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Client : The New Romney Consortium Flood Risk Assessment for Proposed Development on the Land at Cockreed Lane, New Romney September 2010

Herrington Consulting Limited

Unit 6 – Barham Business Park

Elham Valley Road

Barham

Canterbury

Kent, CT4 6DQ

Tel/Fax +44 (0)1227 833855

www.herringtonconsulting.co.uk

This report has been prepared by Herrington Consulting Ltd in accordance with the instructions of their client, The New Romney Consortium, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk.

© Herrington Consulting Limited 2010

Herrington Consulting Limited

Unit 6 – Barham Business Park

Elham Valley Road

Barham

Canterbury

Kent, CT4 6DQ

Tel/Fax +44 (0)1227 833855

www.herringtonconsulting.co.uk

This report has been prepared by Herrington Consulting Ltd in accordance with the instructions of their client, The New Romney Consortium, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk.

© Herrington Consulting Limited 2010

Client : The New Romney Consortium Flood Risk Assessment for Proposed Development on the Land at Cockreed Lane, New Romney Contents Amendment Record This report has been issued and amended as follows:

Issue Revision Description Date Written by Checked by

1 0 Draft report issued by email 11 Sept 2010 SPH SAB

2 0 Final 13 Sept 2010 SPH SAB

This page is left intentionally blank

Contents

1 Background and Scope of Appraisal 1

2 Development Description and Planning Context 2 2.1 Site Location 2 2.2 The Development 2 2.3 The Sequential Test 2 2.4 The Exception Test 6

3 Definition of Flood Hazard 8 3.1 Site Specific Information 8 3.2 Potential Sources of Flooding 9 3.3 Existing Flood Risk Management Measures 11

4 Probability and Consequence of Flooding 12 4.1 Existing Likelihood of Flooding 12 4.2 The Extent of Flooding 13 4.3 Depth and Velocity of Flooding 13 4.4 Flooding Characteristics 14

5 Climate Change 15 5.1 Potential Changes in Climate 15 5.2 Impacts of Climate Change on the Development Site 16

6 Flood Mitigation Measures 17 6.1 Application of the Sequential Approach at a Local Scale 17 6.2 Raising Floor Levels & Land Raising 18

7 Offsite Impacts 19 7.1 Proximity to Watercourse and Flood Defence Structures 19 7.2 Displacement of Floodwater 19 7.3 Impact on Fluvial Morphology & Impedance of Flood Flows 19

8 Surface Water Management 21 8.1 Site Characteristics 21 8.2 Sustainable Drainage Systems (SUDS) 22 8.3 Requirements for Surface Water Discharge 24 8.4 Preferred Surface Water Management Strategy 24

9 Residual Risk 28 9.1 Flood Resistance and Resilience 28 9.2 Public Safety and Access 29

9.3 Flood Warning 31

10 Conclusions 32 10.1 Recommendations 33

A Appendices 35

Land at Cockreed Lane, New Romney Flood Risk Assessment

1

1 Background and Scope of Appraisal

Flooding is a major issue in the United Kingdom. The impacts can be devastating in terms of the

cost of repairs, replacement of damaged property and loss of business. The objectives of the FRA

are therefore to establish the following:

whether a proposed development is likely to be affected by current or future flooding

from any source

whether the development will increase flood risk elsewhere within the floodplain

whether the measures proposed to deal with these effects and risks are appropriate

whether the site will be safe to enable the passing of part c of the Exception Test if this is

appropriate (paragraph D9 of PPS25)

Herrington Consulting has been commissioned by the New Romney Consortium and Evelyn Frith

to prepare a Flood Risk Assessment for the proposed development on the land at Cockreed

Lane, New Romney, Kent, TN28 8TE.

This appraisal has been undertaken in accordance with the criteria set out for Flood Risk

Assessments in the Planning Policy Statement 25 – Development and Flood Risk (PPS 25) and

to ensure that due account is taken of industry best practice, it has been carried out in line with

the CIRIA Report C624 ‘Development and flood risk - guidance for the construction industry’.

Reference is also made to the Practice Guide to PPS25 (December 2009) that has been

published by the Department for Communities and Local Government.


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2 Development Description and Planning Context

2.1 Site Location The site is located on the northwestern flank of the town of New Romney and lies approximately

2km inland of the Littlestone-on-Sea shoreline. In total the site covers an area of approximately 12

hectares, and is predominantly a greenfield site at this point in time. The location of the site in

relation to the surrounding area and the shoreline is shown in Figure 2.1. The site plan included in

Appendix A.1 of this report gives a more detailed reference to the site location and layout.

Figure 2.1 – Location map (Contains Ordnance Survey data © Crown copyright and database right 2010)

2.2 The Development The development proposals for the site are currently in the very early master planning stage and

therefore details of scheme layouts etc have not been finalised. The proposals are, however, to

develop the site predominantly for residential use with the school playing fields remaining as

existing.

2.3 The Sequential Test Local Planning Authorities (LPA) are encouraged to take a risk-based approach to proposals for

development in or affecting flood risk areas through the application of the Sequential Test and the

objectives of this test are to steer new development away from high risk areas towards those at

lower risk of flooding. However, in some areas where developable land is in short supply there


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can be an overriding need to build in areas that are at risk of flooding. In such circumstances, the

application of the Sequential Test is used to ensure that the lower risk sites are developed before

the higher risk ones.

PPS25 states that the Sequential Test should be applied at all stages of the planning process and

generally the starting point is the Environment Agency’s flood zone maps. These maps and the

associated information are intended for guidance, and cannot provide details for individual

properties. They do not take into account other considerations such as existing flood defences,

alternative flooding mechanisms and detailed site based surveys. They do, however, provide high

level information on the type and likelihood of flood risk in any particular area of the country. The

flood zones are classified as follows:

Zone 1 – Low probability of flooding – This zone is assessed as having less than a 1 in 1000

annual probability of river or sea flooding in any one year.

Zone 2 – Medium probability of flooding – This zone comprises land assessed as having

between a 1 in 100 and 1 in 1000 annual probability of river flooding or between 1 in 200 and

1 in 1000 annual probability of sea flooding in any one year.

Zone 3a – High probability of flooding - This zone comprises land assessed as having a 1 in

100 or greater annual probability of river flooding or 1 in 200 or greater annual probability of

sea flooding in any one year.

Zone 3b – The Functional Floodplain – This zone comprises land where water has to flow or

be stored in times of flood and can be defined as land which would flood during an event

having an annual probability of 1 in 20 or greater. This zone can also represent areas that are

designed to flood in an extreme event as part of a flood alleviation or flood storage scheme.

The location of the site is shown on the Environment Agency’s flood zone map in Figure 2.2 and

the information provided by this map has been interrogated and summarised in Table 2.2 below.


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Key to flood map

Zone 3 - Extent of flooding from the sea by a flood that has a 0.5% (1 in 200) or greater chance of happening each year or from a river by a flood that has a 1% (1 in 100) or greater chance of happening each year.

Zone 2 - Additional extent of an extreme flood from rivers or the sea. These outlying areas are likely to be affected by a major flood, with up to a 0.1% (1 in 1000) chance of occurring each year.

Flood defences

Areas benefiting from flood defences

Location of development site

Figure 2.2 – Flood zone map showing the location of the development site ( © Environment Agency)

The above mapping shows the development site to be within Flood Zone 3 and not to be

benefiting from existing flood defences that have been constructed in the last 5 years. This

mapping does not distinguish between high risk areas and the functional floodplain, i.e. Zones 3a

and 3b. This is an important differentiation that needs to be made by the FRA because PPS25

states that development other than essential transport and utilities infrastructure should not be

located within the functional floodplain.

The functional floodplain is defined by PPS 25 as land where water has to flow or be stored in

times of flood during events that have a probability of occurrence of 1 in 20 (5%) or greater in any

one year. The Practice Guide to PPS25 goes on to further clarify this by adding the following

definitions:

(a) Areas which would naturally flood with an annual exceedance probability of 1 in 20 (5%)

or greater, but which are prevented from doing so by existing infrastructure or solid

buildings will not normally be defined as functional floodplain.

(b) Developed areas are also not generally considered to comprise functional floodplains,

however, areas such as car parks that have been designed to provide a flood storage

and conveyance function may be.

(c) The functional floodplain may also include areas intended to provide transmission and

storage of water from other sources of flooding (e.g. surface water)


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Based on information provided by the Environment Agency and that derived as part of this

appraisal, the following functional floodplain test is applied:

Do predicted flood levels show that the site will be affected by an event having a return period of 1 in 20 years or less?

x

Is the site defended by flood defence infrastructure that prevents flooding for events having a return period of 1 in 20 years or greater?

Does the site provide a flood storage or floodwater conveyance function? x

Does the site contain areas that are ‘intended’ to provide transmission and storage of water from other sources?

x

Is site within the functional floodplain (Zone 3b). No

Table 2.1 – Functional floodplain test

The flood zone mapping and associated information has been summarised in Table 2.2 below.

