The key tool for assessing whether developments comply with the Strategy is the Developer Checklist which is reproduced below. This summarises the findings of the Strategy into a simple checklist which guides Developers in the assumptions to make and the data to be provided.

A flowchart has also been devised to guide Developers through the overall process and both this and the checklist are contained in the Protocol document in Appendix A.

The approach of the Checklist is to ensure that flood risk and mitigation are adequately assessed (the Developer can undertake a Flood Risk Assessment) and that there is sufficient water supply and sewage treatment capacity to accommodate that development (letters from the appropriate service providers are required to demonstrate that this is the case).

Developers’ proposals will be assessed by Halcrow against the Strategy using all of the information available above. This assessment will confirm compliance or request additional information/state the reasons why compliance has not been confirmed.

Once compliance has been confirmed a letter will be issued which will allow the Developer to progress the application from a Water Cycle Strategy perspective. The overall Protocol document has been set up with this in mind and allows the Environment Agency and hence the other parties to remove their objections to planning agreement on water cycle grounds.



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13.2 Developer Checklist

Flood Risk Management

1 Confirm that there is no net increase in flood risk to upstream or downstream communities and provide evidence

2 Run-off attenuated to greenfield rates i.e. 2l/s/ha or less

3 Confirm Flood Estimation handbook (FEH) rainfall is used to size any balancing facilities and river flow estimates (calculations must include adequate sensitivity tests to determine the effect of changing parameters)

4 Details of attenuation measures (including maintenance requirements) and supporting calculations

5 New river crossings/weed screens to be designed to minimise risk of blockage (refer to Environment Agency guidelines)

6 No development proposed on floodplain (referring to Environment Agency floodmaps)

7 Sufficient access to watercourses to be maintained to allow for future maintenance

Sustainable Urban Drainage Systems

8 Driveways and other suitable hard surfaces constructed from permeable paving

9 Detail of additional SUDs proposals

Water Supply and Sewage Treatment

10 Letter provided from water supply undertaker confirming demand can be met in accordance with the Water Cycle Strategy

11 Letter provided from sewerage supply undertaker confirming demand can be met in accordance with the Water Cycle Strategy

Water Consumption

12 Confirm development can meet water consumption target of 120l/h/d and enclose supporting details (e.g. proposals for water efficient appliances)

13 Confirm that development will utilise rainwater harvesting from roofs (minimum tank size 2.5m3 per house)

Green Infrastructure (developments > 100 ha)

14 Confirm 25% minimum flood attenuation ponds/wetlands will be designed for multifunctional uses (access, footpaths, cycle-ways, recreational uses etc) and submission of outline details

15 Details of any additional green infrastructure proposals enclosed



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14 Conclusions and Recommendations

14.1 Conclusions

A strategic approach is required to the water cycle in Corby to ensure that existing infrastructure is not overwhelmed and that constraints are addressed at the earliest opportunity. It also allows areas to be earmarked for strategic solutions, especially for flood balancing ponds, and ensures that Corby is developed to consistent standards throughout.

Approximately £72 million will need to be spent on water infrastructure within Corby to ensure that it meets the growth rate set out in Catalyst Corby’s Regeneration Framework. This expenditure will need to be phased between now and 2031. Table 14.1 below gives a summary breakdown of this cost.

Table 14.1 Water Cycle Strategy Cost Breakdown

Infrastructure Required Cost (£million)

Water Supply 12.51

Sewage Treatment 21.64

Sewage Network 7.50

Flood Management - Strategic 10.33

Flood Management – Developers own sites 20.0

Total 71.98

Of this £72 million, some £10.3 million will be required for strategic flood defence works for which Developers will meet the cost via the Cost Apportionment mechanism that has been derived.

Provided compliance with this Strategy can be demonstrated, via the Developer Checklist which sets out strategic requirements, the Environment Agency will remove their standing objection to development. This will allow the planning process to restart, albeit with certain conditions, and Developer contributions, as set down in Section 106 agreements will be levied at the appropriate time.

Developer contributions have been estimated at approximately £600 per house to cover cost of the strategic flood defence works and this includes an allowance for maintenance. Industrial developers will be given the choice of on site mitigation to a 1 in 100 year standard or contribution to the Strategy along with domestic Developers.

Infrastructure required comprises: flood storage facilities (new and improved existing); channel and watercourse improvements; foul network improvements (additional storage and new pipework); uprating of the existing sewage treatment works and the construction of a new sewage



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treatment works at Priors Hall/Weldon; additional water supply, water mains and a new storage reservoir to the west of Corby. They are summarised on Figure 14.1 below.

Figure 14.1 Proposed Strategic Water Infrastructure



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Limiting factors are sewage treatment capacity and possibly water treatment capacity if AWS are unable to progress the Rutland Water and Wing WTW schemes as quickly as programmed. The capacity of Corby STW is the most critical factor. Current estimates show that the STW will be at capacity in 2007 and the WTW will be at capacity in 2010 given predicted rates of development.

AWS are unable to respond in a strategic fashion to development as they are a regulated business and have strict charging regimes. They can only respond to requisitions form Developers and this is a hindrance to the development of this Strategy.

14.2 Recommendations

The recommendations of this Phase 1 Strategy are as follows:

• The findings of Phase 1 Reports are reviewed and are used to revise the phase 2 scope

• The Phase 2 works are commenced immediately to refine the Strategy and to finalise the costs that Developers must pay by having a better understanding of the works required

• Water Company funding is discussed at high level between Ofwat (the Water Regulator), the ODPM and the water companies themselves. The output must be a funding stream that allows water companies to respond to strategic development

• Sewage treatment and water supply capacities are discussed with AWS in more detail so that the impacts on development are fully understood

• Implementation of this Strategy for all forthcoming developments not identified in Corby Borough Council’s Local Plan/Urban Housing Capacity Study occurs

• An integrated flood risk model is produced to assess impacts and to be used as a tool for subsequently helping to assess development proposals.

Appendices Appendix A Protocol



Appendix D Dwellings Completion Spreadsheet


Appendix F Technical Notes

Appendix A Protocol

Corby Water Cycle Strategy Planning Agreement (Land Drainage) Protocol This Protocol is dated 29 July 2005 and is agreed by:

(1) Catalyst Corby (“CC”) which is the urban regeneration company whose office is at Allied House, 1 Spencer Court, Corby, Northants, NN17 1NU.

(2) Environment Agency (“the Agency”) whose office is at Nene House, Pytchley Road, Pytchley Lodge Industrial Estate, Kettering, NN15 6JA.

(3) Corby Borough Council (“the Council”) whose principal office is Deene House, New Post Office Square, Corby, Northants NN17 1GD and together referred to as “the Parties”.

Statement of purpose This Protocol is intended to provide guidance to developers on the measures that must be taken to satisfy the requirements of the Corby Water Cycle Strategy Study. As the Strategy develops this Protocol will be converted to a formal land drainage agreement between the Parties. Provided that the requirements of the strategy are met, the Parties agree to remove any objections to proposals on land drainage and flood risk grounds. This Protocol will also incorporate where possible requirements of other interested parties such as Anglian Water Services Ltd (AWS) in support of the strategic approach being taken to the development in Corby. AWS are not a party to this Protocol for regulatory reasons. Developer(s) will still be required to pay separate levies to AWS to cover water supply and sewerage services, where appropriate, as set down by the Water Act and determined by Ofwat. 1. Recitals

1. The Council is a drainage authority and a local planning authority for the drainage catchment within which the development sites are situated and as the drainage authority is seeking to adopt Byelaws to enable a levy to be raised on development, and as a local planning authority will seek to use its powers under section 106 of the Town and Country Planning Act 1990 as amended for the same purpose.

2. The Agency and the Council are the authorities responsible for the watercourses into which the Development Sites ultimately drain. The attached figure in Appendix 1 shows the delineation of responsibilities.

3. The Development Sites are identified in the Regeneration Framework and are also shown in Appendix 1. The Council as local planning authority wishes to ensure they are capable of being developed as soon as reasonably practicable in order to facilitate regeneration and local economic growth.

4. CC, the Agency and the Council accept that the development of the Development Sites will be facilitated by the Drainage Works.

5. The Parties have therefore agreed that the Drainage Works shall be carried out and completed at the expense of the Parties (where appropriate) and the Developers in the manner set out in this Protocol. Appendix 2 contains an indicative list of ALL the proposed works under the Strategy.

2. Definitions “Completion Certificate” means a certificate of completion issued under the Works Contract upon completion of the construction works. “Contractor” means the contractor or contractors who will be appointed by the Parties to undertake the Drainage Works. “Defects Certificate” means a certificate issued under the Works Contract to confirm that defects have been corrected or that there are no defects, on completion of the Defects Correction Period. “Developer(s)” are those organisations proposing to develop the Developments Sites. “Flood Risk Management Fund” means the account set up and administered by the Agency to receive Developer and other contributions which will fund the Drainage Works. This fund is ring fenced and will operate on a non profit making basis. “Development Sites” means the sites shown on the figure in Appendix 1. “Drainage Works” means the works detailed in Appendix 2. “Regeneration Framework” means the strategy to attract new economic investment and diversify the economic base for Corby, which is lead by CC. “Works Contract” means the ECC Conditions of Contract current at the time. 3. Statutory Instruments

1. Water Resources Act 1991; Section 111 Local Government Act 1972 and all other enabling powers including the Local Government Act 2000; Town and Country Planning Act 1990 (others??) are relevant to this Protocol.

4. Construction of Drainage Works

1. The Agency shall consult with the Council and CC on the employment of a project manager or equivalent to oversee the design and construction of the Drainage Works.

2. The Council will be responsible for serving all land entry notices under the relevant powers to enable the Drainage Works to commence and be completed including where necessary in the opinion of the Council the use of compulsory purchase powers.

3. The Parties will appoint a Contractor or Contractors to undertake the whole or part of the Drainage Works under the Works Contract(s) as may be the case. This is subject to the Flood Risk Management Fund having received the appropriate monies from the Developers to allow those works to take place.

4. The parties will allow the Contractor(s) all reasonable access to any area required for the Works Contract(s), subject to obtaining any necessary consent required in order to carry out and complete the Drainage Works subject to normal

supervision and inspection requirements of the Agency and the Council as land drainage authorities.

5. In accordance with the conditions set out in the Section 106 agreement the Developer(s) shall forthwith pay the sum required to the Flood Risk Management Fund to cover the cost of design, supervision and construction of the Drainage Works and land acquisition costs.

6. In accordance with the conditions set out in the Section 106 agreement the Developer(s) shall forthwith pay the commuted sums required to the Flood Risk Management Fund for the maintenance of the Drainage Works.

7. As part of the Drainage Works, a commuted sum for maintaining the Drainage Works shall be calculated over a 30 year maintenance period (or such other period as the Parties may agree). Following such calculation, the commuted sum shall be apportioned by the Agency between the Agency, the Council and any other relevant drainage authorities in relation to their responsibilities. The apportionment shall not be paid to the Agency or any other relevant authority until the adoption by them of the relevant Drainage Works. If the Parties agree that any part of the commuted sum is not required for the cost of the works, the unused part shall be repaid to the Developer(s).

8. Alternatively, the Developer(s) may wish to engage a third party to undertake the maintenance works for the specified period in which case sub-clause 7 above would not apply.

9. The Developer(s) and their successors in title in the Development Sites referred to in Appendix 1, shall have no financial liability for further Drainage Works provided that the development of the Development Sites is within the design parameters on which the Drainage Works have been constructed under the Works Contract(s).

10. Prior to the letting of the Works Contract(s) the Parties will consult on the Drainage Works to be carried out in accordance with those specified in Appendix 2. If the costs of the Drainage Works (following design and tender) are found to be above the sum allowed in the Flood Risk Management Fund the parties agree to meet and discuss the options for moving the project forward.

