sworders flood risk assessment conceptual drainage...

222 The Long Shoot, Nuneaton

SWORDERS

FLOOD RISK ASSESSMENT

&

CONCEPTUAL DRAINAGE DESIGN

10th October 2013

Final Version 1.0

RAB: 588

Ms. Heidi Smith

Sworders

The Gatehouse

Hadham Hall

Little Hadham

Ware

Hertfordshire

SG11 2EB

The Long Shoot, Nuneaton FRA

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Revision History

Version Date Amendments Issued to

Draft v1.0 October 2013 Ms. Heidi Smith,

Sworders

Quality Control

Action Signature Date

Prepared Peter O’Flaherty BSc MSc 10/10/2013

Checked Gavin Wilson BSc PhD MCIWEM MInstP 10/10/2013

Approved Ray Pickering DipCE CEng MCIWEM C.WEM MCGI MEPS 10/10/2013

Disclaimer

This document has been prepared solely as a FLOOD RISK ASSESSMENT for Sworders. RAB

Consultants accepts no responsibility or liability for any use that is made of this document other than by the client for the purposes for which it was originally commissioned and prepared. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Managing Director of RAB Consultants Ltd. Any advice, opinions, or recommendations within this document should be read and relied upon only in the context of the document as a whole.

RAB Office

RAB Consultants Kingsbrook House, 7 Kingsway, Bedford, MK42 9BA


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Executive Summary

This flood risk assessment (FRA) has been prepared by RAB Consultants on behalf of Sworders in support of the proposed development at 222 The Long Shoot, Nuneaton. This FRA has been prepared in accordance with the Environment Agency’s FRA Guidance Note 1 (Development within a Critical Drainage Area or greater than 1 hectare in Flood Zone 1) and National Planning Policy Framework Technical Guidance.

The development proposes the demolition of the existing agricultural buildings at the site and the construction of up to 120 residential properties, access roads, parking bays, and associated residential curtilage. The existing residential building will remain. The proposed development will result in an approximate 1.02 – 1.555ha increase in impermeable surfacing (dependent on whether or not drives and footpaths at the site will be constructed with permeable material).

The site is entirely located within Flood Zone 1 of the Environment Agency’s Flood Map; at low

risk of fluvial flooding. The site is also shown to be at low risk of flooding from all other sources including groundwater and surface water.

The Greenfield runoff rate for the proposed development site has been estimated using the IH124 method as 4.4 l/s per ha for a QBAR flow. The proposed development will require sustainable surface water management to ensure that this existing runoff rate is not increased as a result of the proposed development. This will require the use of appropriate SuDS

techniques to manage surface water runoff from the site.

In the conceptual drainage scheme rainwater on the proposed development sites hard surfacing will be directed to an attenuation pond to be constructed in Field 4 on site where it will be stored before being directed into the existing watercourse running along the south-west boundary of the site. This is naturally where surface water runoff is directed to from the site.

Outflow will be limited in the scheme to the pre-existing QBAR (4.5/6.8l/s based on 1.02ha and 1.555ha impermeable area respectively) during a 100 year storm event including the effects of climate change.

The scheme uses a Hydrobrake MD5-SW to limit outflow from the attenuation pond to the watercourse. If the proposed developments drives and footpaths are constructed with permeable material, Microdrainage calculations indicate that the attenuation pond would need

to occupy an area of 1,400m2 with a normal water level 0.5m below the general ground level to control outflow to the existing QBAR during a 100 year storm event including the effects of climate change. Conversely, if the proposed developments drives and footpaths are constructed with impermeable material, Microdrainage calculations indicate that the attenuation pond would need to occupy an area of 2,100m2 with a normal water level 0.5m below the general ground level to control outflow to the existing QBAR during a 100 year storm event including the effects of climate change.

The drainage impact study found that with each of the conceptual drainage scheme outputs, the developments impact is mitigated in terms of discharge and general surface water flow direction. Despite this, the net effect on the volume of surface water entering the watercourse will be an increase of 242.4m3 and 369.6m3 during the 100 year storm with a 1.02 and 1.555ha impermeable area respectively. With both impermeable areas proposed, this increase in volume will be limited to the pre-existing QBAR rate of discharge during a 100 year storm event including the effects of climate change in line with recommendations from the SuDS manual. As a result, the development provides betterment to the existing situation.


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It is likely that the net volume increase in surface water discharge from the site is lower than that calculated in the conceptual drainage scheme because they have been calculated under

worst case scenarios with no infiltration and no network pipe storage capacity. While the site visit observations indicate that the site is fairly permeable, the viability of infiltration SuDS at the proposed development cannot be confirmed until formal infiltration testing has been carried out at the site.

On the completion of more firm proposals for the residential development, it is recommended that a detailed SuDS scheme is designed that compliments the conceptual design in this report.

The final SuDS scheme should address the quality of runoff in addition to the quantity.