P

PS25 states that the Local Planning Authority should apply the sequential approach as part of the

identification of land for development in areas at risk from flooding. Without having

comprehensive knowledge of the land that is available in the district it is not possible for this FRA

to comment on whether the subject site will pass the Sequential Test. However, from the work

that has been undertaken as part of this site specific appraisal it is possible to provide evidence

that can help in the application of the Sequential Test and this is summarised in the conclusions

of this report.

The second level of appraisal is through the application of the more detailed and refined flood risk

information contained within the Strategic Flood Risk Assessments (SFRA). Such a document

has been prepared for the Shepway District and this has been referenced as part of this site-

specific FRA.

Flood Zone (percentage of

site within zone)

Source of

flooding

Probability of flooding

occurring in any one year *

Benefiting from existing flood

defences**

Zone 1 : 0%

Zone 2 0%

Zone 3a 100% Tidal Significant No

Zone 3b 0%

Table 2.2 – Flood zone classification

(*)

Significant: the chance of flooding in any year is greater than 1.3% (1 in 75)

Moderate: the chance of flooding in any year is 1.3% (1 in 75) or less, but greater than 0.5% (1 in 200)

Low: the chance of flooding in any year is 0.5% (1 in 200) or less

(**) the flood zone maps only recognise defences constructed within the last 5 years


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The most detailed stage at which the Sequential Test can be applied is at a site based level.

Careful consideration of the site’s topography and development uses can provide opportunities to

locate more vulnerable buildings on the higher parts of the site and placing less vulnerable

elements such as car parking or recreational use in the areas exposed to higher risk. This

approach is examined later on in this FRA.

2.4 The Exception Test According to PPS 25, if following the application of the sequential test it is not possible, consistent

with wider sustainability objectives, for the development to be located in zones of lower probability

of flooding, the exception test can be applied.

As part of this process it is necessary to consider the type and nature of the development. Table

D.2 in PPS25 defines the type and nature of different development classifications in the context of

their flood risk vulnerability. This has been summarised in Table 2.3 below.

Flood Risk Vulnerability Classification Zone 1 Zone 2 Zone 3a Zone 3b

Essential infrastructure – Essential transport infrastructure, strategic utility infrastructure, including electricity generating power stations

e e

High vulnerability – Emergency services, basement dwellings caravans and mobile homes intended for permanent residential use

e

More vulnerable – Hospitals, residential care homes, buildings used for dwelling houses, halls of residence, pubs, hotels, non residential uses for health services, nurseries and education

e

Less vulnerable – Shops, offices, restaurants, general industry, agriculture, sewerage treatment plants

Water compatible development – Flood control infrastructure, sewerage infrastructure, docks, marinas, ship building, water-based recreation etc.

Shaded cell represents the classification of this development

Key :

Development is appropriate Development should not be permitted

e Exception test required

Table 2.3 - Flood risk vulnerability and flood zone compatibility

From Table 2.3 above it can be seen that the development falls into a classification that requires

the Exception Test to be applied. For the Exception Test to be passed there are three criteria that

must be satisfied and these are listed below:

a) that it can be demonstrated that the development provides wider sustainability benefits to the

community that outweigh flood risk.


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b) that the development is on developable (defined by PPS3 as a site that is in a suitable

location for housing) or previously developed land (commonly known as brownfield land).

c) that a FRA demonstrates that the development will be safe, without increasing flood risk

elsewhere, and where possible, will reduce flood risk overall.

Demonstrating that the development provides wider sustainability benifits and is on ‘developable’

land is outside the scope of this appraisal and is generally a decision that is made by the planning

authority. The primary focus of this FRA is therefore to satisfy criterion (c) of the above.


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3 Definition of Flood Hazard

3.1 Site Specific Information In addition to the high level flood risk information shown in the Environment Agency flood zone

maps, additional data from detailed studies, topographic site surveys and other information

sources is referenced. This section summarises the additional information collected as part of this

FRA.

Information provided by Local Authority – Shepway District Council’s Strategic Flood Risk

Assessment has been referenced as part of the development of this site-specific FRA. The

Council has also been consulted with regards to historic flooding issues and site drainage.

Information provided by Internal Drainage Board (IDB) – The Romney Marshes Area IDB has

been consulted as part of the development of this FRA. The IDB has identified the ownership of

specific watercourses within the general area and has also confirmed the greenfield runoff rate for

this site as being 4 l/sec/ha. A copy of the IDB’s response is included in Appendix A.2.

Site specific topographic surveys - A topographic survey has been undertaken for the site and

a copy of this is included in Appendix A.1. From this it can be seen that the level of the site varies

between 2.3mOD and 3.6mOD.

Geology – Reference to the Geological Survey map for this location shows that the underlying

solid geology in the location of the subject site is Hastings Bed. Overlying this are superficial

deposits of Alluvium.

Soils - Soil type provides a generic description of the drainage characteristics of soils. This will

dictate, for example, the susceptibility of soils to water logging or the capacity of a soil to freely

drain to allow infiltration to groundwater. Soil type may only be fully determined after suitable

ground investigations, although the mapped soil types (soil association) found beneath the study

area may be used as an indicator of permeability and infiltration potential. Reference to the

National Soil Resources Institute mapping shows that the general soil type in this location is

‘Loamy and clayey soils of coastal flats with naturally high groundwater’.

Historic flooding – Shepway District Councils Strategic Flood Risk Assessment highlights areas

of historic flooding within the district. These maps do not highlight any areas of historic flooding in

close proximity to the site.

Other Information – No further information has been provided.


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3.2 Potential Sources of Flooding The main categories of flooding have been assessed as part of this appraisal. The specific issues

relating to each one and its impact on this particular development are discussed below. Table 3.1

at the end of this section summarises the risks associated with each of the flooding sources.

Flooding from Rivers – Areas within the marshes are at risk of flooding from fluvial events.

These events arise as a consequence of long duration high intensity rainfall, which lasts for

several days. The Wallingham Sewer is the closest of these watercourses to the site and has

bank levels set at approximately 2.5mOD. The average land level of the low-lying marsh to the

north of New Romney is around 2.3mOD and consequently any flooding from one of these

watercourses will propagate widely out across the marsh. The lowest parts of the subject site are

also at around 2.3mOD and therefore there is potential for shallow flooding of these lower-lying

areas. However, given that the flows in the Wallingham sewer are derived from water drained

from the surrounding land, it is highly unlikely that anything other than very shallow flooding will

occur in these areas.

Flooding from the Sea - The site lies within a large coastal floodplain and this flooding

mechanism is considered to represent the primary source of risk. Consequently this risk is

examined in greater detail later on in this appraisal.

Flooding from Land (overland flow and surface water runoff) - Overland flooding typically

occurs in natural valley bottoms as normally dry areas become covered in flowing water and in

low spots where water may pond. This flooding mechanism can occur almost anywhere, but is

likely to be of particular concern in any topographical low spot, or where the pathway for runoff is

restricted by terrain or man-made obstructions.

Inspection of the site and its surrounding area shows that land levels fall naturally across the site

towards Cockreed Lane. The catchment immediately above the site is predominantly urban and

therefore it is possible that during an extreme event the surface water and highway drains will

become overwhelmed. This will result in surface water flows in roadways.

Detailed examination of the area topography shows that there are a number of potential flow

paths from the urbanised area above the site. These could lead to overland flows entering the site

during extreme rainfall events. Further examination of the site does, however, show that the any

overland flow entering the site will continue to flow in a north-west direction where it will eventually

enter the drainage ditch on the southern side of Cockreed Lane.

During extreme events, it is possible that the flow from this ditch into the Wallingham Sewer could

be restricted, resulting in water levels in the ditch backing up. At this stage it is not possible to

accurately quantify the impacts of such an event, however, based on the existing topographic

information it is considered unlikely that anything other than shallow flooding could occur in the

lowest lying parts of the site.


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Flooding from Groundwater - Water levels below the ground rise during wet winter months, and

fall again in the summer as water flows out into rivers. In very wet winters, rising water levels may

lead to the flooding of normally dry land, as well as reactivating flow in ‘bournes’ (streams that

only flow for part of the year). Where land that is prone to groundwater flooding has been built on,

the effect of a flood can be very costly, and because groundwater responds slowly compared with

rivers, floods can last for weeks or months. Groundwater flooding generally occurs in rural areas

although it can also occur in more urbanised areas where the process known as groundwater

rebound can cause localised flooding of basements. This increase in the water table level is

occurring as a result of the decrease in groundwater extraction that has taken place since the

decline in urban aquifer exploitation by heavy industry.

