micklehurst road, mossley pps25: assessment · matt travis, bsc (hons), msc, mciwem, cenv, csci -...

Micklehurst Road, MossleyAddendum PPS25:

Flood Risk Assessment

March 2011

© Enzygo Ref:

SHF. 135.001.R.002.A

‘Experience and expertise working in union’

STEP Business Park

Wortley Road, Deepcar, Sheffield S36 2UH

Tel: +44(0)114 290 3677 Fax: +44(0)114 290 3688

www.enzygo.com

Enzygo Limited

STEP Business Centre

Wortley Road

Deepcar

Sheffield

S36 2UH

Tel: 0114 290 3677

Tel/Fax: 0114 290 3688

www: www.enzygo.com

Addendum PPS25: Flood Risk Assessment

Micklehurst Road, Mossley

Project: Micklehurst Road, Mossley

Site: Micklehurst Road, Mossley, Tameside

For: Steelstone Properties Ltd

Status: Final

Date: March 2011

Author:

Reviewer:

Keelan Serjeant, BSc (Hons), MSc - Senior Hydrologist

Matt Travis, BSc (Hons), MSc, MCIWEM, CEnv, CSci - Director

Disclaimer:

This report has been produced by Enzygo Limited within the terms of the contract with the client and taking account of the resources devoted to it by agreement with the client.

We disclaim any responsibility to the client and others in respect of any matters outside the scope of the above.

This report is confidential to the client and we accept no responsibility of whatsoever nature to third parties to whom this report, or any part thereof, is made known. Any such party relies on the report at their own risk.

Enzygo Limited Registered in England No. 6525159

Registered Office Stag House The Chipping Wotton-Under-Edge Gloucestershire GL12 7AD

Steelstone Properties Ltd

© Enzygo Ltd 2011 i SHF.135.001.R.002.A

CONTENTS

TABLES, DRAWINGS & APPENDICES ............................................................................... II 1.0 INTRODUCTION ...................................................................................................... 1 1.1 Background .............................................................................................................. 1 1.2 Report Structure ....................................................................................................... 1 2.0 HISTORICAL FLOODING ....................................................................................... 3 2.1 2004 Flood Event ..................................................................................................... 3 3.0 RIVER MODELLING................................................................................................ 6 3.1 Hydrological Modelling ............................................................................................. 6 3.2 Hydraulic Modelling .................................................................................................. 8 3.2.1 Boundary Conditions .............................................................................................. 10 3.2.2 Hydraulic Roughness ............................................................................................. 11 3.3 Culvert Blockage .................................................................................................... 11 3.4 Flood Outline Mapping ........................................................................................... 12 4.0 MODEL RESULTS ................................................................................................ 13 4.1 1 in 100 year (plus climate change) event ............................................................. 13 4.2 1 in 1000 year event............................................................................................... 17 4.3 Culvert Blockage .................................................................................................... 21 4.4 Sensitivity Analysis ................................................................................................. 23 4.4.1 Mannings ‘n’ ........................................................................................................... 23 4.4.2 Downstream Boundary ........................................................................................... 25 4.5 Discussion .............................................................................................................. 26 5.0 SITE DRAINAGE ................................................................................................... 28 5.1 Surface Water Drainage......................................................................................... 28 5.2 Geology .................................................................................................................. 28 5.3 Soil ......................................................................................................................... 28 5.4 Groundwater .......................................................................................................... 28 5.5 Infiltration Methods ................................................................................................. 28 5.6 Surface Water Management Strategy .................................................................... 29 5.6.1 Attenuation Requirement ....................................................................................... 30 5.7 Designing for Exceedence ..................................................................................... 30 6.0 SUMMARY AND CONCLUSION ........................................................................... 31 DRAWINGS ......................................................................................................................... 33 APPENDIX 1 ........................................................................................................................ 34 APPENDIX 2 ........................................................................................................................ 35 ABBREVIATION/ACRONYMS ............................................................................................ 36 GLOSSARY ......................................................................................................................... 39


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TABLES, DRAWINGS & APPENDICES

TABLES

1. Relevant catchment descriptors from the FEH CD-ROM v2.0 2. ReFH Peak Discharge 3. Chainage and location of surveyed river cross sections and structures 4. Downstream water levels (mAOD) used in the HTBDY unit 5. Mannings ‘n’ roughness coefficient values 6. Maximum water levels for the 1 in 100 year (plus climate change) event for the

Micklehurst Brook 7. Maximum water levels for the 1 in 1000 year event for the Micklehurst Brook 8. Maximum water levels for the 1 in 1000 year event for the Micklehurst Brook with a

75% blockage in the downstream culvert 9. Maximum water levels for the 1 in 1000 year event for Micklehurst Brook with

Mannings ‘n’ roughness values increased by 20% 10. Maximum water levels for the 1 in 1000 year event for Micklehurst Brook with

Mannings ‘n’ roughness values decreased by 20% 11. Maximum water levels for the 1 in 1000 year event for Micklehurst Brook with the

downstream boundary 400 m further downstream

DRAWINGS

1. Surface Water Flowpaths Pre-development 2. Surface Water Flowpaths Post-development 3. Section Drawings 4. Flood Outlines

APPENDICES

1. Environment Agency Correspondence 2. Model Results

FIGURES

1. Flooding approximately 30 m to the north of the site 2. High water levels in Micklehurst Brook at the confluence with Staley Brook 3. High water levels in Micklehurst Brook just below the confluence with Staley Brook 4. Flooding of Micklehurst Brook below the site 5. Micklehurst as shown on the FEH CD-ROM v2.0 6. Schematic of the ISIS model 7. Maximum water level at chainage MICK_0307 8. Maximum water level at chainage MICK_0305 9. Maximum water level at chainage MICK_0275 10. Maximum water level at chainage MICK_0235 11. Maximum water level at chainage MICK_0225 12. Maximum water level at chainage MICK_0185 13. Maximum water level at chainage MICK_0145


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14. Maximum water level at chainage MICK_0135 15. Maximum water level at chainage MICK_0135c1 16. Maximum water level at chainage MICK_0307 17. Maximum water level at chainage MICK_0305 18. Maximum water level at chainage MICK_0275 19. Maximum water level at chainage MICK_0235 20. Maximum water level at chainage MICK_0225 21. Maximum water level at chainage MICK_0185 22. Maximum water level at chainage MICK_0145 23. Maximum water level at chainage MICK_0135 24. Maximum water level at chainage MICK_0135c1 25. Maximum water level at chainage MICK_0145 26. Maximum water level at chainage MICK_0135 27. Maximum water level at chainage MICK_0135c1


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1.0 INTRODUCTION

1.1 Background

This document forms an addendum to the Enzygo Flood Risk Assessment (FRA) dated January 20111 at the proposed development site at Micklehurst Road, Mossley, Tameside.

The report details the flood risk at the site and how this could be managed and mitigated to allow the site to be development for housing in support of the enclosed detailed planning application for residential development and access.

Correspondence has been received from the Environment Agency (see Appendix 1) which recommends refusal of the planning application. After a meeting with Sandrine Thomas the Environment Agency’s, Development and Flood Risk Engineer for the area it was agreed that further assessment of the flood risk at the site was needed to be undertaken including the development of a 1-Dimensional (1D) Model of Micklehurst Brook, for the planning application to gain approval from the Environment Agency.

The following scope was agreed with the Environment Agency:

To assess the historical flooding of Micklehurst Road and its impact on the proposed properties based upon photographic evidence from the Environment Agency.

Quantify the flood risk from Micklehurst Brook taking into account the impacts of climate change.

Considers the effect of a range of flooding events including extreme events (e.g. 1 in 1000 year) on people and property.

Assess the risk of blockage on the downstream culvert and its affect on flooding.

Provide further information with regards to the surface water management strategy.

1.2 Report Structure

This addendum has the following report structure:

Section 2 provides details of the 2004 flood event;

Section 3 details the river modelling undertaken;

Section 4 outlines the model results;

1 Enzygo, PPS25: Flood Risk Assessment, Micklehurst Road, Mossley, SHF. 135.001.R.001.D.


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Section 5 provides further details of the surface water management strategy; and

Section 6 presents a summary and conclusions.


