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North Wiltshire District Council Strategic Flood Risk Assessment Level 1
Draft Report October 2007
Prepared for:
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North Wiltshire District Council Strategic Flood Risk Assessment
Revision Schedule
Strategic Flood Risk Assessment October 2007
Rev Date Details Prepared by Reviewed by Approved by ’ 01 25/10/07 Draft Strategic Mark Crussell Carl Pelling Damon O Brien Flood Risk Assistant Hydrologist Senior Consulltant Technical Director Assessment
Scott Wilson Ltd., The Crescent Centre, This document has been prepared for the titled project or named Temple Back, part thereof and should not be relied upon or used for any other Bristol, project without an independent check being carried out as to its BS1 6EZ suitability and prior written authority of Scott Wilson being obtained. Scott Wilson accepts no responsibility or liability for the consequence of this document being used for a purpose other than Tel. 0117 917 1221 the purposes for which it was commissioned. Any person using or Fax. 0117 930 0342 relying on the document for such other purpose agrees, and will by [email protected] such use or reliance be taken to confirm his agreement to indemnify Scott Wilson for all loss or damage resulting there from. www.scottwilson.com Scott Wilson accepts no responsibility or liability for this document to any party other than the person by whom it was commissioned. North Wiltshire District Council Strategic Flood Risk Assessment
Table of Contents
1 Introduction ...... 1 1.1 Specific Overview ...... 2 1.2 The North Wiltshire District Study Area...... 2 1.3 Aim of SFRA...... 4 1.4 SFRA Objectives ...... 4 1.5 SFRA Structure ...... 4
2 Policy Context ...... 6 2.1 National Policies ...... 6 2.2 Regional Policies ...... 7 2.3 Local Policies...... 9 2.4 Environment Agency Policies ...... 10 2.5 Other Relevant Policies ...... 13
3 Data Collection & Review...... 14 3.1 Overview ...... 14 3.2 Regulatory & Administrative Areas ...... 14 3.3 Environment Agency Flood Zone Map...... 15 3.4 Hydraulic Modelling ...... 16 3.5 Flood Defences ...... 17 3.6 Topographic Data ...... 17 3.7 Surface Water...... 18 3.8 Ground Water...... 18 3.9 Sewer Flooding...... 18 3.10 Data Presentation...... 19
4 Methodology...... 20 4.1 Fluvial Flooding ...... 20 4.2 Sewer Flooding...... 23 4.3 Pluvial Flooding (Overland Flow) ...... 23 4.4 Groundwater Flooding ...... 24 4.5 Artificial Sources (Infrastructure Failure)...... 24
5 Flood Risk in North Wiltshire District ...... 26 North Wiltshire District Council Strategic Flood Risk Assessment
5.1 Fluvial Flooding ...... 27 5.2 Sewer Flooding...... 28 5.3 Pluvial Flooding (Overland Flow) ...... 29 5.4 Groundwater Flooding ...... 30 5.5 Artificial Sources (Infrastructure Failure)...... 30
6 Guidance on Applying the PPS25 Sequential Test...... 31 6.1 What is the Sequential Test? ...... 31 6.2 How should the SFRA be used to apply the Sequential Test? ...... 33 6.3 Additional Guidance...... 35
7 Guidance on Applying the PPS25 Exception Test ...... 37 7.1 What is the Exception Test? ...... 37 7.2 Why is there an Exception Test? ...... 37 7.3 What is Required to Pass the Exception Test? ...... 37
8 Flood Risk Management...... 41 8.1 Flood Defences ...... 41 8.2 Flood Warning Areas...... 41
9 Drainage of Development Sites...... 43 9.1 Principles...... 43 9.2 What are SuDS?...... 43 9.3 SuDS Policies...... 45 9.4 SuDS Methods ...... 45 9.5 Where can SuDS can be utilised? ...... 48
10 Policy Recommendations...... 51 10.1 Flood Risk ...... 51 10.2 Sustainable Drainage Systems...... 52 10.3 Flood Mitigation ...... 53 10.4 Water Environment...... 54
11 Site Specific FRA Guidance ...... 55 11.1 When is a Flood Risk Assessments Required?...... 56 11.2 FRA Requirements ...... 57
12 Emergency Planning ...... 62 North Wiltshire District Council Strategic Flood Risk Assessment
13 References...... 63
14 Figures ...... 64
Appendix A: Data Register...... 65
Appendix B: Sewer Flooding Data...... 66
Appendix C: Sequential and Exception Test ...... 67
Appendix D: Coarse Assessments...... 72
Appendix E: Flood Risk Assessment Guidance ...... 73 North Wiltshire District Council Strategic Flood Risk Assessment
1 Introduction
The Planning and Compulsory Purchase Act 2004 (PCPA) (HMSO, 2004) requires Local Planning Authorities to produce Local Development Frameworks (LDFs) that will replace the system of Local Structure and Unitary Development Plans. LDFs are a portfolio of documents (Local Development Documents (LDDs)) that collectively deliver the spatial planning strategy for the authority area. The PCPA 2004 requires LDDs to undergo a Sustainability Appraisal (SA), which assists Planning Authorities in ensuring that their policies fulfil the principles of sustainability. Strategic Flood Risk Assessments (SFRAs) constitute a component of the SA process and should be used in the review of LDDs or in their production.
The release of Planning Policy Guidance Note 25: Development and Flood Risk in July 2001 (PPG25) (DTLR, 2001) introduced a new emphasis on flood risk. This increased the responsibility that Local Authorities have to ensure that flood risk is understood and managed effectively using a risk-based approach as an integral part of the planning process.
PPG25 has recently been superseded (7th December 2006) by Planning Policy Statement 25: Development and Flood Risk (PPS25) (CLG, 2006). This re-emphasises the active role Local Authorities should have in ensuring that flood risk is considered in strategic land use planning. PPS25 requires Local Planning Authorities to undertake ‘ SFRAs and to use the findings to inform land use planning. In February 2007, a living ’ draft of the Practice Guide Companion to PPS25 was released for consultation. Although this is a consultation document, it provides a suggested approach to the production of SFRAs that should be considered.
To assist local Authorities in their strategic land use-planning, SFRAs should present sufficient information to enable Local Authorities to apply the Sequential Test to their proposed development sites. The SFRA should have regard to catchment wide flood “ issues and also involve a: - process which allows the Local Planning Authority to determine the variations in flood risk across and from their area as the basis for preparing ” appropriate policies for flood risk management for these areas .
In addition, where development sites cannot be located in accordance with the Sequential “ Test as set out in PPS25 (i.e. to steer development towards areas of lowest risk): The scope of the SFRA should be increased to provide the information necessary for the ” application of the Exception Test.
The spatial planning of these new developments must be considered with regard to the current and future risk of flooding from a number of sources. These flood sources include fluvial, stormwater management systems and groundwater. It is therefore vitally important that flood risk is considered at a strategic scale to inform land allocations and future developments within the emerging LDFs. To ensure the above targets are accommodated and that the emerging policies for the area are compatible, North Wiltshire District Council has commissioned a Strategic Flood Risk Assessment.
North Wiltshire District Council SFRA 1 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
In accordance with the recently released Practice Guide Companion to PPS25, Strategic Flood Risk Assessments may be completed in two consecutive stages. The Level 1 SFRA should present sufficient information to enable the Local Planning Authority to apply the Sequential Test (Appendix D) to potential development areas and to assist in identifying whether application of the Exception Test (Chapter 7) will be necessary through a Level 2 SFRA. In addition, the Level 1 SFRA provides background information and a preliminary review of available data, sufficient to scope the type of assessment necessary should a Level 2 SFRA be required.
The Level 1 SFRA should be used by the North Wiltshire District Council to undertake the Sequential Test to identify where potential development sites can be located within Flood Zone 1. Development that cannot be located within Flood Zone 1 should be assessed through a Level 2 SFRA where required. This report presents information generated during the Level 1 SFRA and provides recommendations for a Level 2 SFRA should this be required.
1.1 Specific Overview
North Wiltshire District Council commissioned Scott Wilson to undertake a Strategic Flood Risk Assessment. The SFRA identifies flood risk issues relevant to both existing and proposed developments within the North Wiltshire District planning authority in order to allow a direct input into the strategic planning of the North Wiltshire District through local development frameworks.
The study area is defined by the administrative boundary of North Wiltshire District (Figure 1) within the county of Wiltshire and covers an area of approximately 767 km2. The study area has boundaries with the areas administered by Swindon Borough Council (SBC), Kennet District Council (KDC), West Wiltshire District Council (WWDC), Bath and North East Somerset Council (BNESC), South Gloucestershire Council (SGC) and Cotswold District Council (CDC). North Wiltshire has a population of approximately 129,410 and is a predominantly rural area with six main settlements. Chippenham is the major urban settlement within the district. Calne, Wootton Bassett, Corsham, Malmesbury, and Cricklade are the main market towns.
1.2 The North Wiltshire District Study Area
The North Wiltshire District boundary includes land located within the Upper Thames Catchment which skirts across the northern edge of the district. There are five main rivers that drain North Wiltshire into the Upper Thames:
Swill Brook Churn;
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Ampney Brook; Ray; River Key; Derry Brook; Marston Meysey Brook.
The Upper Bristol Avon Catchment drains the majority of North Wiltshire. There are five main rivers that drain North Wiltshire into the Upper Bristol Avon: Sherton Avon; Tetbury Avon; By Brook; River Marden; Brinkworth Brook.
The main rivers and their catchment areas are displayed in Figure 2.
1.2.1 Upper Thames Catchment
The River Thames enters the district to the west of Ashton Keynes and flows in an easterly direction, south of the Cotswold Water Park situated inside the northern boundary of the district. As the river flows through the district it is joined by a number of tributaries from the north and south. The River Thames flows past the north of Cricklade before crossing the North Wiltshire Swindon district boundary. This area is relatively low – lying land (50 100m AOD). The catchment is predominately impermeable, with the exception of the Thames River Corridor where there are vast deposits of Alluvium. The land use is mainly Agricultural with Mineral extraction in certain areas.
1.2.2 Upper Bristol Avon Catchment
The Bristol Avon rises in the Cotswolds to the north west of the district. Initially the Avon comprises of two tributaries, the Sherston Avon and Tetbury Avon. The confluence of the two rivers is at Malmesbury, where flooding is liable to occur from both rivers. The Bristol Avon flows south through chippenham, before flowing west out of the district towards Bath and then to its outlet on the Bristol Channel. Typically the tributaries of Bristol Avon – are located on low lying land (50 100m AOD) which form open floodplains. An exception is By Brook which originates in deep incised valleys, forming confined – floodplains (100 150m AOD). The market towns of Calne and Corsham are situated in close proximity to By Brook and the River Marden respectively. The main land use within the catchment is Agriculture.
North Wiltshire District Council SFRA 3 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
1.3 Aim of SFRA
A suitable SFRA should present sufficient information to enable the Local Planning Authority to apply the Sequential Test (Chapter 6) to potential development sites and to assist in identifying if application of the Exception Test (Chapter 7) will be necessary and can be satisfied. Where the Exception Test is required the SFRA should present sufficient information to demonstrate that development will be safe from the risks of flooding for the lifetime of the development.
1.4 SFRA Objectives
Specifically the principle objectives related to the aims of the North Wiltshire District SFRA are outlined below:
1. To provide an assessment of the impact of all potential sources of flooding in accordance with PPS25, including an assessment of any future impacts associated with climate change;
2. To provide the information needed to apply the Sequential Test for identification of land suitable for development in line with the principles of PPS25;
3. To provide evidence based reports to inform the Sustainability Appraisal of Development Plan Documents (DPDs) with regard to catchment-wide flooding issues which affect the Study Area;
4. To allow North Wiltshire District Council to assess the flood risk for specific development and proposal sites, thereby setting out the requirements for site specific Flood Risk Assessments (FRAs);
5. Enable planning policies to be identified to minimise and manage flood risks
6. To provide specific advice regarding Sustainable Drainage Systems (SuDS) and a review of suitable techniques, in terms of the geology throughout North Wiltshire District Council;
7. To enable the relevant authorities to use the SFRA as a basis for decision making at the planning application stage;
1.5 SFRA Structure
The SFRA approach is in line with current guidance documents (PPS25 and the – accompanying Practice Guide Companion to PPS25 Living Draft). The Practice Guide ’ Companion to PPS25 recommends SFRA s are completed in two consecutive stages, this provides local planning authorities with tools throughout the LDF and SFRA process sufficient to inform decisions regarding development sites. The two stages are: – Level 1 SFRA Study Area Flood Source Review & Data Review
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– Level 2 SFRA Development Sites Assessments for Exception Testing
The results of the Level 1 SFRA will enable a prompt start to the commencement of Level 2 (where required). The data review element of Level 1 also enables a robust specification and programme to be developed for Level 2.
In addition this Level 1 SFRA provides background information and a preliminary review of available data, sufficient to scope the type of assessments necessary should a Level 2 SFRA be required. The deliverables from Level 1 should be used by the local planning authority to identify where sites cannot be located in Flood Zone 1 and therefore require further investigation through a Level 2 SFRA. This report presents the information generated during Level 1 of the SFRA. – 1.5.1 Level 1 SFRA Study Area, Flood Source Review and Data Review
The objective of the Level 1 SFRA is to collate and review available information on flood risk for the study area. The information has been sourced from a variety of stakeholders including the Environment Agency, North Wiltshire District Council, Thames Water and Wessex Water. The collation and review of these data provides information for a broad- scale assessment of flood risk (Chapter 5). In addition to the review of data and consultation with local stakeholders, Level 1 also reviews the available data to meet the requirements of a Level 2 SFRA where required.