Data on groundwater flooding has been compiled by the British Geological Society and is

illustrated on mapping, which is the product of integrating several datasets: a digital model of the

land surface, digital geological map data and a water level surface based on measurements of

groundwater level made during a particularly wet winter. This dataset provides an indication of

areas where groundwater flooding may occur, but is primarily focussed on groundwater flooding

potential over the Chalk of southern Britain as Chalk shows some of the largest seasonal

variations in groundwater level, and is thus particularly prone to groundwater flooding incidents.

Inspection of this groundwater flood risk mapping data shows that the general area in which the

development site lies is identified as being at low risk from groundwater flooding. The more

detailed mapping on groundwater emergence provided as part of the Defra Groundwater Flood

Scoping Study (May 2004), which shows areas where groundwater flooding has occurred in the

past and also areas that are potentially vulnerable to groundwater emergence has also been

referenced as part of this FRA. This shows that no groundwater flooding events were recorded

during the very wet periods of 2000/01 or 2002/03 and that the site itself is not located within an

area where groundwater emergence is predicted.

Groundwater flooding is most likely to occur in low-lying areas that are underlain by permeable

rock (aquifers). The underlying geology in this area is Alluvium, which can be associated with

groundwater flooding. However, when the topography and nature of the site and surrounding land

are taken into consideration it is considered that the site specific risk of groundwater flooding is

low. There are also no records of groundwater flooding affecting this area in the past.

Flooding from Sewers – 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 result when the sewer is overwhelmed by heavy rainfall, becomes blocked or is of

inadequate capacity, and will continue until the water drains away. When this happens to

combined sewers, there is a high risk of land and property flooding with water contaminated with

raw sewage as well as pollution of rivers due to discharge from combined sewer overflows.

This part of New Romney is not served by any mains sewers and therefore the risk of flooding

from this source is considered to be low.


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Flooding from Reservoirs, Canals and other Artificial Sources - Non-natural or artificial

sources of flooding can include reservoirs, canals 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. The potential effects of flood risk management infrastructure and other structures also

need to be considered. Reservoir or canal flooding may occur as a result of the facility being

overwhelmed and/or as a result of dam or bank failure. Also, any man-made drainage system

such as a drain, sewer or ditch could potentially cause flooding.

Inspection of the Ordnance Survey mapping for the area shows that there are no artificial sources

of flooding within close proximity to the site.

Source of

flooding

Initial Level

of risk

Appraisal method applied at the initial flood risk assessment

stage

Rivers/fluvial

watercourses

Low/Moderate Environment Agency flood zone maps

Sea/Estuaries Significant * Environment Agency flood zone maps

Overland flow Low Site based appraisal and historical evidence

Groundwater Low BGS groundwater flood hazard maps, Defra Groundwater Flood

Scoping Study and site specific geological data

Sewers Low Site based appraisal

Artificial sources Low Site based appraisal and historical evidence

Table 3.1 – Summary of flood sources and risks (* denotes the principal flood risks to the site)

3.3 Existing Flood Risk Management Measures Shepway’s shoreline is approximately 42km long and much of this is defended to protect the

lower-lying, rich and fertile land that forms part of the Romney, Walland and Denge Marshes. The

land levels in these marsh areas are generally below the mean high water springs (MHWS) level

and consequently without the protection of the existing sea defences much of this land would be

permanently inundated.

The town of New Romney is located on the Romney Marsh, and is approximately 2km from the

eastern shoreline at Littlestone-on-Sea, which does benefit from a formal flood defence scheme.

The defences along the south facing shoreline are generally formed from natural shingle ridges.

The standard of protection varies greatly around the coast, however, it is the weakest link in the

chain of defence that is the most significant when examining flood risk. For the majority of the

Romney Marsh it is likely that a breach in the managed shingle ridge between Jury’s Gap and

Dungeness poses the greatest threat.


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4 Probability and Consequence of Flooding

4.1 Existing Likelihood of Flooding Due to the length of Shepway’s shoreline and its juxtaposition with respect to the tidal flows in this

part of the English Channel, the predicted extreme sea levels vary depending on location. The

extreme sea levels that have been used by this appraisal are based on those published in the

Extreme Sea Levels - Kent, Sussex, Hampshire and Isle of Wight Report (JBA, December 2004)

Revision 10. These are summarised in Table 4.1 below.

Location Extreme 1 in 200 year

sea level (mOD)

Dungeness 5.06

Dymchurch 4.96

Folkestone 4.96

Table 4.1 – Predicted Extreme Sea Levels

When these water level elevations are compared with that of the site, it can be seen that the

majority of the site is lower than the extreme 1 in 200 sea level. However, given the presence of

the existing sea defences, flooding from the sea at the site will only result from either the existing

defences breaching or being overtopped by wave action.

Recently, the whole Romney Marsh area has been modelled using TUFLOW hydraulic modelling

software as part of the Strategic Flood Risk Assessment commissioned by Shepway District

Council. As part of the SFRA, 14 scenarios were considered in order to look at the risks

associated with coastal flooding and these comprise a number of breach and wave overtopping

events.

The outputs from the SFRA indicate that the proposed development site is only affected as a

result of a breach in the sea defences on the south facing shoreline of the Dungeness peninsular,

(Scenarios 1 & 2). Here the sea defences are currently managed by the Environment Agency and

comprise a natural shingle bank that extends along the whole length of the south coastline. Whilst

this part of the sea defences does offer the town of Lydd some protection, it currently only

provides a low standard of protection against breaching and if the shingle bank were to fail during

an extreme storm event there is potential for tidal inundation.

The SFRA indicates that the most likely mechanism by which sea flooding could affect the site is

as a result of a breach in the Dungeness defences. On the south facing Dungeness peninsular

the defences provide a much lower standard of protection than the formal seawalls between

Dymchurch and Littlestone. If a breach occurred here and remained open for a sufficient duration

then it is possible that the vast Romney Marsh flood compartment could fill up to a level that

threatened the development site.


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Predicting the likelihood of a breach in the Dungeness defences is outside the scope of this

appraisal, however, in line with the precautionary approach promoted by PPS25 it is assumed

that there is potential for these defences to be breached over the lifetime of the proposed

development. This residual risk is therefore examined in detail in the following sections of this

report.

4.2 The Extent of Flooding Figurer 4.1, below, shows the extent of the flooding at the development site that would occur as a

result of breach in the flood defences. This figure shows the predicted flood extents from the

SFRA modelled Scenario 2. This scenario represents breaches at South Brooks, Galloways,

Broomhill Sands and at the Hythe Ranges frontage and is the worst case of all modelled breach

and overtopping scenarios.

Figure 4.1 – Predicted flood extents from Breach Scenario 2 - 1 in 200 +cc scenario

From this flood extents map it can be seen that the lower-lying, western end of the site is shown

to be affected, however, the eastern half, which is up to 1m higher in elevation, is shown to be

unaffected.

4.3 Depth and Velocity of Flooding Interrogation of the modelling outputs shows that the depth of flooding ranges across the site from

zero to a maximum of 0.7m. The maximum water surface elevation associated with the worst

case scenario is 2.93mOD.


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The model has also been interrogated to determine the maximum flow velocities within the site.

These were shown to be 0.15m/sec.

4.4 Flooding Characteristics As well as the source of flooding, the size and nature of the flood compartment has a major

influence on the rate of rise of the floodwaters. For a small, steep sided compartment, the rate of

rise will be rapid, whereas for a compartment that is relatively large with respect to the source of

flooding and has shallow sloping sides, the rate of rise will be more gradual. The flood

compartment in which the development site is located fits the description of the latter.

The site is located approximately 2km from the sea defences and therefore there will be a

residual delay between the defences breaching and the floodwater reaching the site. Given that

the volumes of water flowing through the breach are likely to be very large, the flow velocities in

close proximity to the breach will be very high and are likely to cause structural damage.

However, these velocities will decrease with distance away from the breach. By the time any

water reaches the development site peak velocities have decreased from 2m/s at the breach

location, to just 0.15m/s at the site.

The modelling results presented in the SFRA identify that it will take approximately 4 hours from

the point of breach for floodwater to reach the site. By hour 7, the floodwater had reached its

maximum depth within the site.

The SFRA model had a run-time of 56 hours and during this time the breaches remained open.

Consequently floodwater continues to enter and leave the flood compartment as the tide rises and

falls. The simulation stops at hour 56, however, at which point floodwater is shown to remain

within the boundaries of the site. Assuming that after 56 hours the breaches are repaired, it would

still be a significant amount of time before all floodwater can be evacuated from the Marsh,

however, at this stage this timeframe has not been quantified.


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5 Climate Change

When the impact of climate change is considered it is generally accepted that the standard of

protection provided by current defences will reduce with time. The global climate is constantly

changing, but it is widely recognised that we are now entering a period of accelerating change.

Over the last few decades there have been numerous studies into the impact of potential changes

in the future and there is now an increasing body of scientific evidence which supports the fact

that the global climate is changing as a result of human activity. Past, present and future

emissions of greenhouse gases are expected to cause significant global climate change during

this century.