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2.0 HISTORICAL FLOODING

2.1 2004 Flood Event

Photographs of the 2004 flood event which affected Micklehurst Road have been provided by the Environment Agency. These photographs were taken by a Tameside Metropolitan Borough councillor. These photographs show that areas within the vicinity of the site flooded in 2004. However, no photographs show the site flooding, therefore it is unknown if the site flooded in 2004.

The extent of the 2004 flood event is unknown and the return period is also unknown therefore, it is not possible to make an accurate assumption from the photographs of the flood risk at the site, as they depict downstream and offsite flooding.

The Environment Agency has confirmed that they do not hold any other details of historic flood events in the area. No other records of historic flood events are available for the site; this has included a search of the Chronology of British Hydrological Events website2, the local newspapers and the internet.

The photograph that shows the nearest flooding to the site is shown in Figure 1; this is located approximately 30m to the north of the site. This shows an area inundated with floodwater from Micklehurst Road at the junction with the access road to the works located to the north east of the site.

Figures 2 and 3 show high water levels in Micklehurst Brook at the confluence with Staley Brook approximately 130 m downstream of the site at the culvert outlet. Figure 4 shows flooding occurring on Micklehurst Road just downstream of the site.

Therefore, after assessing historical flood events especially the 2004 it can be seen that the majority of the site has not flooded historically. The only proposed houses that are at risk of the flooding from Micklehurst Road are the three proposed properties (no. 1, 2 and 3) adjacent to Micklehurst Road next to no.73 Micklehurst Road.

Drawing 1 shows the surface water flowpaths associated with surface water runoff at the site and down Micklehurst Road pre-development and Drawing 2 shows the surface water flowpaths post-development. The flowpaths have been modelled to show the surface runoff generated across the site, and surrounding area using the topographical survey of the site and contours of the surrounding area.

Drawing 2 shows that the surface water flowpath that would affect the three proposed properties (no. 1, 2 and 3) adjacent to Micklehurst Road next to no.73 Micklehurst Road has been directed down the access road to the east of the properties and then follows the pre-development flowpath (see Drawing 1) to Micklehurst Brook. Therefore, reducing the flood risk posed by surface water runoff down Micklehurst Road.

2 http://www.dundee.ac.uk/geography/cbhe/#searching


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To further mitigate the flood risk from surface water runoff down Micklehurst Road, the finished floor levels of these three properties (no. 1, 2 and 3) will be raised above current ground levels and the level of Micklehurst Road.

The overall flow route has not been altered it has just been directed around the three properties by the use of the access road and finished floor levels of the properties.

Figure 1 – Flooding approximately 30 m to the north of the site

Figure 2 – High water levels in Micklehurst Brook at the confluence with Staley Brook


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Figure 3 – High water levels in Micklehurst Brook just below the confluence with Staley Brook

Figure 4 – Flooding of Micklehurst Road below the site


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3.0 RIVER MODELLING

3.1 Hydrological Modelling

Hydrological modelling provides the input for hydraulic models by converting rainfall into runoff. Micklehurst Brook flows through the site near the northern boundary from east to west.

It is important to understand the hydrological nature of the Micklehurst Brook due to its implications on fluvial flood risk at the site.

Such an investigation was undertaken using ‘industry standard’ techniques such as the Centre for Ecology and Hydrology (CEH) Flood Estimation Handbook (FEH) CD-ROM v2.03 and the Revitalised Flood Estimation Handbook (ReFH)4 method. It has been agreed with the Environment Agency that the ReFH method can be used to model the flood flows on Micklehurst Brook up to the 1 in 1000 year event. This method is also regarded as being precautionary in the estimation of flood peaks.

The ReFH method is based on robust hydrological modelling techniques and is considered to be an improvement over the FEH/FSR Rainfall Runoff method (FEH), as described in the Flood Estimation Handbook (FEH)5. This method uses catchment descriptors from the FEH CD-ROM v2.0, to generate design flood events of specific return periods.

Catchment descriptors from the FEH CD-ROM v2.0 can be used to infer the physical nature of the catchment and its possible response to a rainfall event. Table 1 sets out these descriptors for the study catchment upstream of the site (see Figure 5). A definition of each can be found at http:www.environment-agency.gov.uk/hiflows/97768.aspx.

The results show that the study catchment covers a small area, has a low baseflow, a high annual average rainfall, is slightly urbanised and a fairly high soil runoff potential. It would be expected that the catchment has a rapid, flashy response to a rainfall event due to the long duration that the soil moisture deficit remains close to field capacity.

Using the catchment descriptors extracted from the FEH CD-ROM v2.0, a critical storm duration of 3 hours, and a winter storm profile (these parameters give the worst case flow scenario at the site) have been used within the ReFH method to calculate the flood flows for the catchment of Micklehurst Brook upstream of the site.

3 Flood Estimation Handbook CD-ROM v2.0 2006, Centre for Ecology and Hydrology. 4 Flood Estimation Handbook, Supplementary Report No. 1 2007, The revitalised FSR/FEH rainfall-runoff method, Centre for Ecology and Hydrology. 5 Flood Estimation Handbook 1999, Restatement and application of the Flood Studies rainfall-runoff method, Volume 4, Institute of Hydrology.


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Table 1 – Relevant catchment descriptors from the FEH CD-ROM v2.0

Catchment Descriptors Value Explanation NATIONAL GRID REFERENCE 398000, 401950

AREA 1.29 Small catchment area PROPWET 0.57 BFIHOST 0.367 Low baseflow

SAAR 1221 High annual average rainfallURBEXT1990 0.0271 Slightly urbanised

SPRHOST 46.29 High soil runoff potential FEH DDF design rainfall parameters

C -0.02559 D1 0.37657 D2 0.43347 D3 0.31419 E 0.30588 F 2.50998

Figure 5 – Micklehurst as shown on the FEH CD-ROM v2.0

Table 2 shows that the peak flows for Micklehurst Brook range from 2.0 m3s-1 for the 1 in 5 year event up to 8.0 m3s-1 for the 1 in 1000 year event. Climate change allowances must be taken into account over the whole lifetime of the development, which has been recommended by the Environment Agency to be 100 years for residential uses. Therefore, the peak river flows have been increased by 20% to account for the affects of climate change in accordance with Table B2 of PPS25.

With an allowance of 20% to account for climate change the peak flow, for the Micklehurst Brook upstream of the site, would be 5.0 m3s-1 for the 1 in 100 year plus climate change event.

Micklehurst Brook


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These peak flows have been taken forward and used as the inflow (upstream boundary) to the model, these represent the flood event flows (including the effects of climate change) for the catchment of Micklehurst Brook upstream of the site.

Table 2 – ReFH Peak Discharge

Return Period (years) Peak Discharge (m3s-1)

1 in 5 2.0

1 in 10 2.4

1 in 25 3.0

1 in 50 3.5

1 in 75 3.9

1 in 100 4.2

1 in 100+20% 5.0

1 in 1000 8.0

3.2 Hydraulic Modelling

Hydraulic modelling is used to convert the hydrological modelling outputs from the ReFH method and converts the runoff into flow and water levels within the river. A hydraulic model of Micklehurst Brook has been developed using the 1D ISIS modelling software. The model is based on river cross section and structure surveys of Micklehurst Brook.

ISIS is a software package for the simulation of river flows and is extensively used throughout the world for flood forecasting, flood alleviation scheme designs, flood risk mapping, flood risk assessments and catchment management planning projects. Version 3 of ISIS was used in this assessment.

The upstream boundary of the model is located near the eastern boundary of the site above the culvert which enters the site from the pond to the east of the site at National Grid Reference 398303, 402003 and the downstream boundary is located just downstream of the confluence with Staley Brook at National Grid Reference 397957, 401948.

Table 3 shows the chainage and location of river cross sections and structures surveyed for inclusion in the model. Drawing 3 shows the location of the river cross sections and long section. A schematic of the model is shown in Figure 6.

The culvert located at the upstream boundary has not been included within the ISIS model as it has been calculated, using the Colebrook-White method, that this culvert has a capacity of greater than 10 m3s-1 which is larger than the peak flow of 8 m3s-1 for the 1 in 1000 year event and therefore will not affect the flow discharging into the site.