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Contents

EXECUTIVE SUMMARY ......................................................................... III

1.0 INTRODUCTION ............................................................................. 1

1.1. Terms of Reference ................................................................................................ 1

1.2. FRA Requirements .................................................................................................. 1

1.3. Site Details ........................................................................................................... 2

1.4. Site Description ..................................................................................................... 3

1.5. Site History and Development Proposals ................................................................... 3

1.6. Existing Drainage Network ...................................................................................... 3

2.0 SITE VISIT 14TH AUGUST 2013 ....................................................... 4

2.1. Observations ......................................................................................................... 5

3.0 DEVELOPMENT AND FLOOD RISK ................................................... 9

3.1. Planning Context .................................................................................................... 9

3.2. NPPF Flood Zones................................................................................................... 9

4.0 ASSESSMENT OF FLOOD RISK ...................................................... 13

4.1. Anecdotal Information .......................................................................................... 13

4.2. Previous Flood History .......................................................................................... 13

4.3. Fluvial Flood Risk ................................................................................................. 13

4.4. Coastal Flood Risk ................................................................................................ 14

4.5. Flood Defence Breach and Overtopping ................................................................... 14

4.6. Canal Flood Risk .................................................................................................. 14

4.7. Reservoir Flood Risk ............................................................................................. 14

4.8. Geological Context ............................................................................................... 14

4.9. Groundwater ....................................................................................................... 15

4.10. Surface Water Flood Risk ................................................................................ 15

4.11. Drainage and Sewage Infrastructure ................................................................ 15

4.12. Climate Change ............................................................................................. 15

5.0 SURFACE WATER RUNOFF PRE-DEVELOPMENT CONDITION ......... 17

5.1. Pre-development runoff rate for proposed development site with permeable drives and footpaths .................................................................................................................. 17

5.2. Pre-development runoff volume for proposed development site with permeable drives and

footpaths .................................................................................................................. 17

5.3. Pre-development runoff rate for proposed development site with impermeable drives and footpaths .................................................................................................................. 18


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5.4. Pre-development runoff volume for proposed development site with impermeable drives and footpaths ............................................................................................................ 18

6.0 DRAINAGE IMPACT STUDY ........................................................... 19

7.0 POST-DEVELOPMENT SURFACE WATER RUNOFF .......................... 21

7.1. SuDS .................................................................................................................. 21

7.2. Conceptual Drainage Scheme ................................................................................ 21

7.3. Conveyance......................................................................................................... 25

7.4. Infiltration SuDS .................................................................................................. 25

8.0 CONCLUSION ............................................................................... 27

9.0 RECOMMENDATIONS .................................................................... 28

APPENDIX A DEVELOPMENT PROPOSALS ............................................... I

List of Figures

Figure 1 - Summary of site details ..................................................................................... 2

Figure 2 - Site layout ....................................................................................................... 4

Figure 3 - Entrance to the existing site ............................................................................... 6

Figure 4 - Parking area adjacent to residential property ....................................................... 6

Figure 5 - View of 222 The Long Shoot taken from rear ....................................................... 6

Figure 6 - Garden of 222 The Long Shoot ........................................................................... 6

Figure 7 - Sheds immediately east of residential property .................................................... 7

Figure 8 - View of Field 1 taken from entrance looking north east .......................................... 7

Figure 9 - View of Field 2 taken from north east corner looking south west ............................. 7

Figure 10 - View of Field 3 taken from entrance looking south east ........................................ 7

Figure 11 - View of Field 4 taken from entrance looking south west ....................................... 8

Figure 12 - Wooden storage facility in Field 4 ...................................................................... 8

Figure 13 - View of ditch running to a culvert entrance at the south west corner of the site ...... 8

Figure 14 - Informal field drain entering the ditch watercourse from neighbouring land to the west of Field 4 ................................................................................................................ 8

Figure 15 Map displaying estimated surface water flow paths based on topographic data and site visit observations .......................................................................................................... 19

Figure 16 Basic outline of the conceptual drainage scheme ................................................. 22

List of Tables

Table 1 - NPPF Flood Zones and Requirements .................................................................... 9

Table 2 - Flood Risk Vulnerability Classification ................................................................. 11


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Table 3 - Flood Risk Vulnerability and Flood Zone 'compatibility' .......................................... 12

Table 4 - NPPF Technical Guidance recommended national precautionary sensitivity ranges for peak rainfall intensities and peak river flows ..................................................................... 16

Table 5 Hierarchy of SuDS techniques.............................................................................. 21


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1.0 Introduction

1.1. Terms of Reference

RAB Consultants Ltd was appointed by Sworders, to prepare this Flood Risk Assessment (FRA) in support of a proposed development for the land at 222 The Long Shoot, Nuneaton. This report also considers the impact of development at the site on surface water drainage and a conceptual design of a sustainable surface water drainage scheme

using SuDS principles.

The National Planning Policy Framework requires a FRA to be carried out to ensure flood risk to the proposed development is considered as well as the impact the development will have elsewhere on people and property.

This FRA has been prepared in accordance with the Environment Agency’s Flood Risk

Assessment (FRA) Guidance Note 1 (Development within a Critical Drainage Area or greater than 1 hectare in Flood Zone 1).

1.2. FRA Requirements

It is a requirement for development applications to consider the potential risk of flooding to a proposed development over its expected lifetime and any possible impacts on flood risk elsewhere, in terms of its effects on flood flows and runoff.

Where appropriate, the following aspects of flood risk should be addressed in all planning applications in flood risk areas:

The area liable to flooding.

The probability of flooding occurring now and over time.

The extent and standard of existing flood defences and their effectiveness over time.

The likely depth of flooding.

The rates of flow likely to be involved.

The likelihood of impacts to other areas, properties and habitats.

The effects of climate change.

The nature and currently expected lifetime of the development proposed and the extent to which it is designed to deal with flood risk.

This FRA follows government guidance on development and flood risk (National Planning Policy Framework).


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1.3. Site Details

Figure 1 - Summary of site details

Site name Land at 222 The Long Shoot, Nuneaton

Site area 5.8 Hectares (approx.)

