evidence paper 6: flood risk and surface water … this paper also provides a strategic overview of...

Chippenham Site Allocations Plan

Evidence Paper 6:Flood Risk and Surface Water

Management

Interim report

December 2014

Wiltshire LocalDevelopmentFramework

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Wiltshire Council

This is one of 6 evidence papers prepared to support the development of the Chippenham

Site Allocations Plan. The evidence papers are:

Evidence Paper 1: Economy

Evidence Paper 2: Housing and Community Facilities

Evidence Paper 3: Transport and Accessibility

Evidence Paper 4: Landscape Assessment

Evidence Paper 5: Biodiversity

Evidence Paper 6: Flood Risk and Surface Water Management

The evidence in these papers need to be read as a whole in order to gain a clear picture of

the emerging preferred areas for development and thereafter the preferred sites within those

areas.

Contents

1. Introduction .................................................................................................................... 1

2. Rationale........................................................................................................................ 3

National planning policy ..................................................................................................... 3

The Wiltshire Core Strategy ............................................................................................... 4

Strategic Flood Risk Assessment ...................................................................................... 4

3. Summary ....................................................................................................................... 5

The extent of flood risk areas (zones 2 and 3) ................................................................... 6

Surface Water Management Issues ................................................................................... 7

Groundwater Flooding Susceptibility ................................................................................. 8

Further evidence requirements .......................................................................................... 9

Conclusion ...................................................................................................................... 10

4. Strategic areas ............................................................................................................. 11

Area A ............................................................................................................................. 11

Area B ............................................................................................................................. 11

Area C ............................................................................................................................. 12

Area D ............................................................................................................................. 14

Area E ............................................................................................................................. 15

Chippenham Site Allocations Plan Evidence Paper 6: Flood Risk and Surface Water Management

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1. Introduction

1.1 The Chippenham Site Allocations Plan will set the long term pattern and direction of

growth for the town’s expansion. Its main purpose is to identify strategic mixed use

sites for businesses, new homes and the infrastructure necessary to support them.

1.2 The Wiltshire Core Strategy sets a minimum amount of additional housing and

employment for the town between 2006 and 2026. It also establishes a set of six

criteria to guide the town’s expansion (the CP10 criteria). These form the central basis

for selecting ‘strategic sites’ expanding the town.

1.3 The Core Strategy identifies, diagrammatically, a set of strategic areas east of the

A350 as potential directions for future expansion by the identification of strategic mixed

use sites. The ‘strategic areas’ are defined by barriers such as main roads, rivers and

the main railway line.

1.4 The Core Strategy has determined that strategic sites will be east of Chippenham

(strategic areas A – E).

1.5 Each of the strategic areas will be assessed to see how they perform against the CP10

criteria and sustainability appraisal objectives and then a preferred area is selected.

1.6 The Chippenham Strategic Sites Assessment Framework sets out in more detail how

each of these criteria will be used. It lists a set of indicators by which an area or site

should be measured, the rationale explaining why it is included and what evidence will

be used to describe how well a site or area performs against that measure.


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1.7 The indicators will therefore be used in the first instance to assess the relative merits

of strategic areas A to E as shown on the Chippenham strategic areas diagram.

Thereafter they will be used to assess individual sites within a preferred area.

1.8 One of the six criteria relates to flood risk1. Areas for growth and site allocations within

the Plan should:

“Avoid all areas of flood risk (therefore within zone 1) and surface water management

reduces the risk of flooding elsewhere.”

1.9 The Strategic Site Assessment Framework2 provides further detail by the evidence

requirement of

‘reliable mapping of flood zones and identification of surface water management

requirements.’

1.10 The strategic site assessment framework assesses preferred areas and site options

against the amount of flood zone 1, 2 and 3 affecting their area.

1.11 In accordance with the Strategic Site Assessment Framework this paper therefore sets

out to:

Flood risk zones: Establish the amount of land within each of flood risk zone 2-3

and assess the location of flood risk areas as a constraint to development.

