Welwyn Hatfield Borough Council Strategic Flood Risk Assessment Final Level 1
Final Report May 2009
Prepared for:
Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment
Revision Schedule
Welwyn Hatfield Strategic Flood Risk Assessment
Rev Date Details Prepared by Reviewed by Approved by
01 16/11/07 WHBC Draft Anna Samuel Stephen Riley Jon Robinson Level 1 Hydrology Specialist Senior Consultant Associate Director SFRA_rev01
Gemma Costin Assistant Flood Risk Specialist
Gemma Hoad Assistant Water Scientist
02 03/0708 WHBC FINAL Anna Samuel Stephen Riley Jon Robinson Level 1 Hydrology Specialist Senior Consultant Associate Director SFRA_rev 2 issued 01-08-08 03 08/01/09 WHBC FINAL Anna Samuel Jon Robinson Jon Robinson Level 1 Hydrology Specialist Associate Director Associate Director SFRA_rev 3 issued 08-01- Clare Postlethwaite 09.doc Consultant Water Scientist
Gemma Hoad Assistant Water Scientist
04 19/05/09 WHBC FINAL Helen Judd Jon Robinson Jon Robinson SFRA_rev 4 Assistant Hydrologist Associate Director Associate Director
Scott Wilson Scott House Alençon Link Basingstoke
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Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment
Table of Contents Abbreviations ...... i Glossary...... ii Executive Summary ...... iv Introduction ...... iv The Welwyn Hatfield Borough Council Study Area...... iv Level 1 SFRA Methodology...... iv Flooding in the Borough ...... v PPS25 Sequential Test and Exception Test ...... v Flood Risk Management...... vi Drainage of Development Sites...... vi Policy Recommendations ...... vi Site-Specific FRA Guidance ...... vi Emergency Planning ...... vi Recommendations ...... vii 1 Introduction ...... 1 1.1 The Welwyn and Hatfield Borough Council SFRA...... 1 1.2 SFRA Structure ...... 2 1.3 Aim of the SFRA...... 3 1.4 Level 1 SFRA Objectives...... 3 1.5 Level 1 SFRA Report Synopsis...... 4 2 The Welwyn Hatfield Borough Council SFRA Study Area ...... 5 2.1 Study Area and Land Use...... 5 2.2 Local Watercourses...... 5 2.3 Geology...... 8 2.4 Hydrogeology ...... 8 2.5 Flood Sources in the Study Area ...... 8 3 Policy Context ...... 11 3.1 European Policies (EU) ...... 11 3.2 National Policies ...... 12 3.3 Regional Policies ...... 14 3.4 Local Policies...... 14 3.5 Environment Agency Policies ...... 18 3.6 Other Relevant Policies ...... 22 Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment
4 Data Collection & Review...... 24 4.1 Overview ...... 24 4.2 Fluvial Flood Data...... 25 4.3 Historic Data/Records...... 28 4.4 Flood Defences ...... 29 4.5 Topographic Data ...... 29 4.6 Pluvial Flooding ...... 29 4.7 Groundwater...... 30 4.8 Sewer Flooding...... 31 4.9 Artificial Sources / Infrastructure Failure ...... 32 4.10 Flood Risk Assessments...... 32 5 Strategic Flood Risk Mapping...... 34 5.1 Requirements of PPS25 ...... 34 5.2 GIS Mapping...... 34 5.3 Fluvial Flooding ...... 35 5.4 Sewer Flooding/ Surface Water Flooding...... 37 5.5 Pluvial Flooding (Overland Flow) ...... 37 5.6 Groundwater Flooding ...... 38 5.7 Artificial Sources...... 39 5.8 Historical Flooding ...... 39 5.9 Residual Risk...... 41 6 Guidance on Applying the PPS25 Sequential Test...... 44 6.1 What is the Sequential Test? ...... 44 6.2 How should the SFRA be used to apply the Sequential Test? ...... 46 6.3 Additional Guidance...... 48 7 Guidance on Applying the PPS25 Exception Test ...... 50 7.1 What is the Exception Test? ...... 50 7.2 Why is there an Exception Test? ...... 50 7.3 What is Required to Pass the Exception Test? ...... 50 8 Flood Risk Management...... 53 8.1 Flood Warning Areas...... 53 8.2 Flood Defences ...... 53 8.3 Flood Risk Operational and Water Level Management ...... 55 8.4 Flood Proof Construction ...... 55 9 Drainage of Development Sites...... 57 Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment
9.1 Principles...... 57 9.2 What are SuDS?...... 57 9.3 SuDS Policies...... 58 9.4 SuDS Methods ...... 61 9.5 Where can SuDS be utilised? ...... 63 9.6 SuDS Constraints ...... 63 10 Policy Recommendations...... 66 10.1 Flood Risk ...... 66 10.2 Flood Risk Management...... 67 10.3 Sustainable Drainage Systems...... 68 10.4 Flood Risk & Environment ...... 69 11 Site Specific FRA Guidance ...... 71 11.1 When are Flood Risk Assessments Required?...... 72 11.2 FRA Requirements ...... 72 11.3 Specific Requirements within the Flood Risk Guidance Table ...... 74 11.4 Consequences of Failing to Adequately Consider Flood Risk ...... 77 12 Emergency Planning ...... 79 12.1 Developing an Emergency Flood Plan...... 79 13 Recommendations ...... 82 13.1 Further Work...... 82 13.2 How and when the SFRA should be updated?...... 83 13.3 Level 2 SFRA ...... 83 14 References...... 85 Appendices ...... 87 Appendix A – Figures...... 88 Appendix B – Historical Records of Flooding ...... 89 Appendix C - Data/Document Registers...... 94 Appendix D - Tables...... 102 Appendix E - SuDS Suitability based on Geology, Hydrogeology and Soils in the SFRA Study Area...... 106 Appendix F: Policy Text ...... 110 Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment
Abbreviations ACRONYM DEFINITION ABD Areas benefiting from defences AONB Area of Outstanding Natural Beauty AOD Above Ordnance Datum BREEAM2 BRE Environmental Assessment Method 2 CFMP Catchment Flood Management Plan DEM Digital Elevation Model DPD Development Plan Documents EA Environment Agency FAS Flood Alleviation Scheme FRA Flood Risk Assessment GIS Geographical Information Systems IDB Internal Drainage Board LDDs Local Development Documents LDF Local Development Framework LDS Local Development Scheme LiDAR Light Detection and Ranging LPA Local Planning Authority ODPM Office of the Deputy Prime Minister PCPA Planning and Compulsory Purchase Act 2004 PPG25 Planning Policy Guidance Note 25: Development and Flood Risk PPS25 Planning Policy Statement 25: Development and Flood Risk RFRA Regional Flood Risk Appraisal RPG Regional Planning Guidance RSS Regional Spatial Strategy SAR Synthetic Aperture Radar SA Sustainability Assessment SFRA Strategic Flood Risk Assessment SPG Supplementary Planning Guidance Location where the Convention on Wetlands was signed in 1971. This is an intergovernmental treaty which provides the framework for national RAMSAR action and international cooperation for the conservation and wise use of wetlands and their resources. SSSI Site of Special Scientific Interest SuDS Sustainable Drainage Systems WHBC Welwyn Hatfield Borough Council
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Glossary
TERM DEFINITION A source of groundwater comprising water-bearing rock, sand or gravel capable Aquifer of yielding significant quantities of water. The process of providing temporary water storage capacity during flood events to Attenuation reducing peak flows Catchment A high-level planning strategy through which the Environment Agency works with Flood key decision makers within a river catchment to identify and agree policies to Management secure the long-term sustainable management of flood risk. Plan Natural processes and human actions causing long term variations in global Climate Change temperature and weather patterns. Culvert A covered channel or pipe that conveys water below the ground surface. Engineered structures and/or managed areas used to protect an area against Flood defence floods. Can take several forms including floodwalls, embankments, floodplains alleviation and storage areas. They are typically ‘designed’ to a standard of protection (design standard). Floodplain Area adjacent to river, coast or estuary that is naturally susceptible to flooding. Flood storage A temporary area that stores excess runoff or river flow often ponds or reservoirs. areas Fluvial flooding Flooding by a river or a watercourse. Groundwater Water that is in the ground, below the water table. A map that delineates the areas that have been predicted to be at risk of flooding Flood plain map during an event of specified probability. Functional An area that has a 5% annual probability of flooding ( 1 in 20 years) from tidal or floodplain fluvial sources. A method by which surface water runoff is disposed to ground through the use of Infiltration structures such as soakaway pits and swales. Thus reducing runoff from the site. Independent bodies set up under the Land Drainage Act 1991. Internal Drainage Internal Boards have permissive powers for the management of drainage on ordinary Drainage Board watercourses within a specified district. Inundation Flooding. Local The LDF is a folder of the Local Development Documents prepared by district Development councils, unitary authorities or national park authorities that outline the spatial Framework planning strategy for the local area. (LDF) Local Planning Body that is responsible for controlling planning and development through the Authority planning system. Watercourses defined on an Environment Agency Main River Map, managed for Main River flow regulation purposes. Mitigation An element of development design which may be used to manage flood risk or Measure avoid an increase in flood risk elsewhere. Surface water flow from rainfall that is unable to soak into the ground or enter Overland Flow drainage systems, and which literally flows over land. Permissive Statutory powers that may be exercised at the discretion Powers of a competent authority.
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TERM DEFINITION Flooding derived from excess rainfall, either that which is not able to infiltrate the Pluvial flooding ground or which exceeds the capacity of piped drainage systems. Risk is the product of the probability of and event occurring and the consequence Risk of that. A risk based approach in to assessing flood risk, which gives priority in ascending Sequential Test order of flood risk, i.e. lowest risk first. Flooding caused by a blockage or overflowing in a sewer or urban drainage Sewer Flooding system. Stakeholder A person or organisation that has a vested interest in, or is affected by a decision. SAs promote sustainable development through the integration of social, Sustainability environmental and economic considerations into the preparation of revisions of Appraisal Regional Spatial Strategies (RSS) and for new or revised Development Plan Documents (DPD) and Supplementary Planning Documents (SPD). SuDS objectives are, to minimise the impacts from the development on the Sustainable quantity and quality of the runoff using a variety of techniques, and maximise drainage system amenity and biodiversity opportunities. Sustainable Development that meets the needs of the present without compromising the development ability of future generations meeting their own needs. Flood defence that is designed for an event, which is expected to occur 1 in x Design standard number of years. In events more severe than this the defence would be expected to fail or to allow flooding.
RETURN PERIOD (1 ANNUAL EXCEEDENCE IN X YEARS ) PROBABILITY (AEP) 2 50.0% 5 20.0% 10 10.0% 20 5.0% 25 4.0% 30 3.3% 50 2.0% 75 1.3% 100 1.0% 200 0.5% 500 0.2% 1000 0.1%
These return periods are the classifications used in PPS25 (Reference 2) to define the Flood Zones:
• Flood Zone 1 – Return Period of >1000 years, AEP <0.1%; • Flood Zone 2 – Return Period of up to 1000 years, AEP 0.1%; • Fluvial Flood Zone 3a – Return Period of up to 100 years, AEP 1%; • Flood Zone 3b – Return Period of up to 20 years, AEP 5%.
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Executive Summary
Introduction
Scott Wilson Limited was commissioned in 2007 to prepare a Level 1 Strategic Flood Risk Assessment (SFRA) for Welwyn Hatfield Borough Council (WHBC). According to Government guidance document Planning Policy Statement 25: Development and Flood Risk (PPS25) all land-use planning should have regard to flood risk. In allocating future developments the Local Planning Authority should consider the flood risk to and arising from potential development sites, both under the existing climate and taking into account the predicted effects of climate change.
The Welwyn Hatfield Borough Council Study Area
The Welwyn and Hatfield Borough Council SFRA study area is defined by the administrative boundary of WHBC. It is situated approximately 25km to the north of London, covering an area of 129km² and an estimated population of 105,500 (2006). Revisions to the East of England Plan now give the borough a development target of 10,000 dwellings up to 2021 with the detailed distribution of this development to be left to the local council to decide.