The information provided in the Level 1 SFRA will allow the application of the Sequential Test for proposed site allocations based on Flood Zones (see Chapter 4 and Table D1, Annex D, PPS25). In addition, the preferred development type within each flood zone is provided through the Flood Risk Vulnerability Classification (Table D2, Annex D, PPS25) and Flood Zone Compatibility (Table D3, Annex D, PPS25) tables.
The information presented in this Level 1 SFRA should not be considered as an exhaustive list of all available flood related data for the study area. The Level 1 SFRA report is a presentation of flood sources and risk, based on data collected following consultation with and input from the local authority and stakeholders within the available timeframe. SFRAs are 'live' documents and should be updated on a regular basis as new information becomes available to the North Wiltshire District Council.
1.5.2 Level 2 SFRA - Development Sites Assessments for Exception Testing
The Level 2 SFRA will provide sufficient information to facilitate the application of the Exception Test where required. This would be based on information collected for the Level 1 SFRA and additional works where necessary.
North Wiltshire District Council SFRA 5 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
2 Policy Context
This section provides an overview of the policy context relevant to flood risk within the North Wiltshire District. For the SFRA to be successful the sustainable flood risk management recommendations within this document need to be considered by North Wilshire District Council with respect to future planning decisions. The national and regional planning policy framework, together with the Environment Agency Catchment Flood Management Plan objectives provides appropriate guidance to the Local Planning Authorities.
2.1 National Policies
2.1.1 Making Space for Water ’ In 2004 the Government s Making Space for Water strategy set out a new national direction for flood risk management planning in England over the subsequent 20 years. The report recognised the requirement for a holistic approach between the various responsible bodies, including flood defence operating authorities, sewerage undertakers and highways authorities, to achieve sustainable development. The report also highlighted the need for a more integrated approach to urban drainage. The protection of the functional floodplain forms an integral aspiration of the strategy.
In January 2007 details of 15 new pilot studies were released that will aim to identify the causes and consider the most suitable ways to manage urban drainage and reduce future flooding from this particular source and improve the resilience of towns and cites across the country to climate change. The outcome of these studies will culminate into guidance upon urban flood risk and integrated drainage, which will be released in Autumn 2008.
Amongst several other key drivers1, the Making Space for Water document intended to improve the manner in which land use planning was undertaken. Since 2004 the ’ particular goals alluded to have been achieved. The Environment Agency s role as a statutory consultee has been extended in areas that are at risk of flooding. In essence, an objection made by the Environment Agency has the same weighting as that of the LPA. An integral part of this new direction for flood risk management planning in England was the production of a new Planning Policy Statement (PPS). As discussed within the ‘ Making Space for Water document itself, the intention was to replace and improve the operational effectiveness of, Planning Policy Guidance Note (PPG) 25. The overriding document PPS25 was released in December 2006 and is discussed below.
1 Including coastal erosion, management of water in a rural setting, improved provision of data and research and an improved incorporation of the three pillars of sustainable development (i.e. economic, social and environmental) in risk management activities.
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2.1.2 Planning Policy Statement 25: Development & Flood Risk
This policy document establishes the national policy for development and flood risk. The ’ overarching aim of PPS25 is to support the Government s objectives for sustainable ‘ development. The aims of planning policy on development and flood risk are to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away ’ from areas at high risk . Therefore, PPS25 seeks towards steering of new development to sites of lower flood risk.
1. Allocate all sites in accordance with the Sequential Test reduce the flood risk and ensure that the vulnerability classification of the proposed development is appropriate to the flood zone classification;
2. Flood Risk Assessments (FRAs) should be undertaken for all developments within Flood Zones 2 and 3 and sites with identified flood sources to assess the risk of flooding to the development and identify options to mitigate the flood risk to the development, site users and surrounding area;
3. Flood Risk Assessments are required for all major developments in Flood Zone 1. These are residential developments consisting of sites greater than 0.5 ha or greater than 10 dwellings and commercial developments that are greater than 1 ha or have a floor area greater than 1000 m2.
4. Flood Risk to development should be assessed for all forms of flooding;
5. Where floodplain storage is removed, the development should provide compensatory storage on a level for level and volume for volume basis to ensure that there is no loss in flood storage capacity.
2.2 Regional Policies
2.2.1 Regional Spatial Strategies (RSS)
The South West Regional Assembly published the Draft South West Regional Spatial Strategy (Draft RSS) in June 2006. A Panel Report is expected in November 2007, which follows a period of consultation and Examination in Public (EiP). The Panel Report will be sent to the Secretary of State who will then issue proposed changes to the Draft RSS. The RSS will supersede County Structure Plans as the statutory development planning document, and covers a period up to 2026. One of the important roles of the Draft RSS is ‘ to translate strategy into proposals for the provision of new homes.
The Draft RSS sets out the dwelling requirements for the wider area including Chippenham (Policy SR22), over the period 2006-2026. These are described in turn as follows: an average provision of about 225 dwellings per annum within Chippenham;
North Wiltshire District Council SFRA 7 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
‘ The Draft RSS makes a provision for job growth each of the settlements Travel to Work ’ Area (TTWA;) of about 6,300, 11,700 between 11,000 and 13,500 over the period 2006- 2026, for Chippenham, Trowbridge and Salisbury, respectively. ’ ‘ ’ The Draft RSS sets out the region s approach to the quality of life in the region. These are culture and environment. Flood risk forms one of many key drivers of the pursuit to enhance the quality of life. Policy F1 priorities the sequential approach outlined in PPS25 (see Section 7) by directing growth to areas where it can be accommodated with little or no risk of flooding, whilst existing development needs to adapt and defend against the increasing risk from flooding. Policy F1 seeks to: Defend existing properties and, where possible, locate new development in places with little or no risk of flooding; Protect flood plains and land liable to tidal or coastal flooding from development; Follow a sequential approach to development in flood risk areas; Use development to reduce the risk of flooding through location, layout and design; Identify areas of opportunity for managed realignment to reduce the risk of flooding and create new wildlife areas.
2.2.2 Sub Regional Strategy
Wiltshire and Swindon Structure Plan 2016 (Adopted April 2006) ‘ The Structure Plan provides a strategic policy framework for land use planning, development and transport across the administrative areas of Wiltshire (incorporating Wiltshire County and Swindon Borough) up to 2016. This framework is used to inform the more detailed Local Development Frameworks produced by the Borough and District ’ Councils against which decisions on development are made.
The Structure Plan will be replaced by the new Regional Spatial Strategy for the South West which is currently at the draft submission stage; however, the following policy is of relevance to the SFRA until such a time as the RSS for the South West is formally adopted: Policy C5 of the Structure Plan relates to the Water Environment:
The water environment, including surface waters, floodplains and groundwater resources, should be protected by the control of development. The strategic planning authorities will support initiatives which seek to protect, restore or enhance the natural elements of the river or waterway environment, and which improve the quality and efficient use of water.
In relation to flood risk management the Policy C5 identifies the importance of protecting the flood plains by ensuring that development does not increase risk. Any proposed
North Wiltshire District Council SFRA 8 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
development in areas of flood risk should be assessed in accordance with a risk-based sequential approach guided by the Environment Agency. ’ In addition North Wiltshire District Council s LDDs should contain policies on flood risk, including the identification of flood plain boundaries and the requirements of flood risk assessments.
2.2.3 Regional Flood Risk Appraisal (RFRA)
The South West RFRA was published in February 2007. The purpose of the RFRA is to provide a broad regional understanding of the risk that flooding creates in South West England, and was undertaken following the stipulation within PPS25. The RFRA is a descriptive document, intended to feed into the Strategic Sustainability Assessment (SSA) and the RSS in order to help determine broad regionally significant locations for development.
An appraisal of flood risk was undertaken within 9 sub-regions of the South West. The outcome of the RFRA regarding these sub-regions pertaining to the Local Authorities of West Wiltshire, North Wiltshire and Kennet District Council was considered not to be significant. The main concerns upon flood risk were associated with settlements located elsewhere within the Southwest. However, with regards Chippenham the RFRA states:
There are flood risks in the town and flood incident management is provided in the form of flood warning to properties. There is flood management infrastructure in the town. Due to the nature of the rivers around Salisbury, they are slow to respond initially but once water levels are raised flood events are sustained and could last many weeks.
2.3 Local Policies
2.3.1 Local Plan
North Wiltshire Local Plan 2011 (Adopted June 2006)
The North Wiltshire Local Plan sets out the districts Spatial Strategy and associated Core Policies until 2011. Due to a change in the planning system the North Wiltshire Local Development Framework (LDF) Core Strategy will replace the Local Plan when it is due to be adopted in 2009. The relevant policies within the current local plan document with regards to flood risk management are displayed below: Policy NE21 (Development in Flood Risk Areas) states: Development in areas of flood risk will be expected to be in accordance with a sequential approach whereby sites are developed in order of risk, with a preference for the development of sites with no flood risk. Policy NE21 highlights the necessity for the sequential approach to future development. The policy also links the proposed development with the provision of maintaining a minimum standard of flood defence for the lifetime of the development.
North Wiltshire District Council SFRA 9 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
Policy NE22 (Surface Water Run Off) states: New development should not increase run-off from the undeveloped situation and for redevelopment reduce run-off. Development should not deplete or pollute ground water flows or result in discharges of pollutants into water courses. Policy NE22 seeks to prevent future development increasing the amount of surface water runoff by ensuring the use of Sustainable Drainage Systems (SuDS). The policy also highlights the need to ensure measures are taken to protect water source protection zones. Policy NE23 (Water Courses) state: Development adjacent to any watercourse will only be permitted where retention of a natural corridor is provided to cater for channel maintenance, enhancement and environmental interests. Policy NE23 requires that water courses and their riverbanks are preserved in their natural state. Culverting of watercourses should be avoided and bank protection works should be carried with great care. Policy H1 (Required Level of Residential Development) states: In the Planarea, between 1991 to 2011, provision will be made for the development of about 13,500 dwellings. ‘ ’ North Wiltshire District Council has published their Second Issues and Options Core Strategy consultation document. The document sets out four possible options for the future spatial development of the district. This SFRA will form part of the evidence base informing the consultation process on the implications of each option on future flood risk within the district.
2.4 Environment Agency Policies
2.4.1 Catchment Flood Management Plan (CFMP)
Catchment Flood Management Plans are high level strategic planning document that provide a catchment overview of the main sources of flood risk and how these can be managed in a sustainable way in the next 50 to 100years. The Environment Agency engages stakeholders within the catchment in order to produce policies in terms of sustainable flood management solutions whilst also considering the land use changes and effects of climate change.
North Wiltshire District Council is considered within two Catchment Flood Management Plans namely: Bristol Avon Thames region ’ An overview of each of the CFMP s is provided below and thereafter any policies produced as a result of these studies will be considered inline with planning policies.
North Wiltshire District Council SFRA 10 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
Thames Regional Catchment Flood Management Plan (Summary document January 2007)
The Thames Regional CFMP covers the northern part of North Wiltshire. The Thames Regional CFMP has summarised the future approach to flood risk management into four main messages: Flood defences cannot be built to protect everything Current flood defences will be maintained but it is unrealistic to continue to build defences to protect all of the peoples and properties at risk of flooding. The focus should be placed on the consequences of flooding rather than the likelihood. Climate change will be the major cause of increased flood risk in the future. The predicted change in weather patterns due to Climate change suggests that winter floods will happen more often. The flood plain is our most important asset in managing flood risk. Many flood plains in the Thames region have no flood defences and can serve their natural function of storing water during times of flood. The value of this flood plain needs to be recognised in reducing the impacts of flooding. By improving the effectiveness of the flood plain it can reduce flooding to properties both locally and further downstream. Development and urban regeneration provide a crucial opportunity to manage the risk. The consequences of flooding can be managed through forward planning. By considering the location, layout and the design of the development flood risk to properties can be reduced. For example, locate more vulnerable properties out of the flood plain, open up urban river corridors to provide more flood plain and make new buildings flood resilient in combination with the use of SuDS.
Bristol Avon Catchment Flood Management Plan (Draft Plan)
The Bristol Avon catchment includes a large majority of the North Wiltshire District. The CFMP proposes a number of objectives which form the foundations for the CFMP policies targeting specific areas. The objectives are based around three main categories, people the environment and the economy.
Flood risk to people: PE1. Reduce flood risk to human health and public safety associated with deep and/or fast flows. PE2. Reduce risk to life associated with fast flows in upstream parts of the catchment. PE3. Reduce disruption to emergency services.
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Flood risk and the environment: ENV1. Increase area of BAP habitats including wetlands and wet woodland for upstream storage. ENV2. Make better use of recreational facilities for flood risk management. ENV3. Ensure that changes within the catchment minimise the impact on the environment and designated sites.
Flood risk to property and the economy: ECON1. Reduce property damages due to flooding in policy units where flood risk needs to be reduced. ECON2. Minimise disruption to communications in the event of a flood on the principal trunk roads and rail lines. ECON3. Reduce disruption to local industry and critical infrastructure. ECON4. To reduce the economic impacts of flooding in locations outside of the main urban areas, from fluvial sources and muddy floods.