Surface Water Management Issues: Identify other sources of flood risk in each of

the strategic areas and their impact, then comment on their cause and the potential

to remedy them.

Further evidence requirements: Identify where and what further detailed

assessment of flood risk and surface water management may be necessary to

progress options in preferred areas.

1.12 Based on this evidence the paper suggests preferred areas, solely in terms of criterion

6 of CP10, within which strategic sites should be selected.

1.13 This paper also provides a strategic overview of flood risk and surface water

management issues, looking at sources of flooding from all potential sources. In

addition to suggesting preferred strategic areas in terms of flood risks, it sets out what

further work would be needed by developers to establish and manage site specific

flood risks. Where appropriate these requirements would be set out as a part of

detailed plan proposals.

1 1 Wiltshire Core Strategy Pre-submission Document, Tracked Changes Version, Wiltshire Council,

April 2014 2 Chippenham Site Allocations Plan Strategic Site Assessment Framework, Wiltshire Council, July

2014


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2. Rationale

National planning policy

2.1 National planning policy directs all new development to zone 1 areas of least risk of

flooding. This is reflected in criterion 6 of Core Policy 10. Zone 1 areas are those at

least risk of flooding; at worst 1 in 1000 years. Only if there is insufficient appropriate

land within zone 1 need other areas be considered. For the purposes of this paper

they are termed flood risk areas (representing zones 2 and 3 of the areas used by the

Environment Agency (EA). There is a sequential approach. The Government’s

practice guidance3 explains in more detail how to apply this approach to site selection

and the evidence required.

2.2 The approach is set out in paragraph 100 of the National Planning Policy Framework4:

“Inappropriate development in areas at risk of flooding should be avoided by directing development away from areas at highest risk, but where development is necessary, making it safe without increasing flood risk elsewhere. Local Plans should be supported by Strategic Flood Risk Assessment and develop policies to manage flood risk from all sources, taking account of advice from the Environment Agency and other relevant flood risk management bodies, such as lead local flood authorities and internal drainage boards. Local Plans should apply a sequential, risk-based approach to the location of development to avoid where possible flood risk to people and property and manage any residual risk, taking account of the impacts of climate change, by:

applying the Sequential Test

if necessary, applying the Exception Test

safeguarding land from development that is required for current and future flood

management

using opportunities offered by new development to reduce the causes and impacts

of flooding

where climate change is expected to increase flood risk so that some existing

development may not be sustainable in the long-term, seeking opportunities to

facilitate the relocation of development, including housing, to more sustainable

locations.”

2.3 Planning policy5 requires that local planning authorities understand flood risk from all

sources. Other water management issues, such as limited capacity in the drainage

network, need to be incorporated into the consideration of development proposals so

that these they can be remedied rather than possibly compounded. Flooding can

occur for a number of reasons in addition to rivers and watercourses over flowing.

3 Planning Policy Statement 25:Development and Flood Risk Practice Guide, DCLG, December 2009

4 National Planning Policy Framework, DCLG, March 2012

5 National Planning Policy Framework, DCLG, March 2012 para 100


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The Wiltshire Core Strategy

2.4 Core Policy 67 of the Wiltshire Core Strategy requires all new development to include

measures to reduce the rate of rainwater run-off and improve rainwater infiltration to

soil and ground (sustainable urban drainage) unless site or environmental conditions

make these measures unsuitable.

2.5 Core Policy 68 ensures that Development must not prejudice the delivery of the

actions and targets of River Basin Management Plans, and should contribute towards

their delivery where possible.

Strategic Flood Risk Assessment

2.6 There are two levels of assessment (Strategic Flood Risk Assessment (SFRA)), Levels

One and Two, the second of which is more detailed than the first. Both potentially

have a part to play in planning future development.

2.7 Generally, SFRA Level 1 establishes flood risk zones so that all new development can

be guided to zone 1: the areas of least risk.