Level 1 SFRA Methodology
This Level 1 SFRA is based on existing published information held by local stakeholders including the Environment Agency, WHBC, Hertfordshire County Council, Three Valleys Water, Thames Water, Hertfordshire Highways and Hertfordshire Fire and Rescue. PPS25 requires that SFRAs examine the risk of flooding to the study area from a number of sources, which include: • Fluvial flooding • Sewer flooding • Groundwater flooding • Pluvial flooding • Flooding from artificial sources Data concerning each source of flooding has been analysed and requirements for the assessment of the probability of flooding have been applied where possible. The flooding probability categories follow the same principles as those set out in PPS25 for fluvial and tidal flooding, providing a three stage classification of high, medium and low probability. Climate change is predicted to increase peak river flows and rainfall intensity. Consequently it is predicted that the risk from flooding will increase in the future consequently the impacts of climate change have been assessed where possible. As part of the Level 1 SFRA, national, regional and local policy documents were reviewed for flood and drainage related policies. There are a number of polices that guide development but they predominately follow the principles of PPS25. The policies reviewed were used in advising WHBC of future policy recommendations for inclusion in the Local Development Framework (LDF).
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Flooding in the Borough Fluvial Flooding
The study area contains a number of watercourses, predominately draining from the north west to the south east. The principle watercourses in the study area include the River Lee, the Upper Colne, Mimmshall Brook and the River Mimram. These watercourses are therefore the main sources of fluvial flooding. Due to the topography of the borough the floodplains associated with these watercourses are relatively constrained. Current Flood Zones have been mapped for all of these watercourses, climate change scenarios have only been mapped for Flood Zone 3a. Sewer Flooding
Thames Water provided only one record of sewer flooding across the whole borough in the last 10 years. Due to the coarse resolution of the data it was decided in consultation with the Environment Agency to discount this source of flooding on the Sequential Test maps as it would prejudice planning in an area of the borough, the size of which is disproportional to the flood risk. Pluvial Flooding
There are no records of pluvial flooding affecting the study area and therefore this risk of flooding from this source is currently assessed as low. Due to the lack of recorded incidents of pluvial flooding, definition of flooding probability cannot be defined. Groundwater Flooding
The limited number of groundwater flooding records for the study area prevents the classification of probability bands (as required by PPS25) on this data alone. Artificial Sources
There are no records of flooding associated with artificial sources and no data on which to classify flooding probability. Consequently flood risks from artificial sources should be considered as low.
PPS25 Sequential Test and Exception Test
The Level 1 SFRA presents guidance to WHBC on applying the Sequential Test. It outlines the criteria required under PPS25 and its Practice Guide and provides additional guidance that expands on the Sequential Test flow diagram and makes recommendations to WHBC on the steps in applying the Sequential Test. In some circumstances where the Sequential Test has not been fully satisfied the Exception Test may need to be applied. The Exception Test is a set of three criteria outlined in PPS25 that seeks to ensure that where development in the floodplain is necessary. The benefits of doing so should outweigh the risks.
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Flood Risk Management
Flood management measures are those measures put in place to reduce (not remove) the risk to people and property from the hazard of flooding. The SFRA identifies that currently there are no flood defences to the design standard of the 1% annual probability flood event located within the study area and there are no proposals to construct any in the future.
Drainage of Development Sites
The SFRA provides details on sustainable Drainage Systems that can be incorporated into development specific SUDS policies. SuDS seek to manage surface water from new developments as close to its source as possible, mimicking the surface water flow regime present on a site prior to development. Typically this approach involves a move away from conventional piped systems to softer engineering solutions inspired by natural drainage processes. As part of planning applications for both allocated and non-allocated sites, site specific FRA will be required in areas at risk of flooding, 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.
Policy Recommendations
To ensure developments promoted under the Local Development Framework achieve the aims of PPS25 and in light of the information generated through the Level 1 SFRA a number of policies have been recommended. These address flood risk in the WHBC by ensuring that the Sequential Test requirements are considered and provide guidance on which planning applications should be accompanied by Site-Specific Flood Risk Assessments.
Site-Specific FRA Guidance
The Level 1 SFRA provides information on flood risk issues at a strategic level. The presence of a Level 1 SFRA does not preclude the requirement for site specific Flood Risk Assessments. PPS25 identifies several instances where a Site-Specific FRA should be prepared and where the Environment Agency should be consulted. However, this does not cover all flood sources, consequently guidance is presented to advise developers and Development Control officers of the instances, outside of the requirements of PPS25, where a Site-Specific FRA may be required. The SFRA provides guidance for Site-Specific FRAs based on the knowledge gathered during the preparation of the Level 1 SFRA; the criteria set out in PPS25; and the existing local policies. A Flood Risk Assessment Guidance table is presented that acts as a reference for those involved with preparing or approving planning applications.
Emergency Planning
Proactive planning is an essential aspect in reducing the overall flood risk to communities, however for those established communities already at risk of flooding. Local Authorities should ensure flood risks are appropriately understood and that robust emergency plans are in place to deal with flood events. The information presented in the Level 1 SFRA should be used by the Local Authority and the Local
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Emergency Planning Forum to identify those vulnerable members of the communities and institutions that may be at risk during a flood event.
Recommendations
Recommendations have been made to WHBC based on the findings of the Level 1 SFRA. It advises that the recommended policies should be included in the emerging Local Development Framework which includes the consideration of SuDS and the preservation of floodplain. It highlights that as well as an impact on flood risk, development in WHBC will also impact on water resources. It recommends that the water resource policies in current legislative documents should be referred to when planning land-use.
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1 Introduction
The Planning and Compulsory Purchase Act 2004 (PCPA 2004) (Reference 1) 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 a council administered area. The PCPA 2004 (Reference 1) requires LDFs and their contributing documents to undergo a Sustainability Appraisal (SA), which assists a Local Planning Authority (LPA) in ensuring that their policies fulfil the principles of sustainability. Strategic Flood Risk Assessments (SFRAs) form part of the evidence base used in the SA process and should be used in the review of LDDs or in their production. The release of Planning Policy Statement 25: Development and Flood Risk in December 2006 (PPS25) (Reference 2) emphasised the responsibility that LPAs have to ensure that flood risk is understood and managed effectively using a risk-based approach as an integral part of the strategic planning process. PPS25 encourages LPAs to undertake SFRAs and to use their findings and those of other studies to inform strategic land use planning.
To assist LPAs in their strategic land use planning, SFRAs should present sufficient information to enable the LPA to apply the Sequential Test (detailed in PPS25) to their proposed development sites. The Sequential Test seeks to guide development to areas of lowest flood risk or, to where necessary, to ensure development vulnerability is appropriate to the flooding probability of an area. To achieve this, SFRAs should have regard to river catchment wide flood issues and also involve a process which allows the LPA 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 (Reference 2) (i.e. to steer development to low risk sites): “The scope of the SFRA should be increased to provide the information necessary for the application of the Exception Test.” (Reference 2)
1.1 The Welwyn and Hatfield Borough Council SFRA
The Regional Spatial Strategy (RSS) for the East of England (Reference 3) has identified growth areas within the area administered by Welwyn Hatfield Borough Council (WHBC). Revisions to the East of England Plan now give the borough a development target of 10,000 dwellings up to 2021 with the detailed distribution of this development to be left to local council to decide. The growth areas are principally focused on the areas of Welwyn Garden City and Hatfield. In addition Welwyn Garden City and Hatfield have been designated as a key centre for development and change in the East of England Plan. Policy SS7 Green Belt requires a strategic Green Belt review involving land in the borough of Welwyn Hatfield and potentially St Albans District. The spatial planning of the administrative area of WHBC must have regard to the current and future risk of flooding. Flooding can occur from a number of sources, it is therefore vitally important that flood risk is considered at a strategic scale to inform land allocations and future developments proposed by the emerging Local Development Frameworks. The growth areas also contain significant areas of Green Belt; 79% of land within the borough is countryside and designated as Green Belt, consequently, it may be necessary to accommodate some
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development in areas of Green Belt. Robust justification will be required before development in Green Belt areas can occur; however, the need to avoid future flood risk may be a contributing factor to development in Green Belt areas.
1.2 SFRA Structure
The Practice Guide Companion to PPS25 (Reference 4) recommends that SFRAs are completed in two consecutive stages. This provides the LPA with tools throughout the LDF and SFRA process sufficient to inform decisions regarding proposed development sites. The two stages are: • Level 1 SFRA – Study Area Flood Source Review & Sequential Test • Level 2 SFRA – Development Site Assessments for Exception Testing. When applied by the LPA, 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.
1.2.1 Level 1 – Study Area Flood Source Review & Sequential Test
A Level 1 SFRA should present sufficient information to enable the LPA to apply the Sequential Test to potential development sites and to assist in identifying if application of the Exception Test will be necessary. The Level 1 SFRA also provides background information and a review of local policies and information on the potential for application of Sustainable Drainage Systems (SuDS). The review of polices is allied to guidance on the requirements for site-specific Flood Risk Assessments (FRA) throughout the study area. The deliverables from Level 1 should be used by the LPA to identify the most suitable locations for development (in line with PPS25 and other planning drivers). Where sites cannot be located in line with the principles of the Sequential Test further investigation may be required through a Level 2 SFRA. The Level 1 SFRA is based on existing published information held by local stakeholders. In some parts of the study area insufficient information is available for a complete assessment in line with the full requirements of PPS25. Due to this lack of data, in some cases and depending on the location of allocations promoted by WHBC, further investigation may be required during the Level 2 SFRA.
1.2.2 Level 2 - Development Site Assessments for Exception Testing
The objective of a Level 2 SFRA is to use information obtained in the Level 1 SFRA where suitable, and additional works where necessary, to reduce uncertainty in available information regarding flood risk to those proposed developments/development sites that cannot be located in low flood risk zones. This information should be used to supplement the LPA in developing justification for development in flood risk areas through application of the PPS25 Exception Test. The information presented for each development site should be sufficient to: ‘demonstrate a development site is ‘safe’, in line with the requirements of the Exception Test’ (Reference 2).
Due to the challenging growth targets faced by WHBC some of the growth may need to be accommodated in areas of Green Belt and potentially areas currently at risk of flooding. The Level 2
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SFRA should present sufficient information to enable the LPA to fully understand the flood risk to developments they are proposing in flood risk areas.
1.3 Aim of the SFRA
The aim of the WHBC SFRA is to present sufficient information to enable WHBC to apply the Sequential Test (explained further in Section 6) to site allocations and to assist in identifying if application of the Exception Test (Section 7) will be necessary. In addition the SFRA should form a reference document for use by development control officers for advising and determining decisions on windfall sites. If, following the application of the Sequential Test, development cannot be located in areas of flood risk which are immediately compatible with the development’s vulnerability classification, the LPA may be required to justify the development through application of the Exception Test, as defined in paragraph 18 of PPS25 (Reference 2) . The Exception Test allows for necessary development while managing flood risk. A Level 2 SFRA will aim to assist the LPA in understanding the flood risk posed to developments in the floodplain, by presenting information sufficient to determine if proposed developments will be safe from the risks of flooding for their lifetime.
1.4 Level 1 SFRA Objectives
In keeping with guidance presented in the PPS25 Practice Guide (Reference 4) a staged approach to the SFRA has been adopted to achieve the aim of the SFRA. The objectives of the Level 1 WHBC SFRA are: • To provide an assessment of flood risk to the Borough of Welwyn Hatfield; • Identify the extent of all PPS25 Flood Zones; • Identify areas at risk of flooding from all flood sources present in the study area; • Provide an evidence-based report which informs WHBC’s Local Development Framework and other Development Planning Documents about managing potential flood risk which are also suitable to inform the Sustainability Appraisal of related documents; • Advise WHBC on suitable polices to address flood risk management in their administrative area; • Advise WHBC on the requirements of site specific flood risk assessments based on local flooding issues and policy recommendations; • Advise WHBC on the principles and applicability of Sustainable Drainage Systems throughout the study area; and, • Present sufficient information to inform WHBC of the flood considerations necessary in emergency planning.