The CFMP has divided the catchment into policy units. These are areas that experience similar types of flooding and have similar assets that are vulnerable to flooding. The different policy units that have been assigned to the various parts of North Wiltshire are listed below: Policy 3 Continue with existing or alternative actions
Area: Wootton Bassett
The current scale of flood risk management for this area is sufficient and future increases in risk are acceptable. Therefore the current flood risk management in Wootton Bassett is considered to be an appropriate level.
Policy 4 Take further action to sustain the current scale of flood risk into the future
Area: Malmesbury, Calne and Corsham
This area is deemed to currently have acceptable levels of flood risk management, how future changes are expected to have a significant impact. Flood risk management needs to address the increases in flood risk posed by urban development, land use change and climate change. Increasing the size of the defences is an option for areas within this policy unit.
North Wiltshire District Council SFRA 12 October 2007 North Wiltshire District Council Strategic Flood Risk Assessment
Policy 5 Take further action to reduce flood risk (now/or in the future)
Area: Chippenham
Flooding within this area is currently felt to be to high and will continue to be high in the future. The existing level of flood risk management is not sufficient with significant risk to people and assets or environmental sites located within the policy area. Policy 6 Take action to increase the frequency of flooding to bring benefits locally or elsewhere
Area: Rural Areas within the district
By increasing flooding in areas that may benefit from increased flood waters flood risk can be reduced in the catchment overall. This policy is likely to apply to the upper catchment where there is opportunity to restore the flood plain and store water in these areas.
2.5 Other Relevant Policies
2.5.1 Sewerage Authority Policies
No policies have been presented by the Sewer Authorities. However, there is national guidance on new sewer infrastructure which should be adhered to as part of any new development.
Sewers for Adoption (A Design and Construction Guide for Developers)
The Sewers for Adoption Guide is to be used by developers undertaking new development when planning, designing and constructing conventional foul and surface water gravity sewers, lateral drains and pumping stations intended for adoption under an Agreement made in accordance with Section 104 of the Water Industry Act 1991. The developer should consult the sewage undertaker and all other relevant bodies at the earliest opportunity before a planning application has been made, so that drainage arrangements can be agreed.
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3 Data Collection & Review
3.1 Overview
The objective of this Level 1 SFRA is to collect, collate and review the information available relating to flooding in the study area. This information is then presented in a format to enable North Wiltshire District Council to apply the Sequential Test to determine potential sites for development and if necessary to apply the Exception Test. The review of data collected allows gaps in the data/information to be identified in order to ascertain additional requirements that may be needed to meet the objectives of the Level 2 SFRA. Appendix A provides a data register listing the data requested and received.
Tasks
The sequences of tasks undertaken in the preparation of this Level 1 SFRA were:
1. Inception meeting with North Wiltshire District Council and the Environment Agency on 23rd August, 2007;
2. Identification of the local stakeholders;
3. Issue of letters to stakeholders requesting data/information;
4. Followed-up data requests (where necessary);
5. Collation and review of available data (Appendices A);
6. Review of received data against the SFRA objectives;
7. Identification of gaps in data;
8. Provision of options to address gaps in data, and;
9. Undertaking of a broad-scale and focused assessment of flood risk.
All tasks were completed between July and the beginning of October 2007.
3.2 Regulatory & Administrative Areas
3.2.1 Environment Agency ’ The study area falls within the Environment Agency s Thames Region and South West Region. Thames Region (West area) is managed by the Wallingford office. South West Region is administered by the North and South Wessex offices at Bridgewater and Blandford respectively. The boundary with EA South West region runs from north of
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Malmesbury to Swindon in North Wiltshire, and from north of Devices to Ham in Kennet District in a line parallel with the A4 south of Marlborough and north of the Vale of Pewsey. The area office located in Bridgewater was the initial contact point for the provision of data for the catchments within the study area.
3.2.2 Drainage
Management of storm water/foul water for the study area is the responsibility of Thames Water, Wessex Water Ltd and North Wiltshire District Council. There are no internal drainage boards present within the study area. In addition, private individuals may be responsible for drainage systems that operate prior to discharge either into a watercourse or into a public sewer.
3.2.3 Stakeholder Meetings
An Inception meeting was held with the Environment Agency and North Wiltshire District Council at Wiltshire County Council offices, Trowbridge on the 23rd August 2007. This meeting allowed discussion on the format of the SFRA and reviewed the data/information received from the various stakeholders.
3.2.4 Stakeholder Consultation
The following stakeholders were contacted to provide the data/information for the SFRA: -
1. North Wiltshire District Council;
2. Wiltshire County Council;
3. Environment Agency (North and South Wessex Area Offices & Thames Region West Area); Wessex Water
4. Thames Water.
3.3 Environment Agency Flood Zone Map ’ Flood zones are defined in the government s planning policy for England and are produced ignoring the presence of existing flood defences, since defences can be ‘ ’ overtopped if a flood occurs which is higher than the defences are designed to withstand. Defences can even fail in extreme events. The Flood Map gives the location of raised flood defences such as embankments and walls, as well as land designated and operated to store flood water.
The Environment Agency has provided an extract of their Flood Map for the study area (Figure 3A - 3F). The Flood Map shows the estimated extent of Flood Zones 2 and 3 for all main rivers and/or watercourses. The Flood Map gives a good indication of the areas at risk of flooding in England and Wales; however it does not provide detail on individual properties.
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The Flood Map has been developed by the Environment Agency using a combination of detailed information from appropriate hydraulic models (where available) and outputs ’ from the Environment Agency s National Generalised Model. Hydraulic models use detailed topographic data and rigorously derived flow estimates to derive flood extents. The National Generalised Model outputs are derived from less accurate topographic data (SAR or LiDAR data) and national data for river flows.
The Flood Map does not provide information on flood depth, speed or volume of flow. It ’ also doesn t show flooding from other sources, such as groundwater, direct runoff from fields, or overflowing sewers.
The coarse estimates of floodplain extents derived from the National Generalised method can be further refined using more detailed modelling techniques and data. Where the Environment Agency produce detailed models, these generalised Flood zones are replaced. The Environment Agency nationally updates the Flood Zone maps every three months.
3.4 Hydraulic Modelling
Existing hydraulic models have been identified from information provided by the ’ Environment Agency. The model s structures and results have been reviewed to assess their suitability for application within the Level 2 SFRA were they may be required to determine the rate of onset, and depth of flood waters for the purposes of the Exception Test. Existing models that have been identified as being held by the Environment Agency are:
Table 3.1 Detailed hydraulic modelling undertaken in the North Wiltshire District
Location Model Return Period Limitation Upper Thames ISIS/ONDA 20 Year Does not account for flood Mike11 defences Calne ISIS 5, 10, 20, 50 Does not account for flood and 100 Year defences Chippenham ISIS 5, 10, 20, 50 Does not account for flood and 100 Year defences Corsham ISIS 5, 10, 20, 50 Does not account for flood and 100 Year defences Lyneham ISIS 5, 10, 20, 50 Does not account for flood and 100 Year defences Malmesbury ISIS 5, 10, 20, 50 Does not account for flood and 100 Year defences Wootton Bassett ISIS 5, 10, 20, 50 Does not account for flood and 100 Year defences
The detailed flood extents produced from these models will be used above the generic Flood Zone modelling. The Upper Thames ISIS/ONDA/MIKE11 model outline of the 1 in
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20 year flood has been used within the flood maps (Figure 3A-3F) to define the functional flood plain (PPS25 Flood Zone 3b).
3.5 Flood Defences
The Environment Agency has provided a GIS layer of the National Flood and Coastal Defence Database (NFCDD) listing details of structures and flood defences which aims to provide the following information: the location, composition and condition of fluvial defences and watercourse referenced to identified risk areas the types of asset (i.e. property, infrastructure, environmental) at risk within identified risk areas and including those protected by fluvial, tidal and coastal defences the extent of floods related to different flooding scenarios (e.g. different return periods and different types of flood event such as overtopping or embankment failure).
The NFCDD details the asset reference, the location, level of protection that the structure provides and the geographic extent of the structure or defence. The NFCDD will be used to identify where structures may cause increased risk of flooding during a blockage scenario and/or could benefit from replacement or removal. Furthermore, the assets listed within the Environment Agency Wessex area also have provided photographs.
Whilst PPS25 ignores the presence of defences in Flood Zone 3a and 2, it is still important to recognise and acknowledge where a flood defence exists and what residual risk of failure is associated with it.
3.6 Topographic Data
3.6.1 LIDAR
Light Detection and Ranging (LIDAR) is an airborne mapping technique which uses a laser to measure the distance between the aircraft and the ground. Environment Agency has provided LIDAR data coverage for the whole of North Wiltshire District which will be used to create a terrain model of the catchment. This data varies in accuracy depending on the nature of the terrain such as in woodlands, complex urban areas and near lakes, due to the limitations in the technique. However, LIDAR is generally recognised to be accurate to within +/- 300mm when compared to actual vertical levels.
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3.6.2 Channel survey
Channel survey has been provided for each of the river systems where detailed hydraulic modelling has been undertaken as part of an Environment Agency National Flood Risk Mapping Framework.
3.7 Surface Water
Intense rainfall, often of short duration, that is unable to soak into the ground or enter drainage systems can run quickly off land and result in local flooding. In developed areas, this flood water can be polluted with domestic sewage where foul sewers surcharge and overflow. Local topography and built form can have a strong influence on the direction and depth of flow. Overland flow paths should be taken into account in spatial planning for urban developments. Flooding can be exacerbated if development increases the percentage of impervious area.
Surface Water flooding is a difficult flood source to quantify in terms of flood estimation. Records of Surface Water flooding provided by North Wilshire District Council and acknowledgement of the local topography within the catchment provide the basis for flood risk estimation.
3.8 Ground Water
An assessment of the risk of groundwater flooding has been considered. A quantified assessment of risk from groundwater flooding is difficult to undertake. Groundwater flooding occurs when water levels in the ground rise above surface elevations. Groundwater flooding may take weeks or months to dissipate because groundwater flow is much slower than surface flow and water levels thus take much longer to fall.
The Environment Agency, West Area of Thames region covers the north half of North Wiltshire District Council. The Environment Agency, West area has provided an analysis of geology covering the relevant parts of Wiltshire together with reported incidences (Chapter 5). The geology of the southern part of North Wiltshire is not generally associated with ground water flooding. The Environment Agency Wessex area has not been able to provide any records of ground water incidence information within this area. Figure 7 displays the solid and drift geology underlying the North Wiltshire District.
3.9 Sewer Flooding
Sewer flooding records has been provided by Wessex Water.
Wessex Water has provided flood incidence maps, these maps are large scale and therefore identifying the data point accurately is difficult (Appendix B). Therefore, these maps have been used as a reference point to consider sewer source of flooding further in the vicinity of this point.
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3.10 Data Presentation
GIS Mapping
This section highlights all the Geographical Information Systems (GIS) layers produced for the SFRA. Using the information collected from the relevant stakeholders, a series of GIS layers have been produced to aid in the assessment of flood risk within the area. These layers are:
Flood Zone 2 (Fluvial or River) Flood Zone 3 (Fluvial or River) Main Rivers Flood Defences (identified from National Flood and Coastal Defence Database) Historic Flood outlines Flood Warning Areas Locations of Pluvial/Sewer/Groundwater flooding Artificial sources North Wiltshire District Boundary Terrain Information e.g. LiDAR, SAR, river cross-sections;
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4 Methodology
Two sets of large scale flood risk maps have been produced for North Wiltshire District. Figure 3A-3F show the fluvial flood risk to the district based on the data collated. Figures “ ” 4A- 4F are collective Other Flood Risk Sources Maps which comprise of the data collected for sewer, pluvial, groundwater overland and artificial sources. The methodology of how these were produced is detailed in the following section.
4.1 Fluvial Flooding
4.1.1 Requirements
In order to determine land at risk and the degree of risk of flooding from river sources, the Environment Agency have provided Flood Zone Maps. The Flood Zones indicate the geographical extent of Flood Zone 2 and Flood Zone 3 for the administrative area of North Wiltshire District Council. The definitions of Flood Zones are provided in table 4-1 below:
Table 4-1: Fluvial Flood Zone Definitions
PROBABILITY OF FLOOD ZONE DEFINITION FLOODING At risk from flood event greater than the 1 in Flood Zone 1 1000 year event (greater than 0.1% annual Low Probability probability) At risk from flood event between the 1 in 100 and Flood Zone 2 1 in 1000 year event (between 1% and 0.1% Medium Probability annual probability) Flood Zone At risk from a flood event less than or equal to High Probability 3a the 1 in 100 year event At risk from a flood event less than or equal to Flood Zone Functional the 1 in 20 year event or otherwise defined by the 3b Floodplain Local Planning Authority
For the purpose of the Level 1 SFRA definitions of the extent of Flood Zones 1, 2, 3a and 3b are required; this information will then be used to identify land at least risk of flooding from fluvial sources. The Flood Zones for 3a and 2 have provided by the Environment Agency and are a combination of detailed hydraulic modelled extents and the JFLOW (generalised modelling technique) extents as described in section 3. Flood Zone 3b for the Upper Thames has been identified by the Environment Agency Flood Zone Map methodology.
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4.1.2 Functional Floodplain
Functional floodplain (Flood Zone 3b) comprises an area of land where water has to flow or be stored in times of flood. The functional floodplain would flood with an annual probability of 1 in 20 (5%) or greater in any year, or is designed to flood in an extreme flood (0.1%), or at another probability to be agreed between the LPA and the Environment Agency, including conveyance routes.