2.8 An SFRA level 1 study involving Chippenham was carried out in 20076 and has been

updated regularly since to take account of new legislative requirements and new

information. The Environment Agency (EA) has recently confirmed that this remains a

sound basis upon which to base site selection and to apply a sequential approach7.

2.9 The EA is updating its existing flood model for the wider Chippenham area. Their

intention is to consider watercourses including the River Avon and River Marden, in

addition to other watercourses in the town such as the Hardenhuish Brook, Ladyfield

Brook and Pudding Brook. The intention is to complete the project by the end April

2015, but this cannot be guaranteed. However, the EA has said this work is unlikely

to result in any substantial changes to the definition of flood zones 1 -3.

2.10 Guidance8 states that where an SFRA Level 1 Assessment shows that land outside

flood risk areas cannot appropriately accommodate all the necessary development, it

may be necessary to increase the scope of the Assessment to a Level 2. SFRA level 2

is most often needed where there are redevelopment proposals within flood risk areas

and they need to satisfy strict tests to be justified as exceptions to normal planning

policy. Core policy 10 anticipates that development through the selection of strategic

sites will be directed to zone 1. The scale of land requirement and the potential land

available within strategic areas outside flood risk areas can appropriately

accommodate all the necessary development. There is therefore no need for SFRA

level 2 assessments to inform the site selection process.

6 Scott Wilson North Wiltshire Strategic Flood Risk Assessment Level One 2007

http://www.wiltshire.gov.uk/planninganddevelopment/planningpolicy/planningpolicyevidencebase/evidencebasenorth.htm#SFRA_Level_One 7 Note of Meeting EA and WC officers 12

th March 2014

8 NPPG Paragraph: 012 Reference ID: 7-012-20140306

http://www.wiltshire.gov.uk/planninganddevelopment/planningpolicy/planningpolicyevidencebase/evidencebasenorth.htm#SFRA_Level_One

http://www.wiltshire.gov.uk/planninganddevelopment/planningpolicy/planningpolicyevidencebase/evidencebasenorth.htm#SFRA_Level_One


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3.1 For the purposes of selecting strategic sites as a part of the Chippenham Site

Allocations Plan it will be necessary to ensure that estimated scales of development

from proposals can be assured without encroaching out of zone 1 areas. However if

development is proposed that potentially abuts higher risk zones, more detailed

assessment will be necessary to establish where a precise boundary to development

should lie taking account, for example, of smaller watercourses that may not have

featured in the original mapping. This more detailed assessment will be provided by

developers of large sites as a flood risk assessment in support of a planning

application.

3. Summary

3.1 Parts of Chippenham are shown as flood risk areas (see figure one). The Bristol Avon

Catchment Flood Management Plan identified that approximately twenty-five to fifty

properties were at risk9. The Council’s Operational Flood Plan states up to 700

properties are at risk of flooding10. Deep and fast water flows can occur11. Most recent

flooding has affected the bottom of the High Street (Superdrug and WH Smith)

3.2 Flood incident management is provided in the form of flood warning to properties.

Flood defences include a radial gate weir in the centre of the town which helps to

regulate river levels by throttling the river’s flow. A five metre high earth embankment

was built to the standard of the time as a part of the Monkton Park housing scheme in

order to protect it from flooding and a concrete wall protects an area around Bewley

House from waters from Hardenhuish Brook.

3.3 In addition, some parts of the Plan area are considered susceptible to groundwater

flooding and increased discharges to these aquifers through infiltration using

Sustainable Urban Drainage Systems (SUDS) may lead to future groundwater flooding

issues. The use of these measures therefore may need to be carefully managed.12

3.4 Generally SUDS are the preferred method for managing the surface water run-off

generated by developed sites. SUDS seek to manage surface water as close to its

source as possible, mimicking surface water flows arising from the site, prior to the

proposed development (In the case of the Chippenham Site Allocations Plan and

proposals for strategic sites these are therefore termed ‘Greenfield rates’). Typically

this approach involves a move away from piped systems to softer engineering

solutions inspired by natural drainage processes.