5. Programme The Parties will as soon as reasonably practicable devise a programme for the completion of the Drainage Works in consultation with the Developer(s). This will be based upon the outline programme given within the Corby Water Cycle Strategy Study. 6. Adoption of the Works The Agency or the Council will adopt the Drainage Works following the issue of a Defects Certificate under the Works Contract(s), subject to the approved design requirements of the Agency and the Council being met. Alternatively, the Developer(s) may wish to engage a third party to undertake maintenance works for the specified period in which case the above clause will not apply.

7. Acquisition of Land

1. The Developer(s) and the Council shall negotiate with the relevant land owners in order to secure the acquisition of any land necessary to facilitate the construction of the Drainage Works.

2. If the Developer(s) and the Council are unable to conclude successful negotiations in relation to any land required in connection with the Works Contract then the Council would only use compulsory purchase powers to secure such land as a last resort. The Developer(s) agree with the Council that if the Council uses its compulsory purchase powers to secure such land, then any reasonable legal and administration costs incurred by the Council shall be paid by the Developer(s) in addition to the commuted sums.

8. Recovery of Costs

1. On certification of completion of the Works Contract(s) and on payment of the total sums the Parties shall notify the Developer(s) of the sums expended on the Drainage Works and their maintenance.

2. If the Parties agree that any part of the original contribution is not required for the cost of the Drainage Works, the unused part shall be repaid to the Developer(s).

9. Statement of common intent

1. The Council in its capacity as a drainage authority and the Agency whilst operating within their statutory drainage duties will use all reasonable endeavours to assist the Developer(s) in a manner set out in this Protocol in relation to the Drainage Works in consideration of the funding and other steps to be taken by the Developer(s). However, nothing in this Protocol shall be or deemed to be a fetter on either the Council or the Agency in their capacity as drainage authorities or on the Council as a local planning authority.

2. Subject to the Council imposing a planning condition on any planning permission requiring that each Development Site will meet the Local Plan/Local Development Framework policy or provisional agreement requirement for the development not to proceed until the flood defence standard has been improved in accordance with the Corby Water Cycle Strategy (final report August 05), the Council and the Agency will withdraw any objections and will not make further objections to development proposals on the Development Sites on the grounds of land drainage, on the basis of the land drainage strategy and assuming no change in circumstances or material considerations.

10. Assignment None of the parties shall assign this Protocol except to a successor in title or any part thereof without prior written consent of the other parties. 11. Variation None of the Parties shall vary this Protocol or any part thereof without the prior written consent of all the other Parties.

Development Identified

Strategy Unaffected

Review against Strategy

DEVELOPER

<1 ha >1 ha

Conventional planning approval mechanism

Protocol / Checklist to Developers

CORBY BOROUGH COUNCIL

Meet CBC and EA to discuss

No impact on Strategy Potential impact on Strategy

Complete checklist and submit funds for review

Pay Review Cost

CONTRIBUTIONS

DEVELOPER

RING FENCED ACCOUNT

HALCROW

Review against Strategy

Proposals DO NOT meet Strategy

Proposals meet Strategy

Issue letter of compliance Return proposals with qualifications

Submit outline planning application and letter of

compliance to CBC

CORBY BOROUGH COUNCIL

Process planning application / issue S106 agreement detailing contribution, maintenance regime etc.

Developer pays contribution for strategic works /

maintenance

DEVELOPER Pay Contribution

Claw-back Mechanism for unused monies

RING FENCED ACCOUNT

DEVELOPER

CORBY WATER CYCLE STRATEGY

PLANNING AGREEMENT (LAND DRAINAGE) PROTOCOL FLOWCHART

Developer Checklist The following checklist summarises the high level measures that need to be taken to satisfy the Corby Water Cycle Strategy. In compiling this list reference has also been made to the “Milton Keynes & South Midlands Sub-Regional Strategy, Strategic Policy 3: Sustainable Communities” statement. Checklist Items Completed

Y/N?

Flood Risk Management

1 Confirm that there is no net increase in flood risk to upstream or downstream communities and provide evidence

Y/N

2 Run-off attenuated to greenfield rates i.e. 2l/s/ha or less

Y/N

3 Confirm Flood Estimation Handbook (FEH) methodology used to size any balancing facilities and river flow estimates (calculations must include adequate sensitivity tests to determine the effect of changing parameters)

Y/N

4

Details of attenuation measures (including maintenance requirements) and supporting calculations

Y/N

5 New river crossings/weed screens to be designed to minimise risk of blockage

Y/N or N/A

6 No development proposed on floodplain (referring to Environment Agency floodmaps)

Y/N

7

Confirm access to watercourses for maintenance and submit details

Y/N


8 Driveways and other suitable hard surfaces constructed from permeable paving

Y/N

9 Detail of additional SUDs proposals enclosed

Y/N

Water Supply and Sewage Treatment

10 Letter provided from water supply undertaker confirming demand can be met in accordance with the Water Cycle Strategy

Y/N

11 Letter provided from sewerage supply undertaker confirming demand can be met in accordance with the Water Cycle Strategy

Y/N

Water Consumption

12 Confirm development can meet water consumption target of 120l/h/d and enclose supporting details (e.g. proposals for water efficient appliances)

Y/N

13 Confirm that development will utilise rainwater harvesting from roofs (minimum tank size 2.5m3 per house)

Y/N

Green Infrastructure (developments > 100 ha)

14 Confirm 25% minimum of flood attenuation ponds/wetlands will be designed for multifunctional uses (access, footpaths, cycleways, recreational uses, etc) and submit outline details

Y/N

15 Details of additional green infrastructure proposals enclosed

Y/N

APPENDIX 1

Figure 1 Watercourse responsibilities Figure 2 Development Areas

APPENDIX 2

Schedule of proposed Works (for ALL the Water Cycle Strategy)

Weed and vegetation clearance, rubbish removal and dredging where necessary of existing ditches Raise low areas on existing flood banks Protect dwellings currently in flood plain with local flood defences Formalise and implement maintenance arrangements Refurbish existing flood control structures -Civil -M&E Clean up of Deene Park Lake

Channel enlargement Strategic flood banks Flood defences to new developments Flood storage reservoirs / washlands and associated structures Enlarge existing culverts Maintenance regime for new works SUDs -Roof drainage -Road drainage -Other Control access to watercourses Transfer of flood waters to other watercourses (pumps / pipes)

Low water pressure - refer to DG2 Register Ongoing maintenance/ asset replacement

Increase output at Wing WTW Trunk main connections New mains (20km) Upgrade Booster Pumps to Beanfield Tower 2 new Booster Pumping Stations New 15Ml storage reservoir at Beanfield New metering, controls, connections, telemetry etc

Improvements currently required

Strategic improvements required

Improvements currently required

Strategic improvements required

WATER SUPPLY WATER SUPPLY

FLOOD DEFENCE WATER SUPPLY AND SEWERAGE

SEWERAGE SEWERAGE

Sewer flooding - refer to DG5 Register Ongoing maintenance/asset replacement

Expansion of Corby STW New STW to serve Priors Hall/Weldon and new pumping station/rising main west of Stanion Pumping station upgrades and on site storage 3 new attenuation tanks and 1.5km new sewers

COST £? £30.5M £? £41.5M

RESPONSIBILITY Environment Agency / Corby Borough Council

Developers via Strategy and on site works

AWS under AMP4 determination

AWS via Ofwat charging mechanisms

CHARGING REGIME EA / CBC maintenance budgets

Cost Apportionment mechanism and on site works

Statute

Statute

Corby Sustainable Development Strategy

Phase 1 Cost Apportionment Mechanism

Notes1) This cost apportionment mechanism will apply to all residential developments in excess of 10 houses that have NOT been identified in the Corby Borough Local Plan 10 June 19972) Industrial developments will be given the choice of attenutating to a 1 in 100 year standard on site or contributing to the Strategy via this mechanism3) This mechanism divides the cost of strategic flood defence and land drainage works between Developers in proportion to their individual impacts. 4) This mechanism does NOT apply to water supply and sewerage charges nor does it allow for maintenance of Developers' own flood mitigation works5) Other developments will be considered on a case by case basis, such as: amenities (parks etc), the A43 Link Road and Anglian Water (additional effluent discharges adversely affecting flood risk)

Strategic improvement costs are as follows:

No Cost (£) Description

1 61,000 Survey Works to enable strategic improvements

2 6,486,545 Additional flood storage

3 2,580,000 Channel/Culvert improvements

4 700,000 Strategic solution - trash screens to prevent blockage

5 500,000 Non-structural improvements

6 1,032,755 10% Contingency

Total 11,360,300

Note that study costs for the Phase 2 works are NOT included above as funds have already been agreed and allocated

Maintenance costs are considered separately but will be based upon a 30 year commuted sum (as recommended by PPG 25)and will be shared out between Developers in exactly the same proportion as the strategic works above

Residential Nos and Industrial Areas

Residential28,699 Houses on Catalyst Corby Dwelling Completions spreadsheet 12 July 2004 (2002 to 2031)5,072 According to Corby Borough Council's Urban Housing Capacity Study (derived from Local Plan document)

23,627 Remaining unallocated houses

Industrial204 Hectares of land to 2031253 Hectares of land in Corby Borough Council Local Plan 1997

See above for comments on industry above current allocation

Maintenance Costs

1) Verified against Northampton FAS maintenance costs at 1.2% of capital works value2) Assume 1.75% given disparate nature of flood defence structures and need for a management company3) The maintenance cost per house over a 30 year period (discounted at 6%) is £122.77

Phase 1 Cost per House

Phase 1 Cost Apportionment

Cost per industrial hectare 0

Cost per house 481

Maintenance per house 123

Total cost per house 604

Environment Agency Contribution

Responsibility for new main river lengths complete with maintenance budget (not included in above costs)Responsibility for existing main river lengths complete with maintenance budget (not included in above costs)

Corby Borough Council Contribution

Responsibility for all other water courses complete with maintenance budget (not included in above costs)

Phase 2 Cost Apportionment Mechanism

It is proposed that a mechansim is modelled during Phase 2 which assessed individual Developer's impact on the Corby Water Cycle Strategy.It will incentivise best practice and will assess developments against three key criteria:

1 Location2 Receiving watercourse3 Sustainability

The formula below gives outline details as to what is proposed

Cost Formula

Cost = {C1*C2*C3(Qd)/(Qt)}*Total Cost of Works

where:

C1 is a location co-efficient which assesses the impact of development location on the flood hydrographC2 is a watercourse co-efficient which assesses the capacity of the watercourse to accept the development outflowsC3 is a sustainability co-efficient which considers how well a Developer had adhered to the Developer Checklist in the ProtocolQd Equivalent run-off from specific development (impermeable area x rainfall rate) - (attenuated flowrate) - (losses from SUDs + losses from rainwater harvesting)Qt Total equivalent run-off from all developments Summation of all the Qd values

Coefficient Description Range Notes(to be confirmed)

C1 Location co-efficient Favours downstream developments with little or no impact upon flood risk in the town

C2 Watercourse co-efficient Favours lesser used watercourses with capacity to accept development

C3 Sustainability co-efficient Favours more sustainable developments over those who have done the minimum

Note that the final mechanism will be subject to scenario testing and may change

0.5 to 1.0 (tbc)1 is do minimum

0.5 to 1.0 (tbc)

0.75 to 1.0 (tbc)1 is worst case

1 is worst case

The following documents have been reviewed during the preparation of this Strategy:

General

• “The Regeneration Framework” – Catalyst Corby, July 2004

• “Corby – The Next Five Years” – Catalyst Corby, September 2004

• “Dwellings Completion Spreadsheet” – Catalyst Corby, 12 July 2004

• “Corby Borough Local Plan” – Corby Borough Council, 10 June 1997

• “Corby – Major Growth Options Study” diagram and “Latest List of significant applications” spreadsheet – Corby Borough Council, March 2005

• “Urban Housing Capacity Study” – Roger Tym & Partners, June 2005

• “Milton Keynes and South Midlands Sub-Regional Strategy” – Office of the Deputy Prime Minister, March 2005

• “Corby Utilities and Strategy Study – Final Study Report” – WSP, undated copy

• “A43 Corby Link Road” – Atkins, July 2003

• “North Northants Local Development Framework, Draft Local Development Scheme” – North Northants Joint Planning Committee, March 2005

• “Creating Better Places through Growth” – Office of the Deputy Prime Minister, January 2005

• “Milton Keynes and South Midlands Development Scoping Report” – Halcrow Group Ltd, May 2004

• “Business Case to the Office of the Deputy Prime Minister, Integrated Water Cycle Studies – Aylesbury and Corby” - Halcrow Group Ltd, April 2004

• “Milton Keynes and South Midlands Scoping Study, Scope of Works for Aylesbury and Corby – Appendix to the Business Case” – Halcrow Group Ltd, April 2004

• “Options for North Northamptonshire – Towards a joint Core Spatial Strategy” – North Northamptonshire Joint Planning Unit, June 2005

• Taylor Woodrow Developments Limited: Land West of Stanion R8, Corby Flood Risk Assessment and Water Quality and Hydrology.