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Figure 6 – Schematic of the ISIS model


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Table 3 - Chainage and location of surveyed river cross sections and structures

Chainage Reference

Chainage (m)

Eastings Northing Comment

MICK_0307 307 398242 401989 Downstream of culvert at

upstream limit of site

MICK_0305 305 398240 401990 Weir

MICK_0275 275 398215 401987

MICK_0235 235 398168 401994 Upstream of bridge

MICK_0225 225 398178 401995 Downstream of bridge

MICK_0185 185 398139 401997

MICK_0135 135 398092 401999 Culvert at downstream limit of

site

MICK_0135c1 135 152.91 Culvert inlet

MICK_0135c4 5 147.80 Culvert outlet

3.2.1 Boundary Conditions

The ISIS software requires an upstream and downstream boundary. The upstream inflow peak flow, generated using the ReFH method, has been detailed in Section 3.1.

The Head-Time Boundary (HTBDY) unit has been used as the upstream and downstream boundaries within ISIS. The HTBDY unit allows the input of a water level as a boundary condition. Table 4 shows the water levels used as a boundary condition.

The downstream boundary unit represents the water level at the downstream extent of the model at the confluence with Staley Brook. It has been calculated that the downstream boundary may not be far enough away from the area of interest so as to not affect the results. When the backwater length L6is calculated this gives a distance of 383 m however, due to the limitations in the available topographical survey it has not been possible to model Micklehurst Brook further downstream. The sensitivity of the model results to the downstream distance of this boundary has been explored in detail (see Section 4.4). The model results may give a conservative estimation of maximum water levels at the site

The water level used within the downstream boundary have been taken from information provided by the Environment Agency (see Appendix 1) this allows the representation of a simultaneous flood event on Micklehurst Brook, and on Staley Brook and is therefore considered to be a conservative representation of flood risk at the site.

6 Backwater length L, which can be estimated as 0.7 x depth/gradient (using dimensionally consistent units).


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Table 4 – Downstream water levels (mAOD) used in the HTBDY unit

Return Period (years) Water Level (mAOD)

1 in 100+20% 146.10

1 in 1000 146.20

3.2.2 Hydraulic Roughness

Hydraulic roughness represents the conveyance capacity of vegetation growth, bed and bank material, channel, sinuosity and structures on the floodplain. Within ISIS, the hydraulic roughness is defined using the Mannings ‘n’ roughness coefficient values.

Table 5 details the Manning’s ‘n’ roughness coefficient values. These values have been developed from site visits, site photographs, with reference to industry standard literature7 and past modelling experience. The sensitivity of the model results to the Mannings ‘n’ roughness coefficient values has been explored in detail (see Section 4.4).

Table 5 – Mannings ‘n’ roughness coefficient values

Landscape Mannings ‘n’

Floodplain 0.050

River Channel 0.035

Culvert 0.013-0.015

3.3 Culvert Blockage

An assessment of the residual flood risk of culvert failure or blockage of the downstream culvert has been undertaken. The culvert unit within the ISIS model has been altered to simulate the blockage and/or collapse of the culvert. The total volume of flow area of the culvert has been reduced by 75% within the ISIS model and represents an obstruction or collapse of the culvert. In effect, this will mean that the blockage is assumed to occupy the same proportion of the width of the section at all water levels, i.e. it is a vertical blockage.

The culvert is small and not well maintained with a dilapidated trash screen at the upstream end of the culvert. A section of the culvert has recently been upgraded, therefore the majority of the culvert is thought to be structurally sound and the probability of this section of the culvert collapsing and/or a blockage is low.

It has been calculated that the culvert has a maximum capacity of approximately 1 to 2 m3 s-1. It can be seen that surcharging of the culvert is likely especially if a blockage and/or collapse of the culvert occurs.

7 Open Channel Hydraulics, Chow V.T., McGraw-Hill, 1959.


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3.4 Flood Outline Mapping

The topographical survey of the site has been combined with the model water levels from the ISIS model to produce the flood outline map. The detailed topographical survey of the site has been used to provide detailed information about the topography and ground surface. The ground surface has been overlaid with the water level information from the ISIS model to produce the flood outline map.

The flood outline map has been drawn using the water levels from the 1 in 100 year (plus climate change) event, the 1 in 1000 year event and the 1 in 1000 year with culvert blockage event models. The results from the sensitivity analysis have been used to assess the accuracy of the flood outline map.


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4.0 MODEL RESULTS

4.1 1 in 100 year (plus climate change) event

Table 6 shows the maximum water levels for the 1 in 100 year (plus climate change) event from the ISIS model of Micklehurst Brook for the cross sections adjacent to the site. Drawing 4 shows the flood outline for the 1 in 100 year (plus climate change) event. The full model results are shown in Appendix 2.

The maximum water level in Micklehurst Brook would be 166.16 mAOD at chainage MICK_0307, which is located at the upstream boundary of the site (i.e. to the east). Table 6 shows that the flooding will be either contained within the river channel or river corridor which includes the buffer strip.

The water will not inundate the site and only low levels of flooding will be experienced along the river corridor during the 1 in 100 year (plus climate change) event.

Flooding would be of minor nature with the developable site ground levels being located above the maximum water levels.

Surcharging of the culvert at the downstream boundary of the site would occur to a level of 0.22 m, the maximum water at the culvert would be 151.65 mAOD and the soffit of the culvert is 151.43 mAOD.

No spilling of floodwater over the culvert and downstream will occur as the minimum ground level above the culvert is 152.49 mAOD and is located above the maximum water level of 151.65 mAOD.

The bridge in the middle of the site will not be inundated with floodwater as the bridge has a deck level of 158.85 mAOD and the maximum water level at this location will be 158.62 mAOD.

Figures 7 to 15 show the maximum water level experienced at each of the cross sections of Micklehurst Brook adjacent to the site during the 1 in 100 year (plus climate change) event. These confirm that only a small area of site will be inundated with water from the Micklehurst Brook during the 1 in 1000 year event as the developable site ground levels are located above the maximum water levels.

Flooding of the river corridor is expected at this return period and would not impact on the overall development of the site as a buffer strip along the Micklehurst Brook has been designed into the masterplan for the site and is required by the Environment Agency.


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Table 6 - Maximum water levels for the 1 in 100 year (plus climate change) event for the Micklehurst Brook

Chainage Reference

Chainage (m)

Water Level (mAOD)

Left Bank (mAOD

Right Bank (mAOD)

Top of Bank

Site Level

Top of Bank

Site Level

MICK_0307 307 166.17 167.02 166.00 166.87 N/A MICK_0305 305 164.46 165.20 166.00 167.74 N/A MICK_0275 275 160.15 161.41 165.50 160.87 N/A MICK_0235 235 158.63 158.70 163.00 158.85 N/A MICK_0225 225 158.63 158.70 162.50 158.85 N/A MICK_0185 185 156.15 156.19 160.00 156.23 N/A MICK_0145 145 152.31 152.52 157.50 152.43 157.10 MICK_0135 135 151.98 152.52 157.50 152.43 157.10

MICK_0135c1 135 151.65 152.52 157.50 152.43 157.10

Figure 7 - Maximum water level at chainage MICK_0307



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Figure 15 - Maximum water level at chainage MICK_0135c1

4.2 1 in 1000 year event

Table 7 shows the maximum water levels for the 1 in 1000 year event from the ISIS model of Micklehurst Brook for the cross sections adjacent to the site. Drawing 4 shows the flood outline for the 1 in 1000 year event. The full model results are shown in Appendix 2.

The maximum water level in Micklehurst Brook would be 166.36 mAOD at chainage MICK_0307, which is located at the upstream boundary of the site (i.e. to the east). Table 7 shows that the flooding will be either contained within the river channel or river corridor which includes the buffer strip.

The water will not inundate the developable site and only low levels of flooding will be experienced along the river corridor during the 1 in 1000 year event.

Flooding would be of minor nature with the developable site ground levels being located above the maximum water levels.

Surcharging of the culvert at the downstream boundary of the site would occur to a level of 1.48 m, the maximum water at the culvert would be 152.91 mAOD and the soffit of the culvert is 151.43 mAOD. Minor spilling of floodwater over the culvert and downstream will occur to a maximum water depth of 0.64 m.

The bridge deck in the middle of the site will not be inundated with floodwater as the bridge has a deck level of 158.85 mAOD and the maximum water level at this location will be 158.85 mAOD.