Existing land-use Agricultural with one residential property and small kennels

Purpose of development Residential

Estimated lifespan 100 years

OS NGR 439122 293266

Country England (NPPF applies)

County Warwickshire

Local Planning Authority Nuneaton and Bedworth District Council

Other authorities Environment Agency Midlands Region


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1.4. Site Description

The current site is predominantly composed of four grassland fields in agricultural use. Approximately 3,000m2 of hardstanding impermeable area is used for vehicular access, car storage and buildings including 222 The Long Shoot, Nuneaton, kennels and storage buildings for agricultural equipment. The site bounds residential properties and curtilage on the southern and eastern boundaries while agricultural land is located adjacent to the northern and western boundaries.

1.5. Site History and Development Proposals

The site has historically been used for agricultural purposes with the one residential building on the site presumably used by the farm owners.

The proposed development is for the demolition of the existing agricultural buildings and

the construction of up to 120 residential properties, access roads, parking and garden space. The existing residential building will remain. The proposed hard standing impermeable surfaces will occupy approximately 1.02 or 1.555ha of greenfield land at the site depending on whether or not the proposed drives and footpaths are constructed with permeable materials.

1.6. Existing Drainage Network

The existing residential property is served by rainwater pipes and gullies which presumably drain to the main surface water sewers running along The Long Shoot. There are no field drains throughout the site. The remainder of the site drains naturally to a ditch located along part of the sites western boundary and away from the site through a culverted route into the Harrow Brook, a tributary of the River Anker.


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2.0 Site Visit 14th August 2013

The site visit was undertaken by RAB Consultants on a clear, sunny day. RAB Consultants undertook a photographic survey and visual assessment of the proposed site (Figure 2).

Figure 2 - Site layout


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2.1. Observations

The site entrance was observed as a stone path suitable for vehicular access sloping down from The Long Shoot (Figure 3). Roadside drains were observed directly adjacent to the site entrance. The stone path is believed to be permeable allowing rainfall to percolate into the soil below. A grass lawn was located immediately to the side of the access road (Figure 3) which moves into a small wooded area before giving way to a car parking area composed of the same stone as the access path (Figure 4).

The access road skirts around the residential property 222 The Long Shoot; observed as a detached house with a paved entrance area and a permeable, grass garden to the rear (Figure 5; Figure 6). The access path then moves along the top of the property and curtilage then past a fenced off kennel area set on surrounding impermeable ground before finally leading to a number of agricultural storage buildings set against the sites western boundary. The agricultural buildings are all built on surrounding impermeable

ground. The measured approximate 3,000m2 of impermeable ground at the existing site was observed to be accurate. To the right of the property, a small grassy piece of land was observed containing two wooden sheds (Figure 7). This is bounded to the east by Field 3.

Located immediately north of the residential property at the site; field 1 is large relatively flat grassland (Figure 8). Field 1 is bounded to the east by Field 2 (Figure 9),

separated by a hedgerow. The two fields are completely undeveloped and in agricultural use.

Field 3 is a small paddock to the south of field 2 (Figure 10) accessed via a gap in the hedgerow. Although predominantly composed of grassland, there is a wooden hay store to the north of the field.

Field 4 is a grassy area of land immediately south of the existing buildings at the site (Figure 11). A small wooden hay store is located in the centre of the field in the approximate location of the former pond marked on the map (Figure 2; Figure 12). The field is bordered to the west by a ditch running from north to south. This was observed to be clear of debris and estimated to have a large capacity (Figure 13). The farmer at the site informed RAB Consultants that all of the land at the site drains naturally to this

ditch and is conveyed into a culvert at the southern boundary of the site (Figure 13) where it then runs beneath the houses along Long Shoot and into the Harrow Brook. A newly constructed field drain was observed leading from the fields to the west of the proposed development into the ditch (Figure 14).

The absence of field drains in any of the fields within the proposed development area indicates good infiltration potential.


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Figure 3 - Entrance to the existing site

Figure 4 - Parking area adjacent to residential property

Figure 5 - View of 222 The Long Shoot taken from rear

Figure 6 - Garden of 222 The Long Shoot


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Figure 7 - Sheds immediately east of residential property

Figure 8 - View of Field 1 taken from entrance looking north east

Figure 9 - View of Field 2 taken from north east corner looking south west

Figure 10 - View of Field 3 taken from entrance looking south east


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Figure 11 - View of Field 4 taken from entrance looking south west

Figure 12 - Wooden storage facility in Field 4

Figure 13 - View of ditch running to a culvert entrance at the south west corner

of the site

Figure 14 - Informal field drain entering the ditch watercourse from neighbouring

land to the west of Field 4


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3.0 Development and Flood Risk

3.1. Planning Context

3.1.1. Applicable Planning Policy

National Planning Policy Framework (NPPF) was issued by the Department for Communities and Local Government in March 2012. NPPF deals specifically with development planning and flood risk using a sequential characterisation of risk based on planning zones and the Environment Agency Flood Map. The main study requirement is to address the management of surface water from the site as a result of the site being over 1ha, therefore meeting the criteria for a flood risk assessment.

3.1.2. Flood Zones

The Environment Agency’s Flood Map shows that the site lies entirely within Flood Zone 1 as described in Table 1 of the Technical Guidance to the National Planning Policy Framework. Flood Zone 1 represents land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%) as outlined in Table 1. However as the size of the proposed development is over 1ha, an FRA is still required to be completed. The proposed development is categorised as a ‘more vulnerable’ development in accordance with Table 2 of the Technical Guidance to the National

Planning Policy Framework.

3.1.3. Sequential and Exception Tests

As the proposed site is in the Environment Agency’s Flood Zone 1, there is no requirement to conduct a sequential or exception test.