3.5 SUDS should be designed to take into account the surface run-off quantity, rates and

also water quality ensuring their effective operation up to and including the 1 in 100

9 Bristol Avon Catchment Flood Management Plan, Summary Report, EA, June 2012, Table 1.

Locations of towns and villages with 25 or more properties at risk in a 1% annual probability river flood, page 7 10

Operational Flood Plan 2012-2013, Wiltshire Council, 2012 page 23 11

Strategic Flood Risk Assessment High Level Executive Summary, Final Report, Scott Wilson, June 2009 para 6.3 12

Chippenham Surface Water Management Plan Intermediate Assessment of Groundwater Flooding Susceptibility, Scott Wilson, November 2011


6

year design standard flood including an increase in peak rainfall of 30% to account

from climate change13.

The extent of flood risk areas (zones 2 and 3)

3.6 Figure one below shows the estimated extent of flood risk zones in the Plan area. It

also shows historic flood mapping recorded by the Environment Agency.

Figure 1: Chippenham Fluvial Flooding Map

3.7 Strategic area A is the only one entirely within zone 1. Other strategic areas abut

rivers and streams and are ranked according to an estimate of the amount of flood

zones 2 and 314. The results are shown in the table below.

Strategic Area

Approximate extent of flood zone 2 and 3 (ha)

A 0

B 40.5

C 76.2

D 18.8

13

Planning Policy Statement 25: Development and Flood Risk, Practice Guide, DCLG, December 2009, paras 5.10 - 5.14 14

Appendix 1 shows the river extents used to make these estimates

A

B

C

D E


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E 41.8

Figure 2: Extent of flood risk zones by Strategic Area

Surface Water Management Issues

3.8 Area A has particular drainage issues because it is raised and flat making it difficult to

create the falls necessary for water to drain by gravity. It is also at the fringes of the

drainage network and this plus the railway limit the scope to improve drainage

connections. Other areas can direct surface water flows to the River Avon, more

straightforwardly and one of the main challenges is controlling rates of flow.

3.9 Additional water flows need to be prevented from arriving more quickly at the radial

gate in Chippenham centre. This is therefore particularly relevant for Areas B and C

immediately upstream. There are high flood risks at the radial gate, evidence of which

was the flooding in past years such as at the High Street affecting Superdrug and WH

Smith’s.

3.10 Drainage effects from areas E and D may also have an impact on downstream

settlements and could be potentially significant and so any developments should also

seek to mimic the green field rates of runoff or preferable improve on. However they

are downstream of Chippenham’s built up area, no properties are in immediate risk

and rural settlements along the river course (such as Lacock) do not straddle the main

river.

3.11 Area D is very flat compared to some other areas creating difficulties for drainage by

gravity.

3.12 Areas ‘barriered off’ by roads and railway embankments interfering with river flows

makes it especially desirable to achieve reductions in surface water rates of run off

compared to their current Greenfield ones. Areas C and D, to be developed, would

each ultimately require bridges over the River Avon, and introduce potential obstacles

into flood risk areas that also need to be carefully considered.

3.13 The SFRA Level 1 reports records the following flood events affecting Chippenham

1980 Fluvial flood event. Areas affected include Littlefield industrial estate, Possibly caused from overflow from Ladyfield Brook.

1995 and 1999

Fluvial flood event. Westmead playing fields. Local road and land affected.

2000 3 properties flooded

3.14 Guidance15 seeks rates of run-off from new developments that should be ‘no greater

than the rates prior to the proposed development’. With climate change predicted to

15

Planning Policy Statement 25: Development and Flood Risk Practice Guide, DCLG, December 2009, paras 3.79 and 5.54


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cause more frequent, short-duration, high intensity rainfall and more frequent

occurrences of long-duration rainfall, surface water flooding is likely to be an

increasing problem.