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1.5 Level 1 SFRA Report Synopsis
This report presents the information generated during Level 1 of the SFRA. Section 2 of the report provides an overview of the WHBC SFRA study area, Section 3 of the report provides an overview of the planning policy framework relevant to the WHBC SFRA. Section 4 of the report describes the data collection process, presents the available data and discusses its benefits and limitations. Section 5 illustrates how the available data has been used in the production of mapping and GIS (Geographical Information Systems) deliverables to meet the requirements of PPS25. Section 6 should be used by the LPA and developers seeking to understand and apply the Sequential Test, and Section 7 should be employed by developers, development control and strategic planning officers where application of the Exception Test is required. Section 8 identifies the flood risk management measures in place within the SFRA study area, and Section 9 summarises where and how SuDS can be utilised. Section 10 provides catchment wide and specific area strategies that have been developed under the headings Flood Risk, SuDS, Flood Mitigation and the Water Environment. Section 11 provides site specific FRA advice, which should be utilised by developers and LPAs. Section 12 illustrates the emergency planning issues that should be considered in tandem with flood risk whilst Section 13 provides recommendations for further work.
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2 The Welwyn Hatfield Borough Council SFRA Study Area
2.1 Study Area and Land Use
The WHBC SFRA study area is defined by the administrative boundary of WHBC, situated approximately 25km to the north of London, covering an area of 129km² (Figure 1). The borough is surrounded by the local authorities of North Hertfordshire, St Albans, Broxbourne, Hertsmere, and East Hertfordshire. The south of the borough borders the London Borough of Enfield. The borough has an estimated population of 105,500 (2006) and includes the major urban areas of Hatfield (in the centre of the borough) and Welwyn Garden City (to the north), which is the largest settlement in the borough with a population of 43,310 (Reference 5). Other urban settlements in the borough include Brookman’s Park, Oaklands, Mardley Heath and Cuffley. Outside of these urban areas there are large tracts of countryside, Green Belt land and smaller settlements and villages. Seventy nine percent (79%) of WHBC is covered by Green Belt land and carries with it nature conservation designations such as those listed below which potentially restrict planning in some areas. Nature designations found in the borough include: • Sites of Special Scientific Interest (SSSI); • Local Nature Reserves; • Wildlife Sites; and • Part of a Special Area of Conservation (SAC).
2.2 Local Watercourses
The study area contains a number of watercourses, predominately draining from north west to south east. The principle watercourses in the study area are the: • River Lee; • Upper Colne; • Mimmshall Brook and • River Mimram; The locations of the watercourses, their associated catchments and responsible drainage authority are illustrated in Figure 2.
2.2.1 River Middle Lee
The River Lee is sourced on the Chilterns at a spring in Leagrave in north Luton. It flows south through Luton receiving contributions from Lewsey Brook, Houghton Brook and Cat Brook. It also receives flows from East Hyde Sewage Treatment Works upstream of Wheathampstead (outside the study area). South of Wheathampstead it is known as the River Middle Lee and remains so throughout the study area.
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The River Lee catchment from its source to its confluence with the River Mimram includes an area of 167 km 2. Within the study area the catchment includes the majority of Hatfield, southern areas of Welwyn Garden City and the rural area in the east, incorporating the settlements of Essendon and Cuffley. The River Middle Lee enters the study area in the west, near Warren Farm and flows in a south- easterly direction towards Welwyn Garden City. It flows through parkland near Lemsford and then under the A1(M) and B197. It flows adjacent to Stanborough Park and the former Splashland Complex in the south of Welwyn Garden City where a number of flow control structures (which include a weir, several culverts and bridges) are located on the watercourse. It continues to flow between Welwyn Garden City and Hatfield under the railway line and under the A414. Here, the Woodhall Farm tributary joins the River Middle Lee from the north. The River Lee continues to flow through Home Park (part of Hatfield House Grade I listed Historic Park and Garden) where it separates and rejoins twice. It continues to flow in an easterly direction, passing isolated farms parallel to the A414. The Hatfield Hyde Brook (from the north) and Essendon Brook (from the south) converge with the River Middle Lee before it flows out of the study area. The River Middle Lee flows for approximately 11 km within the WHBC administrative area. Out of the study area it becomes the River Lower Lee at Fieldes Weir. It is a principle tributary of the River Thames, reaching its confluence with the River Thames approximately 3km upstream of the Thames Barrier.
2.2.1.1 River Mimram
The River Mimram is a chalk stream that rises from its source near Codicote which is to the west of the study area. The River Mimram catchment includes an area of approximately 82km 2. The catchment covers the majority of the north of the study area including the majority of Welwyn Garden City, Welwyn and Oaklands. It flows for approximately 8 km within the study area. The River Mimram flows along the study area boundary in the northwest of the borough from Codicote Bottom to the northwest of Welwyn. Here it flows out of the study area and re-enters approximately 0.3 km downstream. From this location it flows in a south-easterly direction through the historic town of Welwyn and under the A1(M). It continues to flow in an easterly direction between Digswell to the north and Welwyn Garden City to the south. The River Mimram leaves the study area at Digswell Water and converges with the River Lee in Hertford. The Upper Lee CAMS (Reference 16) notes that the River Mimram has been recorded to have insufficient flows to support ecological needs, even in times of higher flows.
2.2.1.2 Upper Colne
The Upper Colne is located in the southwest of the study area. It flows from near the A1(M) in a westerly direction through North Mimms park and exits the study area at Roestock. Only the upper reaches of the River Colne are located within the study area but it has several associated tributaries extending through the study area including the Mimmshall Brook, Ellen Brook and Nast. The Upper Colne itself only flows through the study area for 2 km however the catchment area covers the majority of the southwest of the study area. This includes Brookmans Park, Welham Green, and south and west areas of Hatfield.
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2.2.1.3 Mimmshall Brook
The Mimmshall Brook is a chalk stream and a tributary of the River Colne. It rises to the southwest of the study area near Borehamwood, within the borough of Hertsmere. It flows north from here and enters the study area adjacent to the east of the A1(M) where it joins the River Colne. The Mimmshall Brook catchment is predominately rural and flows through isolated farms and small villages.
Mimmshall Brook has resulted in serious flooding to properties on five occasions since 1928. Studies into the flooding caused by the Mimmshall Brook has resulted in the construction of the Mimmshall Brook Flood Alleviation Scheme (FAS). This FAS provides protection against the 2% annual probability fluvial flood event to 13 bungalows on Warrengate Road and an electricity substation. This standard of protection has been achieved through the construction of a raised flood defence wall along the left bank of the watercourse and a mitred flood gate on the road, the construction of this FAS was completed in March 2004. 2.2.2 Other Main Rivers
The Environment Agency is the responsible authority for Main Rivers within the study area. All the rivers described above are Main Rivers. In addition the following watercourses are also classified as Main Rivers and are tributaries of the rivers discussed previously. Table 2-1 provides a summary of river catchments within the study area and the river catchments that they fall under: • Turkey Brook – Rises to the east of Potters Bar in the southeast of the study area, it flows in an easterly direction and meets the River Lee Navigation at Enfield Lock; • Northaw Brook – Located to the east of Potters Bar in the southeast of the study area and is a tributary of Cuffley Brook; • Hempshill Brook – Located to the east of Potters Bar in the southeast of the study area and is a tributary of Northaw Brook; • Cuffley Brook – Rises in Northaw Country Park and flows east to the outskirts of Cuffley and out of the study area. It is a major tributary of the River Lee converging with the River Lee outside of the study area; • Essendon Brook – A tributary of the River Lee (Upper). It rises near Warrenwood Park and flows north west of Essendon and joins the River Lee; • Wildhill Brook – A tributary of Essendon Brook that rises near Wildhill; • Ellen Brook - A tributary of the River Colne that flows to the south west and west of Hatfield; • Nast – A tributary of the River Colne that flows to the west of Hatfield; and, • Hatfield Hyde Brook – Rises from three sources in the south of Welwyn Garden City before flowing south to its confluence with the River Lee (Upper) near Burnside.
Table 2-1 : Rivers Catchments and Associated Rivers located within Welwyn Hatfield Borough.
RIVER CATCHMENT RIVERS River Lee Middle Lee Woodhall Farm Tributary (section of the River Lee)
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RIVER CATCHMENT RIVERS Hatfield Hyde Brook Wildhill Brook Essendon Brook River Mimram River Mimram Mimmshall Brook River Colne Ellen Brook Nast Turkey Brook Northaw Brook Lee-Stort / Thames Hempshill Brook Cuffley Brook
2.3 Geology
The solid geology of the study area consists of Upper Chalk in the north with the Lambeth Group outcropping through the centre of the borough with outcrops of London Clay to the south (Figure 3). The drift geology consists predominantly of River Terrance Deposits, Alluvium and Till (Figure 4) (Reference 6). Appendix E details the solid geology, drift geology and the soil types found throughout the study area.
There are a number of sinkholes within the WHBC SFRA study area, most notably to the south of Welham Green at Water End. Sinkholes are depressions in the ground of various sizes that capture overland flow. They are normally found in chalk or limestone landscapes connecting to underlying aquifers. They are a permanent feature of the landscape and constitute a drainage outlet for the enclosed karstic (Chalk) basin within the study area. Natural England have noted that ‘at times of flood a lake with surface water overflow accumulates in the sinkhole basin, but eventually it drains back into the natural sinks.’ (Reference 8)
2.4 Hydrogeology
The hydrogeology of the study area is primarily influenced by the unconfined Upper Chalk, which is classed as a major aquifer and is a significant water resource for the surrounding towns and villages. To the south east of the study area the chalk is overlain by London Clay, Pebble Gravel and Reading Beds. The Pebble Gravel and Reading Beds are classified as minor aquifers with the London Clay classified as a non-aquifer (Reference 7).
2.5 Flood Sources in the Study Area
2.5.1 Fluvial
Rivers flood when the amount of water in them exceeds the flow capacity of the river channel or when culverts and bridges are blocked by debris. Climate change is forecast to increase peak flows by 20% by 2115, resulting in higher peak flows within channels and consequently an increase in the occurrence of flooding.
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2.5.2 Sewers
Flooding from sewers occurs when the sewage system (combined and surface water sewers) is exceeded by heavy rainfall, if the system becomes blocked or is of inadequate capacity, and will continue to remain flooded until the water drains away. Sewer systems are typical to all the urban centres in the study area. Modern sewer systems are typically designed to accommodate rainstorms with a 30 year return period (Reference 9). Older sewer systems were often constructed without consideration of a design standard and may in some areas (served by Victorian sewers) have an effective design standard of less than 30 years. Consequently rainstorm events with a return period greater than 30 years would be expected to result in flooding of some parts of the sewer system. In addition, as towns and villages have expanded to accommodate growth, their original sewer systems have rarely been upgraded, eventually becoming overloaded and reducing their effective design standard of 30 years. Compounding this problem are the predicted effects of climate change. Climate change is forecast to result in milder wetter winters and increased rainfall intensity in summer months. This combination will increase the pressure on existing sewer systems effectively reducing their design standard, leading to more frequent flooding.
2.5.3 Groundwater
Groundwater flooding is a result of water levels in the ground rising above the ground surface. Groundwater flooding is most likely to occur in low-lying areas underlain by permeable rocks. A large proportion of the study area is underlain by aquifers which have the potential to cause groundwater flooding.
2.5.4 Pluvial
Pluvial flooding or overland flow is often a result of intense rainfall, which is unable to soak into the ground or enter drainage systems. Areas of steep ground or impermeable geology / pedology (soil) have the potential to generate runoff which can become a flood source. See section 2.3 and Figures 4 and 5 for a description of the geology of the study area. Parts of the study area have been identified as having relatively steep topography. These areas include land around Cuffley, East Hatfield, Ayot St Lawrence and North Welwyn Garden City. Consequently these areas may generate overland flow during rainfall events which could present a flood source to areas down slope of them (see Figure 10).
2.5.5 Artificial Sources
Artificial sources include any water bodies not covered by the previous categories. This typically includes canals, lakes, reservoirs and operational and redundant industrial processes including mining, quarrying and sand and gravel extraction sites. Under the Reservoirs Act 1975 the following raised structures have also been identified in the borough: • Brocket Hall Lake, Lemsford, Nr Welwyn Garden City • The Broadwater, Welwyn Garden City.
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From a review of available information and Ordnance Survey mapping there are lakes and gravel pits located within the WHBC administrative area. Those known are: • Stanborough Lake – located in Welwyn Garden City; and • Sand and gravel pits near Coopers Green.