• Planning Liason Consultation – Anglian, Northern. Priors Hall Urban Extension, Wardell Armstrong.

Flood Defence/River and Reservoir data

• “Strategic Flood Risk Assessment Stage 1 – Data Collection and Evaluation” – Bullen Consultants, March 2004

• “Strategic Flood Risk Assessment – Stage 2 Draft” – Bullen Consultants, April 2004

• “Flood Defence Strategy for Padholme Catchment” – Bullen Consultants, May 2003

• “Padholme Catchment Land Drainage Agreement” – Environment Agency, May 2003

• “Corby Area Flood Storage Reservoirs – Great Oakley” – Environment Agency, November 2003

• “Corby Area Flood Storage Reservoirs – Gretton Brook” – Environment Agency, November 2003

• “Flood Defence and Land Drainage Operational and Emergency Contact Arrangements, EA Anglian Region for area covered by Corby Borough Council, Issue 3.0” – Environment Agency, April 2005

• “Asset Condition Rating – Willow Brook” – Environment Agency, February 2005

• “Strategic Review of Development and Flood Risk – Nene Catchment Northampton and Upstream” – Halcrow Group Ltd, 1999

• “Making Space for Water – 1st Government response to Autumn 04 consultation exercise” – DEFRA, March 2005

• “Planning Policy Guidance Note 25 Development and Flood Risk” – Department of Transport, Local Government and the Regions, July 2001

Green Infrastructure

• “Planning Sustainable Communities – A Green Infrastructure Guide for Milton Keynes and the South Midlands” – Environment and Quality of Life Sub Group, April 2005

Sewage Treatment/Foul Network/Water Quality

• “Corby STW Kaldnes Reactors: Review of Process Headroom” – AWS, September 2004

• “Corby STW Process Audit “ – AWS, June 2003

• “A Water Quality Investigation of the Willow Brook, a River Contaminated with Domestic and Industrial Waste Water” – Imperial College, September 1995

Groundwater

• Geological Survey of Great Britain, Drift Sheet 171 (Kettering) - identification of Major and Minor Aquifers

• Memoirs of the Geological Survey of Great Britain: Geology of the Country around Kettering, Corby, and Oundle, 1963. Description of local and regional geology.

• National Rivers Authority, Groundwater Vulnerability Map Sheet 24 (North Northamptonshire and West Fens), 1994. Assessment of risks to groundwater from leaching contaminants.

• Environment Agency, Nene Catchment Abstraction Management Strategy (CAMS), 2005. Assessment of groundwater resources in the development area.

• Environment Agency website: What’s in your backyard? – Source Protection Zone information. Determination of potable water supply abstractions in the local area.


• Harvesting rainwater for domestic uses: an information guide, Environment Agency, July 2003

• Model agreements for sustainable water management systems: Model agreement for rainwater and greywater use systems, CIRIA, 2004

• Model agreements for sustainable water management systems: Model agreements for SUDs, CIRIA, 2004

• A Review of Published Material on the Performance of Various SUDs Devices prepared for the Environment Agency by Professor C J Pratt, Coventry University, December 2001.

Digital Data

In addition the following digital data were used as part of the Strategy development:

• SAR DTM/DTME tiles for the Corby area – Environment Agency, April 2005

• Flow data for the Willow Brook North, Central and Southern Arms, Harpers Brook, Gretton Brook and the River Welland – Environment Agency, June 2005

• AWS Sewer Network Model (Infoworks), April 2005

• Environment Agency floodmaps (internet site)

• ISIS models from on-going studies for the Willow and Harpers Brooks

Websites

• www.bedzed.org.uk BedZED housing development

• www.bsria.co.uk BSRIA – Building Services Research and Information Association

• www.cat.org.uk Centre for Alternative Technology – suppliers listings and advice

• www.ciria.org.uk/suds CIRIA initiative on promoting good practice within SUDs

• www.edenproject.com Eden project – a large scale rainwater harvesting system.

• www.environment-agency.gov.uk/savewater Environment Agency advice on saving water

• www.hockerton.demon.co.uk Hockerton housing development

• www.wras.co.uk Water Regulations Advisory Service

• www.eng.warwick.ac.uk/DTU/rwh/components4.html Research programme at Warwick University into rainwater harvesting

• www7.caret.cam.ac.uk/guide_suds.htm Engineering design for sustainable development

• www.suds-sites.net UK SUDs database

• www.rainharvesting.co.uk UK supplier of rainwater harvesting systems

• www.savewater.com.au Australian website on water saving devices

• www.atlantiscorp.com.au/projects Australian website giving examples of SUDs projects in Australia

• www.wsud.melbournewater.com.au Australian website providing manuals on water sensitive urban design

Appendix D Dwellings Completion Spreadsheet

DWELLING COMPLETIONSUpdated Rates - All Residential Sites12 July 2004

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 TOTAL2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031

A EXISTING ALLOCATIONS PA

A1 Oakley Vale 350 350 400 400 400 350 350 300 2,900A2 West of Stanion 100 125 125 150 150 100 50 800A3 Other Allocations & Permissions 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 27 627

B URBAN RESTRUCTURING PA

B1 Urban Capacity 25 25 25 25 25 50 50 50 50 50 50 50 50 50 50 51 75 75 75 75 75 75 75 75 75 75 1,426B2 Town Centre Living 50 50 50 100 100 75 50 50 50 50 25 50 50 50 800

C URBAN EXTENSIONS PA

C1 Priors Hall & Weldon 100 150 225 300 350 450 450 450 500 400 400 400 400 400 350 300 275 200 100 115 6,315C2 Oakley - Stanion Extension 100 250 250 250 250 200 200 300 300 350 400 400 450 425 375 4,500C3 Western Extension 100 175 255 250 250 250 300 300 300 325 350 350 400 450 450 500 500 500 543 6,548C4 Northern Extension 50 50 75 150 200 125 38 688C5 Southern Extension 50 50 50 100 150 150 100 100 100 100 100 100 100 100 45 1,395

Additional Capacity 200 500 500 500 500 500 2,700Annual Totals 400 400 450 450 700 800 875 875 875 1,050 1,055 925 975 875 1,025 1,151 1,150 1,175 1,200 1,045 1,100 1,250 1,200 1,190 1,240 1,475 1,493 925 875 500 28,699

CUMULATIVE TOTALS

A1 Oakley Vale 350 700 1,100 1,500 1,900 2,250 2,600 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900 2,900A2 West of Stanion 100 225 350 500 650 750 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800A3 Other Allocations & Permissions 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 575 600 627 627 627 627 627 627B1 Urban Capacity 25 50 75 100 125 175 225 275 325 375 425 475 525 575 625 676 751 826 901 976 1,051 1,126 1,201 1,276 1,351 1,426 1,426 1,426 1,426 1,426B2 Town Centre Living 50 100 150 250 350 425 475 525 575 625 650 650 650 650 650 700 750 800 800 800 800 800 800 800 800C1 Priors Hall & Weldon 100 250 475 775 1,125 1,575 2,025 2,475 2,975 3,375 3,775 4,175 4,575 4,975 5,325 5,625 5,900 6,100 6,200 6,315 6,315 6,315 6,315 6,315 6,315 6,315C2 Oakley - Stanion Extension 100 350 600 850 1,100 1,300 1,500 1,800 2,100 2,450 2,850 3,250 3,700 4,125 4,500 4,500C3 Western Extension 100 275 530 780 1,030 1,280 1,580 1,880 2,180 2,505 2,855 3,205 3,605 4,055 4,505 5,005 5,505 6,005 6,548 6,548 6,548 6,548C4 Northern Extension 50 100 175 325 525 650 688 688 688 688 688 688C5 Southern Extension 50 100 150 150 250 400 550 650 750 850 950 1,050 1,150 1,250 1,350 1,395 1,395 1,395 1,395 1,395 1,395 1,395 1,395 1,395 1,395 1,395

Additional Capacity 200 700 1,200 1,700 2,200 2,700Dwellings Completed 400 800 1,250 1,700 2,400 3,200 4,075 4,950 5,825 6,875 7,930 8,855 9,830 10,705 11,730 12,881 14,031 15,206 16,406 17,451 18,551 19,801 21,001 22,191 23,431 24,906 26,399 27,324 28,199 28,699Site Area 2 3 5 7 25 48 77 108 157 217 279 334 392 444 505 577 649 722 797 862 927 1,002 1,073 1,148 1,225 1,319 1,418 1,480 1,538 1,572

PER YEAR TOTALS

Dwelling Completions 400 400 450 450 700 800 875 875 875 1,050 1,055 925 975 875 1,025 1,151 1,150 1,175 1,200 1,045 1,100 1,250 1,200 1,190 1,240 1,475 1,493 925 875 500 28,699Land Take (Excluding Allocations & Restructuring) 2 2 2 2 18 23 28 32 48 60 62 55 58 52 62 72 72 73 75 65 65 75 72 74 78 93 100 62 58 33 1,572Average Household Size 2.48 2.46 2.45 2.44 2.43 2.42 2.41 2.40 2.39 2.38 2.38 2.37 2.36 2.36 2.35 2.35 2.34 2.34 2.33 2.33 2.33 2.32 2.31 2.31 2.31 2.31 2.31 2.31 2.31 2.31Population Growth (pa) 992 984 1,103 1,098 1,701 1,936 2,109 2,100 2,091 2,499 2,511 2,192 2,301 2,065 2,409 2,705 2,691 2,750 2,796 2,435 2,563 2,900 2,772 2,749 2,864 3,407 3,449 2,137 2,021 1,155 67,484Population Growth (cumulative) 992 1,976 3,079 4,177 5,878 7,814 9,922 12,022 14,114 16,613 19,123 21,316 23,617 25,682 28,090 30,795 33,486 36,236 39,032 41,467 44,030 46,930 49,702 52,451 55,315 58,722 62,171 64,308 66,329 67,484

Total Corby Population 53,000 53,992 54,976 56,079 57,177 58,878 60,814 62,922 65,022 67,114 69,613 72,123 74,316 76,617 78,682 81,090 83,795 86,486 89,236 92,032 94,467 97,030 99,930 102,702 105,451 108,315 111,722 115,171 117,308 119,329 120,484

Option Appraisal for the Corby Catchment

KeyViable Option for consideration into works programme (5 to 10 years). Assume 10 years programme at this stage.Possibly viable Option for consideration into works programme.Not Viable Option for the next 10 years.

No. Option Possible Locations Effect on flooding Environmental and General Impacts/Opportunities/Constraints

1 Creation of storage area(s).

2 Buy and demolish derelict houses,warehouses, factories,disused road or railways to create storage area.

3 Increase storage in existing washlands and natural floodplains by additional excavation or creation of bunds to use all available storage.