As the name implies, functional floodplains have the highest considered probability of flooding and as such are located in Flood Zone 3b of the sequential test. PPS25 requires the exclusion of various vulnerable development types from this flood zone. Therefore it is important to consider the location of functional floodplains in the future spatial planning and emerging LDF documents for an area.
PPS25 states that functional floodplain should be determined considering the effects of defences and other flood risk management infrastructure. The presence of a workable and robust flood defence removes the definition of functional floodplain. The functional floodplain relates only to river and coastal flooding, it does not include areas at risk of flooding solely from other sources of flooding (e.g, surface water, sewers).
In accordance with the PPS25 Practice Guide, in the absence of detailed modeling of functional floodplain or more appropriate data. For the purpose of the Level 1, Flood Zone 3 has been used to define functional floodplain (in the absence of a modeled functional flood plain) as a precautionary approach to unsure that no inappropriate development is placed in areas of high risk. The emphasis is therefore on the developer to undertake modelling to redefine the floodplain in the area and then present this to the Environment Agency for consideration.
4.1.3 Climate change
PPS25 requires that allowances for climate change are made when considering the extent of flooding on a yearly increment scale for the lifetime of proposed developments. One of the main aims of the SFRA is to assist in the Sequential Test and development of local planning policy including the development of the Core Strategy and the Site Allocation plans.
The decisions behind the location of future residential development is a key consideration in the local planning process and the impact of climate change on flooding associated with areas considered for residential development should be considered. Flooding impacts affecting potential residential development should consider a minimum of 100 years for climate change impacts, with commercial and industrial development considering either the design lifetime of the development or 60 years of climate change.
It is noted that on application of the Sequential Test, if areas for development are likely to fall within Flood Zones 2 or 3 and require the application of the Exception Test, then the likely effects of climate change should be investigated for those individual sites.
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In the absence of modeled climate change outlines, the Level 1 SFRA Flood Zone 2 should be treated as Flood Zone 3 with climate change. It is advised for the Level 2 SFRA, subject to the formal agreement of the Environment Agency in areas which detailed hydraulic models are present that the 1 in 100 year plus climate change event is mapped.
4.1.4 Unmapped Watercourses
Flood Zones provided by the Environment Agency are not defined for all watercourses. Typically watercourses with a catchment area less than 3km2 are omitted from Environment Agency mapping unless there is a history of flooding affecting a population. Consequently there will be some locations adjacent to watercourses that on first inspection of the flood maps, it is suggested there is no flood risk.
However, as part of the SFRA, unmapped watercourses have been provided with an 8 metre buffer either side of top of banks designated as Flood Zone 3b in line with the precautionary principle adopted by PPS25 (refer to figures 3A-3F). This should be fully investigated and the floodplain refined as part of any future site specific Flood Risk Assessments. This will not only ensure that more people are not placed at risk through ’ inappropriate development, but also that development is not restricted in an area that isn t at significant actual risk of flooding within the buffer strip.
4.1.5 Data Source
The fluvial data source is Environment Agency Flood Zone maps which incorporates the detailed hydraulic models specified within Section 3.3.
In line with the practice guide, in the absence of a modelled functional floodplain outline, all areas within Zone 3 should be considered as Zone 3b (Functional Floodplain) unless, or until, an appropriate FRA shows to the satisfaction of the EA that it can be considered as falling within Zone 3A (High Probability).
The Upper Thames is the only main river watercourse with detailed outlines for functional floodplain using a 1 in 20 year return period. Therefore the flood maps produced for this watercourse will differentiate between Flood Zone 3a and 3b.
There has been no assessment of the potential impacts of climate within the Level 1 SFRA into the effects of climate change into fluvial functional floodplain sources. There is currently no Flood Zone 2 or 3 with climate change mapped. In the absence of climate change outlines Flood Zone 2 should be treated as Flood Zone 3 with climate change.
4.1.6 Mapping
The Flood Zone Maps, detailed hydraulic model extents and ordinary watercourse layer have been overlaid on an Ordnance Survey base in order to assess the geographic extent of flood risk as seen in Figures 4A- 4F.
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4.2 Sewer Flooding
4.2.1 Requirements “ ” The majority of sewers are built to the guidelines within sewers for adoption (WRC, 2006). As a minimum, these sewers are designed to manage runoff from a storm event of a size and intensity of 1 in 30 years. It is therefore likely that the majority of the existing sewer systems will surcharge to some degree during rainstorm events with a return period greater than 30 years (e.g. 100 years) which can cause flooding of property and land both from surface water but also foul water wastewater sewers. It is therefore important to understand where the existing system is restricted in capacity by considering where historical sewer flooding has occurred and as such, where sewer flooding is considered to be higher risk.
4.2.2 Data Source
Wessex Water has provided point locations of sewer flooding incidents that have occurred in the last 10 years.
4.2.3 Mapping
The points provided by Wessex Water have been plotted on a large scale map and are therefore very difficult to accurately translate into points on a GIS layer without error and are therefore included within an Appendix B in hardcopy.
4.3 Pluvial Flooding (Overland Flow)
4.3.1 Requirements
Overland flow results from rainfall that fails to infiltrate the surface and travels over the ground surface; this is exacerbated by low permeable urban development or low permeability soils and geology (such as clayey soils). Overland flow is likely to occur at the base of an escarpment and low points in terrain. Overland flow incidents along with recorded surface water flooding as provided by Environment Agency. These have been mapped to assist in identifying possible low points in the landscape.
Local topography and built form can have a strong influence on the direction and depth of flow. The design of development down to a micro-level can influence or exacerbate this. Overland flow paths should be taken into account in spatial planning for urban developments. In addition, surface water flooding can be exacerbated if development increases the percentage of impervious area.
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4.3.2 Data Source
Environment Agency has provided information as part of the database defining historic sources of flooding as a result of overland flow. This dataset provides a record of incidences that have occurred within the North Wiltshire district indicating the location and apparent source or cause of flooding. It does not provide an indication of the severity of the flooding and may not be always accurate in the extents.
4.3.3 Mapping
The historic flooding incidences have been mapped using the GIS layer provided on the collective maps in Figures 4A- 4F.
4.4 Groundwater Flooding
4.4.1 Requirements
An assessment of the risk of groundwater flooding needs to be considered; however, a quantified assessment of risk from groundwater flooding is difficult to undertake due to lack of records for groundwater levels, the variability in geological conditions and lack of predictive tools (such as modelling) which can make assessments of the risk of “ groundwater flow and flooding following rainfall events. Groundwater flooding occurs ” when water levels in the ground rise above surface elevations . The nature of the underlying geology of North Wiltshire means that groundwater flooding is not significant within the district.
4.4.2 Data Source
The Environment Agency (Thames region, West Area) has provided a list of reported groundwater incidents and synopsis of the geology and associated likelihood of flooding.
4.4.3 Mapping
The Environment Agency Thames Region (West Area) list of reported ground water incidents have been included in Figure 4A- 4F.
4.5 Artificial Sources (Infrastructure Failure)
4.5.1 Requirements
Artificial flood sources include raised channels such as canals or storage features such as ponds and reservoirs, where water is retained above natural ground level. Artificial sources of flooding within a 1km radius of the any sites should be considered in order to assess the residual risk of a water body overtopping or of retaining wall failure and the
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potential flow paths for flood water that might occur in such an event. Thereafter, an appreciation can be given as to whether it is considered an actual or residual flood risk from artificial sources.
The potential effects of flood risk management infrastructure and other structures also need to be considered. Reservoir or canal flooding may occur as a result of the facility being overwhelmed and/or as a result of dam or bank failure. The latter can happen suddenly resulting in rapidly flowing; deepwater that can cause significant threat to life and major property damage. Industrial flooding can also occur when pumping ceases and groundwater returns to its natural level, for example in former mineral workings and urban areas where industrial water abstraction is reduced from its former rate. Some of this flooding may be contaminated.
4.5.2 Data Source
Ordnance survey Mastermap data is a topographic layer which includes land area classifications such as roads, buildings, terrain and water bodies. Using a GIS system all the water bodies have been extracted from this layer and thereafter artificial water bodies identified and such as raised channels, canals and storage features. For the purpose of this SFRA the technique satisfactorily highlights the perched watercourse.
4.5.3 Mapping
A GIS layer of the artificial sources has been produced and overlain on an OS base with other sources of flood risk data in figures Figure 4A- 4F. In line with PPS25, these have been defined as functional floodplain i.e. areas of land where water has to flow or be stored in times of flood.. No has been assessment of overtopping, structure failure or flood routes has been undertaken during the Level 1 SFRA. This assessment should form part of a site-specific Flood Risk Assessment, see Section 5.5.
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5 Flood Risk in North Wiltshire District
A suitable Level 1 SFRA will collate and review existing information on flood sources and flood risk. This assists the local planning authority in its obligation to consider flood risk in consideration of strategic land allocations and developing future policies. This is achieved by providing sufficient information to enable local planning authorities to apply the Sequential Test (as set out in PPS25 and the Practice Guide Companion, see Chapter 6) by allowing the determination of suitable sites for development in relation to flood risk and the classification of vulnerability.
In accordance with PPS25 and the accompanying Practice Guide, where there are no reasonably available sites in Flood Zone 1, it may be necessary to locate development allocations in Flood Zone 2. Only where there are no reasonably available sites in Flood Zones 1 and 2, should development be considered in Flood Zone 3. If following application of the Sequential Test it is not possible (consistent with wider sustainability objectives) for development to be located in lower flood risk zones, the Exception Test can be applied. The successful application of the Exception Test will require information specific to those development sites and would be provided in a Level 2 SFRA.
The data/information collected from the various stakeholders, including previous reports relating to flooding within the catchment, have been reviewed. The following sections summarises the flood risk within the district from all forms of flooding (fluvial, groundwater, surface water and sewer flooding). The information has been presented in a series of maps to aid the Sequential Test process for the allocation of future sites.
Flood risk in study area maps: Figure 1: Study Area District Map Figure 2: Main Rivers and Sub-catchment Boundaries Figure 3A: Fluvial Flood Risk Map (1) Figure 3B: Fluvial Flood Risk Map (2) Figure 3C: Fluvial Flood Risk Map (3) Figure 3D: Fluvial Flood Risk Map (4) Figure 3E: Fluvial Flood Risk Map (5) Figure 3F: Fluvial Flood Risk Map (6) Figure 4A: Other Flood Risk Sources Map (1) Figure 4B: Other Flood Risk Sources Map (2) Figure 4C: Other Flood Risk Sources Map (3) Figure 4D: Other Flood Risk Sources Map (4)
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Figure 4E: Other Flood Risk Sources Map (5) Figure 4F: Other Flood Risk Sources Map (6)
5.1 Fluvial Flooding
Fluvial flooding is the main source of flooding within the North Wiltshire District. In the north of the district, the Upper Thames and its tributaries have wide open flood plains. Previous flood events in 1995 and more recently July 2007 have resulted in settlements (Cricklade and Marston Meysey) within the district being affected by flood waters. The flood risk within the Bristol Avon Catchment includes a number of the main settlements within the district (Calne, Chippenham, Corsham and Malmesbury. Malmesbury is located at the confluence of the Sherston Avon and Tetbury Avon, flooding here is liable to occur from both rivers. Table 5.1 lists selected historical flood events over the recent future.
Table 5.1: Selected historical flood events within North Wiltshire. Information sourced ’ from CFMP s, Environment Agency database and reviewed reports.
Location Year of Details of affected area flood Calne 1988 Fluvial flood event. Areas affected include the A4, Mill Street, Church Street and a number of properties. Flooding has now improved in area. 1980 Fluvial flood event. Areas affected include Littlefield industrial estate, Possibly caused from overflow from Chippenham Ladyfield Brook. 1995 Fluvial flood event. Westmead playing fields. Local road 1999 and land affected. 2000 3 properties flooded Cricklade 1995 Flooding around Cricklade from Thames Bridge 1982 Fluvial flood event. Holloway Bridge. Buildings, transport Melmesbury 1992 routes and land affected. 1992 Fluvial Flood Event. Goosebridge Cottage and properties 1999 nearby affected. Transport routes also disrupted. 1996 Fluvial Flood Event in Lea, Chink Farm. Transport routes flooded.
1996 Fluvial flood event in Brokenborough, Boakley Farm. Transport routes affected. Malmesbury 1996 14 properties affected in Malmesbury. Fluvial flood event in Charlton, B4040 affected. 2000 Fluvial flood event. 22 properties affected including transport routes. 2007 Fluvial flood event, Malmesbury. From the Tetbury Avon. Land affected by flooding.
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Marston 2007 Fluvial / Surface water runoff. Village street inundated Meysey and two properties flooded.
In addition to historical fluvial flood risk, the Bristol Avon CFMP identifies the approximate number of properties at risk from the 1 in 100 year fluvial flood within the districts main towns: Calne - 60 properties Chippenham - 20 properties Corsham - 5 properties Malmesbury - 60 properties
The fluvial flood risk within the district is displayed in Figures 3 and 4.
Climate Change
As part of this Level 1 SFRA, no allowance for climate change has been made. It is noted that on application of the Sequential Test, if areas for development are likely to fall within Flood Zones 2 or 3 and require the application of the Exception Test, then the likely effects of climate change should be investigated for those individual sites.
It is advised for the Level 2 SFRA, subject to the formal agreement of the Environment Agency in areas which detailed hydraulic models are present that the 1 in 100 year plus climate change event is mapped. In the absence of climate change outlines Flood Zone 2 should be treated as Flood Zone 3 with climate change.