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3 Policy Context
This section provides an overview of the planning policy framework relevant to the WHBC SFRA. Information contained in this SFRA will inform the preparation of sustainable policies for flood risk management to be incorporated into the Core Strategy and Development Plan Documents DPDs. The SFRA should also be used to inform the Sustainability Appraisal of Local Development Documents (LDDs) and will facilitate informed decision-making relating to land use and development allocation within the respective Development Plan Documents (DPDs). PPS25 (Reference 2) was reviewed as the key flood risk and development policy at a national level. This was followed by the draft Regional Spatial Strategy (RSS) for the East of England (Reference 3) and the May 2008 update of this document, and the subsequent Secretary of State’s Proposed Changes to the Draft Revision to the Regional Spatial Strategy for the East of England (Reference 2), the Secretary of State’s Proposed Changes and further Proposed Changes to the Draft Revisions to the RSS for the East of England, October 2007 and May 2008. The RSS for the East of England will supersede the Regional Planning Guidance for the South East RPG 9 (Reference 10), which was also reviewed as part of this SFRA. WHBC must consider a number of planning policies (of which flood risk is only one) to ensure developments are sustainable in the long term. In consideration of these polices WHBC must decide on the ‘weight’ to attribute to each policy in determining the suitability of development in their administrative area.
3.1 European Policies (EU)
3.1.1 EU Water Framework Directive
The EU Water Framework Directive was developed following a review of EU water policy. It seeks to restore and improve water quality in rivers, coastal waters and groundwater in an integrated way. It seeks to achieve ‘good ecological status’ of water bodies through integrated river basin management. This is a method of ensuring all requirements and pressures on the water environment are taken into account within a river basin. The implications of the Water Framework Directive on flood risk are likely to include controls on the type of flood alleviation schemes that can be implemented and that any flood alleviation schemes should also contribute to achieving ‘good ecological status’ through methods such as restoration of floodplains to their natural state and purpose.
3.1.2 European Union Floods Directive
The European Directive on the Assessment and Management of Flood Risks came into force on the 26 th November 2007. Under the directive Member States have 2 years to transpose the directive into domestic law and a further six years to satisfy its requirements. The directive requires Member States to designate competent authorities to implement the Directive; for England, this will be the Environment Agency. The directive requires the following elements to be undertaken: • Preliminary flood risk assessments to identify areas that are at potentially significant flood risk, to be completed by 20 December 2011; • Flood hazard maps (showing the likelihood and flow of the potential flooding) and flood risk maps (showing the impact), to be completed by 20 December 2013;
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• Flood risk management plans (showing measures to decrease the likelihood or impact of flooding), to be completed by 22 December 2015; and • Updates every 6 years thereafter that take into account the impact of climate change. The Environment Agency hope to achieve the requirements of the Directive through the use of existing published information. This may include reference or use of data presented in SFRAs, CFMPs and/or Strategic Flood Risk Mapping projects. In some cases the assessments may require new information to be generated to inform the stages of assessment required by the Directive. This Directive requires member states and the community to consider the potential impacts that policies might have on flood risks and the management of flood risks. It states that objectives regarding management of flood risk should be determined by the Member States themselves and should be based on local and regional circumstances.
3.2 National Policies
3.2.1 Making Space for Water (Reference 11)
Making Space for Water was released by DEFRA in March 2005 after a consultation period. Its intention is to inform the development of a new strategy on the management of issues surrounding flood risk and coastal erosion up to 2025. It does not state specific policies but presents the Government’s objectives on: 1. Land use planning – strongly encourages Flood Risk Assessments to be prepared at all levels of the planning process; 2. Rural Issues – promotes the environmental pillar of sustainable development through the use of wetlands and washlands, and managed realignment of coasts and rivers; 3. Integrated urban drainage management – commits to ensuring that SuDS techniques are incorporated into new developments; 4. Coastal issues – seeks to develop a more strategic and integrated approach to managing coastal flooding and erosion risks; and, 5. Living with flood risk – identifies that there is a need to raise awareness and preparation in local communities for the changing flood and erosion risks resulting from climate change.
3.2.2 Planning Policy Guidance notes (PPG) and Statements (PPS)
The Government publishes a suit of documents that provide further guidance on various planning subjects. This guidance is a material consideration when determining planning applications.
The PPGs and PPSs of most relevance to development and flood risk for WHBC are: • PPS1: Delivering Sustainable Development • PPS: Planning and Climate Change –a supplement to PPS1 • PPS3: Housing • PPS12: Local Development Frameworks • PPS25: Development and Flood Risk
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PPS1 sets out the overarching planning policies on the delivery of sustainable development through the planning system. PPS1 states that in preparing development plans local authorities should take into account ‘the potential impact of the environment on proposed developments by avoiding new development in areas at risk of flooding and sea-level rise’.
Planning and Climate Change sets out how planning, in providing for the new homes, jobs and infrastructure needed by communities, should help shape places with lower carbon emissions and resilience to climate change which is now accepted as inevitable. In particular paragraph 44 states that ‘in their consideration of the environmental performance of proposed development, taking particular account of the climate the development is likely to experience over its expected lifetime, planning authorities should expect new development to… provide public and private open space as appropriate so that it offers accessible choice of shade and shelter, recognising the opportunities for flood storage, wildlife and people provided by multifunctional greenspaces.’
PPS3 underpins the delivery of the Government's strategic housing policy objectives and their goal to ensure that everyone has the opportunity to live in a decent home, which they can afford in a community where they want to live. With regard to the location of new housing development it states that ‘any physical, environmental … and flood risk should be taken into account (paragraph 38).‘
PPS12 sets out the principles under which the LDF should be developed, with regard to flood risk it advises the avoidance of major new development in areas that are likely to be at greatest risk now or over the lifetime of the development (for example of flood risk) page 69 (i).
PPS25 requires that local councils must, when preparing the local development framework, 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. Assess the flood risk to development for all forms of flooding; and, 3. It must be ensured 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. 4. Undertake Flood Risk Assessments (FRAs) 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. FRAs are required in the following circumstances: a. for all development other than minor development within Flood Zones 2 and 3 and in Flood Zone 1 where there are critical drainage problems; b. for any development on land exceeding 1 hectare regardless of flood zone classification c. for development within 20m of the top of bank of a Main River, and d. for any culverting operation or development which controls the flow of any river or stream.
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3.3 Regional Policies
3.3.1 East of England Plan - The Revision to the Regional Spatial Strategy (RSS) for the East of England (Reference 3)
The Regional Spatial Strategy (RSS) for the East of England has identified growth areas within the administrative area of WHBC. The Borough has been allocated a development target of 10,000 dwellings up to 2021 with the detailed distribution of this development to be left to local Councils to decide. The growth areas are principally focused on the areas of Welwyn Garden City and Hatfield. In addition, Welwyn Garden City and Hatfield have been designated as key centres for change and development in the East of England Plan. The following policies of the adopted RSS are of particular relevance to the SFRA and the future policy development of WHBC LDF, listed below and detailed policy text is contained in Appendix F. • Policy WAT1: Water Efficiency • Policy WAT2: Water Infrastructure • Policy WAT3: Integrated Water Management • Policy WAT4: Flood Risk Management • Policy SS1: Achieving Sustainable Development • LA3: Welwyn Garden City and Hatfield Key Centre for Development and Change
3.3.2 Regional Planning Guidance for the South East (RPG9) (Reference 10)
Due to restructuring of regional boundaries, and that the East of England RSS is adopted, RPG9 (and the emerging revised South East Plan) no longer form part of the local development plan.
3.3.3 Sub Regional Strategy
From a review of the RSS for the East of England and South East England, Welwyn and Hatfield borough is not currently covered by a sub regional strategy.
3.3.4 Regional Flood Risk Appraisal (RFRA)
Discussion with the regional planning officer for the East of England has revealed that there is currently no RFRA. Discussions are underway for its preparation.
3.4 Local Policies
3.4.1 District Plan
3.4.1.1 Welwyn Hatfield District Plan 2005 (Reference 12)
The current district plan was adopted in 2005 and is the Welwyn Hatfield District Plan. The policies below are taken from the plan and are relevant to sustainable development, the control of development in flood plains and flood prevention and the protection of ground and surface water. The main proposed housing sites identified in the plan are presented in Appendix F.
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The policies of relevance from the current plan which have been saved are listed below with detailed policy text presented in Appendix F. • Policy SD1 - Sustainable Development; • Policy R6 - River Corridors; • Policy R7 - Protection of Ground and Surface Water; and • Policy R10 - Water Conservation Measures.
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Table 3-1 presents the list of housing allocations in the WHBC administrative area which forms part of the Strategic Housing Land Availability Assessment. Sites are surveyed at the end of March each year to assess their build status. Table 3-1 : Main Proposed Housing Sites in Welwyn and Hatfield 1 st April 2007 to 31 st March 2008
ESTIMATED PERMITTED NO. SITE NO. OF STATUS NO. DWELLINGS 1574 Complete HS1 Hatfield Aerodrome 1700 1709 130 Not Started 5 Under construction Creswick School Site HS2 50 109 Completed WGC
Land at Chequersfield HS3 218 320 Completed WGC
Former Wellfield HS4a Works, Wellfield Rd, 60 70 Completed Hatfield Adj. Factory Site, No further information on this site, development HS4b 22 Wellfield Rd, Hatfield likely towards the end of the plan period
Peartree HS5 Redevelopment area, 94 94 Completed WGC Godfrey Davis Garage Demolition Occurred. Application for 42 dwellings HS6 28 site, Welwyn refused June 2008
The Dairy, Homestead No further information on this site, development HS7 12 Lane, WGC likely towards the end of the plan period
Garage and Depot, HS8 24 24 Completed Lemsford Lane, WGC Sea Cadet Hut, HS9 22 21 Completed Lemsford Lane, WGC Oaklands Campus, HS10 23 23 Completed Lemsford Lane, WGC SKB site, Ridgeway, HS11 128 120 Completed WGC
Mount Pleasant Outline application with indicative layout for 117 HS12 73 Depot, Hatfield dwellings withdrawn November 2007 The Council is the major landowner and Hilltop, High View, HS13 75 redevelopment of the site will be the subject of a Hatfield planning brief following public consultation.
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ESTIMATED PERMITTED NO. SITE NO. OF STATUS NO. DWELLINGS Claregate, Great HS14 North Road, Little 14 Application approved for 24 dwellings Heath 24 units approved around edge of Howe Dell School, HS15 10 24 school (not actually on allocated Hatfield area) Former Allotments, HS16 22 Application received for 24 dwellings Knella Road, WGC Catomance Site, HS17 Bridge Road East, 86 86 Completed WGC Knella Road 24 Not started. Application HS18 22 24 Workshops, WGC approved August 2007
HS19 Hatfield Town Centre 165 275 275 not started
The Forum, Hatfield HS20 84 84 84 under construction Town Centre Hatfield Aerodrome HS21 200 467 Completed District Centre
Welwyn Garden City Development of this site will be the subject of a HS22 50 Town Centre masterplan, which is currently under production.
3.4.2 Local Development Framework (LDF)
Work is underway on the new Local Development Framework, a suite of planning documents that will set out the Council's future planning policies and eventually replace the adopted Welwyn Hatfield District Plan, 2005.
3.4.2.1 Local Development Documents (LDD)
The LDF will comprise Local Development Documents (LDDs), which will be of two types: Development Plan Documents (DPDs), which can deal with different issues or geographical areas, will be subject to independent examination and have the weight of development plan status; and • Supplementary Planning Documents (SPDs), which will not be subject to independent testing and will therefore not have development plan status. SPDs will however be subject to rigorous procedures of community involvement and sustainability appraisals, and can be a material consideration in the determination of planning applications.
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3.4.3 Strategic Housing Land Availability Assessment
WHBC has commissioned Tribal Consulting (formerly Llewelyn Davies Yeang) and Atisreal to carry out a Strategic Housing Land Availability Assessment. The Assessment will identify sites available to enable WHBC to meet the borough's housing requirement for the period up to 2021 and beyond. This work is still ongoing.
3.5 Environment Agency Policies
3.5.1 Catchment Flood Management Plan (CFMP)
A CFMP is a high-level strategic planning tool. It is used to identify and agree policies for sustainable flood risk management when working with other organisations and decision-makers. It takes into account the likely impacts of climate change and future development across the region. The plan does not propose specific or detailed measures but identifies where further work is needed.