The nature of climate change at a regional level will vary: for the UK, projections of future climate

change indicate that more frequent short-duration, high-intensity rainfall and more frequent

periods of long-duration rainfall of the type responsible for the recent UK flooding could be

expected.

5.1 Potential Changes in Climate Global sea levels will continue to rise, depending on greenhouse gas emissions and the

sensitivity of the climate system. The relative sea level rise in England also depends on the local

vertical movement of the land, which is generally falling in the south-east and rising in the north

and west. Annex B of PPS25 provides allowances for the regional rates of relative sea level rise

and these are shown in Table 5.1.

Net Sea Level Rise (mm/yr) Relative to 1990

Administrative Region

1990 to 2025

2025 to 2055

2055 to 2085

2085 to 2115

East of England, East Midlands, London, SE England (south of Flamborough Head)

4.0 8.5 12.0 15.0

South West 3.5 8.0 11.5 14.5

NW England, NE England (north of Flamborough Head)

2.5 7.0 10.0 13.0

Table 5.1 - Recommended contingency allowances for net sea level rise

From these values it can be seen that the extreme flood level at the site will change with time and

that this change is not linear. The 1 in 200 year flood level at the site has therefore been

calculated for four time steps between the current day and the year 2115 and these values are

shown in Table 5.2 below.


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Year 1 in 200 year extreme water level (mOD)

Current day

(year 2009) 5.06

2025 5.14

2055 5.39

2085 5.75

2115 6.20

Table 5.2 – Climate change impacts on extreme flood levels

To ensure that any recommended mitigation measures are sustainable and effective throughout

the lifetime of the development it is necessary to base the appraisal on the extreme sea level that

is commensurate with the planning horizon for the proposed development. For residential

development this is taken as 100 years.

Annex B of PPS25 also provides guidance on sensitivity allowances for other climatic changes

such as increased rainfall intensity and peak river flows. These are shown in Table 5.3 below and

where appropriate have been applied as part of this appraisal.

Parameter 1990 to 2025 2025 to 2055 2055 to 2085 2085 to 2115

Peak rainfall intensity +5% +10% +20% +30%

Peak river flow +10% +20%

Offshore wind speed +5% +10%

Extreme wave height +5% +10%

Table 5.3 - Recommended national precautionary sensitivity ranges

5.2 Impacts of Climate Change on the Development Site The SFRA modelling results that have been referenced in this FRA are based on detailed flood

modelling which takes in to account an allowance for climate change up to the year 2115. The

flood depths, velocities and extents quoted in Section 4 of this report are based on the 2115

climate change scenarios and therefore include for the impacts of sea level rise and increased

storminess etc.


17

6 Flood Mitigation Measures

The key objectives of flood risk mitigation are:

to reduce the risk of the development being flooded

to ensure continued operation and safety during flood events

to ensure that the flood risk downstream of the site is not increased by increased runoff

to ensure that the development does not have an adverse impact on flood risk elsewhere

Up to this point in the report the risks to the site have been appraised and the consequences of

these risks occurring have been considered. The following section of this report examines ways in

which flood risk can be mitigated.

Mitigation Measure Appropriate? Comment

Careful location of development

within site boundaries See Section 6.1

Raising floor levels See Section 6.2

Land raising See Section 6.2

Flood Warning See section 9.3 of this report for further detail

Flood proofing See Section 9.1

Alterations/ improvements to

channels and hydraulic structures x Not required/appropriate

Flood defences x Not required/appropriate

Compensatory flood plain storage x Not required (see Section 7.2)

Management of development runoff See Section 8.4

Table 6.1 Appropriateness of mitigation measures

6.1 Application of the Sequential Approach at a Local Scale The sequential approach to flood risk management can also be adopted on a site based scale

and this can often be the most effective form of mitigation. For example, on a large scheme this

would mean locating the more vulnerable dwellings on the higher parts of the site and placing

parking, recreational land or commercial buildings in the lower lying and higher risk areas.

There are some areas, such as those adjacent to the watercourses that are present within the

site, that are significantly lower than the main body of the site. In these areas the scheme layout


18

avoids development, which is in line with the site-specific sequential approach recommended by

PPS25.

6.2 Raising Floor Levels & Land Raising The Environment Agency recommends that the minimum floor level of buildings at risk of flooding

should be 300mm above the design flood level, which is the 1 in 200 year extreme water level

plus the appropriate allowance for climate change. The Environment Agency’s guidance also

requires that all sleeping accommodation be raised a minimum of 600mm above the design flood

level. In addition, if the development is a single storey dwellings or ground floor flats where there

is no safe refuge on an upper floor, then floor levels should be set 300mm above the 1 in 1000

year level.

The design flood level for this development is 2.93mOD and therefore based on the above

guidance, the minimum floor levels are as follows:

Living accommodation = 3.23mOD

Sleeping accommodation = 3.53mOD

In many areas of the site it will be possible to achieve the above floor levels without the need to

any significant floor raising, especially given that there are no proposals for bungalows or flats

with sleeping accommodation on the ground floor. There are, however, some areas of the site

where ground levels are as low as 2.5mOD and in these areas it is proposed to raise the land

levels slightly as part of a landscaping exercise. At this stage it is anticipated that land levels will

be raised by around 400mm.

Where necessary, it is also proposed to raise land levels along the routes of all roadways within

the site so that a minimum level of 2.93mOD is achieved.


19

7 Offsite Impacts

7.1 Proximity to Watercourse and Flood Defence Structures Under the Water resources Act 1991 and Land Drainage Byelaws, any proposals for development

in close proximity to a ‘main river’ would need to take into account the Environment Agency’s

requirement for an 8m buffer zone between the river bank and any permanent construction such

as buildings or car parking etc. This is to allow access for maintenance, to provide biodiversity

opportunities and also to provide room for the river banks to erode without threatening any

development. Consequently, prior consent of the Environment Agency is required for any

development within the bye-law distance and this consent is in addition to planning permission.

The development site is significant distance from the coastal defences at Littlestone, however, the

watercourse running along the northwestern boundary of the site is shown by the map provided

by the IDB (including in appendix A.2) to be designated as a main river. Consequently it will be

necessary to maintain a buffer zone of up to 8m between the bank of the watercourse and any

development.

7.2 Displacement of Floodwater The construction of a new building within the floodplain has the potential to displace water from

that area and to increase flood risk elsewhere by raising flood levels. Whilst the impact of a single

development within a large floodplain such as this is negligible, it is the cumulative affect of all

development in the area that PPS 25 seeks to prevent. It achieves this by requiring any

displacement that has the potential to increase risk elsewhere to be compensated for as part of a

compensatory flood storage scheme.

In defended tidal areas such as this one, it is generally considered that there is only potential for

new development to have an adverse impact on flood risk if the floodplain in which it is to be built

is confined. For example, if the defences were to breach, the extent of flooding would be

restricted by geographical features such as railway embankments or higher ground. However,

when the extent of flooding that would result from a breach in the defences that protect this

particular site is considered, it can be seen that the floodplain is not confined and does in fact

extend for some considerable distance. It is therefore concluded that the proposed development

will not have an adverse impact on maximum flood levels and therefore the provision of

compensatory storage is not required.

7.3 Impact on Fluvial Morphology & Impedance of Flood Flows As a result of the recommended surface water management measures, the peak hydraulic

loading on the receiving watercourse will not be increased. As such it is considered that the

development will not affect the morphology of the receiving watercourse.

In terms of the way in which the development would interact and modify flood flows, its location

and size with respect to the flood risk area and the flow path has to be considered. Given the


20

distance from the source of flooding, and the nature and topography of the surrounding area, it is

considered that the proposals will not significantly impede or change flood flow regimes.


21

8 Surface Water Management

8.1 Site Characteristics The requirements of PPS25 for managing rainfall runoff from developments depends on the pre-

developed nature of the site. If it is an undeveloped greenfield site then the impact of the

development will need to be mitigated so that the runoff from the site replicates the natural

drainage characteristics of the pre-developed site. In the case of brownfield sites, drainage

proposals will be measured against the existing performance of the site, although it is preferable

for solutions to provide runoff characteristics that are similar to greenfield behaviour.

The relevant characteristics of the site and the proposed development are set out in Table 7.1.

Site Characteristic Value

Total area of site 12.02 ha

Impermeable area (existing) 0.61 ha

Impermeable area (proposed) 3.29 ha

Current site condition Predominantly greenfield

Greenfield runoff rate 2.7 l/sec/ha (IoH Report 124 methodology) 4.0 l/sec/ha (recommended by IDB)

Infiltration coefficient 0.01 to 0.001 m/hr (assumed, based on typical soil conditions)

Standard Percentage Runoff (SPR) 36%

Current surface water discharge method None

Is there a watercourse within close proximity to site?