Figures 16 to 24 show the maximum water level experienced at each of the cross sections of Micklehurst Brook adjacent to the site during the 1 in 1000 year event. These confirm that only a small area of site will be inundated with water from the Micklehurst Brook during the 1 in 1000 year event as the site ground levels are located above the maximum water levels.


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Table 7 - Maximum water levels for the 1 in 1000 year event for the Micklehurst Brook

Chainage Reference

Chainage (m)

Water Level (mAOD)

Left Bank (mAOD

Right Bank (mAOD)

Top of Bank

Site Level

Top of Bank

Site Level

MICK_0307 307 166.36 167.02 166.00 166.87 N/A MICK_0305 305 164.63 165.20 166.00 167.74 N/A MICK_0275 275 160.27 161.41 165.50 160.87 N/A MICK_0235 235 158.86 158.70 163.00 158.85 N/A MICK_0225 225 158.82 158.70 162.50 158.85 N/A MICK_0185 185 156.54 156.19 160.00 156.23 N/A MICK_0145 145 153.13 152.52 157.50 152.43 157.10 MICK_0135 135 153.13 152.52 157.50 152.43 157.10

MICK_0135c1 135 152.91 152.52 157.50 152.43 157.10 Note: Water levels highighted in yellow show the cross-sections where the modelled water level is above the top of bank level.



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4.3 Culvert Blockage

The risk of blockage on the downstream culvert has been investigated. The total flow area of the culvert has been reduced by 75% within the ISIS model.

Table 8 shows the maximum water levels for the 1 in 1000 year event from the ISIS model of Micklehurst Brook if a 75% blockage and/or collapse of the culvert was to occur. Drawing 4 shows the flood outline of the 1 in 1000 year event with the culvert blockage. The full model results are shown in Appendix 2.

The maximum water levels would increase by a maximum of 1.03 m just upstream of the downstream culvert. The maximum water levels experienced across the site would be unaffected.

Figures 25 to 27 show the maximum water level experienced at the cross sections where an increase in water level would be experienced. These confirm that only the buffer strip will be inundated with water from Micklehurst Brook during the 1 in 1000 year event with


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culvert blockage as the site ground levels are located above the maximum water levels and would not be greatly different to the 1 in 1000 year event flood outline.


Table 8 - Maximum water levels for the 1 in 1000 year event for the Micklehurst Brook with a 75% blockage in the downstream culvert

Chainage Reference

Chainage (m)

Water Level (mAOD) Left Bank

(mAOD Right Bank

(mAOD)

1 in 1000 year

event

Culvert Blockage

DifferenceTop of Bank

Site Level

Top of Bank

Site Level

MICK_0307 307 166.36 166.36 0.00 167.02 166.00 166.87 N/A MICK_0305 305 164.63 164.63 0.00 165.20 166.00 167.74 N/A MICK_0275 275 160.27 160.27 0.00 161.41 165.50 160.87 N/A MICK_0235 235 158.86 158.86 0.00 158.70 163.00 158.85 N/A MICK_0225 225 158.82 158.82 0.00 158.70 162.50 158.85 N/A MICK_0185 185 156.54 156.54 0.00 156.19 160.00 156.23 N/A MICK_0145 145 153.13 153.50 0.37 152.52 157.50 152.43 157.10MICK_0135 135 153.13 153.50 0.37 152.52 157.50 152.43 157.10

MICK_0135c1 135 152.91 153.94 1.03 152.52 157.50 152.43 157.10Note: Water levels highighted in yellow show the cross-sections where the modelled water level is above the top of bank level.



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4.4 Sensitivity Analysis

Sensitivity analysis has been undertaken on the model in order to develop an understanding of the relationship between key input factors, to assess whether the model is behaving normally and to assess the accuracy of the model results.

An assessment of the sensitivity of the model results to the choice of the Mannings ‘n’ roughness values and the location of the downstream boundary was undertaken by carrying out additional runs on the 1 in 1000 year event.

4.4.1 Mannings ‘n’

Mannings ‘n’ roughness values were uniformly increased by 20% for the first run (see Table 9) and uniformly decreased by 20% for the second run (see Table 10). The full model results are shown in Appendix 2.

The results of the sensitivity analysis show that a variance in the Mannings ‘n’ roughness values of +/-20% show corresponding changes in the water levels of a maximum +/-0.55 m or 0.36% showing that the model is relatively sensitive to changes in the roughness values and is behaving normally and shows a high confidence in the model results.


© Enzygo Ltd 2011 24 SHF.135.001.R.002.A

Table 9 shows that if the Mannings ‘n’ roughness values were to uniformly increase by 20% due to seasonal changes in the vegetation the maximum water levels experienced within Micklehurst Brook may increase by a maximum of 0.22m or 14% however, this would have very little affect on the overall water levels, flood outline and therefore flood risk at the site.

The flooding will be either contained within the river channel or river corridor and would not impact on the developable site and proposed properties.

Table 9 - Maximum water levels for the 1 in 1000 year event for Micklehurst Brook with Mannings ‘n’ roughness values increased by 20%

Chainage Reference

Chainage (m)


(mAOD Right Bank

(mAOD)

1 in 1000 year

event

Increase by 20%

Difference Top of Bank

Site Level

Top of Bank

Site Level

MICK_0307 307 166.36 166.36 0.00

(0.00%) 167.02 166.00 166.87 N/A

MICK_0305 305 164.63 164.68 0.05

(0.03%) 165.20 166.00 167.74 N/A

MICK_0275 275 160.27 160.32 0.05

(0.03%) 161.41 165.50 160.87 N/A

MICK_0235 235 158.86 159.08 0.22

(0.14%) 158.70 163.00 158.85 N/A

MICK_0225 225 158.82 158.90 0.08

(0.05%) 158.70 162.50 158.85 N/A

MICK_0185 185 156.54 156.59 0.05

(0.03%) 156.19 160.00 156.23 N/A

MICK_0145 145 153.13 153.12 -0.01

(0.01%) 152.52 157.50 152.43 157.10

MICK_0135 135 153.13 153.13 0.00

(0.00%) 152.52 157.50 152.43 157.10

MICK_0135c1 135 152.91 152.91 0.00

(0.00%) 152.52 157.50 152.43 157.10


© Enzygo Ltd 2011 25 SHF.135.001.R.002.A

Table 10 - Maximum water levels for the 1 in 1000 year event for Micklehurst Brook with Mannings ‘n’ roughness values decreased by 20%

Chainage Reference

Chainage (m)


(mAOD Right Bank

(mAOD)

1 in 1000 year

event

Decrease by 20%

Difference (m)

Top of Bank

Site Level

Top of Bank

Site Level

MICK_0307 307 166.36 166.36 0.00

(0.00%) 167.02 166.00 166.87 N/A

MICK_0305 305 164.63 164.55 -0.05

(0.05%) 165.20 166.00 167.74 N/A

MICK_0275 275 160.27 160.21 -0.06

(0.04%) 161.41 165.50 160.87 N/A

MICK_0235 235 158.86 158.74 -0.12

(0.08%) 158.70 163.00 158.85 N/A

MICK_0225 225 158.82 158.73 -0.09

(0.06%) 158.70 162.50 158.85 N/A

MICK_0185 185 156.54 156.48 -0.06

(0.04%) 156.19 160.00 156.23 N/A

MICK_0145 145 153.13 152.98 -0.15

(0.10%) 152.52 157.50 152.43 157.10

MICK_0135 135 153.13 152.82 -0.31

(0.21%) 152.52 157.50 152.43 157.10

MICK_0135c1 135 152.91 152.36 -0.55

(0.36%) 152.52 157.50 152.43 157.10

4.4.2 Downstream Boundary

As noted in Section 3.1.2, the downstream boundary may not be far enough away from the area of interest so as to not affect the results. Therefore, the sensitivity of the model results to the distance downstream of this boundary has been explored.

The downstream boundary has been moved downstream by 400 m which is greater than calculated backwater length of 383 m.

Table 11 shows the maximum water levels for the 1 in 1000 year event from the ISIS model of Micklehurst Brook if the downstream boundary is moved 400 m further downstream. The full model results are shown in Appendix 2.

The results of the sensitivity analysis show that model is not very sensitive to changes in the distance of the downstream boundary downstream. The only changes experienced are at the downstream culvert with an increase in the maximum water level within Micklehurst Brook of 0.15 m or 36% however, this would have negligible effect on the overall water


© Enzygo Ltd 2011 26 SHF.135.001.R.002.A

levels, flood outline and therefore flood risk at the site. The flooding will be either contained within the river channel or river corridor and would not impact on the proposed properties.