3.5.1.1 Thames Catchment Flood Management Plan (Reference 13)
‘The Thames Region CFMP covers the fluvial part of the River Thames basin and its tributaries. This is from the source of the Thames in Gloucestershire, through to Teddington weir, as well as the Lee Basin and the smaller rivers in London. It includes some extensive rural areas such as Wiltshire and Oxfordshire, and densely populated urban areas including London, Swindon, Reading, Slough, Guilford and Luton.’ The Thames CFMP identifies the following key flooding characteristics of WHBC: • The Lee and its tributaries are a fluvial river system; • The centre of this district (sic) is urbanised and development is generally set back from the river; • The north west and eastern corner of the catchment is predominantly rural; • The main river crossing through WHBC is the River Lee. The Mimram drains the north of this borough and the Colne the southern corner; • The rivers in district (sic) are relatively natural.
The types of flood risk in the Upper Lee and tributaries consist of: • Overtopping of river banks. • Overflow of surface water drains. • Rapid surface water runoff from urban areas. • In-channel blockages and constrictions. • Possible foul and groundwater flood risk problems.
Currently the flood risk management measures employed in WHBC involve in channel maintenance and removal of blockages. To alleviate flood risks to future development in WHBC the Thames CFMP advocates that:
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• In Welwyn Hatfield the floodplain needs to be seen as the most important asset for reducing flood risk. As the extent of the floodplain in this district is small and generally confined to existing green space, avoiding development in the floodplain should therefore be possible, this is the most effective manner to reduce future flood risk. • Reducing flood risk in the Middle Lee catchment needs to look to reduce the consequences of flooding, this will involve a spectrum of activity, from spatial planning policies at a borough level, through to flood resilience at the individual property level. The aim of this is to enable sustainable development throughout the catchment that is increasingly resilient to flooding.
Risk reduction to developments and people should be sought through: • Use of the Sequential Test to locate new development in least risky areas, giving highest priority to Flood Zone 1. • If the Sequential Test is undertaken and a site in a floodplain is identified as the only site for development, after application of Exception Test use the sequential approach to design the site and seek opportunities to reduce risk (e.g. change to a less vulnerable land use, reduce footprint, replace existing building with a development on stilts, make space for water). • Build resilience into a sites design (e.g. flood-proofing, raised floor levels). • Ensure development is ‘Safe’. For residential developments to be classed as ‘safe’ dry access without crossing through the floodplain will be required.
In terms of riverside development the CFMP seeks to: • Preserve existing undeveloped river corridors from further development. • Set development back from rivers, seeking an 8 metre wide undeveloped buffer strip. This will make space for water and will help to accommodate climate change. • Look at opportunities for river restoration/enhancement as part of a development to make space for water and to reduce the legacy costs associated with the maintenance of hard engineering structure. • Assess the condition of existing assets (e.g. bridges, culverts, river walls) and renew so that their lifetime is commensurate with the lifetime of the development. Enhancement opportunities should be sought when renewing assets e.g. deculverting, bioengineered river walls, raising bridge soffits to take into account climate change. More sustainable solutions to flood risk management will be easier and less costly to maintain and ultimately less expensive to replace. • Prevent further culverting and building over of culverts. All new developments with culverts running through their site should seek to deculvert rivers for flood risk management and conservation benefit.
Requirements for drainage for new developments suggested by the CFMP include that: • SuDS are used on all new development. • All sites greater than 1 ha in size require the following: • SuDS, • Greenfield discharge rates, • 1 in 100 year on-site attenuation taking into account climate change.
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• On land allocations in the SFRA space shall be set-aside for SuDS.
Where Flood Alleviation Schemes are required, they must: • Define the functional floodplain and protect greenfield functional floodplain from future development. They must also develop appropriate flood risk management policies for the brownfield functional floodplain, focusing on risk reduction. • Protect remaining greenfield floodplain from future development. • Identify sites where developer contributions could be used to fund future flood risk management schemes. • Look at opportunities to make space for water to accommodate climate change.
Emergency planning is obliged to: • Use the SFRA to inform the emergency planning process. • Use the SFRA to educate local people to improve flood awareness.
Long Term Planning: • A long-term approach to managing flood risk in this catchment is required. To manage flood risk in the future, long-term land use planning will need to consider the following: • Land swaps, removing vulnerable development from the floodplain.
• Change the vulnerability classification of existing developments in areas at risk.
Integration of these suggested policy considerations into the LDF / LDD should ensure that the objectives and aspirations of the EA and national policy are met whilst strengthening the position of WHBC with regard to Flood Risk.
3.5.2 Water Level Management Plan (WLMP)
The Ministry of Agriculture Fisheries and Food (MAFF) launched its WLMP initiative in 1991, with the aim of providing a means by which the water level requirements for a range of activities in a particular area, including agriculture, flood defence and conservation, can be balanced. With the integration of MAFF into DEFRA the responsibility for producing WLMPs has also changed, broadly (although not exclusively) as follows: • the EA are responsible for preparing WLMPs for Main Rivers, • Internal Drainage Boards are responsible for preparing WLMPs for those watercourses they manage; and • Local Authorities are responsible for preparation of WLMPs for other watercourses. Water Level Management Plans provide a framework by which the water level requirements of a particular site can be discussed in order to incorporate and integrate a range of activities. The Environment Agency has a responsibility to be involved in the production of these plans in consultation with other interested bodies such as Natural England, Internal Drainage Boards, conservation groups and landowners. There are no water level management plans within the borough.
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3.5.3 Strategic Flood Risk Mapping (SFRM)
The Environment Agency (EA) has adopted a strategic approach to flood risk mapping. Rather than look at one watercourse or an individual reach of a watercourse in isolation, the EA now seek to assess, review and manage flood risk on a catchment wide basis. Not only does this enable the EA to assess the impact of proposed works at sites up and downstream, but the catchment wide approach also enables the EA to better meet the EU Water Framework Directive and work towards river basin management plans. These Flood maps will show the 100 year return period floodplain, the areas benefiting from defences, the increased extent of the 1 in 100-year flood due to predicted climate change and the worst historic flood outline. The Environment Agency is to produce high-level, strategic river catchment plans across England and Wales to identify and agree policies for sustainable flood risk management for the next 100 years.
3.5.3.1 Upper Lee
The lower part of the Upper River Lee Flood Risk Management Strategy (Reference 14) has identified that an integrated approach to flood risk management should be adopted within the Upper Lee catchment, including structural, non-structural, maintenance, and further studies. A combination of delivery plan elements have been proposed; separated into short, medium and long term actions (defined as the next 3, 10, and 100 year).
3.5.3.2 Upper Colne
The Upper Colne Flood Risk Management Strategy (Reference 15) incorporates the Mimmshall Brook, River Ver, River Gade, River Bulbourne, River Chess, River Misbourne and the Upper Colne. The first stage began in 2001 from which priority areas where identified and the Upper Colne catchment was divided into 26 sub-catchments.
3.5.4 Water Cycle Strategies (WCS)
Water Cycle Strategies (WCS) provide plans and programmes for Water Services Infrastructure implementation. They are determined through an assessment of the environment and infrastructure capacity for: • Water supply, • Sewage disposal, • Flood risk management, and • Surface water drainage. Water Cycle Studies also consider the impact of efficiency measures and provide an estimate of the cost and timescales for implementation for identified solutions and for the identified infrastructure improvements required to facilitate development. There are no Water Cycle Strategies currently covering the entire borough. However, the Rye Meads Water Cycle Study does include part of the borough and is in production and due to complete in Spring 2009.
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3.5.5 Catchment Abstraction Management Strategy (CAMS) (Reference 16)
The study area is covered by the Upper Lee CAMS. A CAMS is a strategy for the management of water resources at a catchment scale. It takes a strategic view of water resources in an area and assesses how much can be abstracted to meet local requirements (potable water, industry etc), whilst leaving sufficient water for ecological uses, and identifies where water is and is not available for future abstraction. It seeks to manage abstractions through a strict licensing system that is frequently reviewed. The Upper Lee CAMS has identified that both the Upper Lee and the River Mimram are over abstracted and no further abstraction licences will be granted on the River Mimram , with the overall aim of decreasing the number of licences issued. In addition to this, the strategy has identified that no more consumptive licences will be granted for the Upper Lee groundwater resources.
3.6 Other Relevant Policies
3.6.1 Water Utility Policies
Thames Water are the sewage providers in the borough. One of their objectives is to achieve sustainable development through the policy below: Sustainable Development Policy Sustainable development at Thames Water means an increasing focus on the balanced consideration of economic, social and environmental aspects in making business decisions. Within and beyond the regulatory framework of the water industry, Thames Water believes adopting Sustainable Development principles will: • Ensure the continued supply of quality drinking water and the provision of wastewater services in line with European standards; • Safeguard water supplies and enable the effective management of water resources; • Ensure the safe recycling of wastewater and sludge to the environment; • Improve services by providing them in a sustainable and cost-effective way; and, • Involve our communities and influence our business partners in working towards more sustainable futures.
3.6.2 Hertfordshire Highways Policies
Hertfordshire Highways have been contacted and requested to provide policies which they wish to see included in the WHBC SFRA. No policies have been made available for this study.
3.6.3 Building Regulations 2002 H3 Rainwater Drainage
Building Regulations 2002 states that: 1. Adequate provision shall be made for rainwater to be carried from the roof of the building; 2. Paved areas around the building shall be so constructed as to be adequately drained;
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3. Rainwater from a system provided pursuant to sub-paragraphs 1) or 2) shall discharge to one of the following, listed in order of priority: • an adequate soakaway or some other adequate infiltration system; or, where that is not reasonably practicable; • a watercourse; or, where that is not reasonably practicable; and, • a sewer.
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4 Data Collection & Review
This section describes the data collection process, presents the available data and discusses its benefits and limitations. A comprehensive record of all the data collected through the production of the Level 1 SFRA is presented in a document register in Appendix C. One of the objectives of the Level 1 SFRA is to collate and review available information on flood risk for the study area, and present this in a manner suitable for WHBC to apply the PPS25 Sequential Test. The information has been sourced from a variety of stakeholders including the Environment Agency, WHBC, Hertfordshire County Council, Three Valleys Water (potable water supply), Thames Water (sewerage undertaker), Hertfordshire Highways and Hertfordshire Fire and Rescue. The information presented in this Level 1 report should not be considered as an exhaustive list of all available flood related data for the study area. The Level 1 SFRA is a presentation of the data collected following consultation with and input from the above stakeholders within the timeframe available.
4.1 Overview
As outlined in Section 1.4 an objective of the 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 the local planning authority to apply the Sequential Test across their administrative area to determine suitability of sites for development and identify sites that may need to be justified through successful application of the Exception Test. The sequence of tasks undertaken in the preparation of the Level 1 SFRA were, in order: • Inception meeting with the Environment Agency, Welwyn Hatfield Borough Council on 15 tth August, 2007; • Established the local stakeholders; • Issued letters to stakeholders requesting data/information; • Followed-up data requests and arranged stakeholder meetings (where necessary); • Collated and reviewed data and populated a data register; • Presentation of available salient information on flood sources and flood risk; • Submission of draft Level 1 report to Welwyn Hatfield Borough Council and the Environment Agency in November 2007; and, • Review of the draft Level 1 report in line with comments provided by the Environment Agency and Welwyn Hatfield Borough Council.
4.1.1 Stakeholder Consultation
The stakeholders that were contacted to provide the data/information for the SFRA were: • Environment Agency - Thames region, North East Area Office; • Welwyn Hatfield Borough Council;
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• Hertfordshire County Council; • Thames Water; • Three Valleys Water; • Hertfordshire Highways; and, • Hertfordshire Fire and Rescue Services.
4.2 Fluvial Flood Data
Main rivers and their catchments were provided by the Environment Agency as a GIS layer. (Figure 2).
4.2.1 Environment Agency Flood Zone Maps
The Environment Agency has provided an extract of their Flood Map for the study area (Figure 5). The Flood Map shows the estimated extent of Flood Zones 2 and 3 (ignoring the presence of flood defences) for all Main Rivers and/or watercourses with identified critical drainage problems. 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. 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. Site specific hydraulic models use relatively detailed topographic data and rigorously derived flow estimates to derive flood extents. The National Generalised Model outputs are derived from SAR or LiDAR data and national data for river flows. The Environment Agency delineates ‘Areas that Benefit’ from defences through the modelling of watercourses. However it has been confirmed that no Areas Benefiting from Defences (ABDs) exist within the WHBC study area. The Flood Map does not provide information on flood depth, speed or volume of flow. It also does not show flooding from other sources, such as groundwater, direct runoff from fields/urban areas, overflowing sewers or the effect of climate change on these sources.