Yes

Is site within groundwater source protection zone? No

Table 7.1 – Site characteristics affecting rainfall runoff

Synthetic rainfall data has been derived using the variables obtained from the Flood Studies

Report (FSR) and the routines within the Micro Drainage Source Control software. The peak

surface water flows generated on site for the existing and post-development conditions have been

calculated by using the Modified Rational Method. Runoff rates have been calculated for a range

of annual return probabilities including the 100 year return period event with a 30% increase in

rainfall intensity to account for future climatic changes.


22

These values are summarised in Table7.2 for a storm duration of 30 minutes. Where further

analysis has been undertaken to calculate source control requirements, a full range of storm

durations has been tested.

Peak runoff (l/sec) Return period

(years) Existing site Developed site

1 58 426

30 194 1046

100 254 1368

100 + 30% 330 1778

Table 7.2 – Summary of peak runoff

The total volume of water discharged from the site from the 100 year 6 hour event (including for a

30% increase for climate change) is summarised in Table 7.3 below for both the existing and

proposed site conditions.

Site condition Total volume discharged

Existing site 496m3

Proposed site 2676m3

Table 7.3 – Total volume discharged from the 100 yr+30%cc 6 hour event

From the above figures it can be seen that the proposed development will increase the

percentage of impermeable area within the boundaries of the site and consequently this will

increase the surface water runoff from the site significantly.

8.2 Sustainable Drainage Systems (SUDS)

Appropriately designed Sustainable Drainage Systems (SUDS) can be utilised such that they not

only attenuate flows but also provide a level of improvement to the quality of the water passed on

to watercourses or into the groundwater table. This is known as source control and is a

fundamental part of the SUDS philosophy.

A range of typical SuDS components that can be used to improve the environmental impact of a

development is listed in Table 8.1 below along with the relative benefits of each feature and the

appropriateness for the subject site.


23

SuDS

Feature

Environ-

mental

benefits

Water

quality

improve-

ment

Suitability

for low

permeability

soils (k<10-6)

Ground-

water

recharge

Suitable

for small /

confined

sites?

Site specific restrictions Appropriate

for subject

site?

Wetlands x x Limited space No

Retention ponds x x

None Yes

Detention basins x x

None Yes

Infiltration basins x x

None, depending on infiltration potential Yes?

Swales x None Yes

Filter strips x None Yes

Rainwater harvesting x None Yes

Permeable paving x

Only suitable for non-adopted highways and hardstanding

Yes

Green roofs x

Unsuitable for proposed roof construction No

Table 8.1 – Environmental improvements achievable through SUDS

The Standard Percentage Runoff (SPR) value has been established for this site from the Flood

Estimation Handbook (FEH) database. This parameter is used to indicate the percentage of

rainfall which becomes direct response runoff to a watercourse. A higher runoff percentage

means that less rainfall is infiltrated into the soil, indicating lower permeability soil.

The SPR for this site is 36%, suggesting that the soils have a relatively low permeability, which is

supported by the geology and soil characteristics referenced in Table 7.1. The groundwater table

is also at around 1.6mOD, which is between 1m and 2m below the ground level across the site.

Consequently, it is unlikely that any SuDS elements that rely entirely upon infiltration will be

appropriate in this location. Notwithstanding this, there will be a degree of infiltration that can be

utilised, especially within shallow features such as swales and permeable paving.

Part H of the Building Regulations recommends that wherever practicable, appropriate SuDS

elements should be incorporated into the drainage system. From Table 8.1 it can be seen that

there is a wide range of SuDS elements that are potentially suitable for this site. The proposed

surface water management strategy is therefore highly biased towards SuDS.

Whilst rainwater harvesting only has a peripheral benefit in terms of flood risk reduction due to the

fact that in most cases the storage tank could potentially be full at the time of extreme rainfall,

there are still significant sustainability benefits associated with this technique. In particular it

reduces demand on already stretched potable water resources. Based on an assumed rate of

consumption of harvested rainwater of 52% of the overall typical consumption per person (125

litres) and 4 people occupying each dwelling, a storage tank of around 3m3 would provide

optimum water saving benefits.


24

8.3 Requirements for Surface Water Discharge PPS25 and its supporting guidance sets out two key criteria for surface water management on

new developments. These are as follows:

1. For all new development the peak discharge rate and the discharge volume of surface water

runoff shall not exceed that of the existing site.

2. Flood flows up to the 1% annual probability (1 in 100 years) event should preferably be

contained within the site at designated temporary storage locations unless it can be shown to

have no material impact in terms of nuisance or damage, or increase river flows during

periods of river flooding (Preliminary rainfall runoff management for developments -

EA/DEFRA W5-074/A).

Part H of the Building Regulations sets out a hierarchy for surface water disposal and infiltration is

the preferred method for achieving this. If this is not possible, the next favoured option is to

discharge to a watercourse. Only if neither of these options are possible should the site discharge

rainwater to a sewer.

8.4 Preferred Surface Water Management Strategy This location is not shown by the Environment Agency’s groundwater source protection zone

maps to be an area where infiltration is restricted; however, the low-lying nature of the site and

the relatively high groundwater level does restrict the use of traditional soakaways as a primary

means of discharging surface water. The superficial geology in this location is Alluvium which will

allow a small degree of infiltration and this assumption is supported by other generic data such as

the SPR value for this location.

Given that there is a watercourse within the boundaries of the site it is logical to assume that

discharging to this will be the preferred option. Initial consultations with the Internal Drainage

Board (IDB) has confirmed that this is acceptable in principle, although consent will be required

from the Environment Agency as the watercourse that runs along the northwestern boundary is

designated a main river and is therefore managed by the Agency rather than the IDB.

In accordance with the general principals of PPS25 and the specific requirements of the IDB, it

will be necessary to ensure that the peak rate of runoff does not exceed the greenfield runoff rate

of 4 l/sec/ha. Consequently the surface water management system will need to include flow

control devices at the point at which it discharges into the watercourse, and will also need to

include sufficient storage to ensure that the design event (100 year plus 30% increase in peak

rainfall intensity) does not result in flooding to areas other than those intended to be flooded.

From Section 8.2 it can be seen that there is a range of SuDS features that could be used in the

surface water management train. The availability of space, the permeability of the soils and the

gradient of the site all play an important role in selecting the most appropriate SuDS feature. At

this stage the development is in the very early master planning stages and therefore the surface


25

water and SuDS design is very much at a strategic level. Notwithstanding this, in order to satisfy

the requirements of PPS25 it is necessary to demonstrate that the preferred option is feasible.

Based on the key site constraints mentioned above and the need to provide a degree of

conservation gain, a surface water management strategy has been developed for the site. This

has been discussed at the early planning stages of the scheme, which has enabled sufficient

space for the SuDS features to be incorporated within the scheme design. The preferred surface

water management strategy is as follows:

Runoff from adopted highways will be discharged into ‘enhanced dry swales’. These

incorporate an underdrain set within a granular material directly below the base of the

swale, which keeps the swale dry most of the time.

Runoff from roof areas will feed directly into rainwater harvesting systems included within

each dwelling. The overflow from the storage tank will discharge into the underdrain of

the swale.

All non-adopted highways, areas of hardstanding and private drives will be surfaced with

permeable paving, including an appropriate granular sub-base.

The network of swales will discharge into one of three detention ponds. It is likely that

ponds will be used rather than dry basins as it is felt that there are significant ecological

benefits associated with appropriately designed ponds.

The ponds will discharge via a flow control device (Hydrobrake or similar) into the

watercourse that runs along the northwestern boundary of the site.

The general principal of this strategy has been tested using the routines in the Micro Drainage

Source Control software, using the design event conditions associated with the 1 in 100 year

storm with an increase of 30% in rainfall intensity to account for climate change. The following

assumptions have been made:

Discharge into the watercourse will be restricted to a maximum rate of 4 l/sec/ha

Infiltration from the swales will be at a rate of 0.005 m/hr

Storage within the swales has been taken into account

The gradient of the swales is based on the assumption that the lower parts of the site will

be raised as part of the flood risk mitigation measures.

Rainfall onto non-adopted highways and drives surfaced with permeable paving will not

contribute to flows within the swales


26

Calculations are based on the assumption that rainwater harvesting tanks are full at the

time of the design event and therefore residual storage is not taken into account.

Based on the above assumptions the volume and size of the three detention ponds has been

determined using the Source Control software package. The normal water level in the pond has

been assumed to be 1.8mOD. With side slopes set at 1 in 4 the overall footprint area of the ponds

is as follows:

Pond 1 = 1150m2

Pond 2 = 430m2

Pond 3 = 305m2

The approximate location of the ponds is shown in Figure 8.1 below. Detailed outputs from the

Source Control analysis are included in Appendix A.3 of this report.