5.2 Sewer Flooding
Wessex Water Ltd and Thames Water Ltd are the statutory water undertakers for the North Wilsthire administrative area and maintain a register of historical sewer flooding events (DG5 Register). Within their respective areas they are responsible for public sewers systems within the North Wiltshire District. Table 5.2 shows the current number of properties at risk of foul sewer flooding within the North Wiltshire District as of September 2007. Such flooding may be internal or external of properties and will be caused by a variety of operational and meteorological conditions. A map representing the data shown in Table 5.2 can be found in Appendix B.
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Table 5.2 Number of properties at risk of foul sewage flooding as of September 2007 – (DG5A flooding frequency 2 in 10 years, DG5B - flooding frequency 1in 10 years, DG5C - flooding frequency 1 in 20 years)
Location DG5A DG5B DG5C Hankerton 1 Minety 5 1 Malmesbury 1 Little Somerford 1 Wootton Bassett 1 Lyneham 1 Broad Town 2 Dauntsey 1 Castle Combe 3 Biddestone 1 Corsham 2 Box 1 Chippenham 1 Calne 1 Compton Bassett 1
There are likely to be a greater number of properties affected from storm events that have larger return periods (e.g. 1 in 50 and 1 in 100 year) due to design standards of most sewers being for 1 in 30 year events (Sewers For Adoption, 2006). However, information is unavailable for these properties or the design standard for which the sewer system has been designed. In addition, information on privately owned and maintained stormwater management systems is unavailable.
Climate Change
Across the Wessex Water region there is an annual increase of approximately 40 dwellings per annum of new incidents of flooding being recorded. Wessex Water is currently undergoing an extensive programme of work to eliminate the majority of foul sewage flooding incidents by 2010. As such, Wessex Water anticipates to see more or less all of the locations to be removed from the register by 2010.
5.3 Pluvial Flooding (Overland Flow)
Surface water flooding occurs when intense heavy rainfall causes excess water to runoff from surrounding areas either as a result of impermeable surfaces (natural or man made) or a combination of saturated ground and steep slopes. In some cases surface water runoff is linked to the incapacity of storm water management systems. The Bristol Avon CFMP which covers the majority of the district indicates that just over 10 % of property flooding is the result of surface water runoff. Notable surface water events have occurred in Calne, Chippenham, Malmesbury and Wootton Bassett (Figures 4A-4F).
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Climate Change
It is anticipated that climate change trends and patterns indicate that more frequent short- duration, high-intensity rainfall and more frequent periods of long-duration rainfall is to be expected. These high intensity rainfall events are likely to lead to overwhelmed drains on a more frequent basis.
5.4 Groundwater Flooding
The nature of the underlying geology of the North Wiltshire District means that groundwater flooding is not significant. The Environment Agency records show only one confirmed groundwater incident, which occurred winter 2003 in Purton where the underlying geology is Corallian. The Corallian is a mix of inter-bedded sandstones, limestones and clays and is classified as a minor aquifer. This type of geology can also give rise to perched groundwater and spring flow.
Clays underlie much of the central area of the northern tip of North Wiltshire area (West Walton & Oxford Clay). The Environment Agency classifies these deposits as non-aquifer because they do not readily store or transmit groundwater. Therefore these deposits would not normally be prone to groundwater flooding.
Limestone members of the Great Oolite Group (Forest Marble and Cornbrash) are present in the northwest of the North Wiltshire area. Forest Marble is an inter-bedded limestone and mudstone and is mostly impermeable. Cornbrash Limestone is classified as a minor aquifer. Springs could occur from perched groundwater within these strata. Limestones and some Mudstones are present to the southwest of the North Wiltshire area. Figure 7 displays the solid and drift geology within the district.
5.5 Artificial Sources (Infrastructure Failure)
Artificial sources of flooding within a 1km radius of the any sites should be considered in order to assess the residual risk of a water body overtopping and potential flow paths. Thereafter, an appreciation can be given as to whether it is considered an actual or residual flood risk from artificial sources. Figure 4A- 4F provides the location of all perched water bodies within the catchment.
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6 Guidance on Applying the PPS25 Sequential Test
6.1 What is the Sequential Test?
PPS25 requires application of the Sequential Test at all stages of the planning process to ensure that developments are removed from areas with a high probability of flooding where possible. Through application of this risk based approach local planning authorities are encouraged to guide new development towards areas of the lowest flood probability.
In applying the Sequential Test planners should also bear in mind the vulnerability classification of their proposed development. Table D2 of PPS25 describes types of development according to their flood vulnerability. By using this information in tandem with the Sequential Test planners should guide developments to those areas where the flooding probability is appropriate to the vulnerability of the proposed development as presented in Table 6-1.
Table 6-1 : Flood Risk Vulnerability Classification (from PPS25, Appendix D, Table D2) . Essential transport infrastructure (including mass evacuation routes), Essential which has to cross the area at risk, and strategic utility infrastructure, Infrastructure including electricity generating power stations and grid and primary substations. . Police stations, Ambulance stations and Fire stations and Command Centres and telecommunications installations required to be operational during flooding. Highly . Emergency dispersal points. Vulnerable . Basement dwellings. . Caravans, mobile homes and park homes intended for permanent residential use. . Installations requiring hazardous substances consent. . 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; and hotels. More – . Non residential uses for health services, nurseries and educational Vulnerable establishments. . Landfill and sites used for waste management facilities for hazardous waste. . Sites used for holiday or short-let caravans and camping, subject to a specific warning and evacuation plan.
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. Buildings used for: shops; financial, professional and other services; restaurants and cafes; hot food takeaways; offices; general industry; – storage and distribution; non residential institutions not included in ‘ ’ more vulnerable ; and assembly and leisure. . Land and buildings used for agriculture and forestry. Less . Waste treatment (except landfill and hazardous waste facilities). Vulnerable . Minerals working and processing (except for sand and gravel working). . Water treatment plants. . Sewage treatment plants (if adequate pollution control measures are in place). . 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. Water- . MOD defence installations. compatible . Ship building, repairing and dismantling, dockside fish processing and Development 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 and essential facilities such as changing rooms. . Essential ancillary sleeping or residential accommodation for staff required by uses in this category, subject to a specific warning and evacuation plan.
PPS25 acknowledges that some areas will (also) be at risk of flooding from flood sources other than fluvial or tidal systems. Consequently all sources of flooding must be considered when looking to locate development in one of the flood zones described above. The other sources of flooding requiring consideration when situating new development allocations include:
. Pluvial; . Groundwater; . Sewers; and . Artificial Sources.
These sources (as sources of flooding) are typically less understood than tidal and fluvial sources. Consequently data often only exists as point source data or through interpretation of local conditions. In addition there is no guidance on suitable return periods to associate with floods arising from these sources. For example modern storm water drainage systems are constructed to a 1 in 30 year standard. Any storm event in excess of the 30 year return period storm would be expected to cause flooding. Consequently when assessing these sources through the Sequential Test, if a location is recorded as having experienced repeated flooding from the same source this should be investigated further.
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6.2 How should the SFRA be used to apply the Sequential Test?
The Sequential Test should be undertaken by the LPA and accurately documented to ensure decision processes can be transparently communicated and reviewed where necessary. The Sequential Test should be carried out on all development sites, seeking to balance the flood probability and development vulnerability of sites throughout a planning authority area.
The recommended steps required in undertaking the Sequential Test are detailed below. This is based on the various constraints placed on the types of vulnerable development presented in Table D3 of PPS25, reproduced below (Table 6-2).
Table 6-2 :PPS25 Table D3 Flood Risk Vulnerability and Flood Zone 'Compatibility' (DCLG, 2006)
FLOOD RISK ESSENTIAL WATER HIGHLY MORE LESS VULNERABILITY INFRASTRUCTURE COMPATIBLE VULNERABLE VULNERABLE VULNERABLE CLASSIFICATION 1
Exception 2 Test
ONE
Z Required Exception Exception Test LOOD LOOD 3A Test F Required Required Exception Test 3B Required
- Development is appropriate - Development should not be permitted
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Figure 6.1: Application of the Sequential Test (from Figure 3.1 of PPS25: Practice Guide, A ‘ ’ Living Draft )
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6.3 Additional Guidance
The sequence of steps presented below in tandem with Figure 6.1 is designed to guide the LPA and developers through the Sequential Test. The steps are designed to ensure land allocations are primarily allocated in line with the principles of the Sequential Test or failing this the requirement for application of the Exception Test is clearly identified.
Recommended stages for LPA application of the Sequential Test.
1. The developments (i.e. housing, hospitals, industrial etc) that need to be accommodated by the LPA should be assigned a vulnerability classification in “ ” accordance with Table D.2 Flood Risk Vulnerability Classification in PPS25;
2. The Flood Zone classification of all development sites should be determined based on a review of the Environment Agency Flood Zones for fluvial sources. This should consider the effects of climate change on flood zone definition for the design life of any development that the site may be suitable for i.e: – 60- years up to 2070 for commercial / industrial developments; and – 100 years up to 2110 for residential developments ‘ ’ 3. In the first instance the highly vulnerable developments the LPA is required to accommodate should be located in those sites it has identified as being within ‘ ’ Flood Zone 1. If the highly vulnerable developments cannot be located in Flood Zone 1, because the identified sites are unsuitable or there are insufficient sites in Flood Zone 1 then sites in Flood Zone 2 can be considered. If sites in Flood Zones 1 and 2 are inadequate then to accommodate the development the LPA may have to identify additional sites in Flood Zones 1 or 2 or seek opportunities to locate the development outside their administrative area. ‘ ’ 4. Once all highly vulnerable developments have been allocated to a development ‘ ’ site, the LPA can consider those development types defined as more vulnerable . ‘ ’ In the first instance more vulnerable development should be located in any unallocated sites in Flood Zone 1. Where these sites are unsuitable or there are insufficient sites, sites in Flood Zone 2 can be considered. If there are insufficient ‘ ’ sites in Flood Zone 1 or 2 to accommodate the more vulnerable development ‘ ’ types, sites in Flood Zone 3a can be considered. However, any more vulnerable developments in Flood Zone 3a will require application of the Exception Test. ‘ ’ ‘ ’ Responses to parts a and b of the Exception Test should be prepared and ‘ ’ agreed through consultation with the Environment Agency before part c is tackled. ‘ ’ 5. Once all more vulnerable developments have been allocated to a development ‘ ’ site, the LPA can consider those development types defined as less vulnerable . ‘ ’ In the first instance less vulnerable development should be located in any remaining unallocated sites in Flood Zone 1, 2 or 3a. Less vulnerable – development types are not appropriate in Flood Zone 3b Functional Floodplain.
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‘ ’ 6. Essential infrastructure developments should also be preferentially located in the lowest flood risk zones, however this type of development can be located in Flood Zones 3a and 3b, where necessary, through application of the Exception Test. Where these types of development are located in Flood Zone 3a or 3b ‘ ’ ‘ ’ ‘ ’ responses to parts a and b of the Exception Test will be required before part c is tackled.
7. Finally, it is recommended that water compatible development is located. As these developments typically have the least flood risk constraints it is considered appropriate to consider them last when allocating development sites.
8. For decisions made through stages 4 to 8 it will also be necessary to consider the risks posed to the site from other flood sources and where comparable development sites in the same flood zone may be more suitable due to: flood risk management measures, the rate of flooding, flood water depth, or, flood water velocity.
Where the development type is highly vulnerable, more vulnerable, less vulnerable or essential infrastructure and a site is found to be impacted by a recurrent flood source (other than fluvial), the site and flood sources should be investigated further regardless of any requirement for the Exception Test. This should be discussed with the Environment Agency to establish the appropriate time for the assessment to be undertaken, (i.e. Exception Test through a Level 2 SFRA or assess through a site specific flood risk assessment).
To encourage the local planning authority to consider the relevant questions and to guide them through the Sequential Test it is recommended that the following questions are considered for each development.
It is recommended that North Wiltshire District Council complete Table 1 (Appendix C) to assist in completion of the Sequential Test to provide a transparent framework and justification of sites that may need to be exception tested.
The potential allocation sites are identified as part of the Core Strategy identifies the location where development is acceptable in principle. Reviews of these specific allocated areas in relation to flood risk are presented in Appendix D. This provides a starting point for the Local Authority to consider whether the allocated sites in the Local Plan should be retained as the benefits outweigh the flood risk or whether these should be removed.
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7 Guidance on Applying the PPS25 Exception Test
7.1 What is the Exception Test?
After application of the sequential test, if it is found to be impossible for a development to be located in a lower flood risk zone, then it may be possible to apply the Exception Test to the allocation, providing the development is consistent with the wider sustainability objectives of the area. Table 6-1 provides guidance on the vulnerability of types of development and in conjunction with Table D1 where various types of development are appropriate with regards to flood risk and where it may be appropriate for the Exception Test to be applied.
7.2 Why is there an Exception Test?
The Exception Test is essential in cases where the sequential test is unable to deliver acceptable sites for allocations. In some areas development is required to ensure social or economic, blight does not occur, thus ensuring continued sustainable development, and to open up areas protected by nature conservation designations for development if they restrict the availability of sites within the lower flood risk areas.