4.2.2 Hydraulic Modelling
Hydraulic models enable the estimation of accurate floodplains and flood depths based on detailed topographic data of river channels including structures (bridges, culverts etc) and flood defences. The floodplains are also compiled using rigorously developed statistically derived flow estimates. Hydraulic models were requested and provided by the Environment Agency. Table 4-1 provides a summary of the hydraulic model data available for the study area with the coverage of those models shown in Figure 6. Models were provided for the following water course and flood scenarios:
4.2.2.1 River Lee Modelling and Mapping Study (Halcrow, 2007)
The hydraulic model for the River Lee within the study area has been constructed using ISIS by Halcrow, as part of the Strategic Flood Risk Management (SFRM) Framework (Reference 14). The model has been run for the 2 year, 5 year, 10 year, 20 year, 25 year, 50 year, 100 year, 100 year + climate change, 200 year and 1000 year return periods. The hydraulic model has not been made available for the Level 1 SFRA, however, a selection of floodplain outlines from this model have been provided.
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4.2.2.2 Upper Colne Strategy Model (Halcrow, 2004)
The hydraulic model for the Upper Colne has been constructed using ISIS by Halcrow, as part of the Upper Colne Flood Risk Management Strategy. The model was run for a number of return periods. The hydraulic model has not been made available for the Level 1 SFRA but flood outlines have been provided for the 20 year, 100 year and 200 year return periods.
4.2.2.3 Mimmshall Brook
The Mimmshall Brook is included in the hydraulic model for the Upper Colne constructed using ISIS by Halcrow, as part of the Upper Colne Flood Risk Management Strategy. The model was run for a number of return periods. The hydraulic model has not been made available for the Level 1 SFRA but flood outlines have been provided for the 20 year, 100 year and 200 year return periods.
4.2.2.4 Un-modelled Watercourses
No hydraulic models exist for the River Mimram, Turkey Brook, Northaw Brook, Wildhill Brook, Nast, Essendon Brook, Hatfield Hyde Brook, Cuffley Brook, Hempshill Brook. These watercourses have been mapped as part of the National Generalised Modelling (NGFM). The NGFM is derived from catchment scale modelling using JFlow modelling software. JFlow is a simple model based on the Manning’s equation, which for shallow flows over topographically driven catchments can provide a good estimation of flood propagation.
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Table 4-1 : Hydraulic Modelling Summary for the Welwyn and Hatfield SFRA study area
HATFIELD WILDHILL ESSENDON UPPER MIMMSHALL ELLEN RIVER TURKEY NORTHAW CUFFLEY HEMPSHILL RIVER RIVER LEE HYDE NAST * BROOK * BROOK * COLNE BROOK BROOK * MIMRAM * BROOK * BROOK * BROOK * BROOK * BROOK * River Lee – Stort / Lee – Stort / Lee – Stort / Lee – Stort / Lee Lee Lee Lee Colne Colne Colne Colne Mimram Catchment Thames Thames Thames Thames
Modelled × × × × × × × × × × All of the Complete River Lee Complete Model within the within the N/A N/A N/A within the N/A N/A N/A N/A N/A N/A N/A Coverage study study study area area area River Lee Upper Upper Modelling Study Colne Colne and Name Strategy Strategy Mapping Model Model Study Modelled Halcrow N/A N/A N/A Halcrow Halcrow N/A N/A N/A N/A N/A N/A N/A By Date March June N/A N/A N/A June 2005 N/A N/A N/A N/A N/A N/A N/A Modelled 2007 2005 Modelling ISIS N/A N/A N/A ISIS ISIS N/A N/A N/A N/A N/A N/A N/A software 2, 5, 10, 20, 25, All Model 50, 100, 20, 100, 20, 100, N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A Runs 100 + cc, 200 year 200 year 200 and 1000 Including N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A Defences Excluding N/A N/A N/A × × N/A N/A N/A N/A N/A N/A N/A Defences *Note that all of the watercourses named (apart from a section of Hatfield Hyde Brook) have Flood Zones 2 and 3 as modelled using JFLOW by the Environment Agency (Section 4.2).
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4.3 Historic Data/Records
Locations throughout the study area that have experienced flooding between 2004 and 2007 have been provided by Hertfordshire Fire and Rescue Services which details point source locations of flooding. This information is provided without details of the source of the flooding incident, records of the antecedent conditions which gave rise to the flooding or reference to a return period for the floods. The locations that experienced flooding are detailed in Appendix B. 4.3.1 Hertfordshire Highways Data
Hertfordshire Highways Agency has provided data on the historical flooding incidents reported to them within the WHBC area. The database was queried to extract information on the flooding frequency as well as the type and effect of flooding. The sources were categorised into the following: • Carriageway flooding; • Property Damage; • Ditch problems; • Blocked gullies; • Sandbags; and, • Subway flooded.
Hertfordshire Highways Agency has also provided a list of planned drainage improvement works for the borough, but there is only a broad indication of the location of the works planned, no record of when or if these works are planned to take place and no details of the proposed improvements. 4.3.2 Environment Agency Data
Historical flood outlines for flood events on the River Mimram, River Lee, River Colne and the Mimmshall Brook have been provided by the Environment Agency. Table 4-2 indicates the data provided. The data includes the river name and the date of flood event, but does not give information regarding the intensity of the storm event (e.g. 1 in 100 years) that produced the flooding. Table 4-2 : Summary of Anecdotal Historical Flood Outlines
GIS REFERENCE * WATERCOURSE EVENT HO_EA_RC_87 River Colne July 1987 HO_EA_RCMB_92 River Colne and Mimmshall Brook September 1992 HO-EA_RCMB_00 River Colne and Mimmshall Brook December 2000 HO_EA_RN_03 River Nast January 2003 HO_EA_RL_07 River Lee (east of study area) March 2007 HO_EA_RL_00 River Lee (east of study area) December 2000 HO_EA_RL_78 River Lee May 1978 HO_EA_RL_47 River Lee March 1947 HO_EA_RM_47 River Mimram March 1947
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The nomenclature used for the GIS reference refers to ’Data type_data source_watercourse_year of flood event’, e.g. Historic Outline_Environment Agency_River Colne_1987. All data is mapped in Figure 7.
4.4 Flood Defences
The Environment Agency has provided outputs from the National Flood and Coastal Defence Database (NFCDD) for the study area in the form of GIS layers presenting the location and details of flood defence structures within the study area. This database contains, in electronic format, details of flood defences covering man-made, natural and maintained channels. Further information on each of the defences is sometimes presented including the type of structure; asset description; asset location; length; height; width; depth; diameter; design standard; operator responsible; last inspection date; next inspection date; condition and asset comments. The EA does not have a regular maintenance program associated with the rivers within the study area. The efficiency of in channel structures is generally maintained by the riparian owners through the removal of obstructions or blockages. The EA removes large blockages and maintains the system where necessary through their permissive powers. Details of all NFCDD flood defences in the study area are presented as a GIS layer and in.Figure 8 In-channel structures which could experience a blockage are also presented in Figure 8.
4.5 Topographic Data
Topographic data in the form of LiDAR has been provided by the Environment Agency. LiDAR data is an airborne mapping technique that uses a laser to measure the distance between an aircraft and the ground surface. The data varies in accuracy depending on the nature of the terrain such as in woodlands, complex urban areas and near lakes. Since 2004, LiDAR is generally recognised to be accurate to within+/- 150mm and all LiDAR data within the study area has an accuracy specification of less than +/- 100mm. The LiDAR data covers approximately half of the study area, with gaps to the north of the study area over Welwyn Garden City and in the southeast of the study area (Figure 9).
4.6 Pluvial Flooding
Pluvial flooding (overland flow) typically arises as a result of intense rainfall, often of short duration, that is unable to soak into the ground or enter drainage systems. It can run quickly off land and result in local flooding. Flooding from overland flow is not reported or recorded to be a flood source within the study area, and liaison with the stakeholders has not identified any records of historical overland flow flooding within the borough. See Figure 10: Sources of Potential Overland Flow.
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4.7 Groundwater
The study area is underlain by extensive aquifers, with many used for potable and/or industrial water supply. In addition most of the watercourses flowing through the study areas are spring-fed, indicating groundwater levels are at or very close to the ground surface in some locations throughout the study area. Despite this, groundwater flooding is not a regular or frequently occurring source of flooding within the study area. The Environment Agency only hold four records of groundwater flooding within the study area since records began in February 2000. This information has been provided by the Environment Agency from their groundwater flooding database. Table 4-3 presents the Environment Agency’s records for the area, Groundwater flooding locations are also illustrated in Figure 11. Groundwater Vulnerability (Figure 12), Source Protection Zones (Figure 13) and solid and drift geology have also been provided by the Environment Agency as GIS shapefiles. Table 4-3 : Summary of Anecdotal Historical Groundwater Flood Events
GIS NATIONAL GRID APPROXIMATE DESCRIPTION OF FLOODING (TAKEN REFERENCE REFERENCE LOCATION DIRECTLY FROM EA RECORDS ) Water beneath floor due to London Clay, poor drainage, House at bottom GW_EA_1 TL 25777 04187 Brookmans Park of slope. Land drains up gradient of gravity flow. Flooded garden where groundwater GW_EA_2 TL 30103 02833 Cuffley seepage through a neighbours wall into garden. Flooding of indoor tennis courts. Water previously noticed, but went away. Has since returned. Site lies on Head deposits over unconfined chalk. Depth to ground water 6 -10mbgl. No GW GW_EA_3 TL 21281 08856 East Hatfield Flooding nearby. Local OBH 2.2km away so no significant rising trends. Building work nearby. Most likely to be local building works disrupting local perched water in the head deposits. Waterlogged Garden may be due to recent building works to rear of property. Site on edge of sands and GW_EA_4 TL 29916 03198 Cuffley gravel deposits which overly London Clay. Site lies in a valley that indicates a preferential course for water [Possibly tributary to River Cuffley
The nomenclature used for the GIS reference refers to ’Data type_data source_Reference Number’, e.g. Groundwater Flooding_Environment Agency_1.
The Environment Agency has also provided groundwater monitoring data for the study area. The data includes monthly borehole level data from 17 monitoring stations located across the WHBC study area, from March 1986 to October 2007. The locations of these boreholes and the associated groundwater trends (determined over the length of the available record) are shown in Figure 11.
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Table 4-4 : Summary of the Groundwater Monitoring Data March 1986 to October 2007 AVERAGE BOREHOLE NATIONAL GRID GIS REFERENCE LEVEL (M GROUNDWATER TREND * REFERENCE REFERENCE AOD) GWM_EA_1 TL20/14 TL2104217518 61.74 Rising GWM_EA_2 TL20/49 TL2156016650 63.94 Rising GWM_EA_3 TL20/62 TL2191003950 47.39 Falling GWM_EA_4 TL20/6D TL2257017010 44.33 Rising GWM_EA_5 TL20/7 TL2302016220 8.21 Rising GWM_EA_6 TL21/103 TL2356019123 64.37 Falling GWM_EA_7 TL21/105 TL2371015500 58.39 Falling GWM_EA_8 TL21/106 TL24111418 60.21 Falling GWM_EA_9 TL21/107 TL2459015030 71.60 Falling GWM_EA_10 TL21/108 TL2494003960 65.36 Falling GWM_EA_11 TL21/109 TL2535009770 64.82 Falling GWM_EA_12 TL21/110 TL2572514639 68.83 Falling GWM_EA_13 TL21/17 TL2629017680 67.43 Falling GWM_EA_14 TL21/2 TL2719010320 89.03 Rising GWM_EA_15 TL21/3 TL27401568 55.54 Rising GWM_EA_16 TL21/87 TL2946609588 45.86 Falling GWM_EA_17 TL21/97 TL29750224 62.78 Rising * Groundwater trend refers to whether the levels in the water table are on average rising or falling over the period March 1986 to October 2007.
The nomenclature used for the GIS reference refers to ‘Data type_data source_Reference Number’, e.g. Groundwater Montioring_Environment Agency_1.