3.2. NPPF Flood Zones

Table 1 shows how the Flood Zones relate to a sequential planning process.

Table 1 - NPPF Flood Zones and Requirements

Zone 1: Low Probability

Land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%).

Appropriate uses

All uses of land are appropriate in this zone.

FRA requirements

For development proposals on sites comprising one hectare or above the vulnerability to flooding from other sources as well as from river and sea flooding, and the potential to increase flood risk elsewhere through the addition of hard surfaces and the effect of the new development on surface water runoff, should be incorporated in a FRA.

Policy aims

Developers and local authorities should seek opportunities to reduce the overall level of flood risk through the layout and form of the development, and the appropriate application of


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sustainable drainage techniques.

Zone 2: Medium Probability

Land assessed as having between a 1 in

100 and 1 in 1000 annual probability of river flooding (1% - 0.1%) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% - 0.1%) in any year.

Appropriate uses

The water-compatible, less vulnerable and more vulnerable uses of land and essential infrastructure in Table2-2 are appropriate in this zone.

Highly vulnerable uses in Table 2-2 are only appropriate in this zone if the Exception Test is passed.

FRA requirements

All proposals in this zone should be accompanied by a FRA.

Policy aims

Developers and local authorities should seek opportunities to reduce the overall level of flood risk through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

Zone 3a: High Probability

Land assessed as having a 1 in 100 or greater annual probability of river flooding (<1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

Appropriate uses

The water-compatible and less vulnerable uses of land in Table 2-2 are appropriate in this zone.

The highly vulnerable uses (Table 2-2) should not be permitted in this zone.

The more vulnerable and essential infrastructure uses in Table 2-2 should only be permitted in this zone if the Exception Test is passed.

FRA requirements


Policy aims

Developers and local authorities should seek opportunities to:

reduce the overall level of flood risk through the layout and form of the development and the appropriate application of sustainable drainage techniques;

relocate existing development to land with a lower probability of flooding;

create space for flooding to occur by allocating and safeguarding open space for flood storage.


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Zone 3b: Functional Floodplain

Land where water has to flow or be stored in times of flood.

(Land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another

probability to be agreed between the local planning authority and the Environment Agency, including water conveyance routes).

Appropriate uses

Only the water-compatible uses and the essential infrastructure listed in Table 2-2 that has to be there should be permitted. It should be designed and constructed to:

remain operational and safe for users in times of flood;

result in no net loss of floodplain storage;

not impede water flows;

not increase flood risk elsewhere.

FRA requirements


Policy aims

In this zone, developers and local authorities should seek opportunities to:

reduce the overall level of flood risk through the layout and form of the development and the appropriate application of sustainable drainage techniques;

relocate existing development to land with a lower probability of flooding.

Source: NPPF Technical Guidance Table 1

Table 2 - Flood Risk Vulnerability Classification

Essential Infrastructure

Essential transport infrastructure and strategic utility infrastructure, including electricity generating power stations and grid and primary substations.

Highly Vulnerable Police stations, Ambulance stations and Fire stations and Command Centres and telecommunications installations and emergency dispersal points.

Basement dwellings, caravans, mobile homes and park homes intended for permanent residential use.

Installations requiring hazardous substances consent.

More Vulnerable Hospitals, residential institutions such as residential care homes, children’s homes,

Social services homes, prisons and hostels.

Buildings used for: dwelling houses, student halls of residence, drinking establishments, nightclubs, hotels and sites used for holiday or short-let caravans and camping.

Non–residential uses for health services, nurseries and education.

Landfill and waste management facilities for hazardous waste.


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Less Vulnerable Buildings used for shops, financial, professional and other services, restaurants and cafes, offices, industry, storage and distribution, and assembly and leisure.

Land and buildings used for agriculture and forestry.

Waste treatment (except landfill and hazardous waste facilities), minerals working and processing (except for sand and gravel).

Water treatment plants and sewage treatment plants (if adequate pollution control measures are in place).

Water-compatible

Development

Flood control infrastructure, water transmission infrastructure and pumping stations.

Sewage transmission infrastructure and pumping stations.

Sand and gravel workings.

Docks, marinas and wharves, navigation facilities.

MOD defence installations.

Ship building, repairing and dismantling, dockside fish processing and refrigeration and compatible activities requiring a waterside location

Water-based recreation (excluding sleeping accommodation).

Lifeguard and coastguard stations.

Amenity open space, nature conservation and biodiversity, outdoor sports and recreation.

Essential sleeping or residential accommodation for staff required by uses in this category, subject to a warning and evacuation plan.


Table 3 - Flood Risk Vulnerability and Flood Zone 'compatibility'


Key:

Vulnerability Classification

(Table 3)

Essential Infrastructure

Water Compatible

Highly Vulnerable

More Vulnerable

Less Vulnerable

Flo

od Z

one

(Table

2)

Zone 1

Zone 2 Exception

Test

Zone 3a

Exception

Test

Exception Test

Zone 3b

Exception

Test

Development is appropriate

Development should not be permitted


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4.0 Assessment of Flood Risk

4.1. Anecdotal Information

The residents of 222 The Long Shoot report that there has never been a flood risk issue at the proposed development site in the time that they have lived there. Even during periods when rainfall has been particularly high, the residents report that it has never caused overland flow issues as it drains away into the ditch running along the west

boundary of Field 4 and consequently doesn’t affect the site.

Anecdotal information provided for the proposed development site can be a valuable additional source of information to published research.