Table 11 - Maximum water levels for the 1 in 1000 year event for Micklehurst Brook with the downstream boundary 400 m further downstream

Chainage Reference

Chainage (m)


(mAOD Right Bank

(mAOD)

1 in 1000 year

event

Decrease by 20%

Difference (m)

Top of Bank

Site Level

Top of Bank

Site Level

MICK_0307 307 166.36 166.36 0.00

(0.00%) 167.02 166.00 166.87 N/A

MICK_0305 305 164.63 164.63 0.00

(0.00%) 165.20 166.00 167.74 N/A

MICK_0275 275 160.27 160.27 0.00

(0.00%) 161.41 165.50 160.87 N/A

MICK_0235 235 158.86 158.86 0.00

(0.00%) 158.70 163.00 158.85 N/A

MICK_0225 225 158.82 158.82 0.00

(0.00%) 158.70 162.50 158.85 N/A

MICK_0185 185 156.54 156.54 0.00

(0.00%) 156.19 160.00 156.23 N/A

MICK_0145 145 153.13 153.16 -0.03

(0.02%) 152.52 157.50 152.43 157.10

MICK_0135 135 153.13 153.13 0.00

(0.00%) 152.52 157.50 152.43 157.10

MICK_0135c1 135 152.91 153.06 +0.15

(0.10%) 152.52 157.50 152.43 157.10

4.5 Discussion

The results show that only the buffer strip and non the built development site would be inundated with water from Micklehurst Brook during the 1 in 100 year (plus climate change) event and the 1 in 1000 year event.

If a 75% blockage and/or collapse of the downstream culvert was to occur during the 1 in 1000 year event the maximum water levels would increase by a maximum of 1.03 m just upstream of the culvert. The maximum water levels experienced across the majority of the site would be unaffected.

The results from the three model runs show that flooding will be either contained within the river channel or river corridor (see Drawing 4). The flooding would be of minor nature with the developable site ground levels being located above the maximum water levels.


© Enzygo Ltd 2011 27 SHF.135.001.R.002.A

Low levels of flooding would be experienced on the buffer strip along the river corridor.

Flooding of the river corridor is expected at this return period and would not impact on the overall development of the site as a buffer strip along Micklehurst Brook would be required by the Environment Agency and has been designed in the masterplan for the site (see Section 6 of the FRA for more details).


Any change in Manning’s ‘n’ roughness due to seasonal changes in the vegetation may increase the maximum water levels experienced within Micklehurst Brook. This would have very little effect on the overall water levels, flood outline and therefore flood risk at the site.


The results of the sensitivity analysis show that model is not very sensitive to changes in the distance of the downstream boundary downstream. The only changes experienced are at the downstream culvert with an increase in the maximum water level within Micklehurst Brook however, this would have very little affect on the overall water levels, flood outline and therefore flood risk at the site. The flooding will be either contained within the river channel or river corridor and would not impact on the developable site and proposed properties.

All these results confirm that none of the proposed properties on the site would be affected by flooding from Micklehurst Brook including the three proposed houses (no. 1, 2 and 3) adjacent to Micklehurst Road next to no.73 Micklehurst Road (see Drawing 4).


© Enzygo Ltd 2011 28 SHF.135.001.R.002.A

5.0 SITE DRAINAGE

5.1 Surface Water Drainage

Further information is provided with regards to the surface water management strategy for the site.

5.2 Geology

An examination of the 1: 50 000 scale Geological Survey map of the area8 indicates that Glacial Till9 underlies this site. On penetrating the till the solid geology is shown to comprise Lower Kinderscout Grit of the Carboniferous Millstone Grit Series. The BGS Lexicon of named rock units10 describes this material as medium to very coarse-grained feldspathic, massive or cross-bedded sandstone with shale pellets, frequently pebbly sandstone, shales and sandy shales, siltstone and sandstone with shale. The map further indicates that there is a series of north - south trending faults a short distance to the east of the site.

5.3 Soil

Borehole records taken during the site investigations as part of the geotechnical assessments for the site has concluded that the soil over the majority of the site consists of topsoil with little made ground evident. Areas of sand and gravel were also found to be abundant at depth.

Areas of made ground are described as very loose, predominantly brown, yellow and grey sand with scattered angular to well rounded gravel of mixed lithology and some ash and clinker inclusions. On penetrating the sand and beneath the topsoil in the remainder of the site was predominantly Till with gravels.

5.4 Groundwater

Small amounts of groundwater are known to be located beneath the site. Water strikes were encountered in most of the boreholes with a minimum depth below the ground surface of 1.20 m up to a maximum depth below the ground surface of 14.80 m.

The proposed development site is not located within a Source Protection Zone.

5.5 Infiltration Methods

It has therefore been concluded that infiltration methods, such as permeable paving, filter strips and soakaways, which are proposed for use of the work will work to a satisfactory standard. As once the overlying made ground and till is penetrated the underlying geology

8 Sheet 86, solid and drift edition, Glossop. 9 Formerly known as Boulder Clay. 10 BGS Lexicon of Named Rock Units.


© Enzygo Ltd 2011 29 SHF.135.001.R.002.A

has an acceptable infiltration rate. This would be confirmed prior to construction via a BRE 365 Infiltration test.

5.6 Surface Water Management Strategy

Sustainable water management measures (SUDS) will be used to control the surface water runoff from the proposed development site therefore, managing the flood risk to the site and surrounding areas from surface water runoff.

A surface water management strategy for the site has been developed to manage and reduce the flood risk posed by the surface water runoff from the site. The surface water management strategy has sufficient capacity for the entire site.

The Environment Agency and Local Planning Authority (LPA) require that the surface water run-off from the development site does not exceed the surface water run-off from the site in its present use (i.e. greenfield runoff rate).

A number of the SUDS options will be used on the site in combination to attenuate the surface water runoff. These are

Permeable paving of all driveways,

Filter strips and soakaways to drain retaining structures;

Creation of a wet woodland/marsh; and

Oversized pipes.

Permeable paving will be used on all of the driveways of the residential dwellings and filter strips discharging to soakaway will be used to drain the retaining structures and landscaped areas where needed.

The creation of a wet woodland/marsh will store and attenuate a proportion of the rainfall and will allow infiltration of the surface water into the soil substrate. A number of other landscaped areas and gardens have also been incorporated into the masterplan for the site, which will allow a proportion of the rainfall to infiltrate into the soil substrate.

These will provide an attractive scheme that enhances the site provides other benefits such protecting groundwater, ecology, landscape, improve the sustainability of the site, while also recycling a valuable resource.

Surface water runoff would be directed to the drainage system through drainage gullies located around the perimeter of the buildings and through contouring of the hardstanding areas.

Oversized pipes in the main access road will attenuate the surface water runoff from the site.


© Enzygo Ltd 2011 30 SHF.135.001.R.002.A

The attenuation volume required to restrict runoff from the developed site has been calculated using the industry standard InfoWorks software suite in accordance with ‘Sewers for Adoption 6th Edition’.

5.6.1 Attenuation Requirement

The attenuation volume required for the 1 in 100 year rainfall event plus climate change (+ 30%) through the use of an oversized pipe has been determined.

The attenuation volume required to restrict runoff to the 1 in 1 year greenfield runoff rate of 15.6l/s (see FRA) has been determined to be approximately 230m3.

The system was modelled within InfoWorks base upon an impermeable contributing area of 0.51ha (i.e. 0.34ha of roadways and 0.21ha of roofs) using the 1 in 100 year rainfall event plus climate change (+ 30%).

It is proposed that the detailed design of the final scheme would be agreed with the Environment Agency and LPA prior to works commencing.

5.7 Designing for Exceedence

It is not economically viable or sustainable to build a drainage system that can accommodate the most extreme events. Consequently, the capacity of the drainage system may be exceeded on rare occasions, with excess water flowing above ground11.

The attenuation requirements have been designed to accommodate the 1 in 100 year rainfall event plus climate change (+ 30%). To prevent flooding of the proposed properties if the design event is exceeded the surface flows generated will be managed.

The design of the site layout provides an opportunity to manage this exceedance flow and ensure that indiscriminate flooding of property does not occur.