3.15 Surface water from developed areas abutting flood risk areas is likely to need the most

careful management to ensure it does not increase risks of flooding elsewhere. New

development can need relatively extensive attenuation measures to achieve the

surface water run-off position of undeveloped land. This may involve intervention and

reliance on man-made measures and this carries an intrinsic risk; such as upstream

storage being overtopped. On the other hand, national policy encourages plans to

exploit opportunities presented by new development to reduce flood risks and its

causes16.

3.16 Figure one shows areas of land within the existing built up area of Chippenham within

flood zones 2 and 3 although much of the most vulnerable areas are protected by flood

defences and river management. Areas A, B, C, upstream, are the source of surface

water that, to some degree, flows immediately through the town. It is essential that

these flows do not increase and add to flood risks within the built up area. A first step

in a risk based approach is to direct development to flood zone 1, areas of least risk.

In general, a reasonable next step is to direct development to areas where the impacts

of flooding, should it happen, in terms of risk to lives and property, are less harmful; in

other words in areas downstream of the built up area. Therefore Areas E and D are

preferable on this account.

Groundwater Flooding Susceptibility

3.17 Work involved in the preparation of a surface water management plan for Chippenham

identified geological conditions around the Plan area that may be a cause of

groundwater flooding. Groundwater flooding occurs sporadically in both location and

time, and tends to last longer than fluvial, pluvial or sewer flooding. Basements and

tunnels can flood, buried services may be damaged, and storm sewers may become

ineffective, exacerbating the risk of surface water flooding. Groundwater flooding can

also lead to the inundation of farmland, roads, commercial, residential and amenity

areas.

3.18 High groundwater level conditions may not lead to widespread groundwater flooding.

However, they have the potential to exacerbate the risk of pluvial and fluvial flooding

by reducing rainfall infiltration capacity, and may increase the risk of sewer flooding

through sewer / groundwater interactions.

3.19 Work carried out on behalf of the Council has identified very broad areas where there

is a greater susceptibility to groundwater flooding. Figure 2 is an extract from the final

report showing those areas. Generally, water management by SUDS, necessary to

achieve Greenfield rates of run-off, need to be more carefully considered in areas C, D

and E and may make effective management of surface water more complex and

higher risk.

16

NPPF paragraph 100


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Further evidence requirements

3.20 Each of the strategic sites will require planning applications for comprehensive

development to be supported by a detailed flood risk assessment. For any sites

immediately abutting flood risk areas, (potentially all areas except Area A) there will

need to be a detailed assessment to ascertain the precise boundaries to zone 2.

3.21 In particular, if sites are allocated requiring new bridges in flood zones 2 and 3 then

this will require proposals to satisfy the exception test in accordance with NPPF

paragraph 102. Proposals for development will need to pass the exception test by:

“it must be demonstrated that the development provides wider sustainability benefits to

the community that outweigh flood risk, informed by a Strategic Flood Risk

Assessment where one has been prepared; and

a site-specific flood risk assessment must demonstrate that the development will be

safe for its lifetime taking account of the vulnerability of its users, without increasing

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

3.22 Both elements of the test will have to be passed for development to be permitted.

3.23 This test will arise if strategic sites are identified in either strategic areas C or D.

Essentially, the selection of one or other of these as preferred areas will need to be

demonstrated by wider sustainability benefits to the community that outweigh the flood

risk of building connections to the town. In these circumstances the Plan and evidence

supporting such proposals would therefore need to set out such benefits and how they

convincingly outweigh risks.


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3.24 Developers promoting sites within strategic areas C or D, where bridges across the

river Avon form a part of their scheme, must demonstrate the development will be safe

for its lifetime without increasing flood risk elsewhere. This level of information will

need to be equivalent to the detail required of an SFRA level 217. The Environment

Agency has indicated18 that developing an appropriate design for a river crossing

would be likely to require detailed hydraulic modelling that has not been done to date

of river flows, depths and timings. This evidence needs to be sufficient to pass the

second part of the exception test above.