4.8 Sewer Flooding
Records of sewer flooding were obtained from Thames Water through a query of their DG5 register. In order to fulfil statutory commitments set by OFWAT, water companies must maintain verifiable records of sewer flooding. This is achieved through maintenance of DG5 registers. Water companies are required to record flooding arising from public foul, combined or surface water sewers and identify where properties suffered internal or external flooding.
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Regrettably the data provided by the Thames Water is limited to postcode data, resulting in the coverage of relatively large areas by comparatively limited and isolated recorded flood events. The data also only covers the last ten years. In addition, the records of flooding do not account for the affect of any capital works designed to alleviate the recorded flooding. In areas exposed to frequent flooding from overloaded sewers, water companies will undertake alleviation works to reduce the severity and/or frequency of the flood events. Thames Water has not provided details of any capital works within the study area or an indication of where recorded flood events have been investigated and mitigated. Thames Water records indicate that there has been 1 sewer flooding incident in WHBC due to an overloaded combined sewer (Table 4-5 and Figure 14). However, the data does not indicate where a flooding incident is a recurring problem, or the severity of flooding that occurred. Consequently a return period and severity cannot be assigned to the incident. Through discussions with the Environment Agency it has been decided to ignore this source of flooding as it would prejudice planning in an area of the borough, the size of which is disproportional to the flood risk. Table 4-5 : Sewer Flooding Records in WHBC Study Area.
NUMBER OF PROPERTIES FOUL / GIS SEWER SURFACE WATER / LOCATION AFFECTED IN LAST 10 COMBINED REFERENCE PROBLEM STORM SEWER YEARS SEWER Welwyn SW_TW_1 Hatfield 1 Combined 0 1 BC
The nomenclature used for the GIS reference refers to ‘Data type_data source_Reference Number’, e.g. Sewer Flooding_Thames Water _1.
4.9 Artificial Sources / Infrastructure Failure
Artificial sources of flooding can include reservoirs, canals and lakes where water is retained above natural ground level or at ground level where large expanses of water could exacerbate flooding from other sources. There are several lakes and gravel pits throughout the study area which are all at ground level (Figure 16). There are also two reservoirs designated under the 1975 reservoirs act, these are classified as raised structures. None of the stakeholders contacted hold any records of flooding arising from artificial sources and/or infrastructure failure.
4.10 Flood Risk Assessments
Table 4-6 provides details of the Flood Risk Assessments that have been carried out in WHBC.. The location of these is illustrated in Figure 17:
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Table 4-6 : Flood Risk Assessments undertaken in WHBC
GIS RIVER GRID DATE SITE ADDRESS CONSULTANT REFERENCE CATCHMENT REFERENCE COMPLETED Textron Site, Mundells, URS FRA_EA_1 Mimram TL 248 138 Welwyn Garden City, Herts Corporation Ltd March 2005 Black Fan Road, Welwyn November FRA_EA_2 Mimram TL 252 126 Bloor Homes Garden City, Herts 2004 Watchmead, Welwyn Waterman FRA_EA_3 Mimram TL 252 128 Garden City, Herts Burrow Crocker February 2005 A T Pepper Postern Gate Farm, Engineering FRA_EA_4 Lower Lee TL 308 047 Newgate Street, Herts Consultancy Ltd April 2005 Wormald Sir Frederic Osborn School, FRA_EA_5 Mimram TL 257 131 Burrow WGC Partnership October 2005 Paddock Farm, Warrengate December FRA_EA_6 Colne TL 229 041 Plandescil Rd, N Mymms Hatfield 2005 Lemsford Mill, Welwyn Hydro FRA_EA_7 Upper Lee TL 219 124 Garden City Generation Ltd April 2005 Bedwell Park, Essendon FRA_EA_8 Lower Lee TL 276 085 WSP Herts July 2006 Wormald Mount Pleasant Depot FRA_EA_9 Upper Lee TL 233 100 Burrows Hatfield Partnership June 2006 Mundlles Welwyn Garden FRA_EA_10 Mimram TL251 137 WSP City February 2007 PEP Civil & Broadwater Road, Welwyn FRA_EA_11 Upper Lee TL 242 126 Structures Garden City, Hertfordshire Limited March 2007 University of Herts Student September FRA_EA_12 Colne TL 215 071 WSP Forum 2007 Milford Nursery, Fulling Mill FRA_EA_13 No details No details JBA Consulting Lane, Welwyn /11/04
The nomenclature used for the GIS reference refers to ‘Data type_data source_Reference Number’, e.g. Flood Risk Assessment_Environment Agency_1.
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5 Strategic Flood Risk Mapping
This section describes the data used in the production of the Strategic Flood Risk Maps and GIS deliverables with which the Sequential Test should be undertaken.
5.1 Requirements of PPS25
Planning Policy Statement 25 (Reference 2) and its accompanying Practice Guide (Reference 4) requires SFRAs to present sufficient information on all flood sources to enable local planning authorities to apply the Sequential Test across their administrative area. In order to apply the Sequential Test information is required on the probability associated with flooding from the different flood sources present across the area. In addition, the assessment of probability should also account for the effects of climate change on a flood source for the lifetime of any development that would be approved under the emerging Local Development Framework. For all but fluvial flood sources the current paucity of data makes definition of robust classifications of flood probability unreliable. For example to define high, medium and low probabilities for groundwater flooding within the study area based on four reported incidents (with no corresponding record of the severity of that flood) is not robust. Consequently for all flood sources other than fluvial sources, where only anecdotal or relatively poor evidence of flooding is available qualitative assessments of probability have been made where the data allows. In some cases qualitative definitions of probability are not practical; in these situations the flood risk from a particular source should be considered as ‘medium’ until proven otherwise and should be investigated through a site specific assessment of flood risk submitted as part of a planning application or a Level 2 SFRA. Details of the requirements for Flood Risk Assessments are presented in Section 11. The following section explains how the available data has been used to achieve the requirements of PPS25 (Reference 2) and the Practice Guide (Reference 4).
5.2 GIS Mapping
Geographical data such as flood extents, watercourse routes, for use in determining appropriate planning decisions are best presented using Geographical Information Systems (GIS). GIS acts as an effective management tool for the coordinated capture, storage and analysis of data of a geographical nature. GIS handles data in a hierarchical manner by storing spatial features within various layers, which are allied to an underlying database. GIS is a recognised tool for the efficient collation, storage and analysis of information and is also an increasingly valuable resource for local planning authorities. The data presented in the GIS layers should be used by WHBC to apply the Sequential Test to developments in their administrative area, either as part of the strategic planning process or to determine the suitability of windfall applications. Table 5-4 provides a summary of the GIS layers included in the SFRA, the data sources and any benefits and limitations of the data.
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5.3 Fluvial Flooding
5.3.1 Requirements
PPS25 requires SFRAs to define fluvial flood zones, these are described in Table 5-1: Table 5-1 : Definition of fluvial flood zones
PROBABILITY OF FLOOD ZONE DEFINITION FLOODING At risk from an undefended fluvial flood event greater Flood Zone 1 Low Probability than the 0.1% annual probability flood event At risk from an undefended fluvial flood event between Flood Zone 2 Medium Probability the 1% and 0.1% annual probability flood event At risk from an undefended fluvial flood event less and Flood Zone 3a High Probability equal to the 1% annual probability flood event At risk from a defended fluvial flood event less than and Flood Zone 3b Functional Floodplain equal to the 5% annual probability flood event
PPS25 states that functional floodplain should be determined considering the effects of defences and other flood risk management infrastructure. Where the functional flood plain has been defined through hydraulic modelling this has been part of a detailed mapping study and consequently these areas are defended. In areas where the 1 in 20 year return period has not been modelled the functional floodplain (Flood Zone 3b) has been defined by a 20m buffer around the watercourses. Where the functional floodplain is mapped using a buffer strip, this provides a standardised estimate of the Flood Zone 3b, consequently, detailed modelling may be required as part of the Level 2 SFRA or a site specific FRA to accurately define the extent of Flood Zone 3a and 3b. Where a buffer strip has been utilised to delineate Flood Zone 3b it is possible that a site may be shown to be undevelopable depending on the vulnerability of the development. Where Flood Zone 3b is defined with a buffer strip the Environment Agency have acknowledged that they would accept opportunities to challenge the extent of the flood zone, however, this would not be accepted for rivers where detailed mapping exists. In instances where a development site appears to be located within Flood Zone 3b it is recommended that the developer enters into pre-application discussions with the Environment Agency’s Development Control team. 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). The main sources of fluvial flooding in the study area are the: • River Lee • River Mimram • Upper Colne; and • Mimmshall Brook
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5.3.1.1 Climate Change
Climate change is predicted to increase peak river flow by 20% up to 2115 and as a result increase floodplain volume and area, which will present a risk to an increased number of properties. Modelled climate change scenarios are available for the 1 in 100 year event (Flood Zone 3a) for the River Lee. There are no climate change scenarios available for the River Colne, therefore the 0.5% annual probability flood event scenario has been used as a surrogate to define Flood Zone 3a). Further surrogates have been utilised on the watercourses which do not have detailed modelling, these are described further in Section 5.3.2
5.3.2 Data Source
Section 4.2.2 identifies the data available for use in mapping the fluvial flood zones required by PPS25 (Reference 2). The mapping has been produced through the use of flood outlines generated by hydraulic models or use of the Environment Agency’s Flood Map. In some cases surrogate data has been used to define Flood Zones where the required data is not available. For example, if the extent of Flood Zone 3a with climate change is required, but the required model (a 1 in 100 year plus climate change) has not been run, it may be suitable to use the 1 in 200 year model, if this is available. It is likely that the surrogate model would give a larger Flood Zone 3a than the required model and this would therefore be considered to be a precautionary approach. .
The suitability of surrogates for use in Flood Zone mapping has been based on the type of data that is available in each case. Where it has been determined that there is no suitable surrogate data available, an alternative method of defining the Flood Zones has been used. In some of these cases, buffer zones have been generated using a combination of site topography and the available modelled data.
Table 5-2 and Table 5-3 below indicate how the Flood Zones were defined in this SFRA, including where either surrogate flood models or buffer zones have been used. Table 5-2 – 2008 Fluvial Flood Zone Mapping Data Sources for Modelled Watercourses
FLOOD UPPER COLNE & UNMODELLED UPPER LEE ZONE MIMMSHALL BROOK WATERCOURSES
100 year undefended 100 year defended outline from Lee outline from Upper Colne 3a modelling & mapping Strategy (Halcrow, 2004) 100 year JFlow study (Halcrow, 2007) plus 100 year JFlow where larger
20 year defended outline 20 year defended outline from Lee modelling & from Upper Colne 3b 20m buffer mapping study (Halcrow, Strategy (Halcrow, 2004) 2007)
1000 year JFlow plus 1000 year JFlow plus 1000 year undefended 100 year defended 2 worst historic flood outline from Lee outline from the Upper outline modelling & mapping Colne Strategy (Halcrow,
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FLOOD UPPER COLNE & UNMODELLED UPPER LEE ZONE MIMMSHALL BROOK WATERCOURSES study (Halcrow, 2007) 2004) where larger plus plus worst historic flood worst historic flood outline outline
NB. Where JFlow is included in the Flood Zones this is a combination of original JFlow (2004) and improved JFlow (2008) data provided by the Environment Agency. Table 5-3 - 2115 Fluvial Flood Zone Mapping Data Sources for Modelled Watercourses
FLOOD UPPER COLNE & UNMODELLED UPPER LEE ZONE MIMMSHALL BROOK WATERCOURSES
100 year + 20% 200 year defended defended outline from outline from the Upper Same as Current 3a Lee modelling & Colne Strategy (Halcrow, Scenario Flood Zone 2 mapping study (Halcrow, 2004) 2007)
It has been agreed with the Environment Agency that climate change scenarios 3b will not be mapped for Flood Zones 3b and 2 due to a lack of suitable data or surrogates. It should also be noted that available climate change outlines for the Upper Colne and Lee are defended outlines, these should be compared with the 2 defended 100 year outlines (in 2008) to get an accurate representation of the impacts of climate change.
5.3.3 Mapping
Using the data set out above in Table 5-2 and Table 5-3 the flood outlines have been overlain on the Ordnance Survey base mapping to provide the PPS25 Flood Zones for use in undertaking the Sequential Test (Figure 18 and Figure 19). The Sequential Test Maps indicating the fluvial floodplains and flood sources for 2007 and 2115 are shown in Figure 20 and Figure 21 respectively.