In particular the design includes preferential flow paths that convey water away from buildings (see Drawing 2). Drawing 2 shows that the resultant surface water flows are directed away from the properties down the main access road, into landscaped areas and the wet woodland/marsh areas. This maintains the overall existing flood flow routes (see Drawing 1 and 2)

Therefore, managing and mitigating the flood risk from surface water runoff to the proposed properties.

11 CIRIA (2006) Designing for exceedance in urban drainage – good practice.


© Enzygo Ltd 2011 31 SHF.135.001.R.002.A

6.0 SUMMARY AND CONCLUSION

This document forms an addendum to the Enzygo FRA dated January 2011 for the proposed development site at Micklehurst Road, Mossley, Tameside.

The report details the flood risk at the site and how this could be managed and mitigated to allow the site to be development for housing in support of the enclosed detailed planning application for residential development and access.

The results of the 1D model show that the development site will not be inundated with water from Micklehurst Brook during the 1 in 100 year (plus climate change) event and the 1 in 1000 year event.

If a 75% blockage and/or collapse of the downstream culvert was to occur during the 1 in 1000 year event the maximum water levels would increase by a maximum of 1.03 m just upstream of the culvert. The maximum water levels experienced across the site would be unaffected and the buffer strip would provide increased flood protection in this area.

The results from the three model runs show that flooding will be either contained within the river channel/river corridor as allocated in the development layout as a buffer strip.

The flooding would be of minor nature with all development site ground levels being located above the maximum water levels.

Low levels of flooding would be experienced on the site along the river corridor/buffer strip. Flooding of the river corridor is expected at this return period and would not impact on the overall development of the site as a buffer strip along Micklehurst Brook would be required by the Environment Agency and has been designed in the masterplan for the site.


Any change in Mannings ‘n’ roughness due to seasonal changes in the vegetation may increase the maximum water levels experienced within Micklehurst Brook. This would have very little effect on the overall water levels, flood outline and therefore flood risk at the site.


The results of the sensitivity analysis show that model is not very sensitive to changes in the distance of the downstream boundary downstream. The only changes experienced are at the downstream culvert with an increase in the maximum water level within Micklehurst Brook however, this would have very little affect on the overall water levels, flood outline and therefore flood risk at the site. The flooding will be either contained within the river channel or river corridor and would not impact on the developable site and proposed properties.


© Enzygo Ltd 2011 32 SHF.135.001.R.002.A

All these results confirm that none of the proposed properties on the site would be affected by flooding from Micklehurst Brook including the three proposed houses (no. 1, 2 and 3) adjacent to Micklehurst Road next to no.73 Micklehurst Road.

In addition, further information has been provided with regards to the surface water management strategy for the site. It has been concluded that infiltration methods, such as permeable paving, filter strips and soakaways, which are proposed for use of the work will work to a satisfactory standard.

The attenuation volume required for the 1 in 100 year rainfall event plus climate change (+ 30%) through the use of an oversized pipe has been determined. The attenuation volume required to restrict runoff to the 1 in 1 year greenfield runoff rate of 15.6l/s has been determined to be approximately 230 m3.

To prevent flooding of the proposed properties if the design event is exceeded the surface water flows generated will be managed. The design of the site layout provides an opportunity to manage this exceedance flow and ensure that indiscriminate flooding of property does not occur. In particular the design includes preferential flow paths that convey water away from buildings. The resultant surface water flows are directed away from the properties down the main access road, into landscaped areas and the wet woodland/marsh areas. Therefore, managing and mitigating the flood risk from surface water runoff to the proposed properties, by following the overall historic flood flowpaths.

This addendum to FRA demonstrates that the proposed development would be operated with minimal risk from flooding, would not increase flood risk elsewhere and is compliant with the requirements of PPS25.

The development should not therefore be precluded on the grounds of flood risk.


© Enzygo Ltd 2011 33 SHF.135.001.R.002.A

DRAWINGS


© Enzygo Ltd 2011 34 SHF.135.001.R.002.A

APPENDIX 1

Environment Agency Correspondence

Environment Agency Appleton House, 430 Birchwood Boulevard, Birchwood, Warrington, Cheshire, WA3 7WD. Customer services line: 08708 506 506 Email: [email protected] www.environment-agency.gov.uk Cont/d..

Tameside Metropolitan Borough Council Planning Council Offices Wellington Road Ashton-under-Lyne Lancashire OL6 6DL

Our ref: SO/2011/108700/01-L01 Your ref: 11/00098/FUL Date: 10 March 2011

Dear Sir/Madam CONSTRUCTION OF 36 HOUSES WITH ASSOCIATED ACCESS AND LANDSCAPING LARGE DEVELOPMENT SITE MICKLEHURST ROAD, MOSSLEY, TAMESIDE Thank you, for your letter dated 15 February 2011. We would wish to make the following comments. In the absence of an acceptable Flood Risk Assessment (FRA) we OBJECT to the grant of planning permission and recommend refusal on this basis for the following reasons: Reason The FRA submitted with this application does not comply with the requirements set out in Annex E, paragraph E3 of Planning Policy Statement 25 (PPS 25). The FRA does not provide a suitable basis for assessment to be made of the flood risks arising from the proposed development. In particular, the submitted FRA fails to:

1. Confirm historical flooding affecting Micklehurst Road. We are aware of a flood event which occurred on 24 August 2004. Details could be provided on request. The FRA which supported Phase 1 of this large development also referred to historical flood events.

2. Quantify the flood risks from Micklehurst Brook taking the impacts of climate

change into account. Section 4.1.1 of the FRA states that the site is at least 2m above the elevation of the watercourse. The risk of blockage on the downstream culvert should be investigated further to confirm that the proposed properties off Micklehurst Road adjacent to no. 73 would not be affected by overland flow.

3. Consider the effect of a range of flooding events including extreme events on

people and property. The FRA has not conducted a quantitative assessment of

End

2

the flood risk from Micklehurst Brook, therefore, the risk from a 1000 year rainfall event has not been estimated (See section 4.5).

4. We consider the site to be entirely Greenfield and the proposed development

should attenuate runoff volumes to Greenfield rates as per the SUDS Manual. Two different rates are given in section 5.7.1 leading to two different attenuation volumes. For a full application we consider that the surface water management design should have been advanced to support the proposed layout demonstrating that the attenuation can be provided on site. Also, the FRA should demonstrate that the proposed development or surrounding land would not be affected by an exceedence event on the drainage system.

If the Council is minded to approve the application as submitted, then, in accordance with paragraph 26 of PPS 25, we should be notified in order that further representations may be considered. Please do not hesitate to contact me should you wish to discuss this position. Yours faithfully Mrs SYLVIA WHITTINGHAM Planning Liaison Officer Direct dial 01925 543362 Direct fax 01925 852260 Direct e-mail [email protected]

®f

®f#*#*#*

STBR01_0408STBR01_0407a

STBR01_0407b

WTNFRM30702

WTNFRM28961

FRA Map centred on SD9809701950 - created 19 January 2010 [Ref:WTNFRM30702]

LegendMain River

#* Export_OutputFlood Zone 3Flood Zone 2

Contact Us: National Customer Contact Centre, PO Box 544, Rotherham, S60 1BY. Tel: 08708 506 506 (Mon-Fri 8-6). Email: [email protected]© Environment Agency copyright and / or database rights 2009. All rights reserved. © Crown Copyright and database right. All rights reserved. Environment Agency, 100026380, 2010.