3.25 Separate Flood Defence Consent would also be required from the Environment

Agency. These requirements would be set out in detailed plan proposals.

Conclusion

3.26 In terms of flood risk and surface water management the relative propensity of the

strategic areas to accommodate strategic sites is as follows:

Ranking Strategic Area

Comments

1 A Entirely with flood zone 1 without abutting other flood risk zones though upstream of existing built up area. However, there are potential issues achieving good drainage.

2 D Has the least flood risk area. A developable area abuts zones 2 and 3 and river crossing(s) may constrict flows, but downstream of built up area. It is also flat with reduced scope for gravity led drainage.

3 E Has the second most flood risk area. A developable area abuts zones 2 and 3. It is downstream of existing built up area but tributary watercourses impinge on developable area.

4 B Has the third most flood risk area. A developable area abuts zones 2 and 3 and is upstream of existing built up area.

5 C Has the most flood risk area. A developable area abuts zones 2 and 3 and river crossing(s) may constrict flows, and is also upstream of built up area.

3.27 The main determinant of this ranking is the extent of flood risks zones within each

area. However, a second determining aspect is an area’s location either upstream or

downstream of Chippenham built up area. This has the single effect of reversing the

position of areas E and B. Each area has particular complications to deal with as far

as water management. They have much the same extent of estimated food risk area,

but area B is upstream of the town. Area C is clearly the least desirable area in flood

risk terms not just because of the extent of area subject to flood risk, but the

combination of also being upstream and involving river crossings that may constrict

water flows.

17 NPPG 7-012-20140306

18 Letter from the Environment Agency, 23

rd July 2014


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4. Strategic areas

Area A

4.1 The Area is entirely within flood zone 1.

4.2 The drainage of this area may be problematic. The area is flat so making it difficult to

have the falls necessary for drainage by gravity. A great deal of surface water is

currently sent to a “sinkhole” where it passes through the clay strata. The area is at

the fringes of the town’s drainage network. As pipe work travels away from this lowest

point, its size decreases as it spreads and therefore capacity can be limited.

4.3 Area A is situated at the limit of the drainage system and so draining an area may

have to be limited to the existing rates available at the edge of the drainage system.

The railway acts as a drainage barrier and so this too will have pipes and culverts

which may sterilize the upstream drainage design. Area A is generally not in fluvial

floodplain but has suffered in part from surface water flooding where the A350 arrives

at Chippenham.

Community Views

4.4 Langley Burrell Residents Association reports that the Swindon Road has flooded in

several places19. Poorly maintained highway drains are seen as a common cause,

particularly when land around is saturated. The land does not drain for weeks due to

the clay subsoil. Flash flooding is known locally to occur between November and

February. In their view both better maintenance of the drainage system and better

infrastructure would be needed as part of any development.

Area B

4.5 A developable area is protected from the River Avon and River Marden by being on

higher ground. There would be limited fluvial flooding on the western bank side due to

the natural lie of the land.

4.6 However drainage from this area will be directed to the River Avon so the creation of

large impervious areas here will lead to additional peak flows joining the river and

therefore additional flows arriving at the radial gate weir in Chippenham centre. This

would add to high flood risk at the radial gate.

Community Views

4.7 Chippenham Community Voice reports20 the year-on-year flooding in the River

Avon/River Marden valley is worsening, and any major development in that area to the

east of Chippenham is only going to exacerbate the problem. Serious flooding

occurred over the 2013 Christmas period to the east of Chippenham. Flooding from

19

Response to consultation on the Strategic Site Assessment Framework, Langley Burrell Residents Association, June 2014, photographs of local area showing food incidents and waterlogged land, October-November, 2014 20

Comments received on Wiltshire Local Development Scheme, Wiltshire Council Cabinet Minutes, January 2014


12

this area stretched from Chippenham to Christian Malford, Sutton Benger, and Ratford

and properties in this area were flooded or threatened with flooding.