4 Allow flooding of forest areas in a controlled and distributed manner.

• Possible adverse effects on biodiversity of any existing woodland or forest area by allowing it to flood, particuarly if floodwater could carry pollutants (e.g. discharges from washing machinesand road runoff). • In general, not viable as increasing storage upstream will not always singularly solve the problem of flooding downstream, risk of adverse environmental effects of allowing forest to flood are too high. • Highly urban area downstream may prove option to be unfeasible.

5 Increase channel conveyance by channel de-silting.

6 Hard bed dredging.

Storage

Changes to the Watercourse

• Potential for significant biodiversity benefits, which could be gained by incorporating the flood plain and not compromise existing biodiversity interest. • Potential for wetland creation. • Possible adverse effects for the built environment, in particular dense urban areas.• Compensation payments required to landowners• Very significant areas of appropriate flood storage may be required• Large enough areas in appropriate locations may not be available • Environmental/legal issue of flooding playing fields and grazing land • Contamination constraints if a brownfield site e.g. old factory/warehouse/railway site.• On-line storage, which is already filled with water at the start of a flood event, will not provide any further capacity for flood storage• A change of flooding regime may affect type of plants grazed on by cattle, leading to devaluation of grazing land and possible breach of common law/statute.

• Access for construction will be a problem as watercourse located at the base of many properties in much of the catchment.• Could worsen conditions downstream if conveyance is increased unless combined with storage area.• May increase erosion of adjacent banks• Environmental impacts particularly on biodiversity and conservation



No. Option Possible Locations Effect on flooding Environmental and General Impacts/Opportunities/Constraints7 Re-profiling

8 Removal of concrete channels

9 Raised banks (sheet piling).

10 Raised banks (walls and earth banks).

11 Realign watercourse away from properties at risk (without increasing conveyance).

12 Realign watercourse and increase conveyance.

• As above• Conversion of concrete lined watercourse to wider natural channel could have significant environmental benefits. Could combine with cycle path along watercourse.

13 Revetment/replacement of concrete channels and culverts with, same size but, smoother surfaces to increase conveyance.

• As above

14 Redirection of flows by controlled flooding of car parks/roads.

• NB. Surface Water flooding is not Agency responsibility. Large car parks tend to located at superstores - they are unlikely to allow them to be flooded as there customers will have nowhere to park. Progressive solution - demarcating areas as flood plain, even if it means sacrifice of lower value property (i.e. cars instead of housing) is now being proposed in the UK and Europe.

15 Catchment Transfer - transfer excess flow during flood from one catchment to another.

• Changes to hydrology of adjacent catchment• Adjacent catchment may not be of sufficient capacity

Transfer of flood waters

• Adverse impacts in terms of landscape, aesthetic and recreational value• Can provide a physical barrier to movement• Impacts associated with construction period • Access for construction will be a problem as watercourse located at the base of many properties in much of the catchment• Loss of natural riparian corridor• Loss of natural banks• Compensation required for landowners regarding land take.• Potential for negative impacts on archaeology • Potential for negative impacts on biodiversity and local ecology

• Environmental impacts particularly on biodiversity and conservation. • Potential impacts on archaeology• Constraints of existing infrastructure• Unsustainable and also unlikely to be cost effective • Adverse visual/aesthetic impacts for works.• Waste disposal issues



No. Option Possible Locations Effect on flooding Environmental and General Impacts/Opportunities/Constraints16 Discharge flood waters into

proposed Thames water sewers.

• Thames Water highly unlikely to agree to fluvial water being discharged in sewers. Sewers would need to be designed for extreme fluvial flood event - likely to be very expensive. • Potential impacts on ecology and conservation

17 Construct tunnels to divert peak flow away from flood risk areas.

• Potential impacts on ecology and conservation• Potential technical problems including flow, velocities, weirs and locks. • Will be very expensive - estimated standard of protection at Lewisham is 1 in 20 year flood therefore costs are likely to exceed benefits.

18 Inform all riparian owners of their legal duty to maintain the watercourse.

• Limited viability unless combined with other schemes e.g. conversion of concrete channel to natural channel as done on the Quaggy.

19 Start legal proceedings now to force riparian owners to repair walls in danger of collapse.

• The NRA (now the Agency) used to adopt this route but abandoned it in the late 1980's as it proved unviable - the legal process proved costly, and often culminated in the landowner proving an inability to pay due to the cost of such works (£1500 to £2500 /linear metre run). Leaving the NRA to pay all legal costs and the cost of repair/replacement.• In general not viable as proven not to work in the past and costs of legal action could far exceed benefits.

20 Purchase land alongside watercourses to enable Agency to carry out routine maintenance of the watercourse.

• The Agency already has powers to enter land to carry out inspection, repair and/or reconstruction under their Statutory powers. However access could be increased if combined with conversion of watercourse from concrete channel to natural channel? Bank ownership also presents opportunities for channel/bank improvement for environmental benefit. • Could be viable - however likely that costs will far exceed benefits due to legal costs and purchase of land.

21 Use Compulsory Purchase Order to demolish houses in the floodplain

• Could be very expensive in terms of legal costs, compensation costs and buying up of property and land. Risk of public outcry. Could however set a precedent for truly sustainable strategies. • Not viable - houses cost from £300k to £1M. Purchase of 15 or so houses would probably lead to an un-beneficial C/B ratio.• Opportunities for habitat creation

22 Buying up of properties which flood frequently and appropriate re-use.

• Very few properties have flooded recently, however many properties at risk from blockages. • Not viable - houses cost from £300k to £1M. Purchase of 15 or so houses would probably lead to an un-beneficial C/B ratio. It could be cheaper to let properties flood - having forewarned the landowners first.

23 Prompt repairs of cracks and regular monitoring (CCTV surveys) and frequent clearance of debris at key locations.

• CCTV survey already underway. • For culverts, it may be more beneficial to replace them when considered as part of a strategic solution (e.g. storage and increased conveyance).

24 Replacement of culverts to increase conveyance, reduce risk of blockage and minimise need for trash screens.

• As above • More appropriate to urban lower reaches of the catchment where consequences of flooding from blockages are higher.• Not always sustainable, opportunities for replacement can exist although dependant on location and requirement of the culvert.

25 Replacement of walls that are at risk of collapse.

• Should be paid for by riparian owner. Number of options - work carried out and paid for by owner, work carried out by Agency but paid for by owner, • Could require legal proceedings by Agency to force riparian owner to pay for repair.

26 Redesign/replace trash screens to minimise maintenance.

• Already on-going programme to replace trash screens

Legal

Maintenance/Repairs/Replacements



No. Option Possible Locations Effect on flooding Environmental and General Impacts/Opportunities/Constraints27 Re-lining of culverts and walls

with smoother surfaces.• Could be more cost effective in the long run to replace. • Major H&S implications associated with resurfacing culverts. • Poor habitat for biodiversity

28 New manholes to allow access to culverts for de-silting.

• Major H&S implications of entering culverts to carry out work.

29 Reduce sediment transport. • Unlikely to have a significant effect on flood levels however could increase the life of the wetland and have significant environmental benefits. Combine with other schemes.• Need to establish extent of existing problem. Hydraulic modelling of sediment transport to assess effect on flood levels. Investigate methods of reducing sediment and costs.

30 More regular cutting back of undergrowth.

• Will increase life of concrete walls as saplings growing on the tops of concrete walls undermine stability. Will need to combine with improved access and health and safety. • Viable if combined with increased access. • Need to establish extent of existing problem. Hydraulic modelling to assess effects of increased conveyance. Detailed benefit/cost analysis. • Landscape and recreational user (e.g. angling) impacts

31 Restriction of post-development flows and volumes to pre-development figures

32

Permeable paving for new developments and/or existing hard standing/tarmac areas

33 Grants for greywater re-use

34 Policy of preventing culverts from being reduced in size when they are re-lined or replaced.

• Surface runoff is not Agency responsibility but the Agency needs to influence on 'good practice'. • Policy Change• Opportunities for habitat enhancement

35 Ensure all roads drains are of sufficient capacity.

• Policy change - Agency to ensure Thames water carries these works.

36 Install pumps to pump excess surface water - could be permanent or temporary installations.

• As above • Could worsen the situation elsewhere. • Not an environmentally sustainable solution. • Unlikely to be viable • Cost of pumping

37 Replace all badly designed/placed/old outfalls

• As above • Thames Water responsibility or potentially a combined scheme• Policy change - Agency to ensure Thames Water carry out the work.

38 Remove or re-locate pipe crossings across watercourses that cause blockages.

• Thames Water responsibility or potentially a combined scheme • Need to assess the increased risk of flooding due to this. Give high priority if this is significant.significant, even if it is not Agency responsibility.

39 On-going programme to locate and eliminate illegal connections between surface water and foul sewers.

• Surface runoff is not Agency responsibility but the Agency needs to influence on 'good practice'. Thames Water responsibility or potentially a combined scheme.• Policy Change

Flood Warning/temporary flood defences

Sustainable Urban Drainage Systems (SUDS)

Surface Drainage Improvements



No. Option Possible Locations Effect on flooding Environmental and General Impacts/Opportunities/Constraints40 Increase in flood warning

sites.

41 Flood warning at sites where there is risk of blockage.

42 Installation of flood protection mechanisms in commercial and residential properties.

• As above • Payment/funding problems • Could be combined with increase in flood warning sites and awareness of flooding among public. • Viable however unlikely to have a significant effect - not many houses that flood regularly.

43 Removable bank walls at hot spot locations.

• House owner will have to install where property backs onto watercourse. Could be combined with increase in flood warning sites and raising awareness of flooding among public. • Could be used at the downstream reaches of the catchment (Lewisham?).• Problem of where to install bank walls - at places prone to surface water flooding, within flood extents, or those sites at risk of flooding from blockage.• Consequences of failure or overtopping can be catastrophic.

44 Increase public awareness about importance of watercourses e.g as habitats, impact of trash on flooding, blockages and cost of maintaining watercourse.

• Unlikely to have a significant effect unless combined with opening up the watercourse. If public don’t have access to the watercourse they're unlikely to see the benefits of maintaining it.• Education opportunities.• Opportunities for increased ‘ownership’ of a community watercourse

45 Opening up of Watercourse. • Will have to ensure health and safety is maintained. E.g. No steep banks, graded channel. • Sustainable solution• Biodiversity and Conservation benefits• Improved recreational access

46 Reduce road access to watercourse to prevent dumping of trash from vehicles into watercourse.

• Must not reduce recreational opportunities.• Dependant on the number of road access points.

Reducing Dumping of Trash in the Watercourse

• Already a good network of flood warning sites. Could be combined with providing flood protection mechanisms on properties with high potential damages (supermarkets, etc). • Flood impacts (environmental and economic) if lowered, still remain.• Reliable - accurate forecasting and dissemination is necessary• A member of the individual household may not be available to be warned• Individuals may not be physically able to respond to the warning• Individuals may not know how to respond effectively.• Could create unnecessary alarm if public are continually warned but no blockage occurs. Public could become complacent.

Appendix F Technical Notes Impacts of forecast population increase on Corby STW Technical Note

Sustainable Urban Drainage Design Options Technical Note

Technical note Page 1

Corby Water Cycle Study Impacts of forecast population increase on Corby STW

Project Corby Water Cycle Study Date

Note Impacts of forecast population increase on Corby STW

Ref

Author Andrew McConkey, Halcrow Group Ltd

1 Water quality, foul flooding and wastewater treatment 1234

Corby is a separately sewered catchment, with the majority of flows draining by gravity to Corby STW. Great Oakley catchment in the South West, with an approximate connected population of 8100, is pumped by Great Oakley SPS, and the Weldon area east of the STW is pumped by several pumping stations (see Figure 1 at rear of report).