Scale 1:10,000

Node Ref. Eastings Northings

Max Stage, 20-year (mAOD)



Max Stage, 100-year + 20% Flow [Climate Change]

(mAOD)STBR01_0408 397971 401945 146 146.08 146.2 146.1

STBR01_0407b 397965 401946 146.01 146.14 146.35 146.19STBR01_0407a 397967 401946 146.04 146.18 146.42 146.23

1 in 100 year (plus climate change) event

Label Max Stage

MICK_0315 166.253

MICK_0307 166.167

MICK_0305 164.455

MICK_0295i 162.97

MICK_0285i 161.432

MICK_0275 160.151

MICK_0265i 159.543

MICK_0255i 159.181

MICK_0245i 159.053

MICK_0235 158.63

MICK_0225 158.63

MICK_0215i 157.956

MICK_0205i 157.398

MICK_0195i 156.917

MICK_0185 156.417

MICK_0175i 154.198

MICK_0165i 153.661

MICK_0155i 153.142

MICK_0145 152.312

MICK_0135sp 151.981

MICK_0135out 146.359

MICK_0135in 151.981

MICK_0135c4 146.61

MICK_0135c3 148.266

MICK_0135c2 149.911

MICK_0135c1 151.648

MICK_0135 151.981

MICK_0005sp 146.359

MICK_0005 146.359

MICK_0000 146.1

1 in 1000 year event

Label Max Stage

MICK_0315 166.427

MICK_0307 166.355

MICK_0305 164.631

MICK_0295i 163.129

MICK_0285i 161.584

MICK_0275 160.266

MICK_0265i 159.715

MICK_0255i 159.399

MICK_0245i 159.243

MICK_0235 158.858

MICK_0225 158.82

MICK_0215i 158.136

MICK_0205i 157.561

MICK_0195i 157.056

MICK_0185 156.539

MICK_0175i 154.348

MICK_0165i 153.825

MICK_0155i 153.377

MICK_0145 153.118

MICK_0135sp 153.125

MICK_0135out 150

MICK_0135in 153.125

MICK_0135c4 147.801

MICK_0135c3 149.465

MICK_0135c2 151.129

MICK_0135c1 152.906

MICK_0135 153.125

MICK_0005sp 150

MICK_0005 147.723

MICK_0000 146.2

1 in 1000 year with a 75% blockage of the downstream culvert

Label Max Stage

MICK_0315 166.427

MICK_0307 166.355

MICK_0305 164.628

MICK_0295i 163.322

MICK_0285i 161.645

MICK_0275 160.26

MICK_0265i 159.715

MICK_0255i 159.399

MICK_0245i 159.243

MICK_0235 158.858

MICK_0225 158.82

MICK_0215i 158.134

MICK_0205i 157.561

MICK_0195i 157.055

MICK_0185 156.539

MICK_0175i 154.345

MICK_0165i 153.868

MICK_0155i 153.567

MICK_0145 153.5

MICK_0135sp 153.492

MICK_0135out 146.512

MICK_0135in 153.492

MICK_0135c4 146.456

MICK_0135c3 148.907

MICK_0135c2 151.358

MICK_0135c1 153.935

MICK_0135 153.492

MICK_0005sp 146.512

MICK_0005 146.495

MICK_0000 146.2

1 in 1000 year + 20% mannings n

Label Max Stage

MICK_0315 166.465

MICK_0307 166.355

MICK_0305 164.68

MICK_0295i 163.183

MICK_0285i 161.601

MICK_0275 160.315

MICK_0265i 159.796

MICK_0255i 159.489

MICK_0245i 159.327

MICK_0235 159.076

MICK_0225 158.897

MICK_0215i 158.208

MICK_0205i 157.628

MICK_0195i 157.109

MICK_0185 156.59

MICK_0175i 154.424

MICK_0165i 153.907

MICK_0155i 153.445

MICK_0145 153.118

MICK_0135sp 153.125

MICK_0135out 150

MICK_0135in 153.125

MICK_0135c4 147.801

MICK_0135c3 149.465

MICK_0135c2 151.129

MICK_0135c1 152.905

MICK_0135 153.125

MICK_0005sp 150

MICK_0005 146.504

MICK_0000 146.2

1 in 1000 year ‐ 20% mannings n

Label Max Stage

MICK_0315 166.387

MICK_0307 166.355

MICK_0305 164.549

MICK_0295i 163.056

MICK_0285i 161.489

MICK_0275 160.206

MICK_0265i 159.626

MICK_0255i 159.292

MICK_0245i 159.151

MICK_0235 158.739

MICK_0225 158.73

MICK_0215i 158.054

MICK_0205i 157.485

MICK_0195i 156.985

MICK_0185 156.476

MICK_0175i 154.256

MICK_0165i 153.715

MICK_0155i 153.258

MICK_0145 152.979

MICK_0135sp 152.83

MICK_0135out 146.413

MICK_0135in 152.83

MICK_0135c4 146.581

MICK_0135c3 148.444

MICK_0135c2 150.339

MICK_0135c1 152.361

MICK_0135 152.83

MICK_0005sp 146.413

MICK_0005 146.413

MICK_0000 146.2

1 in 1000 year event with D/S boundary a further 400m D/S

Label Max Stage

MICK_0315 166.427

MICK_0307 166.355

MICK_0305 164.631

MICK_0295 163.129

MICK_0285 161.584

MICK_0275 160.266

MICK_0265 159.715

MICK_0255 159.399

MICK_0245 159.243

MICK_0235 158.858

MICK_0225 158.82

MICK_0215 158.136

MICK_0205 157.561

MICK_0195 157.056

MICK_0185 156.539

MICK_0175 154.348

MICK_0165 153.825

MICK_0155 153.401

MICK_0145 153.155

MICK_0135sp 153.125

MICK_0135out 150

MICK_0135in 153.125

MICK_0135c4 149.318

MICK_0135c3 150.537

MICK_0135c2 151.756

MICK_0135c1 153.057

MICK_0135 153.125

MICK_0005sp 150

MICK_0005 149.268

MICK_0000 146.2


© Enzygo Ltd 2011 37 SHF.135.001.R.002.A

AMP Asset Management Plan

AONB Area of Outstanding Natural Beauty

BGS British Geological Survey

CEH Centre for Ecology and Hydrology

CC Climate Change

CFMP Catchment Flood Management Plan

CIRIA Construction Industry Research Information Association

Defra Department for Environment, Food and Rural Affairs

DC Development Control

EA Environment Agency

FEH Flood Estimation Handbook

FEH CD-ROM v2.0 Flood Estimation Handbook CD-ROM version 2

FCDPAG Flood and coastal defence project appraisal guidance

FRA Flood Risk Assessment (site specific)

FSR Flood Screening Report

FRSA Flood Risk Standing Advice

FRMS Flood Risk Management Strategy

GDPO 1995 Town and Country Planning (General Development Procedures) Order 1995

GIS Geographic Information System

IDB Internal Drainage Board

IPCC Intergovernmental Panel on Climate Change

IUD Integrated Urban Drainage

IUMD Integrated Urban Drainage Management

LDD Local Development Document

LPA Local Planning Authority

NaFRA National Flood Risk Assessment

NFCDD National Flood and Coastal Defence Database


© Enzygo Ltd 2011 38 SHF.135.001.R.002.A

mAOD Metres Above Ordnance Datum

ODPM Office of the Deputy Prime Minister

Ofwat Water Services Regulation Authority

OS Ordnance Survey

PPG Planning Policy Guidance Note

PPS Planning Policy Statement

RBMP River Basin Management Plan

ReFH Revitalised Flood Estimation Handbook method

RFRA Regional Flood Risk Appraisal

RDA Regional Development Agency

RMS Risk Management Solutions

RPB Regional Planning Body

RSS Regional Spatial Strategy

SAC Special Area for Conservation

SFRA Strategic Flood Risk Assessment

SMP Shoreline Management Plan

SPA Special Protection Area

SPD Supplementary Planning Document

SSSI Site of Special Scientific Interest

SUDS Sustainable Drainage Systems

SWMP Surface Water Management Plan

TCPA 1990 Town and Country Planning Act 1990

UKCIP UK Climate Impact Programme

WFD Water Framework Directive


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Term Definition

Annual exceedance probability

The estimated probability of a flood of a given magnitude occurring or being exceeded in any year, e.g. 1 in 100

chance or 1%.

Adoption of sewers The transfer of responsibility for the maintenance of a

system of sewers to a sewerage undertaker.

Attenuation Reduction of peak flow and increased duration of a flow

event.

BGS Flooding Susceptibility Map

This data set is the first national hazard or susceptibility data set of groundwater flooding. The resolution of the modelled output is 50m by 50m cells. This data set is a hazard data

set, not a risk data set, meaning that it does not provide any information about the likelihood of a groundwater flooding

event occurring. It is noted that the BGS Groundwater Flooding Susceptibility Map is to be used as a screening

tool, and should not be used to inform planning decisions. Based on geological and hydrogeological information, digital

data has been used to identify areas where geological conditions could enable groundwater flooding to occur and where groundwater may come close to the ground surface.