7.3 What is Required to Pass the Exception Test?
The Exception Test consists of three sections which are detailed below. All of these sections are required to be passed before it could be deemed that a development would be appropriate within the flood zone. – 7.3.1 Part A Wider Sustainability to the Community
It must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by the SFRA where one has been ‘ ’ prepared. If the DPD has reached the submission stage (Figure 4 of PPS12; Local Development Frameworks the benefits of the development should contribute to the Core ’ Strategy s Sustainability Appraisal.
The planning application should be scored against the sustainability criteria of the Sustainability Appraisal. Where a development fails to score positively against the SA the LPA could consider planning conditions or Section 106 Agreements.
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Developable sites are defined in Planning Policy Statement 3 (PPS3) Housing as those sites be in a suitable location for Housing development and these should be a reasonable prospect that the site is available for, and could be developed at the point envisaged.
North Wiltshire District Council has prepared a Sustainability Appraisal Scoping Report in April 2003 in which it lists Sustainability Criteria namely: Provision and access to safe, healthy and attractive living and working environments for all Provision and access to community facilities Reduced need for travel and reliance on the private car Reduced poverty and social exclusion Quality and character of towns, villages and countryside Natural habitats and species diversity Built and archaeological heritage Water management Land and Soil Quality Quality of environmental health: air, noise, light, odour Reduced use of non-renewable energy sources Land and buildings Water resources Waste management and minerals Maintenance and improvements of the economic viability of the district Maintenance and improvement of attractive working environments
Appraising the proposed developments and producing evidence to support this in assisting North Wiltshire District Council in achieving its Sustainability Objectives should be undertaken by the developer. For each objective it should be demonstrated whether the development meets this objective by classifying it as:
Positive Neutral or Negative
This should formulate a holistic picture of whether the development provides a positive contribution to sustainability. This evidence should be submitted and by the developer and the evidence formally agreed by the Local Planning Authority.
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– 7.3.2 Part B Brownfield Land
PPS3 Annex B provides the following definition of previously developed land (often referred to as brownfield land): ‘ Previously-developed land is that which is or was occupied by a permanent structure, including the curtilage of the developed land and any associated fixed surface ’ infrastructure.
The definition includes defence buildings, but excludes: – Land that is or has been occupied by agricultural or forestry buildings. – Land that has been developed for minerals extraction or waste disposal by landfill purposes where provision for restoration has been made through development control procedures. – Land in built-up areas such as parks, recreation grounds and allotments, which, although it may feature paths, pavilions and other buildings, has not been previously developed. – Land that was previously-developed but where the remains of the permanent structure or fixed surface structure have blended into the landscape in the process of time (to the extent that it can reasonably be considered as part of the natural surroundings).
There is no presumption that land that is previously-developed is necessarily suitable for housing development nor that the whole of the curtilage should be developed.
The development should be provided on previously developed land. However, if the site is greenfield and it is deemed that there is no reasonably available previously developed land this evidence must be presented that are no reasonably available alternative sites on developable previously- developed land.
Reasonably available is defined as being achievable within the next five years and furthermore need to be of the similar scale, nature and appropriate for this type of development. – 7.3.3 Part C Safe from Flood Risk
A FRA must demonstrate that the development will be safe, without increasing food risk elsewhere, and, where possible, will reduce flood risk overall. The PPS25 Companion ‘ ’ – Guide provides details on the definition of safe in Chapter 5 Risk Management by – Design, and Chapter 6 Residual Risk. Details on possible mitigation measures against different forms of flooding area also provided in Table 11-1. ‘ ’ A minimum requirement of the definition of safe should be:
. Dry access for more and highly vulnerable uses;
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. Dry escape for residential dwellings should be up to the 1 in 100 year flood event taking into account climate change; and . Preferably dry for other uses such as educational establishments and less vulnerable land use classifications.
However the definition of safe should be clarified and agreed to by the local Environment Agency Office.
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8 Flood Risk Management
8.1 Flood Defences
The North Wiltshire District is predominately rural with the population focused around three main market towns within the district; therefore flooding can occur in many areas of the district with minimal effect to people or the economy. As a result, flood defences are generally focused around the urban centres within the district where flood waters are liable to cause disruption on a number of levels. The National Flood and Coastal Defence Database (NFCDD) compiled by the Environment Agency holds information on natural and man-made defences. The standard of these flood defences is only available for man-made defences. Table 8.1 below shows the standard of flood defence schemes within North Wiltshire. Figure 5 displays the location of NFCDD defences throughout the North Wiltshire District.
Table 8.1 Flood defence schemes sourced from NFCDD and Environment Agency Catchment Food Management Plans.
Location Standard of protection Years % Annual probability Calne 35 3 Chippenham 50 2 Malmesbury 50 2
Areas of land situated behind these defences are still identified as flood risk areas on the Environment Agency Flood Maps. The presence of flood defences does not remove the complete risk for the areas protected as defences can be breached by flood waters if flood defences are neglected or as the result of an extreme flood event.
8.2 Flood Warning Areas
The Environment Agency operates a flood warning service in all areas at risk of flooding, which is available on their website. It consists of a series of warning codes that indicate the level of danger. The flood warnings are disseminated through a variety of mediums that include TV, radio, Automated Voice Messaging service direct to a phone/fax/pager, internet and/or loudhailer. There is also an emergency Floodline number (0845 988 1188) and a quickdial number for individual rivers.
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The majority of North Wiltshire is situated within the Upper Bristol Avon Flood Watch Catchment. The Flood Warning Areas within the district are displayed in Figure 6 of the report. The Flood warning system is intended to assist residents at risk of flooding prepare for floods by obtaining sand bag, moving valuables, property upstairs and where necessary evacuating the property. The Environment Agency aim to give a minimum of two hours warning prior to the onset of a flood event. However the rapid onset of some flood events, particularly when a period of intense rainfall falls within an urban catchment means sometimes a sufficient warning cannot be raised.
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9 Drainage of Development Sites
9.1 Principles
Traditionally, built developments have utilised piped drainage systems to manage storm water and convey surface water run-off away from developed areas as quickly as possible. Typically these systems connect to the public sewer system for treatment and/or disposal to local watercourses. Whilst this approach rapidly transfers storm water from developed areas, the alteration of natural drainage processes can potentially impact on downstream areas by increasing flood risk and reducing water quality. Receiving watercourses are therefore much more sensitive to rainfall intensity, volume and catchment land uses after a catchment or areas of a catchment have been developed.
Due to the difficulties associated with up rating sewer systems it is uncommon for sewer and drainage systems to keep pace with the rate of development/re-development and there are increasingly stringent controls placed on discharges to watercourses. As development progresses and/or urban areas expand these systems become inadequate for the volumes and rates of storm water they receive, resulting in increased flood risk and/or pollution of watercourses. Allied to this are the implications of climate change on rainfall intensities, leading to flashier catchment/site responses and surcharging of piped systems.
In addition, as flood risk has increased in importance within planning policy, a disparity has emerged between the design standard of conventional sewer systems (1 in 30 year) and the typical design standard flood (1 in 100 year). This results in drainage inadequacies for the flood return period developments need to consider, often resulting in potential flood risk from surface water/combined sewer systems.
A sustainable solution to these issues is to reduce the volume and/or rate of water entering the sewer system and watercourses.
9.2 What are SuDS? ’ Sustainable Drainage Systems (SuDS) are the Government s preferred method for managing the surface water run-off generated by developed sites and PPS25 notes that regional planning bodies and Local Authorities should promote their use for the management of runoff. 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. Typically this approach involves a move away from piped systems to softer engineering solutions inspired by natural drainage processes.
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 year design standard flood including an increase in peak rainfall of 30% to account from climate change.
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Wherever possible, a SuDS technique should seek to contribute to each of the three goals identified below with the favoured system contributing significantly to each objective. Where possible SuDS solutions for a site should seek to:
1. Reduce flood risk (to the site and neighbouring areas),
2. Reduce pollution, and,
3. Provide landscape and wildlife benefits.
These goals can be achieved by utilising a management plan incorporating a chain of techniques, (as outlined in Interim Code of Practice for Sustainable Drainage Systems 2004), where each component adds to the performance of the whole system:
Prevention good site design and upkeep to prevent runoff and pollution (e.g. limited paved areas, regular pavement sweeping)
Source control runoff control at/near to source (e.g. rainwater harvesting, green roofs, pervious pavements)
Site control water management from a multitude of catchments (e.g. route water from roofs, impermeable paved areas to one infiltration/holding site)
Regional control integrate runoff manage from a number of sites (e.g. into a detention pond)
In keeping with the guidance of PPS25, local authorities should encourage the application of SuDS techniques. This chapter presents a summary of the SuDS techniques currently available and a review of the soils and geology of the study area, enabling the local authorities to identify where SuDS techniques could be employed in development schemes.
The application of SuDS techniques is not limited to one technique per site. Often a successful SuDS solution will utilise a number of techniques in combination, providing flood risk, pollution and landscape/wildlife benefits. In addition, SuDS can be employed on a strategic scale, for example with a number of sites contributing to large scale jointly funded and managed SuDS, however, each development site must offset its own “ ” increase in runoff and attenuation cannot be traded between developments.
Detailed design guidance can be found in the SuDS Manual C697, and associated Site Handbook for the Construction of SuDS, C698. These publications provide best practice guidance on the planning, design, construction, operation and maintenance of SuDS, to ensure effective implementation within developments.
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9.3 SuDS Policies
There are a number of policies and planning documents that promote the implementation of SuDS in new developments.
PPS25
In terms of identifying a requirement to consider SuDS on a development project the following general principle (set out in PPS25) should be followed: “ The surface water drainage arrangements for any development site should be such that the volumes and peak flow rates of surface water leaving a developed site are no greater than rates prior to the proposed development, unless specific off-site arrangements are ” made and result in the same net effect.
This is to alleviate the pressure on sewer systems that are often antiquated, serving a catchment area greater than their original design and/or designed to a standard less than that required to mitigate development from a 1% annual probability flood event.
If a proposed development results in an increase in surface water, then the Environment Agency will expect to see SuDS forming part of the proposed mitigation. With their new powers of direction over planning applications in flood zones or for major development, any developments that do not incorporate SuDS can expect them to be required through Section 106 conditions to their planning applications. Where the consented discharge rates are low, this can significantly impact on the viability of development proposals.
Communities for Sustainable Homes
The Code for Sustainable Homes identifies the proactive reduction of surface water run ’ off as a mandatory element worth two credits towards the 57 required for the Code s level 3 rating. Through incorporating suitably designed systems into a development SuDS can also contribute to several other assessment criteria under Code for Sustainable Homes, such as those relating to ecology and potable water consumption, which offer a further 9 and 5 points respectively towards the Level 3 rating.
9.4 SuDS Methods
SuDS techniques can be used to reduce the rate and volume and improve the water quality of surface water discharges from sites to the receiving environment (i.e. natural watercourse or public sewer etc). Various SuDS techniques are available; however the techniques operate on two main principles:
Infiltration Attenuation
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All systems generally fall into one of these two categories, or a combination of the two.
The design of SuDS measures should be undertaken as part of the drainage strategy and design for a development site. A ground investigation will be required to access the suitability of using infiltration measures, with this information being used to assess the required volume of on-site storage. Hydrological analysis should be undertaken using industry approved procedures, to ensure a robust design storage volume is obtained.
During the design process, liaison should take place with the Local Planning Authority, the Environment Agency, Thames Water and Wessex Water in order to establish that the design methodology is satisfactory and to also agree on a permitted rate of discharge from the site.
Table 9-1 provides a summary of the different SuDS techniques, and which techniques are suitable to comply with the three goals of sustainability.
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Management Water Amenity Component Description Water Quality Train Quantity Biodiversity Layer of vegetation or gravel on roof areas providing Green roofs ● ● ● absorption and storage. Capturing and reusing rainwater for domestic or Rainwater harvesting ● ○ ○ irrigation uses.
Prevention Permeable Infiltration through the surface into underlying layer. ● ● ○ pavements Drain filled with permeable material with a Filter drains ● ● perforated pipe along the base. Similar to filter drains but allows infiltration through
Source ● ● Infiltration trenches sides and base. ● ● Soakaways Underground structure used for store and infiltration. Vegetated areas used for treating runoff prior to Bio-retention areas ● ● ● discharge into receiving water or infiltration Grassed depressions, provides temporary storage, Swales ● ● ○ conveyance, treatment and possibly infiltration. Provides treatment by filtering runoff through a filter Sand filters ● ● media consisting of sand. Dry depressions outside of storm periods, provides ● ● ○ Basins temporary attenuation, treatment and possibly
Site infiltration. Designed to accommodate water at all times, ● ● ● Ponds provides attenuation, treatment and enhances site
Regional amenity value. Similar to ponds, but are designed to provide Wetland ● ● ● continuous flow through vegetation. ● – ○ Key: highly suitable, - suitable depending on design
Table 9-1: Summary of SuDS Techniques and their Suitability to meet the Three Goals of Sustainability
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9.5 Where can SuDS can be utilised?
The underlying ground conditions of a development site will often determine the type of SuDS approach to be used at development sites. This will need to be determined through ground investigations carried out on-site, however an initial assessment of a sites suitability to the use of SuDS can be obtained from a review of the available soils/geological survey of the area.
Based on a review of the following maps we can then recommend suitable SuDS techniques that would be compatible with the underlying geology:
The British Geological Survey - GeoIndex, Bedrock and Superficial Deposits. Online: http://www.bgs.ac.uk/magazine/geology/
Soil Survey of England and Wales, 1:250,000 Soil Map of England and Wales
In the design of any drainage system and SuDS approach, consideration should be given to site-specific characteristics and where possible be based on primary data from site investigations. The information presented in Table 9-2 and Table 9-3 is provided as a guide and should not be used to accept or refuse SuDS techniques.