Whilst the system is a separately sewered catchment, there is a significant storm response in the network providing evidence of either misconnections of roof drainage and/or highway drainage to the foul network, or sewer dereliction causing rainfall related infiltration. Hence the sewerage system shows some of the characteristics of a combined system, namely risk of foul flooding and wet weather storm sewage discharges.

Whilst the new development areas will initially be entirely separately sewered, it is important to accept that over time the system will start to deteriorate, and misconnections are inevitable, hence some allowance must be made for rainfall related runoff from these areas entering the sewerage system.

In order to identify what impact the population expansion may have on the sewerage system, particularly with respect to foul flooding and storm discharges, it is first essential to characterise the current performance of the system and identify any constraints. From this it is then possible to assess how to improve the network to maintain the same level of service, or even improve the level of service to higher design standards. This assessment looks at the constraints of the current sewerage system and the options for sewage disposal and treatment to meet future design standards.



2 Constraints of current system

It is expected that a sewerage system be able to convey a minimum of 3 times dry weather flow to the STW with minimal surcharge to ensure that dry weather flow during peak times can be carried by the system. If there is a storm response, then the system will need to convey significantly more than this to ensure that there is no risk of foul flooding or of pollution of watercourses from storm discharges.

The verified model of Corby has been assessed with respect to peak dry weather flow conditions and with respect to its response in storm events to identify ‘at risk’ areas.

2.1 Peak (x3) dry weather flow capacity

The model was run in design mode, with the design baseflow increased threefold for twenty four hours. There are several small sections of sewer that surcharge by flow (shown as blue pipes in Figure 2) which highlight potential risk areas.

2.2 Storm response

(a) Foul flooding The minimum level of service deemed to be acceptable in the future is ‘no foul sewer flooding in a 30year return period storm’.

A series of FEH design rainfall events were generated using the Infoworks rainfall generator to determine the critical duration rainfall event for the Corby catchment. The event that produced the greatest extent and volume of foul flooding was 480 minutes in duration with a Return Period of 30 years. This event was selected as the minimum design criteria.

Figure 3 shows the locations where significant foul flooding is predicted for a 30yr return period event.

• The Pen Green area will not be affected by any predicted increase in growth, and the model is not accurately verified in this area, therefore this area has been ignored for the remainder of the study.

• Any development in the North West area of Corby will cause an increase in the risk of foul flooding in a 30 year return period, in an area where foul flooding is already predicted to occur.

• The area immediately upstream of the treatment works is also at risk of foul flooding in a 30yr return period event.



(b) Storm discharge There are three overflows in the catchment that operate at a return period of less than 30 years assessed by simulating a 30 year, 480 minute design FEH rainfall event. A 118 event time series rainfall dataset was provided by AWS. It was not possible to determine what period of time this dataset was representative of. However, the greatest rainfall event in the series has a total depth of 30.64mm. This compares to a 30.25mm FEH design rainfall for a 10 year 240 minute event. A storm spill analysis was carried on the network model to determine the number spill frequency and volume at each of the overflows, shown below in Table 1.

Table 1 Spill analysis for storm overflows in Corby

Spill frequency (no) Spill volume (m3)

Great Oakley PS 0 0

Pen Green PS 5.0 2459.1

Weldon PS 1.0 34.5

STW storm tank 5.0 2464.0

2.3 River quality

Corby STW discharges to the Sotuhern Stream of the Willow Brook. The river quality objective (RQO) both upstream and downstream of the STW is RE4, and the river currently meets its RQO both upstream and downstream of the works. In fact, the river significantly outperforms its target, with both upstream and downstream recording a General Quality Assessment (GQA – the Environment Agency’s routine monitoring program) of GQA B, equivalent to an RQO of RE2.



3 Scope and constraints for population expansion to 2021

The population forecasts for 2021 have been added to AWS’ model with an impermeable area allowance for sewer deterioration and surface water misconnections. Figure 4 shows how the population has been applied to the new catchment areas based on the population forecast.

The new population areas were added as large subcatchments with boundary and total area covered taken from GIS. The natural drainage connection direction was determined using a 3D visualisation of the existing Infoworks network with a ground model (based on Environment Agency’s SAR data) superimposed. This allowed the most likely gravity drainage direction to be determined and route of connection to the existing system be determined.

Where a large new development area had to be subdivided based on the natural drainage, the population apportioned to each of the subcatchments was determined by the surface area of that catchment as no more detailed information was available.

Figure 5 below shows the 3D visualisation including the new catchments.



Figure 5 Topography of Corby catchment

An allowance had to be made for an impermeable area in each of the new catchments. Even though the catchments will be separately sewered, over time there will be an inevitable creep in the impermeable area connected as misconnections and sewer dilapidation begin to occur. A conservative modelling estimate for separate sewers is that 2% of the total catchment area will in the fullness of time contribute runoff to the sewerage system. It has been assumed that by 2031, 2% of the total new catchment area will contribute runoff to the sewerage system. This is almost certainly a conservative assumption, as the level of misconnection or dilapidation to cause this amount of runoff would be unlikely to occur in this time frame. This figure was then factored by the proportion of the total development in 2031 has been completed by 2021 to give a runoff for the 2021 model.

3.1 Storm response

(a) Foul flooding



There are no new areas at risk of flooding in a 30 year design event created by the predicted developments to 2021. However, the areas already affected will be subject to slightly higher risk of flooding, and the predicted severity of flooding for a 30year event as measured by flood depth and volume increases. There is not predicted to be any increase in the extent of risk of foul flooding by 2021, although if population growth expands as predicted with no improvements in infrastructure, there will be an increase in the severity (depth and volume) of the flood.

Figure 6 below shows the likely extent of flooding in a 30yr design rainfall event for the forecast population growth to 2021. Figure 7 shows in more detail those areas most likely to be affected in the Cottingham Road area.



Figure 6 Likely locations of foul flooding from a 30year design event

Pen Green Area PS

Cottingham Road

Medina Road

Beanfield Ave/

Newark Drive



Figure 7 Cottingham Rd area flooding detail

165_002

400_160

400_170

400_200

400_310 40

430_080

430_100

430_110

430_130

440_110

440_120

440_130

487_000

492_000

495_025

495_050

495_060

495_070

497_020

SP87887903

SP87899001

SP87899002

SP88880801

SP88882801

SP88882802

SP88882803

SP88882901SP88883903

165_002

400_160

400_170

400_200

400_310 40

430_080

430_100

430_110

430_130

440_110

440_120

440_130

487_000

492_000

495_025

495_050

495_060

495_070

497_020

SP87887903

SP87899001

SP87899002

SP88880801

SP88882801

SP88882802

SP88882803

SP88882901SP88883903

(b) Storm spill analysis

The storm analysis below uses the same 118 storm events as used for the baseline spill analysis. The assessment shows that the increase in foul flow has little impact on the number of spills from the intermittent discharges in the catchment.

The spill regime is compared with the baseline in Table 2.



Table 2 Impact of 2021 population on storm discharges

Current frequency

Current spill volume

2021 if PS not upgraded

2021 spill if PS not upgraded

2021 if PS Upgraded by 3DWF

2021 spill if PS Upgraded by 3DWF

Pen Green PS 5.0 2460 5.0 2460 5 2460

Weldon South PS 1.0 34.5 1.0 35.3 1 31

However, the capacity of the system to accept storm sewage is inevitably decreased by the addition of foul flow without upgrading the PS, meaning an increased risk of either storm sewage discharging to local watercourses or an increase in the risk of foul flooding. In Corby, the overflows from PS in catchment are really emergency overflows and not designed to provide hydraulic relief in smaller storm events. For example Great Oakley PS does not have any storm spills for the ten year rainfall series simulated. However, a 15 year 480 minute design event whilst not causing a spill with the present population, does cause a spill when the additional population to 2021 is added if the PS is not uprated (see Figures 8 and 9 below). If the PS is upgraded from 64l/s to 94l/s (equivalent to the peak flows from the new population), there is no spill during the same design rainfall event.

Therefore, it is important that all PS are upgraded as a minimum by a rate equivalent to 3DWF to prevent this deterioration.



Figure 8 Great Oakley PS – spill volume for current population in a 15 year 480 minute design rainfall event

Figure 9 Great Oakley PS – spill volume for 2021 population in a 15 year 480 minute design rainfall event



3.2 Key constraints to development to 2021 population

(a) Pumping station capacity The new pumping stations (West of Stanion and Priors Hall) will only convey foul flows for the immediate future, therefore as stated above, need to be sized to a minimum of 3DWF for the population. However, in order to look strategically to the future, a long term view must be taken to providing Formula A pump rates and the level of storage detailed. Whilst these will not be needed by 2021, a Formula A equivalent pump rate is the greatest that could be required long term to ensure that the PS can cope with future rainfall runoff expected from older sewerage networks.

The storage was calculated at 6hours storage at dry weather flow.

West of St

West of Stanion/Oakley Stanion extension

No development can take place in this catchment without a new foul sewage pumping station and rising main being constructed.

The most direct connection to the existing network is a direct connection to the STW would be for rising main running north (approximately 1km length) followed by a new gravity trunk sewer from the end of the rising main to the STW as shown in Figure 10.

Table 3 PS parameters for 2021 population

Population DWF (l/s) 3DWF (l/s) Formula A (l/s)

Storage (m3)

West of Stanion

4,998 12.15 32.98 97.77 265

Priors Hall 13,338 26.72 70.85 244.4 577 Great Oakley PS (increase)

4,220 29.47 84.17 233

CTC pumping station (increase)

3,451 9.46 26 70.73 200



Figure 10 New West of Stanion PS

Priors Hall/Weldon

A new PS is required to serve the development at Priors Hall and Weldon. Whilst some of the development may be able to be accommodated by the existing Weldon North and South pumping stations, not all of it can gravitate to either of the PSs.

A rising main of approximately 3.5km would be required, with a vertical lift of around 10m.

Southern Extension & south of Western Extension

Both of these areas drain to the existing Great Oakley PS which would need upgrading by a value equivalent to 3DWF.

Oakley Vale North

This area drains to the Corby Technical College (CTC) PS, which would need to be upgraded by 3DWF of the new population (from 30 to 45l/s)

Remaining gravity drainage catchments

New West of Stanion PS

New rising main (1km length, 43m lift)

New gravity sewer (approx 1km length, 400mm diameter)



Development in any of the gravity drained subcatchments will cause an increase in the theoretical risk of foul flooding in the areas already identified to be at risk from foul flooding. However, the ‘trigger’ normally used to identify when a section of sewerage network needs to be reassessed to prevent the risk of foul flooding is when there is a risk of internal foul flooding from a 1 in 5 year design rainfall event. There are no areas that are pushed through this trigger level by the new developments (Pen Green area is predicted to flood during a 5year return period rainfall event but is not affected by new development).

Cottingham Road area, which has a history of combined surface water and foul flooding, does not predict flooding during a 5 year rainfall event, although the extent and duration of flooding for a 10year return period event is increased by the new properties. Figures 11 and 12 below show how the new population increases the depth, volume and duration of flooding manhole SP87887903 for a 10year 480 minute rainfall event.

Figure 11 Predicted flooding with 2021 population at Cottingham Rd (MH SP87887903) for a 10yr480 minute rainfall event



Figure 12 Predicted flooding with current population at Cottingham Rd (MH SP87887903) for a 10yr480 minute rainfall event

Sewage treatment works

DWF for the new population to 2021 is 19,112m3/d

Flow to full treatment (at 3DWF) would need to be provided for 531l/s, which is although is higher than the current consented flow is less than the actual flow to full treatment at present1.

The storm tank size based on 68 l/hd as required by current EA policy is 6,600m3. Current storm tank size is approximately 3000m3. The exact current storm tank capacity has not been possible to determine because of the recent construction of an additional storm tank. The previous capacity was 2125m3, and according to the works manager this has been increased by a factor of 50%, hence the assumed 3000m3.