4.2. Previous Flood History

The 2008 Level 1 Nuneaton and Bedworth Borough Council Strategic Flood Risk Assessment (SFRA) report that there is only one historic flood outline that has been produced by the Environment Agency for the borough, and this pertains to the December 1992 flood event where sections of the River Anker flooded areas in the north of the borough including Attleborough. The flood outline provided within the 2008 SFRA shows that the proposed development site was unaffected by this event. Other flood events are reported including a 1968 major flood event in Nuneaton but without an outline it is

unknown whether this affected the area near to the proposed development site.

Information on flooding from surface water and artificial drainage sources has been provided for the 2008 SFRA by Severn Trent Water in the form of four digit postcode locations as recorded within their DG5 Flood Register. The aim of the DG5 levels of service indicators is to measure the frequency of actual flooding of properties and

external areas from the public sewerage system by foul water, surface water or combined sewage. It should be noted that flooding from land drainage, highway drainage, rivers/watercourses and private sewers is not recorded within the register. The DG5 Register shows that there have been 3 properties affected by flooding from artificial sources in the CV11 6 postcode region in the 10 years prior to production of the 2008 SFRA. Because there is no further information provided it is unknown whether the proposed development site was affected by these flood events.

Anecdotal evidence of surface water flooding locations in the 2008 SFRA does not make any reference to the proposed development site or immediately surrounding area.

British Waterways confirmed within the 2008 SFRA that there are no records of canal breaches within Nuneaton and Bedworth Borough.

Investigation carried out by the 2008 SFRA found that there are no records of reservoir breach or overtopping incidents in the Nuneaton and Bedworth Borough.

Consultation with the Environment Agency conducted by the 2008 SFRA has suggested that there are no known problems with flooding from groundwater within the Borough of Nuneaton and Bedworth.

4.3. Fluvial Flood Risk

The 2008 SFRA notes that the fluvial Harrow Brook has caused flooding along The Long Shoot, with the last major incident reported in 1999. Subsequent to the last incident the


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Council carried out some minor work to a ditch course joining the Harrow Brook, and the Environment Agency also installed flood protection works in the form of a bund and

pumping station to help alleviate the risk of flooding according to the 2008 SFRA. The anecdotal evidence received from Nuneaton and Bedworth Borough Council indicates that all recorded fluvial flood incidents correspond with the Environment Agency’s Flood Zone 2 at this location.

The proposed development site is located entirely within the Flood Zone 1 (low probability) of the Environment Agency’s Flood Map as described in Table 1 of the

Technical Guidance to the National Planning Policy Framework. Consequently the site was not affected by the fluvial Harrow Brook flooding described above. There is no fluvial flood risk associated with this site.

4.4. Coastal Flood Risk

Warwickshire is landlocked with levels typically 90m AOD; therefore, interactions with the sea can be discounted.

4.5. Flood Defence Breach and Overtopping

There are no formal flood defences protecting the proposed development site. Consequently there is no risk of flooding from this source.

4.6. Canal Flood Risk

The 2008 SFRA notes a requirement for the risk from canal flooding for adjacent development. However, the north east boundary of the proposed development site is located approximately 1km from the nearest canal; Ashby de la Zouch Canal. At this distance it is not thought that the canal is a significant risk of flooding to the proposed

development site.

The 2012 Level 2 Nuneaton and Bedworth Borough Council SFRA notes that breach modelling has been undertaken using Jflow+ to produce breach flood extent, depth, velocity and hazard maps. These maps confirm that the proposed development site is not at risk during a canal breach event.

4.7. Reservoir Flood Risk

The proposed development site is not shown to be at risk of reservoir flooding on the Environment Agency’s reservoir flood map.

4.8. Geological Context

Indicative information from British Geological Survey (BGS) shows that the sites underlying bedrock geology is composed of Mercia Mudstone Group mudstone and clay. This is overlain by Thrussington Member diamicton to the east of the site and Wolston Clay silt and clay to the west of the site. This is thought to be a setting with low permeability but the absence of field drains on site and the lack of any evidence of poor infiltration observed during the site visit indicate the opposite. Ultimately the sites infiltration potential would need to be determined through infiltration testing carried out in accordance with BRE Digest 365 to gain an accurate measure of site permeability and the feasibility of infiltration SuDS techniques for onsite surface water management.


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4.9. Groundwater

It is thought that the clay rich bedrock and superficial deposits would act as a barrier to any rising groundwater and therefore the risk of flooding from this source is minimal. This conclusion is supported by an absence of historical flooding at the site according to the Environment Agency records provided to the 2008 SFRA.

4.10. Surface Water Flood Risk

The proposed development site is situated on relatively impermeable soils which could mean that surface water is an issue. This is because there is limited potential for rainfall to infiltrate the soil and it therefore sheets across the land as overland flow. This risk is not quantified. Despite this indicative potential for surface water at the proposed development site, anecdotal evidence provided within the 2008 SFRA does not identify any surface water flood incidents or issues in the area.

One of the main issues with surface water flooding is that in areas with no history, relatively small changes to hard surfacing and surface gradients can cause flooding (garden loss and reuse of brownfield sites for example). As a result, continuing development could mean that surface water flooding can become more frequent.

The 2012 SFRA provides surface water mapping from the Environment Agency. The

information is based on a national scale map (Flood Map for Surface Water) identifying those areas where surface water flooding poses a risk. The mapping is based on a rainfall event with a 1 in 200 chance of occurring in any year. The extents are not sufficiently detailed to allow a full site-specific assessment of future surface water flood risk but an overview is possible. The majority of the site is shown to not be at risk of surface water flooding with the exception being the area of the existing farm buildings and ditch which are within the shallow flood extent (depths under 0.1m). Shallow

depths such as this are extremely unlikely to pose any threat to persons or property on site.