Figure 8.1 – Schematic layout of key drainage features


27

The above calculations are indicative only and are based on a number of assumptions that have

to be made at this early stage in the scheme development process. Whilst they do not comprise a

detailed drainage scheme, they do at this stage demonstrate that the proposed strategy for

surface water management is achievable and that sufficient space has been made available

within the site to accommodate features such as the detention ponds and swale.


28

9 Residual Risk

When considering residual risk it is necessary to make predictions as to the impacts of a flood

event that exceeds the design event, or in the case of areas that are already defended to an

adequate standard, the impact of a failure of these defences.

The mitigation measures discussed in Section 6 of this report will significantly reduce the risk of

the development being affected by flooding; however, they do not completely remove the risk.

This section of the report is therefore associated with the way the residual risk is managed and

the safety of the occupants of the proposed development.

9.1 Flood Resistance and Resilience During a flood event, floodwater can find its way into properties through a variety of routes

including:

Ingress around closed doorways.

Ingress through airbricks and up through the ground floor.

Backflow through overloaded sewers discharging inside the property through ground

floor toilets and sinks.

Seepage through the external walls.

Seepage through the ground and up through the ground floor.

Ingress around cable services through external walls.

Since flood management measures only manage the risk of flooding rather than eliminate it

completely, flood resilience and resistance measures may need to be incorporated into the design

of the buildings. The two possible alternatives are:

Flood resistance or ‘dry proofing’, where flood water is prevented from entering the building. For

example using flood barriers across doorways and airbricks, or raising floor levels. Such

measures are generally only considered appropriate for some ‘less vulnerable’ uses and where

the use of an existing building is to be changed and it can be demonstrated that no other measure

is practicable.

Flood resilience or ‘wet proofing’, accepts that flood water will enter the building and allows for

this situation through careful internal design for example raising electrical sockets and fitting tiled

floors. The finishes and services are such that the building can quickly be returned to use after the

flood.

It is has been shown that the proposed development will have ground floor levels that are raised

well above the 1 in 200 year flood level. However there is always the risk that this event could be


29

exceeded, in which case, by incorporating flood resilience into the design of the building it will be

possible to increase its resilience to flooding and thereby reduce the impact of such an event.

Details of flood resilience and flood resistance construction techniques can be found in the

document ‘Improving the Flood Performance of New Buildings; Flood Resilient Construction’,

which can be downloaded from the Communities and Local Government website.

The applications that are recommended for this development are as follows:

Floors - Solid concrete floors should be used instead of suspended floor construction as they

can provide an effective seal against water rising up through the floor, provided they are

adequately designed. Solid concrete floors generally suffer less damage than suspended floors

and are less expensive and faster to restore following exposure to floodwater. However, in some

cases where floor raising is recommended it is not always possible to construct the ground floor

using a solid base at this would require vast quantities of material. In such circumstances it is

recommended that to enable any voids beneath the building to be cleaned following a flood event,

adequate provision for access should be made.

Walls - The use of stud walls and plasterboard on the ground floor of the development should be

avoided wherever possible as these absorb water and generally have to be removed and rebuilt

after a flood event.

Services - Boilers should be mounted on a wall above the level that floodwater is likely to reach.

Electricity sockets should be located at least one metre above floor (or well above likely flood

level) with distribution cables dropping from an upper level. Service meters should also be at least

one metre above floor level (or well above likely flood level) and placed in plastic housings.

9.2 Public Safety and Access PPS25 states that, where required, safe access and escape is available to/from new

developments in flood risk areas. The Practice Guide goes on to state that access routes should

be such that occupants can safely access and exit their dwellings in design flood conditions and

that vehicular access to allow the emergency services to safely reach the development will also

normally be required.

This FRA has indicated that the development site could be partially affected by floodwater under

the design event conditions. Reference to the Practice Guide shows that in circumstances where

it is not possible to provide dry access, the flood hazard to people under the design flood

conditions needs to be quantified.

In the report ‘Flood Risks to People’ (R&D output FD2320/TR2) a methodology for quantifying

flood hazard is set out using the following equation:

HR = ((v + 0.5) d) + DF


30

where, HR = flood hazard rating

d = depth of flooding (m)

v = velocity (m/sec)

DF = debris factor (see Table 9.1)

Depths Debris Factor (DF)

d = 0 to 0.25m 0.5

d > 0.25m 1.0

Table 9.1 - Guidance on the use of Debris Factor (DF)

The levels within the site have been established and using the design flood event conditions that

include for climate change impacts, the following parameters are derived, i.e. d = 0.7m and v =

0.15m/sec. When these values are entered into the above equation a Hazard Rating of 1.45 is

given.

When this value is compared to the threshold values given in Table 9.2 below it can be seen that

the degree of hazard is classified as ‘significant. Access from and to the dwellings within the site

is therefore potentially dangerous under design event conditions.

Hazard Rating (HR)

Degree of flood hazard

Description

< 0.75 Low Caution – shallow flowing water or deep standing water

0.75 to 1.25 Moderate Dangerous for some, i.e. children – deep or fast flowing water

1.25 to 2.5 Significant Dangerous for most people – deep fast flowing water

> 2.5 Extreme Dangerous for all – extreme danger with deep and fast flowing water

Table 9.2 – Classification of Hazard Rating Thresholds

In order to reduce the hazards associated with traversing floodwater, it has therefore been

proposed that the land in the lower-lying areas of the site will be raised by up to 400mm. In

addition, all access roads will be constructed to a minimum level of 2.93mOD, i.e. the design flood

level. Consequently the depth of flooding along all access routes will be reduce to zero and

therefore access to and from the site will be classified as safe.

Inspection on the wider area topography and the proposed masterplan drawing shows that there

is an access from the site onto the Ashford Road. At this point the level of the highway is at

3.1mOD, which is above the predicted 200 year (plus climate change) flood level. This road rises

towards the south and consequently safe dry access from the site will be available to the main


31

area of the New Romney town. The SFRA states that whilst the towns of New Romney and Lydd

are technically dry islands, they are of sufficient size and contain a wide range of resources to

enable these towns to be classified as safe havens.

9.3 Flood Warning Whilst the probability of an event of sufficient magnitude to cause floodwaters to reach the levels

discussed in this report is very low, the risk of such an occurrence is always present. With the

sophisticated techniques now employed by the Environment Agency to predict the onset of flood

events the opportunity now exists for all residents within the flood risk area to receive flood

warnings.

This forewarning could be sufficient to either allow residents to evacuate the area or prepare

themselves and their property for a flood event. It is therefore recommended that the Environment

Agency’s Floodline Service is contacted to find out if it is possible to register for Floodline

Warnings Direct, which is a free service that provides flood warnings direct by telephone, mobile,

fax or pager.

The site is located within the flood warning area referred to as ‘Coastal areas from Dungeness to

Rye’. For further details call Floodline on 0845 988 1188 and enter area number 0125113.


32

10 Conclusions

The key aims and objectives for a development that is to be sustainable in terms of flood risk are

summarised in the following bullet points:

the development should not be at a significant risk of flooding, and should not be

susceptible to damage due to flooding

the development should not be exposed to flood risk such that the health, safety and

welfare of the users of the development, or the population elsewhere, is threatened

normal operation of the development should not be susceptible to disruption as a result

of flooding and safe access to and from the development should be possible during flood

events

the development should not increase flood risk elsewhere

the development should not prevent safe maintenance of watercourses or maintenance

and operation of flood defences by the Environment Agency

the development should not be associated with an onerous or difficult operation and

maintenance regime to manage flood risk; the responsibility for any operation and

maintenance required should be clearly defined

the development should not lead to degradation of the environment

the development should meet all of the above criteria for its entire lifetime, including

consideration of the potential effects of climate change.

In determining whether the proposals for development at Cockreed Lane are sustainable in terms

of flood risk and compliant with PPS25, all of the above have been taken into consideration as

part of this FRA.

From Table 2.3 it can be seen that the proposed development is situated within a Zone 3a flood

risk areas and is a development type that is classified as being ‘more vulnerable’. Consequently, it

has been necessary to apply the Exception Test to determine whether suitable and appropriate

mitigation can be incorporated into the design of the scheme to ensure that it is sustainable in

terms of flood risk.

The risk of flooding has therefore been considered across a wide range of sources and it is only

the risk of coastal flooding that has been shown to have any significant bearing on the

development. However, when this risk is examined in detail, it has been demonstrated that with

appropriate mitigation, the development will be safe and will not increase flood risk elsewhere.

Consequently, it has been shown that the development can pass requirement (c) of the Exception

Test (PPS25 Annex D, D9) and is therefore appropriate for its location within a flood risk area.


33

In addition to passing the Exception Test, it is also necessary for the planning authority to

demonstrate that the development can pass the Sequential Test. As discussed in Section 2.3,

without having comprehensive knowledge of the land that is available for development in the

district it is not possible for this FRA to comment in detail on the test.