If the population expansion continues unchecked until 2021 without any improvements at the STW, there is a small impact on the predicted spill

1 Modelled flow to full treatment as provided in the AWS model is 650l/s. This was confirmed as the peak pumping rate to full treatment by the works manager during a site visit.



regime from the STW storm tanks, primarily because the FFT is substantially higher than 3DWF (see Table 4 below).

If storage is increased to 6,600m3 and FFT is kept at the current rate, there are no storm discharges from the treatment works up to 2021.

Table 4 Impact of population expansion to 2021 on STW storm tank spill regime

Baseline pop – spill frequency

Baseline pop – spill volume

2021 pop – spill frequency

2021 pop – spill volume

2021 pop with new storm tank volume Spill frequency

2021 pop with new storm tank volume Spill volume

5.0 2464.0 7 4,031 0 0

Figure 13 (rear of report) provides a summary of the infrastructure requirements to 2021.



4 Predicted population to 2031

The final projected population for 2031 was added to the Infoworks model, using the same apportionment method as before where a catchment had to be subdivided. The population forecast by subcatchment is shown in Figure 13 at the back of this report.

The impermeable area for each of the new subcatchments, with the exception of the town centre and urban capacity subcatchments, was increase to 2% of total catchment area. The verified model already allows for impermeable area connections to the network in the town centre area, and it is likely that redevelopment will reduce this impermeable area not increase it.

4.1 Peak (3DWF) capacity

Surcharge under peak dry weather flow conditions across the whole network for the 2031 population is shown in Figure 13 (back of document). There are no issues with peak DWF being conveyed by the existing drainage network.

4.2 Storm response

(a) Foul flooding

The additional population from the western extension creates a new foul flooding issue at the Junction of Gainsborough Road Jubilee Road adjacent to the school as shown in Figure 15 below. Flooding in this area is only occurs at the 1 in 30 year event, and not at any of the smaller return period events. Therefore the population increase does not cause foul flooding at this location at the ‘trigger’ level of 1 in 5 year return period event.



1314

Figure 15 Flooding at Gainsborough Road

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The areas already at risk of flooding (at baseline population and in 2021) will be subject to slightly higher risk of flooding, and the predicted severity of flooding for a 30year event as measured by flood depth and volume increases.

For example at Medina Road, the flood hydrographs for 2006, 2021 and 2031 are shown in Figures17 to 19below. The total flood volume and depth for the three scenarios are given in Table 5 below.

The predicted location of flooding is shown in Figures 16, 20 and 21.



Table 5 Total flood volume and depth at Medina Road Flood volume (m3) Flood depth2 (m)

Current 76 0.26

2021 95 0.19

2031 146 0.45

2 This flood depth is very dependant on the floodable area for the node. This appropriateness of this figure has not been verified, therefore these values must be taken as comparative rather than absolute.



Figure 16 Location of flooding at Medina Road

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Figure 17 Predicted Flood volume and depth at Medina Rd for baseline population for an M30-480 FEH design rainfall event



Figure 18 Predicted Flood volume and depth at Medina Rd for 2021 population for an M30-480 FEH design rainfall event

Figure 19 Predicted Flood volume and depth at Medina Rd for 2031 population for an M30-480 FEH design rainfall event



Figure 20 Predicted flooding for an M30-480 rainfall event for 2031 forecast population at Collingwood Road.

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SP87887903

SP87899001

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SP88880801 SP88881802

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SP88884801

Figure 21 Predicted flooding for a M30-480 rainfall event for Newark Rd area for 2031 population



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(b) Spill analysis

The increase in foul flows and runoff catchment will have an impact on the number of storm spills from the intermittent discharges if there is no improvement in the infrastructure.

The spill regime is compared with the baseline in Table 6.

Table 6 Spill analysis for 2031 forecast population

Current frequency

(No)

Current spill volume

(m3)

Scenario 1

Spill frequency

(No)

Scenario 1

Spill volume

(m3)

Scenario 2

Spill frequency

(No)

Scenario 2

Spill volume

(m3)

Pen Green PS 5.0 2459.1 5.0 2459.1 5

Weldon South PS 1.0 34.5 1.0 34.5 1



Great Oakley PS 0 0 0 0 0

CTC PS 0 0 0 0 0

STW storm tank 5.0 2464.0 12.0 13504.3 0

Scenario 1 – Pumping stations not upgraded to account for extra baseflow, FFT and storm tank size at STW kept at current

Scenario 2 – Pumping stations upgraded by a rate equivalent to 3DWF, and FFT and storm tank size at works increased to the 2031figure.

4.3 Key constraints to development to 2031 population

(a) Pumping station capacity The new pumping stations (West of Stanion and Priors Hall) will only convey foul flows for the immediate future, need to be sized to a minimum of 3DWF for the population. However, in order to look strategically to the future, a long term view must be taken to providing Formula A pump rates and the level of storage detailed.

West of Stanion/Oakley Stanion extension

The capacity at the new West of Stanion PS needed for the 2031 population needs to be increased to allow for the increased population at Oakley Stanion Extension.

Priors Hall/Weldon

The capacity at the new Priors Hall PS needed for the 2031 population needs to be increased.

Southern Extension & south of Western Extension

Both of these areas drain to the existing Great Oakley PS which would need to be uprated.

Corby Technical College (CTC) PS



The capacity at the CTC PS needs to be uprated to accommodate the increased population.


Rising main Storage (m3)

West of Stanion

4,998 12.15 33 97.77 1km, 43m static head

265

Priors Hall 13,338 26.72 71 244.4 3.5km 10m static head

577

Great Oakley PS

4,220 Inc from 64 to 94

Inc. from 94 to 178

750m, 32m static head

Inc. from 200 (current) to 250

CTC pumping station

3,451 9.46 Inc from 30 to 56

Inc. from 56 to 120

1.5km, 23m static head

In from 30 (current) to 200

Remaining drainage catchments

Development in any of the gravity drained subcatchments will cause an increase in the theoretical risk of foul flooding in the areas already identified to be at risk from foul flooding. However, the ‘trigger’ normally used to identify when a section of sewerage network needs to be reassessed to prevent the risk of foul flooding is when there is a risk of internal foul flooding from a 1 in 5 year design rainfall event. There are no areas that are pushed through this trigger level by the new developments (Pen Green area is predicted to flood during a 5year return period rainfall event but is not affected by new development).

Cottingham Road area, which has a history anecdotally of flooding, does not predict flooding during a 5 year rainfall event, although the extent and duration of flooding for a 10year return period event is increased by the new properties.

Table 8 STW requirements to 2031

P PE (load)

DWF 3DWF Peak Storm tank size

Consent

BOD/Ammonia

Table 7 PS requirements for 2031 population



Corby STW

120484 336 783 2.4 8193 RE4 12/4

RE3 9/2

RE2 6/1

Sewage treatment works

DWF for the new population is 29,104m3/d

Flow to full treatment at 3DWF would need to be provided for 783l/s.

The storm tank size based on 68 l/hd is 8193m3 which prevents any modelled storm discharge to the river in the top 118 events in 10 years. In order to prevent foul flooding in the immediate upstream catchment, the inlet works would have to be hydraulically capable of handling peak flows of 2.4 m3/s.

The current downstream RQO is RE5, although this is unlikely to be a satisfactory target, therefore effluent quality to meet RE4, RE3 and RE2 have been provided.

In order to prevent any deterioration in the current river quality, the STW would need to treat to an effluent quality of 6mg/l BOD and 1mg/l Ammonia as a 95 percentile standard. Whilst no account has been taken of likely future standards under the Water Framework Directive, and whilst standards cannot be assumed at this stage because of the difficulty equating a biochemical water quality target with an ecological target, it is likely that RE2 will be the minimum standard acceptable, and a 6/1 effluent quality would meet an RE2 objective.

It must be noted however, any increase in flow at the STW may need to be mitigated with an increase in flood protection on the river.

4.4 Long term outline solutions to foul flooding

As has already been stated, foul flooding from the sewerage system is not a critical issue in Corby when using the standard levels of service. None of the new development would cause the risk of foul flooding to trigger investment when using the standard water company level of service. However, to ensure that Corby is protected to a 30 year design level of service, the following improvements will be needed.

The cost estimates are average estimates extrapolated from the cost estimate mechanism in sewers for adoption (attenuation tanks), and from a water company spreadsheet used to determine the cost of new pipework.



(a) Cottingham Road area In order to achieve a 30year return period storm design for the Corby sewerage system the conveyance at and around Cottingham Road where the foul flooding is most extreme would need to be increased, either by removing the two parallel sewers and flow control device and replacing with a larger diameter pipe. This would require extensive civil works under a section of main road. The length of road affected would be approximately 1.5km and is shown below.

Estimated cost £3.0 M

(b) Newark Road area Initial indications are that the flooding in Newark Rd area would be relieved by the Cottingham Rd solution. However, if the detailed design modelling did not alleviate the risk of flooding, an attenuation tank in the Newark Rd area of approximately 1,500m3 would be required.

Estimated cost £1,5M

Figure 22 Location of likely sewerage improvements at Cottingham Road



(c) Medina road area 30 year foul flood protection for the Medina Road area could be achieved with an attenuation tank of approximately 1,000m3 volume.

Estimated cost £1M

(d) Gainsborough Rd area This level of protection could be achieved in the Gainsborough Road area with an attenuation tank of approximately 2,000m3 volume.

Estimated cost £2M

5 Alternative option 1 – New STW at Priors Hall serving new properties at

Priors Hall & Weldon

An alternative option to pumping the new foul flow from the Priors Hall and Weldon areas back to the existing treatment works, would be to provide a new local treatment plant serving this area. A likely discharge point, identified from the natural drainage of the existing catchment is shown in Figure 23 below.



Figure 23 New STW serving Priors Hall

The benefit of this solution would be that it reduces the pressure on capacity (both hydraulic and process) at Corby STW. In addition, more ‘sustainable’ drainage removes the need for abortive pumping from the Priors Hall area back upstream to Corby STW.

The disadvantages are that there would be more treatment works to operate and maintain and associated problems of new STWs such as the long lead in time needed to overcome local opposition and associated costs of public enquiry . There is also the issue of underload to the plant design capacity as development proceeds.

Table 9 STW parameters P PE

(load) G DWF 3DWF Peak Storm

tank size

Consent

Corby 104,527 205,426 310 713 2.1 7107 (RE3)

Priors Hall STW



STW 10/2.5

(RE2) 6/1

Priors Hall

15,957 15,957 120/145 26 67 0.4 1085 (RE3) 14/3.5

(RE2)8.5/1.5

Peak flows during a 30yr FEH rainfall event at Priors Hall, if it assumed that 2% of the drainage catchment will contribute runoff to the system in future years, would be in the order of 400l/s therefore the inlet works would need to be able to cope with flows up to this figure to prevent any risk of flooding.

Assuming a minimum storage of 68 l/hd would require a tank of 1085 m3.

This option does not reduce the risk of flooding (for a 30year design event) at any locations other than at the STW inlet, therefore the suggested improvements to secure 30yr protection for option 1 are still needed.

If Corby STW is treated to meet an RE4 objective in the downstream Willow Brook, The Willow Brook downstream of Priors Hall STW may not be able to meet its RQO of RE3 because of the combined impact of Corby STW and Priors Hall STW. Therefore Corby STW needs to be treated to a consent standard of 10mg/l BOD and 2mg/l Ammonia as a 95percentile to guarantee RQO compliance in the Willow Brook both downstream of Corby STW and downstream of Priors Hall STW.

To ensure that there is no deterioration in current quality, both treatment works would have to treat to the effluent standards to meet RE2. This is the minimum standard that is likely to be acceptable by 2031 because of the likely stringent demands of the Water Framework Directive.

Table 10 PS parameters


Storage (m3)

West of Stanion

12,461 21.6 56.2 228 465

Great Oakley PS (increase)

5,780 10.02 25.98 111 216

CTC pumping station (increase)

3,451 9.46 26 70.73 200



The pumping station upgrades at CTC and Great Oakley will still be required, and the new pumping station serving west of Stanion Oakley/Stanion extension will still be required, as above.