4.11. Drainage and Sewage Infrastructure

Severn Trent Water has provided details from their DG5 Register to the 2008 SFRA. The

DG5 Register is a record of sewer flood events in post code regions kept by local water companies. The DG5 register shows that there have been 3 properties affected by sewer flooding in the sites post code region in the ten years prior to the production of the 2008 SFRA.

Future development or maintenance works could increase or reduce strain on the sewer network, therefore, the presence or lack of a property postcode on the DG5 Register is not an indication of future flood risk.

4.12. Climate Change

There is clear scientific evidence that global climate change is happening now. Higher greenhouse gas levels in the atmosphere are likely to cause higher winter rainfall totals in the future. Past greenhouse gas emissions mean some climate change is inevitable in

the next 20-30 years. Lower emissions could reduce the amount of climate change further into the future but changes are still projected at least as far ahead as 2115.

In assessing the impacts of climate change on flood risk emanating from the land and rivers, sensitivity ranges given in the Technical Guidance to the National Planning Policy


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Framework (Table 4) may provide an appropriate precautionary response to the uncertainty about climate change impacts on rainfall intensities and river flow.

Table 4 - NPPF Technical Guidance recommended national precautionary sensitivity ranges for peak rainfall intensities and peak river flows

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%


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5.0 Surface Water Runoff Pre-development Condition

The approximate 1.02ha or 1.555ha of site area to be developed is currently greenfield.

5.1. Pre-development runoff rate for proposed development site with permeable drives and footpaths

Using the IH124 method for determining Greenfield runoff rate built into Microdrainage WinDes 2013.1 (including the modification given in the Interim Code of Practice for SUDS, Chapter 6):

AREA = 1.02ha.

SAAR = 700mm

SOIL = 0.45

Pre-development QBAR = 4.5 l/s.

Pre-development peak flow with 1 year return period = 3.7 l/s.



Pre-development peak flow with 100 year return period plus 30% climate change = 14.95 l/s.

5.2. Pre-development runoff volume for proposed development site with permeable drives and footpaths

Using FSR method to determine rainfall and FSSR 16 fixed percentage runoff model for volume (Greenfield runoff volume analysis module built into Microdrainage WinDes 2013.1):

M5_60 = 20.000mm.

Ratio R = 0.400.

Areal reduction factor = 1 (for small site).

Return period = 100 year.

Storm duration = 360 minutes.

Area = 1.020ha.

SAAR = 700mm (obtained from WinDes 2013.1 built in FSR map).

CWI = 105.

Urban = 0.000.

SPR = 47.000

PR% = 45.98.

Pre-development Greenfield runoff volume = 293.151m3.


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5.3. Pre-development runoff rate for proposed development site with impermeable drives and footpaths

Using the IH124 method for determining Greenfield runoff rate built into Microdrainage WinDes 2013.1 (including the modification given in the Interim Code of Practice for SUDS, Chapter 6):

AREA = 1.555ha.

SAAR = 700mm

SOIL = 0.45

Pre-development QBAR = 6.8 l/s.


Pre-development peak flow with 30 year return period = 13.4.2 l/s.


Pre-development peak flow with 100 year return period plus 30%

climate change = 22.88 l/s.

5.4. Pre-development runoff volume for proposed development site with impermeable drives and footpaths

Using FSR method to determine rainfall and FSSR 16 fixed percentage runoff model for volume (Greenfield runoff volume analysis module built into Microdrainage WinDes 2013.1):

M5_60 = 20.000mm.

Ratio R = 0.400.

Areal reduction factor = 1 (for small site).

Return period = 100 year.

Storm duration = 360 minutes.

Area = 1.555ha.

SAAR = 700mm (obtained from WinDes 2013.1 built in FSR map).

CWI = 105.

Urban = 0.000.

SPR = 47.000

PR% = 45.98.

Pre-development Greenfield runoff volume = 446.911m3.


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6.0 Drainage Impact Study

The development involves the construction of up to 120 residential houses with associated driveways, parking bays and access roads with an estimated total impermeable area of either 1.02ha or 1.555ha dependent on whether or not the proposed drives and footpaths are constructed with permeable surfaces.

The existing surface water QBAR flow rate is 4.4 l/s per ha with volumes of 293.2m3 or

446.9m3 leaving the site during a 100 year return period storm event over a duration of 360 minutes for the respective 1.02 and 1.555ha sites.

At present, it is understood from site visit observations and anecdotal evidence that all surface water flow on site follows the general topography of the site trending south-west across the site into the small watercourse running along the south western boundary of the site (Figure 15). The surface water then leaves the site via the watercourse which

enters a culvert in the south west corner of the site and is channelled under neighbouring houses into the Harrow Brook (Figure 15).

Figure 15 - Map displaying estimated surface water flow paths based on topographic data and site visit observations


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The proposed development will see an increase of approximately 1.02 or 1.555ha in hard surfacing across the site dependent on whether or not permeable surfacing is used for

the proposed drives and footpaths.

On a localised scale, landscaping and the creation of new roads may alter the flow of surface water, but it is thought that overall, surface water flow paths will continue to follow the general topographic trend of the site flowing towards the south west boundary (Figure 15).

Any increase in hard surfacing will cause an increase in surface water QBAR flow rates and the volumes of surface water leaving the site. It will be necessary to quantify these increased surface water flow rates and volumes in order to offset them with SuDS.