However, from the evidence that has been put forward in this FRA it is clear that whilst the site is

within a general high risk zone, the detailed evidence shows that the risk of flooding is

significantly less than is depicted by the Environment Agency’s flood zone map. Consequently,

this should be borne in mind when comparing this site with others that are within the same flood

risk zone.

10.1 Recommendations The findings of this report are such that it is recommended that the development is suitable for its

location within the flood risk area. There are, however, a number of mitigation measures and

considerations that are required to reduce the risk to the development and other areas within the

floodplain.

The flood resilience measures outlined in Section 9.1 of this report are to be incorporated

into the design of the building

The finished floor level (FFL) for all living accommodation shall be set at a minimum of

3.23mOD

The finished floor level (FFL) for all sleeping accommodation shall be set at a minimum of

3.53mOD

All access routes within the development site should be set at a minimum level of

2.93mOD.

The surface water management strategy for the development will need to be developed

to a detailed design stage and this will need to take into account the requirements set

out in Section 8.

The use of appropriate SUDS techniques as discussed in Section 8.2 should be

considered for incorporation into the scheme design. For this development the use of

rainfall harvesting and porous paving for all hardstanding surfaces is recommended.

A buffer zone between the existing watercourse and any proposed development should

be maintained. The extent of this zone is likely to be between 4m and 8m and will need to

be agreed with the Environment Agency and the IDB


34

With the above mitigation measures incorporated into the design of the development the

proposals will meet the requirements of PPS 25 and will therefore be acceptable and sustainable

in terms of flood risk.


35

A Appendices

A.1 Appendix A.1 – Drawings

A.2 Appendix A.2 – Correspondence

A.3 Appendix A.3 – Surface Water Management Calculations


36

Appendix A.1 – Drawings


37

Appendix A.2 – Correspondence

Simon Herrington

Subject: FW: Site at New Romney

Page 1 of 1Message

08/09/2010

-----Original Message----- From: Nick Botting [mailto:[email protected]] Sent: 07 September 2010 16:22 To: [email protected] Subject: RE: Site at New Romney Afternoon Simon See attached map for current infrastructure at the site adjacent to Cockreed Lane. Dark blue is IDB maintained watercourse, red is EA Main River, orange is Shepway watercourse and the brown squares are Shepway piped watercourse. General accepted rate for Greenfield run-off on the Marsh would be 4l/s/Ha. We wouldn’t object to a surface water outfall but as all our and SDC’s watercourses discharge ultimately to Main River we are dependant on the EA to keep their watercourses at realistic levels. Please note that the culvert from the Wallingham into the New Sewer we believe has a high invert, also the New Sewer is pumped. It would worth your while finding out what the EA’s future regime on water level control will be on Main River on the Marsh. I’m in a meeting tomorrow morning but will be free to discuss after lunch. Tel 01303 872142 Regards Nick

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38

Appendix A.3 – Surface Water Management Calculations

Cascade Summary of Results for Source Control_Pond 1.srcx

Upstream

Structures

Outflow To Overflow To

Source Control_Swale1.srcx (None) (None)

Source Control_Swale2.srcx

Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

15 min Summer 1.929 0.129 6.4 96.6 O K

30 min Summer 1.978 0.178 11.0 134.3 O K

60 min Summer 2.056 0.256 16.7 197.4 O K

120 min Summer 2.178 0.378 18.0 301.0 O K

180 min Summer 2.273 0.473 18.0 385.1 O K

240 min Summer 2.337 0.537 18.0 444.6 O K

360 min Summer 2.413 0.613 18.0 517.2 O K

480 min Summer 2.455 0.655 18.0 558.1 O K

600 min Summer 2.482 0.682 18.0 585.3 O K

720 min Summer 2.506 0.706 18.0 609.5 O K

960 min Summer 2.544 0.744 18.0 648.4 O K

1440 min Summer 2.587 0.787 18.0 692.9 O K

2160 min Summer 2.587 0.787 18.0 692.4 O K

2880 min Summer 2.534 0.734 18.0 637.6 O K

4320 min Summer 2.367 0.567 18.0 472.6 O K

5760 min Summer 2.182 0.382 18.0 304.0 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

15 min Summer 128.285 33

30 min Summer 84.226 46

60 min Summer 52.662 74

120 min Summer 31.800 132

180 min Summer 23.353 190

240 min Summer 18.644 250

360 min Summer 13.543 366

480 min Summer 10.792 482

600 min Summer 9.043 532

720 min Summer 7.823 588

960 min Summer 6.219 706

1440 min Summer 4.493 972

2160 min Summer 3.241 1372

2880 min Summer 2.568 1768

4320 min Summer 1.847 2556

5760 min Summer 1.461 3176

Herrington Consulting Ltd

Unit 6 - Barham Busine...

Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010

File Source Control_Ca...

Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By

Source Control W.12.4

Page 1

©1982-2010 Micro Drainage Ltd


Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

7200 min Summer 2.092 0.292 17.7 227.3 O K

8640 min Summer 2.050 0.250 16.4 192.7 O K

10080 min Summer 2.026 0.226 15.0 173.1 O K

15 min Winter 1.933 0.133 6.8 99.5 O K

30 min Winter 1.982 0.182 11.4 138.0 O K

60 min Winter 2.063 0.263 17.0 203.3 O K

120 min Winter 2.192 0.392 18.0 312.7 O K

180 min Winter 2.299 0.499 18.0 409.3 O K

240 min Winter 2.380 0.580 18.0 485.6 O K

360 min Winter 2.486 0.686 18.0 589.0 O K

480 min Winter 2.553 0.753 18.0 657.4 O K

600 min Winter 2.599 0.799 18.0 705.7 O K

720 min Winter 2.632 0.832 18.0 740.3 Flood Risk




2880 min Winter 2.546 0.746 18.0 650.6 O K

4320 min Winter 2.228 0.428 18.0 344.6 O K

5760 min Winter 2.067 0.267 17.1 206.1 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

7200 min Summer 1.217 3760

8640 min Summer 1.048 4432

10080 min Summer 0.923 5152

15 min Winter 128.285 33

30 min Winter 84.226 46

60 min Winter 52.662 74

120 min Winter 31.800 130

180 min Winter 23.353 188

240 min Winter 18.644 246

360 min Winter 13.543 360

480 min Winter 10.792 470

600 min Winter 9.043 576

720 min Winter 7.823 666

960 min Winter 6.219 746

1440 min Winter 4.493 1044

2160 min Winter 3.241 1480

2880 min Winter 2.568 1908

4320 min Winter 1.847 2608

5760 min Winter 1.461 3072



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 2



Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

7200 min Winter 2.023 0.223 14.7 170.3 O K

8640 min Winter 1.998 0.198 12.8 150.5 O K

10080 min Winter 1.982 0.182 11.3 137.4 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

7200 min Winter 1.217 3752

8640 min Winter 1.048 4432

10080 min Winter 0.923 5152



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 3


Cascade Rainfall Details for Source Control_Pond 1.srcx

Rainfall Model FSR Winter Storms Yes

Return Period (years) 100 Cv (Summer) 0.750

Region England and Wales Cv (Winter) 0.840

M5-60 (mm) 20.000 Shortest Storm (mins) 15

Ratio R 0.400 Longest Storm (mins) 10080

Summer Storms Yes Climate Change % +30

Time / Area Diagram

Total Area (ha) 0.001

Time

(mins)

Area

(ha)

0-4 0.001



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 4


Cascade Model Details for Source Control_Pond 1.srcx

Storage is Online Cover Level (m) 2.800

Tank or Pond Structure

Invert Level (m) 1.800

Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²)

0.000 721.4 0.400 880.1 0.800 1054.8

0.200 798.8 0.600 965.5 1.000 1148.0

Hydro-Brake® Outflow Control

Design Head (m) 1.000 Hydro-Brake® Type Md4 Invert Level (m) 1.800

Design Flow (l/s) 18.0 Diameter (mm) 152

Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s)

0.100 3.9 1.200 19.7 3.000 31.2 7.000 47.6

0.200 12.9 1.400 21.3 3.500 33.7 7.500 49.3

0.300 17.9 1.600 22.8 4.000 36.0 8.000 50.9

0.400 16.8 1.800 24.2 4.500 38.2 8.500 52.5

0.500 15.1 2.000 25.5 5.000 40.3 9.000 54.0

0.600 14.8 2.200 26.7 5.500 42.2 9.500 55.5

0.800 16.2 2.400 27.9 6.000 44.1

1.000 18.0 2.600 29.0 6.500 45.9



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 5



Upstream

Structures



Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

15 min Summer 1.981 0.181 10.1 50.8 O K

30 min Summer 2.042 0.242 13.4 69.3 O K

60 min Summer 2.129 0.329 14.1 96.3 O K

120 min Summer 2.258 0.458 14.1 138.4 O K

180 min Summer 2.347 0.547 14.1 168.9 O K

240 min Summer 2.403 0.603 14.1 188.9 O K

360 min Summer 2.445 0.645 14.1 204.3 O K

480 min Summer 2.465 0.665 14.1 211.3 O K

600 min Summer 2.479 0.679 14.1 216.8 O K

720 min Summer 2.492 0.692 14.1 221.4 O K

960 min Summer 2.509 0.709 14.1 228.1 O K

1440 min Summer 2.521 0.721 14.1 232.6 O K

2160 min Summer 2.497 0.697 14.1 223.4 O K

2880 min Summer 2.435 0.635 14.1 200.5 O K

4320 min Summer 2.198 0.398 14.1 118.5 O K

5760 min Summer 2.048 0.248 13.6 70.9 O K

7200 min Summer 2.007 0.207 11.8 58.7 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