6 Alternative option 2 – New STW at Priors Hall new STW at Stanion

The development in the Southern Extension, Oakley Vale South, and Oakley/Stanion extension fall hydrologically in the Harpers Brook catchment. The natural drainage therefore will be towards Harpers Brook. There is a very small existing works at Stanion serving the village of Stanion. A new STW at this point would remove the need for pumping stations to serve these areas. However, Harpers Brook is currently a high quality watercourse meeting the RE1 objective. A new treatment works would need to treat to a very high standard just to meet the RE1 objective. However, because the Harper’s Brook currently is significantly better than



RE1, it is not practicably possible to reliably treat to a high enough quality to prevent any deterioration from the actually current quality (and not just deterioration to the RE1 objective) without advanced treatment technologies such as reverse osmosis. However, a high rate tertiary treatment plant with a polishing reedbed/constructed wetland could be designed to produce a very high quality effluent.


The benefit of this solution is that it reduces the pressure on capacity (both hydraulic and process) at Corby STW and more ‘sustainable’ drainage removes the need for abortive pumping from Priors Hall area back upstream to Corby STW, as before.

It would also reduce the upgrade work needed for existing pumping stations as the CTC Pumping Station would not need to be upgraded, and Great Oakley Pumping Station would only need to be upgraded by 10l/s rather than 24l/s.

The disadvantages are that there would be more treatment works to operate and maintain and associated problems of new STWs such as the long lead in time needed to overcome local opposition and associated costs of public enquiry . There is also the issue of underload to the plant design capacity as development proceeds.

In this case, the issue of negative public relations is likely to be a big problem with a new STW discharge into a very high quality watercourse.

Design parameters for each STW are summarised below:

Table 11 STW parameters P PE

(load) G PG DWF 3DWF Peak Storm

tank size

Consent

Corby STW

85,377 186,276 276 630 2 5800 RE4 13/4

RE3 10/2



RE2 6/1

Priors Hall

15,957 15,957 120/145 26 67 400 l/s

1085 14/4 (Corby STW must treat to meet a minimum standard of RE3)

RE2 8.5/1.5 (Corby STW must treat to a minimum objective of RE2)

Stanion 19,150 19,150 120 2873 34 87 370 l/s

1300 RE1 target 8/1

No deterioration

3/0.17



Figure 24 West of Stanion STW 100_460

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WestofStanion

Location of Stanion STW



7 Results and options selection

There are no immediate problems in the catchment with respect to foul flooding, and the current level of service provided by AWS to its customers will not be affected by the new developments. However, the new developments will increase the risk of foul flooding and to protect the population of Corby of to a 30yr design standard, civil engineering works of approximately £7.5M will be needed in the long term. This figure is not affected by the carious options assessed for the location of the treatment plants.

Storm discharges are not currently a problem in the catchment. Whilst a full UPM assessment will not carried out until Phase 2, the costs estimated for each option include an element of storage that is on the conservative side and will reduce the number of spills from the pumping stations and the storm tank from the current situation. It is highly unlikely that the UPM assessment will support the need for further storage.

The water quality DS of Corby STW is currently very high, outperforming the RQO. Assuming that the current quality, and not the RQO target quality will determine the effluent quality consent, it is immediately apparent that a very high quality effluent will be needed from any of the options for treatment works. This is especially true at Stanion STW where the current quality is RE1. From a river quality viewpoint, and from the extra cost needed for a new local treatment option at Stanion, the option for a new STW at Stanion to the Harpers Book option cannot be supported unless there are large environmental or cost savings at Corby STW and on the Willow Brook.

The capital costs are very much dependant on whether the existing Corby STW can accept any additional load or flow and at what level a new STW is triggered.

Assuming that a replacement STW will be needed at Corby STW for any option, the costs per option are as Table 12.



Table 12 Cost comparison of options to 2031 assuming new Corby STW

Rebuild Corby STW

Alt option 1

Alt option 2

£M £M £M

STW 2031 2031 2031

Corby STW 34 33 30

Priors Hall 7.4 7.4

Stanion 11

New PS

Priors Hall PS & rising 1.54

storage 0.45

West Stanion PS & rising 0.46 0.46

storage 0.26 0.26

PS

Great Oakley PS & rising 0.465 0.465 0.465

storage 0.05 0.05 0.05

CTC PS & rising 0.606 0.606

storage 0.2 0.2

Sewerage

Foul flooding 7.5 7.5 7.5

Total 45.5 49.9 56.4

It may be possible to provide additional capacity at the existing works to include the new population up to and including 2031. If the existing STW can be squeezed, there are significant cost savings as shown below. However, there are great difficulties and risks in doing this at Corby

For example, if Corby STW can be squeezed to accept the additional capacity for alternative options 1 and 2, the costs are as follows in Table 13:



Table 13 Cost comparison for 2031 options assuming Corby STW can be upgraded

Rebuild Corby STW

Alt option 1

Alt option 2

£M £M £M STW improvements 2031 2031 2031 Corby STW 34 12.2 9.1 Priors Hall 7.4 7.4 Stanion 11 New PS

Priors Hall PS & rising main 1.54

storage 0.45

West Stanion PS & rising main 0.46 0.46

storage 0.26 0.26 PS improvements

Great Oakley PS & rising main 0.465 0.465 0.465

storage 0.05 0.05 0.05

CTC PS & rising main 0.606 0.606

storage 0.2 0.2 Sewerage improvements Foul flooding protection 7.5 7.5 7.5 Total 45.5 29.1 35.5

Therefore the most cost effective option in this case is to uprate Corby STW to a load population equivalent of 205,426, and provide a new local treatment option at Priors Hall.

A new treatment works at the existing Corby STW site is the most likely long term option given the lack of spare capacity at the STW, the fact that the inlet works are overloaded, and the strict consent that is likely to be needed to meet the WFD requirements by 2031. However, these costs tend towards the local treatment options, if the existing site capacity can be squeezed.

It must be noted however, that these costs do not take into account any improvements that may be needed to flood defences in the receiving waters, and hence any final cost estimate must take both into account.

Comment [M1]: As before.



Figure 1 Location of Corby STW and pumping stations

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CTC pumping station

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Weldon Area

Figure 1 – Location of pumping stations and storm overflows

Pen Green pumping station

East Weldon PS

North Weldon PS



Figure 2 3DWF surcharge in Corby

Figure 2 System surcharge at 3DWF

Blue pipes are surcharged by flow, pink pipes are surcharged above pipe height



Figure 3 Foul flooding during a 30yr rainfall event

Pen green area

Corby STW

Cottingham Rd Beanfield Ave

Medina Road



Figure 4 Population expansion to 2021

Nothern Extension238

collingham north250

coll ingham south500

western expansion NN1250

western expansion N1500

Western extension North C1328

Western expansion SouthC1300

Western expansion_south1000

Sourthn extension3220

New Oakley Vale South 2006!3451

New Oakley Vale North 20063451

west of stanion1904

Oakley Stanion extension3094

Priors Hall & Weldon13388

Towncntre&UC3870



Figure 13 Summary of infrastructure required for 2021 population forecast



Figure 14 Population expansion to 2031 showing surcharge at 3DWF

Nothern Extension238

collingham north250

collingham south500

western expansion NN1250

western expansion N1500

Western extension North C1328

Western expansion SouthC1300

Western expansion_south1000

Sourthn extension3220

New Oakley Vale South 2006!3451

New Oakley Vale North 20063451

west of stanion1904

Oakley Stanion extension3094

Priors Hall & Weldon13388

Towncntre&UC3870



Figure 15 Summary of infrastructure required for 2031 population forecast

Halcrow Group Limited Griffin House 135 High Street Crawley West Sussex RH10 1DQ Tel +44 (0)1293 434500 Fax +44 (0)1293 434599 www.halcrow.com

Technical note

Project Corby Water Cycle Study Date 19 April 2005 Note Sustainable Urban Drainage Design Options Ref Author Nancy Troup

1 Introduction

1.1 An essential component of the Corby Water Cycle Study is the provision of an integrated and sustainable strategy for the long term development of Corby with respect to sustainable urban drainage systems (SUDS).

1.2 PPG 25 advocates use of Sustainable Urban Drainage System (SUDS) for new developments as an alternative concept in planning, design and management of storm drainage systems. The principles of SUDS should be adopted for source control and for attenuation of runoff from the paved surfaces and amenity areas to reduce the surface water run-off levels and alleviate flooding.

1.3 This technical note presents a review of SUDS options and their technical, economical and environmental viability. Calculation methods have been researched and some examples are presented here. Similar case studies have also been investigated in order to establish the short and long term maintenance of such systems as this is essential to their sustainability.

1.4 All relevant websites and publications are listed in Appendix A.

2 SUDS Options 2.1 Many different techniques have been developed to reduce the impact of

urban runoff on the receiving watercourse and downstream catchment. They can be categorised into three techniques as follows:

• Source control techniques – i.e. the reduction of flows entering the drainage system at source.

• Attenuation of flows within the drainage system (using storage tanks, ponds and permeable conveyance systems).

• End of pipe systems – i.e. the passive treatment to collected water before discharge to a watercourse.


Project Corby Water Cycle Study Note Sustainable Urban Drainage Design Options

2.2 Each of the techniques has their advantages and generally their usage depends on the local authority, developer or household owner’s ability to take ownership of the system and maintain it.

2.3 Source Control Techniques - these include below and above ground storage and can be grouped into the following categories:

• Porous pavements – water is allowed to drain through the pavement rather than drain off it, either directly into the subsoil or into a temporary storage area.

• Swales – long shallow grass lined ditches which are used to temporarily store and convey surface runoff.

• Infiltration basins – grass lined dry pond used to temporarily store and convey surface runoff.

• Soakaways – below ground, compact storage area that conveys water directly into the ground, often built of dry brickwork and filled with a permeable material.

• Infiltration trenches – similar to a soakaway but extend linearly across a site therefore provide a greater surface area for a given volume.

• Rainwater storage tanks – can be underground or above ground storage tank, used to either temporarily store runoff or for re-use of water in the home.

• Inlet control – a control fitted to the inlet of the drainage system such that flow is attenuate at source e.g to allow controlled flooding of parks and recreational areas.

2.4 Attenuation of Flows within the Drainage System – these techniques allow

the attenuation of flows in storage areas as follows:

• On-line storage tank – this can be created either by increasing the capacity of a pipe or constructing an on-line detention tank with a flow regulator at the outlet (e.g a hydrobrake).

• Off-line storage tank – a flow regulator is installed at the inlet to the pipe such that additional run-off flows off-line into a storage tank.

• Permeable conveyance systems – allow stormwater to move slowly towards a receiving watercourse, allowing storage, filtering and some loss of water before the outfall. e.g French drains.



2.5 End of pipe systems – these allow temporary storage of stormwater and can also designed such that natural processes remove pollutants from the stormwater. They include:

• Dry ponds – these are grass-lined or porous paving excavated areas that temporarily store stormwater and also allow for the settlement of solids. They are generally off-line.

• Permanent wet ponds – these store water at all times but are constructed such that additional storage is provided to temporarily store stormwater. They are constructed on-line.

3 Suitability of SUDS

Generally a combination of SUDS is chosen for a development. The suitability of SUDS depends on a number of different factors including: • Ground Conditions – depth of the water table, soil permeability,

susceptibility of the aquifer, levels of contamination etc. • Topography • Cost • Maintenance • Land Use • Public perception The feasibility of each of the systems, presented in Sections 2.3 – 2.5, for the Corby development is presented in Tables 3-1 and 3-2. As can be seen from the table many SUDS options could be suitable for the Corby development. It is therefore recommended that a combination of systems is used. Each development could have a different combination of SUDS depending on the factors given above.