The data set defines areas with one of five levels of groundwater susceptibility, ranging from high susceptibility to negligible or no susceptibility. Areas with no data represent

areas with no susceptibility to groundwater flooding.

BGS Indicators of Flooding Map

The map shows areas vulnerable to two main types of flooding - inland (river floodplains) and coastal/estuarine.

The map is based on observation of the types of geological deposit present and does not take into account any man-

made influences such as house building or flood protection schemes. It also does not take into account low-lying areas where flooding could occur but where there are no materials

indicating flooding in the geological past.

Catchment Flood Management Plans (CFMP)

A strategic planning tool through which the Environment Agency will seek to work with other key decision-makers within a river catchment to identify and agree policies for

sustainable flood management.

Climate change (cc) Long-term variation in global temperatures and weather

patterns, both natural and a as a result of human activity.

Design event A historic or notional flood event of a given annual probability

against which suitability of proposed development is assessed and mitigation measures, if any, designed.

Design event exceedance Flooding resulting from an event which exceeds the

magnitude for which the defences protecting a development were designed – see residual risk.

Design flood level The maximum estimated water level during the design event.


© Enzygo Ltd 2011 41 SHF.135.001.R.002.A

Term Definition

Exceedance flood risk assessment

A study to assess the risk of a site or area being affected by exceedance flow, and to assess the impact that any changes

made to a site or area will have on the exceedance flood risk.

Exceedance flow Excess flow that emerges on the surface once the

conveyance capacity of a drainage system is exceeded.

Flood Defence

Flood defence infrastructure, such as flood walls and embankments, intended to protect an area against flooding

to specified standard of protection.

Flood and Coastal Defence Operating Authorities

The Environment Agency, local authorities and Internal Drainage Boards with legislative powers to undertake flood

and coastal defence works.

Flooding due to infrastructure failure

Non-natural or artificial sources of flooding can include reservoirs, canal and lakes where water is retained above natural ground level, operational and redundant industrial

processes including mining, quarrying and sand and gravel extraction, as they may increase floodwater depths and

velocities in adjacent areas.

Flooding from artificial drainage systems

In urban areas, rainwater is frequently drained into surface water sewers or sewers containing both surface and waste water known as “combined sewers”. Flooding can results

when the sewer is overwhelmed by heavy rainfall, becomes blocked or is of inadequate capacity.

Flooding from rising/high groundwater

Groundwater flooding occurs when water levels in the ground rise above surfaces elevations.

Flood Mitigation

All measures to reduce the effect of flooding including flood avoidance, flood resistance and flood resilience.

Flood Map

A map produced by the Environment Agency providing an indication of the likelihood of flooding within all areas of

England and Wales, assuming there are no flood defences. Only covers river and sea flooding.

Floodplain

A Floodplain is flat or nearly flat land adjacent to a watercourse, an estuary or the sea, that experiences

occasional or periodic flooding, or would flow but for the presence of flood defences where they exist.

Flood risk management strategy

A long-term approach setting out the objectives and options for managing flood risk, taking into account a broad range of

technical, social, environment and economic issues.

Flood risk assessment (including regional, sub-

regional/strategic, and site specific)

A study to assess the risk to an areas or site from flooding, now and in the future, and to assess the impact that any changes or development on the site or area will have on

flood risk to the site and elsewhere. It may identify, particularly at more local levels, how to manage those

changes to ensure that flood risk is not increased. PPS25


© Enzygo Ltd 2011 42 SHF.135.001.R.002.A

Term Definition

differentiates between region, sub-regional/strategic and site specific flood risk assessments.

Flood risk management measure

Any measure which reduces flood risk such as flood defences.

Flood Zone A geographic area within which the flood risk is in a

particular range, as defined within PPS25.

Fluvial Flooding

Flooding from rivers, streams, watercourses etc these flood when the amount of water in them exceeds the flow capacity

of the channel.

Freeboard The difference between the flood defence level and the

design flood level.

Greenfield land Land that has not been previously developed.

Hold the line Maintaining the existing flood defences and control

structures in their present positions and increase the standard of protection against flooding in some areas.

Internal Drainage Board (IDB)

Public authority and are responsible for providing a service in land drainage and flood protection in areas of the UK.

Local development framework (LDF)

A non-statutory term used to describe a folder of documents which includes all the local planning authority’s Local

Development Documents (LDDs).

Local Development Documents (LDDs)

All development plan documents which form part of the statutory development plan, as well as supplementary

planning documents which do not form part of the statutory development plan.

Main River

A watercourses designated on a statutory map of Main Rivers, maintained by Defra, on which the Environment

Agency has drainage and flood control management responsibility.

Ordinary Watercourse

All rivers, streams, ditches, drains, cuts, dykes, sluices, sewers (other than public sewers) and passages through which water flows which do not form part of a Main River. Local authorities and, where relevant, Internal Drainage

Boards have similar permissive powers on ordinary watercourses, as the Environment Agency has on Main

Rivers.

Overland flow flooding

Otherwise known as pluvial flooding. Intense rainfall, often of short duration, that is unable to soak into the ground or enter drainage systems can run quickly off land and result in local

flooding.

Planning Policy Statement 25: Development and Flood Risk (PPS25)

A statement of policy issued by central Government on flood risk to replace Planning Policy Guidance 25: Development

and Flood Risk (PPG25).

Precautionary principle Where there threats of serious or irreversible damage, lack


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Term Definition

of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent

environmental degradation.

Previously-developed land (often referred to as

brownfield land)

Land which is or was occupied by a permanent structure, including the curtilage of the developed land and any

associated fixed surfaces infrastructure (PPS3 annex B)

Regional Spatial Strategy (RSS)

A broad development strategy for a region for a 15 to 20 year period prepared by the Regional Planning Body.

Reservoir (large raised) A reservoir that holds at least 25,000 cubic metres of water

above natural ground level, as defined by the Reservoirs Act 1975.

Resilience

Constructing the building in such as way that although floodwater may enter the building its impact is minimised,

structure integrity is maintained and repair, drying and cleaning are facilitated.

Resistance Constructing a building in such as way as to prevent

floodwater entering the building or damaging its fabric. This has the same meaning as flood proof.

Return period The long-term average period between events of a given

magnitude which have the same annual exceedance probability of occurring.

Residual risk The risk which remains after all risk avoidance, reduction

and mitigation measures have been implemented.

Risk Management Solutions (RMS) Pluvial

Flood Map

The RMS data model does not take into account Coastal/Storm Surge Flooding, Dam Failure Flooding, Sewer

Overflow Flooding or risk of flooding from the sea. The source data is created using 0.0005 decimal degree grid

cells which represent the ground surface which have been obtained by flying over the UK and sending a radar signal

down to the ground which then bounces back up to the plane. This has been translated into British National Grid - as

a result of the translation, the data does not appear as a regular grid. Due to this re-projection cell sizes will vary

across the country.

River basin management A management plan for all river basins required by the Water

Framework Directive.

Shoreline management plan (SMP)

A plan providing a large-scale assessment of the risk to people and to the developed, historic and natural

environment associated with coastal processes. It presents a policy framework to manage these risks in a sustainable

manner.

Standard of protection The design event or standard to which a building, asset or

area is protected against flooding, generally expressed as an annual exceedance probability.


© Enzygo Ltd 2011 44 SHF.135.001.R.002.A

Term Definition

Sustainable Drainage Options (SUDS)

SUDS are alternatives to traditional piped drainage systems that utilise natural drainage processes to convey, and

improve the quality of surface water runoff generated by urban development. Including: green roofs, water butts, swales , rainwater harvesting, filter strips, wetland areas,

infiltration basins, detention basins, retention ponds, porous and pervious paving.

Surface water run-off The flow of water from area caused by rainfall.

Tidal Flooding

Flooding to low-lying land from the sea and tidal estuaries is caused by storm surges and high tides.

Vulnerability class PPS25 provides a vulnerability classification to assess which

uses of land maybe appropriate in each flood risk zone.

Washland An area of the floodplain that is allowed to flood or is

deliberately flooded by a river of stream for flood management purposes.

Water Framework Directive A European Community Directive (2000/60/EC) of the

European Parliament and Council designed to integrate the way water bodies are managed across Europe.

Windfall sites Sites which become available for development unexpectedly

and are therefore not included as allocated land in a planning authority’s development plan.

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