Table 9-2: Suitable SuDS Techniques Dependent on Geology
SuDS Location Techniq Permeability Geology Description ue Jurrasic Dominates the Western part of the Moderate Great Oolite Sedimentary district. Skirting Chippenham in the Rock south and Malmesbury in the north. Upper Green Cretaceous Dominates the south east of the Moderate Sand and Sedimentary district, east of the Calne. Gault Rock Jurrasic Located in the west of the district. Moderate Cornbrash Sedimentary Rock Infiltration Attenuation Jurrasic A relative thin strip is located north to Moderate Corallian Sedimentary south, to the west of Calne and Rock Wooton Bassett. or Combined Infiltration and Clay, Sand and These superficial deposits are located Variable Alluvium Silt within the floodplain.
Oxford Clay Jurrasic Dominates the central region of the Low and Kellaways Sedimentary district. Underlying Chippenham and Beds Rocks Malmesbury. Jurrasic Between the Upper Green sand layer Kimmeridge Low Sedimentary and the Corallian layer. Dominate
Attenuation Clays Rock rock type underlying Wotton Bassett.
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Table 9-3: Suitable SuDS Techniques Dependant on Soil Type
SuDS Soil Geology Permeability Description Technique Association Jurassic Shallow well drained brashy limestone calcareous fine loamy soils over limestone. Some similar deeper soils Moderate and some non-calcareous clayey soils. ELMTON 1 Jurassic Shallow, well drained brashy limestone and calcareous fine loamy soils over ELMTON 2 Moderate sandy limestone. Some deeper fine loamy
limestone or fine loamy over clayey soils.
Jurassic Shallow well drained brashy limestone and calcareous clayey soils over SHERBORNE Moderate clay limestone, associated with slowly
permeable calcareous clayey soils.
River terrace Well drained calcareous and non-
Infiltration gravel calcareous fine loamy soils over limestone gravel. Some deep fine BADSEY 1 Moderate loamy soils and fine loamy soils over
gravel, and similar but shallower soils affected by groundwater.
or Combined Infiltration and Attenuation River terrace Well drained calcareous fine loamy BADSEY 2 and lacustrine soils over limestone gravel. Some Moderate gravel similar soils affected by groundwater.
Mesozoic and Deep well drained sandy and coarse Tertiary sands loamy soils. Some ferruginous sandy FRILFORD Moderate and some coarse loamy soils affected
by groundwater. Risk of soil erosion.
Jurassic and Slowly permeable calcareous clayey Cretaceous soils. Some slowly permeable EVESHAM 2 clay seasonally waterlogged non- Low calcareous clayey and fine loamy or fine silty over clayey soils.
Jurassic clay Slowly permeable calcareous clayey and limestone soils associated with shallow well drained brashy calcareous soils over Attenuation Evesham 1 Low limestone. Landslips and associated
irregular terrain locally
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Eocene and Deep fine loamy soils with slowly jurassic loam permeable subsoils and slight and clay seasonal waterlogging associated Bursledon Low with deep coarse loamy soils variably
affected by groundwater.
Drift over Jurrasic and Slowly permeable seasonally Wickham 2 Cretaceous waterlogged fine loamy over clayey, Low clay or fine silty over clayey and clayey soils. mudstone
Drift over Slowly permeable seasonally Mesozoic and waterlogged fine loamy over clayey Wickham 3 Tertiary clay and coarse loamy over clayey soils, Low and loam and similar more permeable soils with slight waterlogging.
Drift over Jurrasic and Slowly permeable seasonally Wickham 2 Cretaceous waterlogged fine loamy over clayey, Low clay or fine silty over clayey and clayey soils. mudstone
Jurassic and Slowly Permeable seasonally Cretaceous waterlogged clayey soils with similar clay fine loamy over clayey soils. Some Denchworth fine loamy over clayey soils with only Low slight seasonal waterlogging and some slowly permeable calcareous clayey soils.
River Alluvium Stoneless Clayey soils, in places calcareous, variably affected by Fladbury 1 Low groundwater. Flat land. Risk of
flooding.
River terrace Calcareous fine loamy soils over drift gravel, variably, affected by groundwater, associated with non- KELMSCOT Low calcareous clayey soils over gravel.
Flat land. Risk of flooding
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10 Policy Recommendations
National and local policies have been reviewed against the local flood risk issues and objectives identified by the Environment Agency in the CFMPs covering the North Wiltshire District. From these policies the following catchment wide and specific area strategies have been developed under the headings Flood Risk, SuDS, Flood Mitigation and the Water Environment. Integration of these suggested policy considerations into LDF / LDD should ensure that the objectives and aspirations of the Environment Agency and national policy are met whilst strengthening the position of the local planning authority with regard to Flood Risk.
10.1 Flood Risk
10.1.1 Catchment Wide Strategies
1. Allocate all sites in accordance with the Sequential Test to reduce flood risk and ensure that the vulnerability classification of the proposed development is appropriate to the flood zone classification;
2. Flood Risk Assessments (FRAs) should be undertaken for all developments within Flood Zones 2 and 3, and sites with identified flooding sources, to assess the risk of flooding to the development and identify options to mitigate the flood risk to the development, site users and surrounding area;
3. Flood Risk Assessments are required for all major developments in Flood Zone 1. These are residential developments consisting of sites greater than 0.5 ha or greater than 10 dwellings and commercial developments that are greater than 1 ha or have a floor area greater than 1000 m2.
4. Flood Risk to development should be assessed for all forms of flooding;
5. Where floodplain storage is removed, the development should provide compensatory storage on a level for level and volume for volume basis to ensure that there is no loss in flood storage capacity.
10.1.2 Area Specific Strategies
1. Surface water flooding should be investigated in detail as part of FRAs for developments located within the Main Settlements. There should be early liaison with the Environment Agency and North Wiltshire District Council for appropriate management techniques.
Through integration of these suggestions, the emerging LDF will comply with PPS25 and the aspirations and policies represented in the following:
Regional Planning Guidance for the South West (RPG10) - Policy RE2
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– Draft Regional Spatial Strategy for the South West Climate Change Policy SD2, Development Policy F and G, and Flood Risk Policy F1 – Wiltshire and Swindon Structure Plan 2016 Policy C5 North Wiltshire Local Plan 2011 - Policy NE22 Bristol Avon Catchment Flood Management Plan and Thames Region Catchment – Flood Management Plan Social, Environmental and Economic Objectives. Building Regulations Part H (Amended 2002)
10.2 Sustainable Drainage Systems
Information on Sustainable Drainage Systems (SuDS) is provided in Chapter 9. Sustainable Drainage Policies should address the following issues as:
10.2.1 Catchment Wide Strategies
1. Sustainable Drainage Systems should be included in new developments unless it is demonstrable that it is not possible to manage surface water using these techniques.
2. PPS25 requires the use of SuDS as an opportunity of managing flood risk, improving water quality and increasing amenity and biodiversity.
3. Flood Risk Assessments are required for all major developments in Flood Zone 1. These are residential developments consisting of sites greater than 0.5 ha or greater than 10 dwellings and commercial developments that are greater than 1 ha or have a floor area greater than 1000 m2.
4. Runoff rates from new developments on greenfield sites should not exceed greenfield runoff rates pre-development and should allow for climate change.
5. Runoff rates from previously developed developable land should not exceed existing rates of runoff and should seek betterment. In addition, an allowance should be made for climate change;
6. Runoff and/or discharge rates should be restricted to greenfield runoff rates in areas known to have a history of sewer and/or surface water flooding.
10.2.2 Area Specific Strategies
1. Flood risk management in Malmesbury, Calne and Chippenham should focus SuDS techniques and land management to account for the effects of climate change. Routes should be established for flood waters to travel with minimal impact to the urban centres.
Through integration of these suggestions, the emerging LDF will comply with PPS25 and the aspirations and policies represented in the following:
Regional Planning Guidance for the South West (RPG10) - Policy RE2 – Draft Regional Spatial Strategy for the South West Climate Change Policy SD2, Development Policy F and G, and Flood Risk Policy F1
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– Wiltshire and Swindon Structure Plan 2016 Policy C5 North Wiltshire Local Plan 2011 - Policy NE22 Bristol Avon Catchment Flood Management Plan and Thames Region Catchment – Flood Management Plan Social, Environmental and Economic Objectives Building Regulations Part H (Amended 2002)
10.3 Flood Mitigation
10.3.1 Catchment Wide Strategies
1. Where an allocation borders an area benefiting from flood defence, opportunities should be sought for the maintenance and upgrading of these flood defences to be partly funded by the development for its lifetime;
2. Opportunities should be sought to open culverted watercourses, where possible, to return them to a natural system, reducing back up of flows and under capacity where this does not exacerbate the flooding elsewhere;
3. River channel restoration should be undertaken where possible to return the river to its natural state and restore floodplain to reduce the impact of flooding downstream;
4. Developments in flood zones should set finished floor levels 300mm above the 1 in 100 year + CC flood level;
5. Developments in flood zones 2 and 3 require the provision of dry access and egress during the 1 in 100 year event + climate change
10.3.2 Area Specific Strategies
1. In Chippenham, where deep and fast flood water flows can occur, opportunities to direct water away from areas of high social impact should be identified.
2. In rural areas upstream of Malmesbury and Chippenham opportunities should be sought to increase flood storage areas capacity, including an additional storage allowance for increases in flow as a result of climate change.
3. The conservation of strategic flood storage areas maintained by North Wiltshire District Council. Opportunities should be sought for maintenance of these areas by developer contribution for the lifetime of the development.
4. Development behind flood defences in principle flood risk areas such as Malmesbury and Chippenham should be controlled to reduce social and economic disruption.
Through integration of these suggestions, the emerging LDF will comply with PPS25 and the aspirations and policies represented in the following:
Regional Planning Guidance for the South West (RPG10) - Policy RE2 – Draft Regional Spatial Strategy for the South West Climate Change Policy SD2, Development Policy F and G, and Flood Risk Policy F1
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– Wiltshire and Swindon Structure Plan 2016 Policy C5 – North Wiltshire Local Plan 2011 Policy NE21 and Policy NE23 Bristol Avon Catchment Flood Management Plan and Thames Region Catchment – Flood Management Plan Social, Environmental and Economic Objectives
10.4 Water Environment
10.4.1 Catchment Wide Strategy
1. Development should not have a detrimental impact on the water environment through changes to water chemistry or resource.
2. Developments should look to incorporate water re-use and minimisation technology;
3. Any development should not be located within 8 metres of the river bank to ensure access for maintenance but also to ensure amongst other things a wildlife corridor for improvement of the riverine environment and allowance of natural processes to operate within the floodplain.
Through integration of these suggestions, the emerging LDF will comply with PPS25 and the aspirations and policies represented in the following:
Regional Planning Guidance for the South West (RPG10) - Policy RE2 – Draft Regional Spatial Strategy for the South West Climate Change Policy SD2, Development Policy F and G, and Flood Risk Policy F1 – Wiltshire and Swindon Structure Plan 2016 Policy C5 – North Wiltshire Local Plan 2011 Policy NE23 and HE1 Bristol Avon Catchment Flood Management Plan and Thames Region Catchment – Flood Management Plan Social, Environmental and Economic Objectives
Flood Risk Management Policies contained within the Catchment Flood Management Plans have been set out by the Environment Agency and assigned to different zones within the SFRA area (Chapter 2).
The area specific strategies should be updated following the application of the sequential test to provide more specific strategies for allocated development sites.
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11 Site Specific FRA Guidance
Flood Risk is a fundamental consideration for any development project regardless of scale or type. Understanding the flood risk to and arising from a development is key to managing the risk to people and property reducing the risk of injury, property damage or even death. Climate change is of particular concern to flood risk. Current predictions suggest the UK will experience milder wetter winters and on average hotter drier summers, whilst sea levels will continue to rise. This will lead to an increase in rainfall and therefore flood events in winter months and increase the risk of large thunderstorms in the summer months, as well as increasing the unpredictability of our weather.
Flooding is not limited to just rivers and sea, in fact flooding can arise from a number of sources, each presenting their own type of risk and requiring management. In addition some areas currently defended from flooding may be at risk in the future as the effects of climate change take hold or defence condition deteriorates with age.
However, development can work with flood risk if it is accurately understood and managed. Using a sound understanding of flood risk to locate and design developments enables flood risks to be managed through positive planning. This positive planning needs to consider the risks to a development from local flood sources but also the consequences a development may have on increasing flood risk. Early identification of flood risk constraints can ensure developments maximise development potential whilst achieving the principles of sustainability.
Level 1 Strategic Flood Risk Assessments present sufficient information to assist Local Planning Authorities to apply the Sequential Test and identify where the Exception Test may be required. These documents are predominately based on existing data. The scale of assessment undertaken for a Strategic Flood Risk Assessment is typically inadequate to accurately assess the risks faced by a particular development at any location with the study area. In addition, the information presented in the Level 1 SFRA does not necessarily fully address all the flood sources. For example, Flood Zones provided by the Environment Agency are not defined for all watercourses. Typically watercourses with a catchment area less than 3km2 are omitted from Environment Agency mapping unless there is a history of flooding affecting a population. Consequently there will be some locations adjacent to watercourses where, on first inspection, it is suggested there is no flood risk. However, as part of the SFRA unmapped watercourses have been provided with an 8 metre buffer either side of top of banks designated as Flood Zone 3b in line with the precautionary principle adopted by PPS25. This should be fully investigated and the floodplain refined as part of a flood risk assessment to ensure more people are not placed at risk through inappropriate development.