Table 3-1

SUDS Option Description How does it work? Maintenance Feasibility Porous Pavements

Porous engineered surface, constructed from either grass concrete, porous concrete blocks, permeable block paving or porous macadam, placed on a high-void ratio aggregate sub-base layer such as gravel.

Surface run-off infiltrates through the surface into the ground, rate of infiltration should be greater than or equal to design rainfall intensity. Pollutant materials are removed in the subsoil.

Silt and pollutants accumulates in the surface layers therefore major maintenance is required to remove silt build-up and other trapped contaminants.

Could be used for driveways and car parks. Water table must be at least 1m below ground level, not to be used in areas of chalk or highly susceptible aquifer due to possible contamination.

Swales and infiltration basins

Vegetated surface features. Swales are long shallow channels while infiltration basins are dry retention ponds within landscaped areas, which are usually grass lined and regularly mown.

Allows rainwater to run in sheets through the vegetation, slowing and filtering the flow. Swales could also include check dam to increase attenuation and where applicable, infiltration.

Little maintenance required - regular mowing, clearing litter, periodic removal of excess silt.

Could be used to attenuate and convey run-off from roads and car parks. Not recommended if leaching potential of underlying soils is high due to possible contamination of aquifer.

Soakaways and Infiltration trenches

Storage area constructed below ground either from dry brickwork or concrete ring units, filled with permeable material.

Stormwater runoff is diverted into the soakaway or trenches and gradually infiltrates into the surrounding soil and eventually reaches the water table.

Needs to be inspected at regular intervals, areas draining to the infiltration device need to be kept clear of silt. Maintenance often left to private individuals or companies

Can be integrated into recreational areas or public open spaces or used as a soakaway for an individual house.

Rainwater storage tanks

Design and material of tank dependent on end user. E.g can be cylindrical, cuboid, made from pre-cast concrete, plastic, brick, ferrocement. Tank can be above or below ground, however if below ground is likely to require a pump to extract water.

Rainwater is collected from the roof of a building via a gutter into a storage tank. Water can then be filtered and used in the home in the home or used directly in the garden.

Tanks should be inspected regularly for cracks or leaks, will need to drained for cleaning. Maintenance often left to private individuals or companies.

Could be used for each household to store rainwater for re-use. Have been many case studies where this has been successfully achieved.

Inlet Control Can be a simple orifice to control flow from an inspection chamber (or water butt) into the drainage system.

Allows either storage in storage tank or controlled flooding of parks and recreational areas

Minimal – Orifice will need to be kept clear.

Could be used at the outlets of rainwater storage tanks to allow some flow to enter the drainage system.



Table 3-2

SUDS Option Description How they work Maintenance Feasibility On-line storage tank

Created by either increasing the size of a storm sewer or incorporating a storage tank into the sewer system.

The excess flow in the flood peak is stored in the storage tank until the downstream sewer or watercourse has spare capacity.

Velocity of flow should be at least 0.75m/s at dry weather flow to allow for self-cleansing of pipes. Tanks should be drained down as rapidly as possible to prevent the risk of siltation.

Can be incorporated into the storm water sewer system at design stage.

Off-line storage tank

Storage tank incorporated into the sewer system however off-line.

The excess flow in the flood peak is diverted off-line into the storage tank until the downstream sewer or watercourse has spare capacity.

Tanks should be drained down as rapidly as possible to prevent the risk of siltation.

Can be incorporated into the storm water sewer system at design stage.

Permeable conveyance system

Can be either a French drain – trench filled with gravel and often wrapped in a geotextile membrane or a swale – shallow grassed depression, dry during dry weather, used to convey flow during wet weather.

They allow stormwater to move slowly towards a receiving watercourse, allowing storage, filtering and some loss of water before the outfall.

Surface should be cleaned twice a year and kept clear of silt and weeds to keep voids clear. Use of grit and salt will affect the drainage potential so should not be used on the surface. Major maintenance could be required to remove silt build-up and other trapped contaminants.

Usually installed for highway drainage and provide a minimum level of mitigation for trunk roads.

Dry pond Area of storage for surface runoff which is free from water during dry weather flow conditions. Consist of excavated, berm-encased or dished areas lined with grass or porous paving.

Peak flows are stored in dry pond and released slowly into the downstream sewer or watercourse once the risk of flooding has passed.

Regular cutting of grass and annual clearance of aquatic vegetation and silt removal. Water quality can be controlled by adsorption, settlement of solids in still water and biological activity.

Can be integrated into recreational areas and public open space. Basins can be used for sport and recreation.

Permanent Wet pond

Area of storage for surface runoff. Contains during water during dry weather but designed to hold more water during wet weather.

Peak flows are stored during wet weather and released slowly into the downstream sewer or watercourse once the risk of flooding has passed.

Regular cutting of grass and annual clearance of aquatic vegetation and silt removal. Water quality can be controlled by adsorption, settlement of solids in still water and biological activity.

Can be integrated into recreational areas and public open space. Visually attractive. Can offer opportunity for provision of wildlife habitat.



4 Calculation Methods 4.1 Detailed calculations have not been carried out at this stage on all of the

SUDS options presented in Section 3. This can be done at a later stage when further details of the proposed developments are available.

4.2 Calculations have however been undertaken to assess the feasibility of rainwater harvesting for the development at Priors Hall in North East Corby. Rainwater harvesting could potentially decrease household water consumption from mains water supply by 30% in addition to reducing surface water runoff.

4.3 Recommend design calculations were taken from the ‘Harvesting rainwater for domestic uses: an information guide’ (Environment Agency July 2003) and www.rainharvesting.co.uk (see Appendix B). A typical layout of a rainwater harvesting system is shown in Figure 1 below.

4.4 The details of the proposed development at Priors Hall were taken from the ‘Environmental Statement Chapter 10: Drainage’ prepared by Wardell Armstrong (Engineering and Environmental Solutions) on behalf of Corby Developments Ltd for this site. It was assumed the total roof area within each development area was 30% of the total impermeable area (this assumption was taken from previous development proposals).



4.5 The annual rainfall yield per house was calculated as follows:

Annual yield = Area of roof x 0.75* x 0.9** x SAAR

* This is a run-off coefficient which allows for the slope of the roof and water loss through wind, evaporation, and the ability of the roofing material to hold onto water. 0.75 is a typical coefficient for pitched tiled roofs.

** This is a filter efficiency coefficient of the downpipe filter through which the rainwater is collected.

4.6 The annual rainfall yield per house within each development area is shown in Table 4-1. The yield varies between development areas due to the housing density and therefore total impermeable area varying between areas.

4.7 According to ‘Harvesting rainwater for domestic uses: an information guide’ (Environment Agency July 2003) ‘on average, each person in England and Wales uses around 150 litre of water every day’. In addition to this ‘on average 46% for purposes that could use rainwater instead of mains supply’.

4.8 The percentage of annual rainwater yield to total consumption per house was calculated for each house in each development area (see Table 4-1). It was assumed that there is an average of 2 occupants per household.

4.9 The annual rainwater yield could be considerably increased if surface runoff from paved areas (e.g the driveway for each house) is also collected.

4.10 The required tank size for each house has also been calculated (see Table 4-1). The recommended method is size the tank at 5% of the annual rainwater yield or of annual demand, using the lowest figure of the two.



Table 4-1 Rainwater harvesting calculation results

Development Area

Development Type

Total Area (Ha)

Housing Density Units/Ha

No. of houses

Assumed % impermeable

Impermeable Area (Ha)

Roof Area per

house (m2)

Rainfall yield

/year (l) Tank

Size (m3) % of

total use R1 Residential 11.13 15 167 30% 3.34 60.00 25515.00 1.28 23%R2 Residential 13.89 15 208 30% 4.17 60.00 25515.00 1.28 23%R3 Residential 1.96 20 39 35% 0.69 52.50 22325.63 1.12 20%R4 Residential 6.91 30 207 44% 3.04 44.00 18711.00 0.94 17%R5 Residential 5.24 20 105 35% 1.83 52.50 22325.63 1.12 20%R6 Residential 6.95 35 243 48% 3.34 41.14 17496.00 0.87 16%R7 Residential 7.32 35 256 48% 3.51 41.14 17496.00 0.87 16%R8 Residential 6.5 40 260 50% 3.25 37.50 15946.88 0.80 15%R9a Residential 2.11 40 84 54% 1.14 40.50 17222.63 0.86 16%R9b Residential 4.07 40 163 57% 2.32 42.75 18179.44 0.91 17%R10 Residential 10.77 35 377 48% 5.17 41.14 17496.00 0.87 16%R11 Residential 1.75 15 26 30% 0.53 60.00 25515.00 1.28 23%R12 Residential 2.19 40 88 50% 1.10 37.50 15946.88 0.80 15%R13 Residential 4.31 50 216 57% 2.46 34.20 14543.55 0.73 13%R14 Residential 8.23 50 412 57% 4.69 34.20 14543.55 0.73 13%R15 Residential 10.42 50 521 57% 5.94 34.20 14543.55 0.73 13%R16 Residential 7.63 50 382 57% 4.35 34.20 14543.55 0.73 13%R17 Residential 0.48 50 24 57% 0.27 34.20 14543.55 0.73 13%R18 Residential 3.5 60 210 62% 2.17 31.00 13182.75 0.66 12%R19 Residential 2.91 60 175 62% 1.80 31.00 13182.75 0.66 12%R20 Residential 2.92 60 175 62% 1.81 31.00 13182.75 0.66 12%R21 Residential 7.71 45 347 54% 4.16 36.00 15309.00 0.77 14%R22 Residential 1.5 45 68 54% 0.81 36.00 15309.00 0.77 14%R23 Residential 5.7 15 86 30% 1.71 60.00 25515.00 1.28 23%



5 Case Studies 5.1 There are many developments that have incorporated SUDS into their

design. The UK SUDS database on www.suds-sites.net provides much information on these developments.

5.2 There have also been a number of developments that been be classed as sustainable developments which incorporate not only SUDS but energy efficient design, building materials from natural, renewable or recycled sources and water saving strategies. These include:

• The Eden Project near St Austell in Cornwall (www.edenproject.com)

• BedZed – an environmentally friendly and energy efficient housing and office development in Beddington, Sutton (see www.bedzed.org.uk).

• The Hockerton Housing Development – an earth sheltered, energy-efficient housing development in Hockerton, Northamptonshire (see www.hockerton.demon.co.uk).



Appendix A – References

Publications:

Harvesting rainwater for domestic uses: an information guide, Environment Agency, July 2003

Model agreements for sustainable water management systems: Model agreement for rainwater and greywater use systems, CIRIA, 2004

Model agreements for sustainable water management systems: Model agreements for SUDS, CIRIA, 2004

A Review of Published Material on the Performance of Various SUDS Devices prepared for the Environment Agency by Professor C J Pratt, Coventry University, December 2001.

Websites:

www.bedzed.org.uk BedZED housing development

www.bsria.co.uk BSRIA – Building Services Research and Information Association

www.cat.org.uk Centre for Alternative Technology – suppliers listings and advice

www.ciria.org.uk/suds CIRIA initiative on promoting good practice within SUDS

www.edenproject.com Eden project – a large scale rainwater harvesting system.

www.environment-agency.gov.uk/savewater Environment Agency advice on saving water

www.hockerton.demon.co.uk Hockerton housing development

www.wras.co.uk Water Regulations Advisory Service

www.eng.warwick.ac.uk/DTU/rwh/components4.html Research programme at Warwick University into rainwater harvesting

www7.caret.cam.ac.uk/guide_suds.htm Engineering design for sustainable development

www.suds-sites.net UK SUDS database

www.rainharvesting.co.uk UK supplier of rainwater harvesting systems

www.savewater.com.au Australian website on water saving devices

www.atlantiscorp.com.au/projects Australian website giving examples of SUDS projects in Australia

www.wsud.melbournewater.com.au Australian website providing manuals on water sensitive urban design

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