Developer submission

4.8 A flood risk assessment was carried out in March 201221. Detailed hydraulic modelling

demonstrated that flood levels for all modelled scenarios are lower than flood zones 2

and 3 designated by the Environment Agency.

4.9 It is proposed that sustainable urban drainage (SUDS) measures such as swales,

balancing ponds with outfalls discharging to the various drains, ditches and

watercourse within the site would achieve Greenfield rates of run-off or less.

Area C

4.10 The flood plain separating Area C from the built up area needs particularly detailed

consideration. A developable area east is protected from the River Avon and River

Marden by being on higher ground as with Area B, but the extent of flood risk zones is

greater. The area has by far the largest amount of immediate flood upstream storage

in effect safeguarding the town centre and with the radial flood defence this area

therefore provides the town centre with its main means of protection.

4.11 The floodplain becomes a ‘hairpin’ route downstream and the flood plain becomes

much narrower. It is this reduction in flood storage capacity that has contributed to

flooding. There is a high flood risk at the radial gate borne out by flooding in past

years in High Street effecting Superdrug and WH Smith’s. Southeast Monkton Park

has flood defences but remains vulnerable to flooding. A priority for any development

would be to avoid any additional peak flows joining the river and therefore additional

flows arriving at the radial gate weir in Chippenham centre.

4.12 Upstream, regular flooding already occurs around Tyherton Lucas from the Cade

Burma in particular. The river Avon’s confluence with the River Marden plus a disused

railway causeway complicates the current flows of flood waters.

4.13 New road and dedicated links across the river for pedestrians and cyclists would be

necessary to properly connect potential development to the town. Such new structures

outside flood zone 1 may displace water, disrupt natural flows or involve the loss of

existing flood storage. None of these aspects present insurmountable problems but do

add a further level of complication.

4.14 Investigations also show that some of area C has a propensity to groundwater

flooding, although much of the affected area is close to the River Avon and as a flood

risk area will not be built on, this may have a bearing on the potential for and design of

SUDS.

4.15 On a positive note, development of area C might deliver greater flood storage to

protect the town and reduce existing risks. National policy expects Local Plans to use

opportunities from new development to reduce the causes and impacts of flooding22.

21

Rawlings Green: Environmental Appraisal, Pegasus Planning Group, March 2012 22

NPPF paragraph 100


13

On a negative note, the consequences of management measures being miscalculated

or failing would be potentially far more serious than other areas because of the amount

of flood zone involved and the area’s immediate relationship to the town.

4.16 The further work being undertaken by the EA on modelling the catchment could

usefully add to detailed knowledge on how to manage flood risk by the optimum

means and with the least risk. Indeed, significantly, it could also provide a clear steer

to develop ways of reducing risk.

4.17 On balance area C appears the least attractive for development in terms of flood risk

and surface water management compared to the others because of the degree to

which flooding is an issue to tackle and the extent of flood risk land.

3.28 Community views

4.18 Residents of Monkton Park and the Riverside Area in particular registered concern that

existing flooding risk will be exacerbated by the run off from any development east of

the River Avon23. In the past, the area has been treated as a flood area by insurers.

4.19 Chippenham Community Voice reports24 the year-on-year flooding in the River

Avon/River Marden valley is worsening, and any major development in that area to the

east of Chippenham is only going to exacerbate the problem.

4.20 The road from Bremhill to Chippenham via Maud Heaths Causeway was closed

recently on two occasions meaning a considerable diversion, the roads through

Christian Malford were also flooded and closed.

4.21 Similar flooding also happened on 30th November 2012 and a clear view is that it

cannot be considered a once in a hundred year flooding event but now an annual

occurrence. The frequency of flooding has been confirmed by a local farmer who

reported water levels up to ten feet or more.

3.29 Developer submission

4.22 Developers promoting development of area C commissioned a hydrological appraisal25

This recognises community concerns about local flood risks. It refers to flood risks to

the Monkton Park Estate to the west and in the area of Tytherton Lucas and Kellaways

to the north. One of the objectives of their assessment was to confirm or otherwise

whether or not these areas are liable to fluvial flood risk.