Site specific flood risk assessments are required to assess the flood risk posed to proposed developments and to ensure that where necessary and appropriate suitable mitigation measures are included in the development.
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This section presents the recommendations for site specific flood risk assessments prepared for submission with planning applications in the North Wiltshire District Council administrative area.
The site specific flood risk assessment guidance presented in the following sections has been developed based on:
1 the recommendations presented in Planning Policy Statement 25 and the consultation draft of the Practice Guide companion to PPS25 2 a review of the policies contained within the existing Local Plans for North Wiltshire DC; and 3 the information gathered through and findings of the Level 1 SFRA process.
11.1 When is a Flood Risk Assessments Required?
When informing developers of the requirements of a flood risk assessment for a development site, consideration should be given to the position of the development relative to flood sources, the vulnerability of the proposed development and its scale.
In the following situations a Flood Risk Assessment should always be provided with a planning application:
1 The development site is located in Flood Zone 2 or 3; 2 The proposed development comprises 10 or more residential dwellings and/or the site areas is greater than 1 hectares (even if the site is located in Flood Zone 1. This is to ensure storm water generated by the site is managed in a sustainable manner and does not increase the burden on existing infrastructure and/or flood risk to neighbouring property); 3 The floor space of proposed non-residential development is greater than 1000m2 or the site areas is greater than 1 hectare; 4 The development site is located in an areas known to have experienced flooding problems from any flood source; and, 5 The development is located within 8m of any watercourse regardless of Flood Zone classification.
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11.2 FRA Requirements
Annex E of PPS25 presents the minimum requirements for flood risk assessment. These include:
1 Consider the risk of flooding arising from the development in addition to the risk of flooding to the development; 2 Identify and quantify the vulnerability of the development to flooding from different sources and identify potential flood risk reduction measures; ‘ ’ 3 Assessment of the remaining residual risk after risk reduction measures have been taken into account and demonstrate that this is acceptable for the particular development; 4 The vulnerability of those that could occupy and use the development, taking account of the Sequential and Exception Tests and the vulnerability classification, including arrangements for safe access; 5 Consider how the ability of water to soak into the ground may change with development, along with how the proposed layout of development may affect drainage systems. 6 Fully account for current climate change scenarios and their effect on flood zoning and risk.
The Practice Guide Companion to PPS25 (consultation document) advocates a staged approach to site specific flood risk assessment with the findings from each stage informing the next iteratively throughout the development process.
The staged approach comprises:
1 Level 1 Screening Study 2 Level 2 Scoping Study 3 Level 3 Detailed Study
11.2.1 Level 1 - Screening Study
A level 1 Screening Study is intended to identify if a development site has any flood risk issues that warrant further investigation. This should be based on existing information such as that presented in the Level 1 SFRA. Therefore this type of study can be undertaken by a development control officer in response to the developer query or by a developer where the Level 1 SFRA is available. Using the information presented in the Level 1 SFRA and associated GIS layers a development control officer could advise a developer of any flooding issues affecting the site. This information can then be used by the developer as the basis to further their understanding of how the flood risks could potentially affect their development.
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11.2.2 Level 2 - Scoping Study
A level 2 Scoping Study is predominately a qualitative assessment designed to further understanding of how the flood sources affect the site and the options available for mitigation. The Level 2 FRA should be based on existing available information where this is available and use this information to further a developers understanding of the flood risk and how it affects their development. This type of assessment should also be used ’ to inform masterplans of the site raising a developer s awareness of the additional elements the proposed development may need to consider. – 11.2.3 Level 3 Detailed Study
Where the quality and/or quantity of information for any of the flood sources affecting a site is insufficient to enable a robust assessment of the flood risks, further investigation will be required. For example it is generally considered inappropriate to base a flood risk assessment for a residential care home at risk of flooding from fluvial sources on Flood Zone maps alone. In such cases the results of hydraulic modelling are preferable to ensure details of flood flow velocity, onset of flooding and depth of floodwater is fully understood and that the proposed development incorporated appropriate mitigation measures.
11.2.4 Flood Zone 1
A Flood Risk Assessment is required within Flood Zone 1 if the proposed development is vulnerable to any type of flood source or comprises 10 or more residential dwellings and/or the site areas is greater than 0.5 hectare (Residential) or 1 hectare (Non- residential). This is to ensure storm water generated by the site is managed in a sustainable manner and does not increase the burden on existing infrastructure and/or flood risk to neighbouring property. This FRA can be brief unless the factors above or local considerations require particular attention. Minimum requirements for a Flood Risk Assessment can be found in Annex E of PPS25 or can also be seen listed above in Section 11.2. “ The PPS25 Policy aim of Flood Zone 1 is developers and local authorities should seek opportunities to reduce the overall level of flood risk to the area and beyond through the layout and form of the development, and the appropriate application of sustainable ” drainage techniques .
11.2.5 Flood Zone 2
All developments proposed within Flood Zone 2 require a Flood Risk Assessment. The minimum requirements can be found in Annex E of PPS25 or can also be seen listed above in Section 11.2. “ The PPS25 policy aim of Flood Zone 2 is developers and local authorities should seek opportunities to reduce the overall level of flood risk to the area and beyond through the layout and form of the development, and the appropriate application of sustainable
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” drainage techniques . The land uses appropriate for this Flood Zone, as classified by Table D2 of PPS25 are Water- compatible, less vulnerable, more vulnerable and essential infrastructure. Highly Vulnerable uses type of land uses are only appropriate in Flood Zone 2 if the Exception Test is passed.
11.2.6 Flood Zone 3a
All developments proposed within Flood Zone 3a require a Flood Risk Assessment. The minimum requirements can be found in Annex E of PPS25 or can also be seen listed above in Section 11.2. The PPS25 policy aims for this zone are:-
i. reduce the overall level of flood risk in the area through the outlay and form of the development and appropriate application of sustainable drainage systems; ii. relocate existing development to land in zones with a lower probability of flooding; and iii. create space for flooding to occur by restoring functional floodplain and flood flow pathways and by identifying, allocating and safeguarding open space for flood storage.
The water compatible and less vulnerable uses of land as classified by Table D2 of PPS25 are appropriate for this zone. The highly vulnerable uses in Table D.2 should not be permitted in this zone. The more vulnerable and essential infrastructure uses in Table D.2 should only be permitted in this zone if the Exception Test is passed. Essential infrastructure permitted in this zone should be designed and constructed to remain operational and safe for uses in times of flood.
11.2.7 Flood Zone 3b
All developments proposed within Flood Zone 3b require a Flood Risk Assessment. The minimum requirements can be found in Annex E of PPS25 or can also be seen listed above in Section 11.2. The PPS25 policy aims for this zone are:-
i. reduce the overall level of flood risk in the area through the outlay and form of the development and appropriate application of sustainable drainage systems;
ii. relocate existing development to land in zones with a lower probability of flooding.
The water compatible uses and essential infrastructure listed in Table D2 of PPS25 that has to be there should be permitted in this zone. It should be designed and constructed to:
remain operational and safe for uses in times of flood; result in no net loss of floodplain storage; not impede flood risk elsewhere.
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Essential infrastructure should pass the Exception Test. Further more, there is a need to clarify the difference between the Greenfield and Brownfield (Previously developed land) functional floodplain. For Greenfield functional floodplain water-compatible development is only appropriate in this zone. ’ Advice from the Environment Agency s National Development Control Policy team on brownfield functional floodplain is that the Environment Agency will consider existing building footprints to be part of the functional floodplain, unless it can be proven that they exclude flood waters. If these buildings do exclude flood waters, then solely the area around these buildings will be deemed functional. When undertaking an FRA this matter should be clarified and ideally preagreed with the Environment Agency.
11.2.8 Greenfield Development and Brownfield Re-development
For greenfield and brownfield development Appendix E provides further guidance on the minimum requirements for a Flood Risk Assessment, a good practice and exemplary practice. Furthermore, when preparing an FRA should be done in conjunction with information in this SFRA in particular using Figures 3, 4 and 5.
11.2.9 Safe Development
Furthermore, the following items should be addressed as part of a Flood Risk ‘ ’ Assessment in order to demonstrate that proposed developments are safe in line with PPS25: ‘ ’ ‘ ’ ‘ ’ 1. Safe access/egress for more vulnerable and highly vulnerable uses is dry. Dry escape for residential dwellings should be up to the 1 in 100 year event taking into account climate change. 2. 'Safe' should preferably be dry for other uses such as educational establishments and 'less vulnerable' land use classifications ‘ ’ ‘ ’ 3. For More Vulnerable and Highly Vulnerable finished floor levels should be at the 1 in 100 year plus climate change level. 4. Where floodplain compensation is undertaken the Environment Agency requires this ‘ ’ is on a Level for Level, Volume for Volume Basis . 5. Flood flow routes should be preserved 6. Flood resilient constructions measures should be incorporated into new developments
Figure 5 demonstrates the location structures and defences using the NFCDD, this should be used as a reference point to identify key structures which may become blocked during times of flood and cause increased flood risk.
11.2.10 Surface Water Flood Risk Areas
In areas at risk of surface water flooding, development should seek to reduce surface water runoff rates as a result of development. Furthermore, the appropriate application of sustainable drainage systems (where possible) to reduce the overall level of flood risk in the area through the outlay and form of the development would be required. Figure 4A-
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4F show these mapped points and should be used as a reference point to further investigate that flood source as part of a Flood Risk Assessment.
11.2.11 Artificial Source/Infrastructure Failure Flood Risk Areas
Artificial sources of flooding within a 1km radius of the any site should be considered in order to assess the residual risk of a water body overtopping and potential flow paths. Thereafter, an appreciation can be given as to whether it is considered an actual or residual flood risk from artificial sources. Reservoir or canal flooding may occur as a result of the facility being overwhelmed and/or as a result of dam or bank failure and therefore a crude assessment of flood flow routes should be assessed. If a perched waterbody is in close proximity, where possible a cross section should be taken in relation to the proposed site, showing level of the waterbody and its water level.
11.2.12 Riverside Development
Main River
Under Section 109 of the Water Resources Act 1991 and/or Byelaws, any works whatsoever in, over, or under within 8metres of top of the channel of any main river watercourse on its banks within would require Environment Agency prior consent. Furthermore the Environment Agency would seek an 8 metre wide undeveloped buffer strip alongside main rivers and would also ask to developers to explore opportunities for river restoration as part of the development.
Ordinary watercourses/canals
The Environment Agency requires a 5 metre undeveloped buffer strip alongside such watercourses. This is to allow access for maintenance and encourage conservation and wildlife habitat.
De-culverting
The Environment Agency would seek deculverting as part of a development. Figure 5 of the NFCDD shows existing structures. Figure 5 should be used in conjunction with the large scale flood risk maps (Figures 3- 4) as a reference point to identify any potential structures that could cause increased flood risk within the area and could benefit from deculverting.
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12 Emergency Planning
Wiltshire County Council Major Incident Plan (MIPs) were compiled in April 2006. The aim of the MIP is to establish clear operational procedures in the case of an emergency.
When extreme flood events occur it is essential to have an emergency plan in place to provide clear procedural instructions. The mobilisation and organisation of the emergency services and supporting agencies, for example the County and District Councils is required to rescue, treat and transport potentially large numbers of casualties. During and after a flood event the role of the local authority includes providing transport for the evacuees and safe rest centres to stay in the event of homes being flooded. Further health and welfare issues are inevitable as a result of serious flood event.
PPS25 classifies Police stations, Ambulance stations, fire stations and command centres as Highly Vulnerable buildings. Hospitals and care homes are classified as More Vulnerable establishments. In the event of an emergency to ensure that those services vital to the rescue operation not also victims the flood waters it is essential that all establishments related to these services are located in the lowest flood risk zones. In addition future development control polices should seek to locate more vulnerable institutes such as schools and care homes in areas of the lowest risk to minimise the potential for flood casualties.
A GIS layer should be produced by the Local Planning Authority of all the institutes such as nursing homes, hospitals and schools and be overlain on the Flood Zones. This can then be provided to the emergency planning team and is superimposed on floodplain maps in order to identify priority institutions which may need evacuating during times of flood.
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13 References
Department for Communities and Local Government, 2006. Planning Policy Statement 25: Development and Flood Risk. TSO publications, Norwich Department for Communities and Local Government, 2007. Development and Flood Risk: ‘ ’ A Practice Guide Companion to PPS25 Living Draft , A Consultation Paper. Department for Communities and Local Government; Development and Flood Risk A Practice Guide Companion to PPS25 Living Draft DEFRA 2005. Flood Risk Assessment Guidance for New Development - Phase 2. Framework and Guidance for Assessing and Managing Flood Risk for New Development – Full Documentation and Tools. DEFRA and Environment Agency R&D Technical Report FD2320/TR2 Environment Agency Bristol Avon Catchment Flood Management Plan, Draft Plan May 2007
Environment Agency- Thames Catchment Flood Management Plan, Summary Document, Consultation 2007 HMSO. 2004. Planning and Compulsory Purchase Act 2004. Published by the Queen's Printer of Acts of Parliament. ISBN: 0 10 540504 3
South West Regional Flood Risk Appraisal, February 2007, South West Regional Assembly
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14 Figures
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