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7.0 Post-development Surface Water Runoff

Post-development surface water runoff must be controlled to the same level as the existing Greenfield runoff rate (4.5/6.8 l/s during a QBAR rainfall event dependent on whether or not the sites drives and footpaths are constructed with permeable surfaces) to ensure that flood risk is no greater to the surrounding area as a result of the development. The use of suitable SuDS to produce a conceptual drainage scheme is a sustainable method of achieving this level of surface water runoff control.

7.1. SuDS

Paragraph 1.3.2 from the SuDS manual (C697) discusses the SuDS ‘management train’ which is intended to mimic the natural catchment process as closely as possible. The hierarchy of techniques used to achieve the management train are shown below in Table 5.

Table 5 - Hierarchy of SuDS techniques

Technique Description

Prevention The use of good site design and housekeeping measures to prevent runoff and pollution (e.g. rainwater harvesting/reuse).

Source control Control of runoff at or very near its source (e.g. soakaways, porous and pervious surfaces, green roofs).

Site control Management of water in a local area or site (e.g. routing water to large soakaways, infiltration or detention basins)

Regional control Management of runoff from a site or several sites (e.g. balancing ponds, wetlands).

The conceptual drainage scheme outlined below follows the management train outlined above.

7.2. Conceptual Drainage Scheme

In the absence of any ground testing a conservative approach has been taken and it is therefore assumed that the hard surface of the site is fully impermeable. For this reason infiltration SuDS were not considered for the site conceptual drainage scheme.

The site is thought to be sufficiently sized to accommodate the use of an attenuation pond to ensure that any increased volume as a result of the development will be discharged at QBAR for the 1 in 100 year rainfall event. Surface water in the pond will then be discharged into the ditch running along the south western boundary of the site as with the existing situation (Figure 16).


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Figure 16 - Basic outline of the conceptual drainage scheme

A conservative approach to the storage capacity of the pipe network leading from the developed area to the pond has been taken; consequently, this value was left as 0m3.

The land currently slopes to the south west towards the ditch running along the south western boundary of the site. Therefore it is recommended to place the attenuation

pond in Field 4, closest to the ditch and in an area of the site where land naturally drains to.


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7.2.1. Conceptual Drainage Scheme – Attenuation pond with Hydro-Brake outlet assuming the sites drives and footpaths will be permeable

The Hydrobrake delivers water more efficiently across the full range of water heads than other outlet controls such as an orifice. Because of this, the use of a Hydrobrake outlet should result in a lower rate of discharge than other outlets including an orifice.

The effectiveness of this conceptual drainage option has been tested using Microdrainage WinDes 2013.1 Source Control module, under the following conditions assuming that the proposed development will construct all drives and footpaths with permeable surfaces:

The critical storm had 960 minute duration.

The final outflow results for the critical storm is:

Post-development peak flow with 1 year return period = 4.1l/s.

Post-development peak flow with 30 year return period = 4.1/s.


Win Des Parameter Value used

Global Variables

Inflow Rainfall Data

Additional inflow None

Storage structure Tank or Pond

Outflow control Hydrobrake

Overflow control None

Climate change (%) 30

Rainfall details

Return period (years) 100

Region England and Wales

M5-60 (mm) 20.000

Ratio R 0.400

Storms Summer and Winter

Cv 0.750 (summer) 0.840 (winter)

Shortest storm duration (mins) 15

Longest storm duration (mins) 10080

Network storage volume (m3) 0

Time area diagram

0 – 10 minutes 0.34 ha

10 - 20 minutes 0.34 ha

10 - 20 minutes 0.34 ha

30 minutes onwards 0 ha

Pond Structure

Cover level (m) 100.000

Invert Level (m) 99.500

Storage (online/offline) Online

Depth (m) - Area (m2) 0 - 1400

Orifice Outflow Control


Design Head (m) 0.500

Design Flow (l/s) 4.5

Hydro-Brake Type MD5 SW Only


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Post-development peak flow with 100 year return period (+CC) = 4.4l/s.

Post-development discharge volume for a 360 minute, 100 year storm from created impermeable area = 535.6m3.

The discharge rates are limited to the pre-development QBAR without a risk of flooding during the 100 year return period rainfall event including climate change.

The additional runoff volume created as a result of this development will be controlled to

4.4l/s and discharged to the nearby watercourse.

7.2.2. Conceptual Drainage Scheme – Attenuation pond with Hydro-Brake outlet assuming the sites drives and footpaths will be impermeable

The conceptual drainage option has been retested using Microdrainage WinDes 2013.1 Source Control module under the following conditions assuming that the proposed development will construct all drives and footpaths with impermeable surfaces:

Win Des Parameter Value used

Global Variables

Inflow Rainfall Data

Additional inflow None

Storage structure Tank or Pond

Outflow control Hydrobrake

Overflow control None

Climate change (%) 30

Rainfall details

Return period (years) 100

Region England and Wales

M5-60 (mm) 20.000

Ratio R 0.400

Storms Summer and Winter

Cv 0.750 (summer) 0.840 (winter)

Shortest storm duration (mins) 15

Longest storm duration (mins) 10080

Network storage volume (m3) 0

Time area diagram

0 – 10 minutes 0.518 ha

10 - 20 minutes 0.518 ha

10 - 20 minutes 0.519 ha

30 minutes onwards 0 ha

Pond Structure

Cover level (m) 100.000


Storage (online/offline) Online

Depth (m) - Area (m2) 0 - 2100

Orifice Outflow Control


Design Head (m) 0.500

Design Flow (l/s) 6.8

Hydro-Brake Type MD5 SW Only


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The critical storm had 960 minute duration.