4.23 Their work concluded that Monkton Park itself lies largely out of the floodplain, in flood

zone 1, and is not susceptible to flooding, with the exception of the most recently

developed south east corner. As with area B detailed modelling confirms that the EA

extent of flood risk areas is quite conservative.

23

Comments received on the Strategic Site Assessment Framework consultation, June 2014 24

Comments received on Wiltshire Local Development Scheme, Wiltshire Council Cabinet Minutes, January 2014 25

Proposed Development of Land At East Chippenham Hydrological Appraisal, Waterman Transport & Development Limited, October 2012


14

4.24 Modelling demonstrates that by reducing the rate of surface water discharge below

that which currently exists, perhaps using a ‘hydro brake’, betterment could be

provided to property downstream of the study area, including the town centre. This

might involve reducing ground levels, producing tangible improvements against the

more frequent and less extreme flood events.

4.25 The developers consider area C equivalent to Rawlings Green (Area B) and South

West Chippenham (Area E) that are also areas for potential strategic development

sites that contain land falling in flood zones 2 and 3. Therefore in their view there

appears to be no sequentially superior site. Attention should therefore focus on which

of those sites could deliver other flood risk benefits, especially for the town centre.

4.26 In this regard area C is considered clearly superior by virtue of engineering measures

that would reduce surface water run-off, possibly increase flood storage and therefore

reduce risks of flooding elsewhere. Land in flood zones 2 and 3 would be used for

formal and informal open space that could benefit the wider community and,

potentially, Abbeyfield School in particular.

Area D

4.27 Area D abuts flood risk areas but has the least area of all those areas that do. A

developable area would lie entirely within flood zone 1 the area of least risk.

4.28 However a similar complication arises in this area as with Area C. Appropriate

development would be at least partially dependent upon creating crossings to the

River Avon in order to ensure proper connections to the town. New road and

dedicated links across the river for pedestrians and cyclists would be necessary to

properly connect potential development. Such new structures outside flood zone 1

may displace water, disrupt natural flows or involve the loss of existing flood storage.

None of these aspects involve insurmountable problems but do add a further level of

complication. However, by comparison to area C, the risk are less because of being

downstream from Chippenham.

4.29 Any development in area D would drain directly to the River Avon and Blackwell Hams

Sewage Treatment Works run by Wessex Water. The drainage effect on water levels

downstream could be significant and so any developments would need to mimic the

green field runoff state or preferably improve on it. However, again the area is

downstream of Chippenham’s built up area, no properties are in immediate risk and

rural settlements in the valley do not straddle the main river.

4.30 Area D is very flat. Runoff water from the Stanley Lane sports facility and the A4

London Road drains south and west to the River Avon through area D.

4.31 Investigations also show that some of area D has a propensity for groundwater

flooding, although much of the affected area is close to the River Avon and as a flood

risk area will not be built on. This may still have a bearing on the potential for and

design of SUDS.


15

Area E

4.32 Area E abuts flood risk zones to the east. It also includes several smaller tributary

watercourses draining to the River Avon. This introduces complications in ensuring

that a sensible scale and pattern of development can be achieved along with

measures to provide for an acceptable surface water management regime.

4.33 Like Areas D, Area E would drain directly to the River Avon and Blackwell Hams

Sewage Treatment Works run by Wessex Water. The drainage effect on downstream

settlements could be significant and so any developments would need to mimic the

green field runoff state or preferable improve on it.

4.34 Investigations also show that some of area E has the highest propensity to

groundwater flooding, although much of the affected area is close to the River Avon

and as a flood risk area will not be built on. It may have a bearing on the potential for

and design of SUDS.


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Appendix 1

Figure 3: Flood risk area estimates

River extent for

flood risk area estimates

evidence paper 6: flood risk and surface water … this paper also provides a strategic overview of...

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