15 min Summer 128.285 24

30 min Summer 84.226 38

60 min Summer 52.662 68

120 min Summer 31.800 126

180 min Summer 23.353 186

240 min Summer 18.644 244

360 min Summer 13.543 344

480 min Summer 10.792 394

600 min Summer 9.043 454

720 min Summer 7.823 518

960 min Summer 6.219 652

1440 min Summer 4.493 920

2160 min Summer 3.241 1312

2880 min Summer 2.568 1700

4320 min Summer 1.847 2380

5760 min Summer 1.461 2952

7200 min Summer 1.217 3672



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 1



Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

8640 min Summer 1.984 0.184 10.3 51.8 O K

10080 min Summer 1.968 0.168 9.1 47.2 O K

15 min Winter 1.990 0.190 10.8 53.7 O K

30 min Winter 2.055 0.255 13.7 73.0 O K

60 min Winter 2.146 0.346 14.1 101.8 O K

120 min Winter 2.281 0.481 14.1 146.2 O K

180 min Winter 2.378 0.578 14.1 179.9 O K

240 min Winter 2.450 0.650 14.1 205.9 O K

360 min Winter 2.534 0.734 14.1 237.4 O K

480 min Winter 2.561 0.761 14.1 247.9 O K

600 min Winter 2.586 0.786 14.1 257.5 O K




2160 min Winter 2.566 0.766 14.1 249.8 O K

2880 min Winter 2.408 0.608 14.1 190.5 O K

4320 min Winter 2.030 0.230 13.0 65.6 O K

5760 min Winter 1.986 0.186 10.5 52.3 O K

7200 min Winter 1.963 0.163 8.7 45.7 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

8640 min Summer 1.048 4408

10080 min Summer 0.923 5136

15 min Winter 128.285 24

30 min Winter 84.226 37

60 min Winter 52.662 66

120 min Winter 31.800 124

180 min Winter 23.353 182

240 min Winter 18.644 240

360 min Winter 13.543 350

480 min Winter 10.792 448

600 min Winter 9.043 478

720 min Winter 7.823 552

960 min Winter 6.219 702

1440 min Winter 4.493 988

2160 min Winter 3.241 1392

2880 min Winter 2.568 1792

4320 min Winter 1.847 2252

5760 min Winter 1.461 2944

7200 min Winter 1.217 3680



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 2



Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

8640 min Winter 1.948 0.148 7.5 41.3 O K

10080 min Winter 1.937 0.137 6.6 38.2 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

8640 min Winter 1.048 4400

10080 min Winter 0.923 5120



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 3









Time / Area Diagram


Time

(mins)

Area

(ha)

0-4 0.001



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 4







0.000 268.5 0.400 327.6 0.800 392.7

0.200 297.3 0.600 359.4 1.000 427.3





0.100 3.7 1.200 16.3 3.000 25.8 7.000 39.4

0.200 11.4 1.400 17.6 3.500 27.9 7.500 40.8

0.300 14.2 1.600 18.8 4.000 29.8 8.000 42.1

0.400 12.6 1.800 20.0 4.500 31.6 8.500 43.4

0.500 11.7 2.000 21.1 5.000 33.3 9.000 44.7

0.600 11.9 2.200 22.1 5.500 34.9 9.500 45.9

0.800 13.4 2.400 23.1 6.000 36.5

1.000 14.9 2.600 24.0 6.500 38.0



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 5



Upstream

Structures



Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

15 min Summer 1.915 0.115 3.7 22.7 O K

30 min Summer 1.969 0.169 5.7 33.9 O K

60 min Summer 2.066 0.266 6.0 54.6 O K

120 min Summer 2.229 0.429 6.0 91.8 O K

180 min Summer 2.323 0.523 6.0 114.6 O K

240 min Summer 2.377 0.577 6.0 127.9 O K

360 min Summer 2.432 0.632 6.0 142.0 O K

480 min Summer 2.468 0.668 6.1 151.6 O K

600 min Summer 2.499 0.699 6.3 159.9 O K

720 min Summer 2.526 0.726 6.4 167.0 O K

960 min Summer 2.563 0.763 6.5 177.1 O K

1440 min Summer 2.584 0.784 6.6 183.0 O K

2160 min Summer 2.526 0.726 6.4 167.2 O K

2880 min Summer 2.422 0.622 6.0 139.6 O K

4320 min Summer 2.239 0.439 6.0 94.0 O K

5760 min Summer 2.035 0.235 6.0 47.9 O K

7200 min Summer 1.973 0.173 5.7 34.6 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

15 min Summer 128.285 27

30 min Summer 84.226 40

60 min Summer 52.662 70

120 min Summer 31.800 128

180 min Summer 23.353 186

240 min Summer 18.644 244

360 min Summer 13.543 348

480 min Summer 10.792 396

600 min Summer 9.043 452

720 min Summer 7.823 516

960 min Summer 6.219 648

1440 min Summer 4.493 914

2160 min Summer 3.241 1308

2880 min Summer 2.568 1704

4320 min Summer 1.847 2468

5760 min Summer 1.461 3064

7200 min Summer 1.217 3680



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 1



Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

8640 min Summer 1.949 0.149 5.1 29.7 O K

10080 min Summer 1.934 0.134 4.5 26.6 O K

15 min Winter 1.917 0.117 3.8 23.1 O K

30 min Winter 1.971 0.171 5.7 34.2 O K

60 min Winter 2.068 0.268 6.0 55.0 O K

120 min Winter 2.240 0.440 6.0 94.4 O K

180 min Winter 2.363 0.563 6.0 124.5 O K

240 min Winter 2.438 0.638 6.0 143.8 O K

360 min Winter 2.533 0.733 6.4 168.9 O K

480 min Winter 2.590 0.790 6.7 184.5 O K





2160 min Winter 2.532 0.732 6.4 168.7 O K

2880 min Winter 2.384 0.584 6.0 129.7 O K

4320 min Winter 2.024 0.224 6.0 45.4 O K

5760 min Winter 1.951 0.151 5.2 30.2 O K

7200 min Winter 1.930 0.130 4.4 25.7 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

8640 min Summer 1.048 4408

10080 min Summer 0.923 5136

15 min Winter 128.285 27

30 min Winter 84.226 40

60 min Winter 52.662 68

120 min Winter 31.800 126

180 min Winter 23.353 184

240 min Winter 18.644 240

360 min Winter 13.543 350

480 min Winter 10.792 444

600 min Winter 9.043 474

720 min Winter 7.823 546

960 min Winter 6.219 694

1440 min Winter 4.493 980

2160 min Winter 3.241 1404

2880 min Winter 2.568 1816

4320 min Winter 1.847 2388

5760 min Winter 1.461 2976

7200 min Winter 1.217 3680



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 2



Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Control

(l/s)

Max

Volume

(m³)

Status

8640 min Winter 1.917 0.117 3.8 23.0 O K

10080 min Winter 1.908 0.108 3.3 21.2 O K

Storm

Event

Rain

(mm/hr)

Time-Peak

(mins)

8640 min Winter 1.048 4408

10080 min Winter 0.923 5112



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 3









Time / Area Diagram


Time

(mins)

Area

(ha)

0-4 0.001



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 4







0.000 191.4 0.400 233.5 0.800 279.9

0.200 211.9 0.600 256.2 1.000 304.6





0.100 2.9 1.200 8.2 3.000 13.0 7.000 19.8

0.200 6.0 1.400 8.9 3.500 14.0 7.500 20.5

0.300 5.2 1.600 9.5 4.000 15.0 8.000 21.2

0.400 5.0 1.800 10.0 4.500 15.9 8.500 21.8

0.500 5.3 2.000 10.6 5.000 16.7 9.000 22.5

0.600 5.8 2.200 11.1 5.500 17.6 9.500 23.1

0.800 6.7 2.400 11.6 6.000 18.3

1.000 7.5 2.600 12.1 6.500 19.1



Elham Valley Road

Barham CT4 6DQ

Date 10 Sept 2010


Micro Drainage

Cockreed Lane

New Romney

Designed By sph

Checked By


Page 5


client : the new romney consortium · client : the new romney consortium flood risk assessment for...

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