The final outflow results for the critical storm is:


Post-development peak flow with 30 year return period = 6.5/s.


Post-development peak flow with 100 year return period (+CC) = 6.7l/s.

Post-development discharge volume for a 360 minute, 100 year storm from created impermeable area = 816.5m3.

The discharge rates are limited to the pre-development QBAR without a risk of flooding during the 100 year return period rainfall event including climate change. However, the pond size required to limit discharge to the existing QBAR and ensure no risk of overtopping during a 1% annual probability storm including the effects of climate change is 700m2 larger than that required if drives and footpaths are constructed with permeable material.

The additional runoff volume created as a result of this development will be controlled to 6.7l/s and discharged to the nearby watercourse.

7.3. Conveyance

The transfer of surface water runoff (conveyance) across the site, between components is essential. There are a variety of approaches that can be used; underground through pipes with little control or water quality treatment, or through vegetated channels on the

surface providing some treatment and attenuation and through more engineered canals or rills. The preference in terms of delivering sustainable drainage objectives is the conveyance of water through vegetated channels or swales. Uncontrolled conveyance to a point of discharge into the environment is discouraged.

Swales are usually shallow grassed or vegetated channels used to collect and/or move water. The shallow side slopes and flat bottom means that for most of the time water flows in a thin layer, some of them can be under drained with the use of perforated pipe. Swales can also provide some storage and filtration as well.

The use of conveyance at the proposed development site would be a tool to transport surface water to the attenuation pond.

7.4. Infiltration SuDS

The flows and discharge volumes calculated above for the two conceptual drainage scheme options are conservative, assuming that no infiltration is possible based on a lack of ground testing undertaken at the site.

Current indications of site permeability have been obtained from 1:50,000 scale British

Geological Survey records, anecdotal evidence and site visit observations. While British Geological Survey records indicate that the site has a low permeability, the anecdotal evidence and site visit observations differ with this and indicate a moderate to high permeability. Infiltration tests must be carried out in accordance with BRE Digest 365 to


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gain an accurate measure of site permeability and the feasibility of infiltration SuDS techniques for onsite surface water management.

If infiltration test results are positive, there are a number of options which would reduce the surface water runoff from the site and consequently would potentially reduce the pond size required to control surface water runoff to pre-development QBAR.

7.4.1. Permeable surfacing

The use of permeable paving or tarmac instead of impermeable surfacing for all drives and footpaths would immediately reduce the proposed developments impermeable area by approximately 5,349.4m2. This is something that could be achieved fairly easily and would significantly reduce volumes of surface water discharged from the site and consequently the size of the attenuation pond required to manage it as demonstrated in Chapter 7.2.

7.4.2. Soakaways

Soakaways are square or circular excavations either filled with rubble or lined with brickwork, pre-cast concrete or polyethylene rings/perforated storage structures surrounded by granular backfill. They can be grouped and linked together to drain large areas including highways. The supporting structure and backfill can be substituted by

modular or geocellular units.

Soakaways easy to construct and operate, require minimal net land, minimal maintenance and are recognised as a good method of volume reduction and peak flow attenuation.


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8.0 Conclusion

This FRA has been prepared to meet National Planning Policy Framework guidelines for a proposed development of up to 120 residential properties at 222 The Long Shoot, Nuneaton. Currently the site is almost entirely Greenfield and functioning as agricultural land. Approximately 3,000m2 of land is impermeable and used as kennels, agricultural stores and a residential property.

The site itself is located within Flood Zone 1 of the Environment Agency’s Flood Map and is at low risk from flooding from all sources. However due to the approximate 5.8ha size of the site, measures must be considered to reduce the impact that the site may have on other areas due to an increased impermeable area.

The Greenfield runoff rate for the proposed development site has been estimated using the IH124 method for determining Greenfield runoff rate revealing a surface water runoff

rate of 4.4 l/s per ha for a QBAR flow.

A conceptual drainage scheme has been designed and tested for effectiveness using Microdrainage. The drainage scheme proposed is for an attenuation pond with outlet control to the watercourse running along the southwest boundary of the site. This attenuation pond will collect water from all proposed impermeable surfacing and discharge at no greater than the pre-existing QBAR for a 100 year return period storm

event including the effects of climate change. Despite an increase in volumes leaving the site via the watercourse, the limitation of the 100 year return period storm event discharge to QBAR is in line with the recommendation given in the SUDS manual and will provide betterment to the existing situation. Thought should be given to conveyance SuDS such as swales which could be used to carry water from the developed site into the pond.

If infiltration tests are carried out and show that the use of infiltration tests are viable for the site they could be used to supplement the attenuation pond, providing opportunities for reduction in volumes leaving the site from the proposed development.

In any case, it can be concluded that the above outline designs for the management of surface water will ensure that the proposed development does not increase flood risk at

the site or in the surrounding area.


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9.0 Recommendations

Post-development surface water runoff from a 100 year return period storm event including the effects of climate change must be controlled to the same level as the existing QBAR runoff rate (4.4l/s per ha) to ensure that flood risk is no greater to the surrounding area as a result of the development. This can be achieved using the provided conceptual drainage design.

Infiltration tests should be carried out in accordance with BRE Digest 365 to

confirm the permeability of the soil.

Informed by the infiltration test results, topographic survey of the site, and post-development surface water runoff calculations a mixture of appropriate SuDS as outlined in Chapter6 should be installed at the site.


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Appendix A Development Proposals


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