5.4 Sewer Flooding/ Surface Water Flooding
The data provided by Thames Water is at a coarse resolution. Through discussions with the Environment Agency it has been decided to discount this source of flooding on the Sequential Test maps as it would prejudice planning in an area of the borough, the size of which is disproportional to the flood risk.
5.5 Pluvial Flooding (Overland Flow)
5.5.1 Requirements
There are no records of overland flow flooding within the study area. However there are several escarpments/steep slopes that have the potential to generate overland flow within the study area (Figure 10).
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As per fluvial flooding, areas with high, medium and low probability should be defined based on the available data. The definition of functional floodplain is not required for flooding from pluvial sources. There are no records of pluvial flooding affecting the study area and therefore this risk of flooding from this source is currently assessed as low. Due to the lack of recorded incidents of pluvial flooding, definition of flooding probability cannot be defined.
5.5.1.1 Climate Change
With the predicted increase in short duration high intensity rainfall and more frequent periods of long duration rainfall, it is predicted that pluvial flooding will increase, therefore posing greater risk to the study area.
5.5.2 Data Source
There are no records of pluvial flooding affecting the borough, Data on future pluvial flood events should be collected to enable a definition of flooding probability to be defined. Several steep slopes have been identified within the study area that are thought to be capable of generating overland flow. These have been identified from a review of local topography (from 1:50,000 mapping) and LiDAR and mapped (Figure 10). To illustrate areas that may have the potential to generate overland flow, and therefore potential locations at risk, buffer areas have been added to cover the areas down slope. As a precautionary approach and until data allows for definition of flooding probability, proposed development in these areas should be considered to be potentially at risk from pluvial flooding, and should be investigated further in a site specific FRA.
5.5.3 Mapping
Areas potentially at risk from pluvial flooding are indicated in Figure 10. The Sequential Test Maps indicating the fluvial floodplains and flood sources for 2007 and 2115 are shown in Figure 20 and Figure 21 respectively.
5.6 Groundwater Flooding
5.6.1 Requirements
The limited number of groundwater flooding records (four) for the study area prevents the classification of probability bands (as required by PPS25 (Reference 2)) on this data alone. However, by comparison of groundwater flooding records with groundwater trends, broad bands of probability can be established as follows: • High Probability - groundwater flooding record in a location of rising groundwater or where there are greater than 5 incidents of groundwater flooding within a 50m radius of the proposed development site • Medium Probability – groundwater flooding record in a location of stable groundwater or where there are between 1 and 5 incidents of groundwater flooding within a 50m radius of the proposed development site
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• Low Probability - groundwater flooding record in a location of falling groundwater or where there are no incidents of groundwater flooding within a 50m radius of the proposed development site For development applications within high or medium probability groundwater flooding areas, a Flood Risk Assessment should be prepared and submitted with the planning application. Details of the requirements for Flood Risk Assessments seeking to assess risks from groundwater sources are presented in Section 11.
5.6.1.1 Climate Change
The Catchment Abstraction Management Strategy (CAMS) (Reference 16) for the area indicates that the study area is currently over licensed and that groundwater recharge is unlikely as it is probable that no water will be available within the system for the predicted future. Consequently groundwater flooding through rising groundwater trends is unlikely to occur as a result of climate change.
5.6.2 Data Source
The Environment Agency has provided point source locations that have experienced groundwater flooding since February 2000 from their groundwater flooding database (Appendix C). Details of these records and their GIS references are shown in Table 4-3.
5.6.3 Mapping
Locations at risk from groundwater flooding are illustrated in Figure 11. The Sequential Test Maps indicating flood sources for 2007 and 2115 are shown in Figure 20 and Figure 21 respectively.
5.7 Artificial Sources
Artificial sources of flood risk are present within the study area (Figure 16). There are no records of flooding associated with artificial sources and no data on which to classify flooding probability. Consequently flood risks from artificial sources should be considered as low but assessed as part of site-specific flood risk assessments for developments within a 100m radius of them. 100m buffers have been applied to the identified artificial sources, within the study area. Consequently areas at risk from flooding from artificial sources are illustrated in Figure 16. The Sequential Test Maps indicating the flood sources for 2007 and 2115 are shown in Figure 20 and Figure 21 respectively.
5.8 Historical Flooding
Historical flood outlines provided by the Environment Agency have been mapped along with the point source data provided by Hertfordshire Fire and Rescue Service of homes flooded in the last few years. No details of the source of the flooding have been provided, however details of these flooding incidents are provided in Appendix B and have been digitised into GIS and are represented in Figure 7. Due to the lack of resolution of the data a definition of flooding probability cannot follow the same approach as that used for fluvial flooding. Therefore based on the available data the following criteria have been used:
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• High Probability - >5 incidents of historical flooding within a 50m radius of a proposed development site; • Medium Probability – between 1 and 5 incidents of historical flooding within a 50m radius of a proposed development site; and • Low Probability - 0 incidents of historical flooding within a 50m radius of a proposed development site. The Sequential Test Maps indicating the fluvial floodplains and flood sources for 2007 and 2115 are shown in Figure 20 and Figure 21 respectively. Hertfordshire Highways data was provided to supplement the poor resolution sewerage flood record provided by Thames Water.
This data has been mapped and presented Figure 15. No details of the source of the flooding have been provided, however details of these flooding incidents are provided in Appendix B and have been digitised into GIS. Due to the poor resolution of the data, the flooding incident has been attributed to whole roads, rather a point source location.
Definitions of flooding probability cannot follow the same approach as that used for fluvial flooding. Therefore based on the available data the following criteria have been used for the frequency of road flooding: • High Probability - >31 incidents of historical flooding on a road; • Medium Probability – between 11 and 30 incidents of historical flooding on a road; and • Low Probability – <11 incidents of historical flooding on a road. The information included within Diagram 1 below is an extract of data from the Hertfordshire Highways flooding database. The diagram indicates the proportion of different types of flooding incidents that have occurred within the study area. Diagram 1 : Pie chart of proportion of different flooding sources
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All development in areas of high risk from historical road flood sources should be subject to a site specific flood risk assessment.
Within areas of medium risk from historical road flood sources, only proposed developments classified as Highly Vulnerable and More Vulnerable should be subject to a site specific flood risk assessment.
5.9 Residual Risk
Section 4.4 and Figure 8 identifies a number of flood defences throughout the study area. From a review of the information presented in the NFCDD it is clear there are several defences in the study area, however, the reported design standard of the defences is predominantly for the 20% annual probability flood event. The NFCDD identifies only one defence in the study area with a design standard greater than the 20% annual probability flood event. This defence protects the area against the 2% annual probability flood event on the Mimmshall Brook (NFCDD_ID_12). The Mimmshall Brook FAS consists of a raised flood defence wall and mitred gate which protects 13 bungalows and an electricity substation. The Environment Agency has confirmed that there are no areas that benefit from defences within the study area and thus on the basis of a lack of data the study area is considered to be undefended. Consequently there is no residual risk associated with a failure in flood defences during the design flood event. The Flood Zone maps (Figure 20 and Figure 21) present a precautionary approach to defining flood risk throughout the study area.
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Table 5-4 : GIS master list
D117462 May 2009 42 Table 5-4 Level 1 SFRA GIS layers Category Source Description of Layer Included (Y/N) Comment Benefits Limitations Fluvial
Flood zones may not give an accurate representation of flood risk. The models do not take into account defences. Typically watercourses with a Environment Agency Polygon layer showing EA flood zone maps A quick and easy reference that can be used as an catchment area less than 3km² are omitted from Environment Agency Provided as GIS layer by EA Y Flood Zone Maps including Flood zone 2 and 3 indication of flood risk. mapping unless there is a history of flooding affecting a population. Consequently there will be some locations adjacent to watercourses that on first inspection, it is suggested there is no flood risk.
Polyline/Point & Polygon layer showing all surface derived from a query of mastermap All Watercourses water features, (ditches, streams, rivers, ponds, Y Displays all of the water features in the study area. Some minor water features in the query can be missed. provided by client lakes, reservoirs, canals derived by cross referencing all Polyline layer showing all watercourses Can see which watercourses are the responsibility Main Rivers watercourses with main river data designated Main Rivers or as Critical Ordinary Y There are other watercourses that may be a significant flood source. of the Environment Agency. provided by EA Watercourses or ordinary watercourses
derived from hydraulic model node point data layer showing the coverage of existing Hydraulic Model Extents Hydraulic model flood plains or levels have been points provided for existing hydraulic hydraulic models to demonstrate where data is Y Limited data There are watercourses that have not been modelled. (node map) provided for all the modelled rivers. models available
derived from hydraulic models where polygon layer showing the functional floodplain (1 Flood Zone 3b has been defined by a 20 m buffer where modelled scenarios Functional Floodplain available or a 20 metre buffer where Y Limited data in 20 year flood plain) for the study area are not available. data is not available The 1 in 100 year +cc is only available for the Upper Lee. The 1 in 200 year polygon layer showing the climate change derived from hydraulic models where is used as a climate change scenario for the Upper Colne and Mimmshall Climate Change floodplain (1 in 100 year plus 20% on peak flows) Y Limited data available Brook. For unmodelled watercourse Flood Zone 2 is to be used as a for the study area surrogate for Flood Zone 3a with climate change There are no areas that benefits from the defences from any of the Defence Locations point layer showing NFCDD entries within the Shows where there are existing defences, heights, watercourses in the study area which would aid in the assessment of land NFCDD Y (NFCDD) study area protecting from fluvial floods type and design standard. allocation. It does not give the depth of the culverts, the date of the last inspection or the next inspection. polygon layer showing the historical floodplain Shows areas that have been historically at risk of Historical Outlines EA Y The flood events do not have return periods associated with them. outlines for the study area flooding in the study area
Groundwater
Broadly shows the trends of groundwater in the The data from the boreholes was gathered on different days and in different study area. Shows those areas with rising trends from a review of groundwater point layers showing trends groundwater levels in years and therefore only a broad comparison can be made. The trends Groundwater level data Y that can be used to determine areas where monitoring data the study area and groundwater monitoring points observed were over the entire monitoring period for each borehole, which groundwater could potentially penetrate the surface differs between boreholes. and cause spring resurgence in dry valleys
review of Hydrogeological maps and Dry Valleys No Dry Valleys N DTM & All watercourse layer Groundwater Vulnerability Groundwater Vulnerability classifications for Indicates the vulnerability of aquifers underlying the From information provided by EA Y Maps underlying aquifers study areas Groundwater Source From information provided by EA SPZs around groundwater abstraction points Y Indicates the locations of SPZs for use in SuDS Protect Zones From records provided by stakeholders Shows areas that have experienced flooding in the Point data layer to be shown on groundwater Evidence is largely anecdotal and does not have a return period or severity Historical records showing evidence/anecdotal evidence Y Limited data past and therefore is likely in the future without maps associated with it. of groundwater flooding only intervention.
43 Overland Flow review of Hydrogeological maps (where Dry Valleys available) and DTM & All watercourse No Dry Valleys N layer The geology of the escarpments was not considered in their identification Shows areas that are most likely to increased and therefore could potentially impede flows. This process was also Escarpment identification Review of DTM/1:50,000 mapping Polygon layer showing escarpments Y overland flow. undertaken through a review of the mapping and LiDAR by 'eye' due to the lack of LiDAR coverage in the area. from a review of BGS 1:50,000 Very precise details on the locations of the geology's has not been provided. Gives details of the locations of soil types and there Geological Maps for the study area and Exact permeability rates cannot be provided as these are extremely Geology review Solid and Drift Geology layer for the study area Y permeability which can be used in advising on the geological memoirs to identify permeable and would require site specific investigation would be required for appropriate SuDS techniques. stratigraphy likely to produce runoff the determination of the geology , permeability and suitable SuDS methods. Historical records None None N
Sewers polygon data layer showing areas of flooding with Indicates areas that are most prone to flooding as The extent and source of the flooding is not known and cannot be displayed records of sewer flooding from Water Historical Record records of date of incident, location, extent, Y have experienced flooding in the last 10 years in this layer. Point sources cannot be applied to the data records provided company records source, cause within a postcode area. either.
Artificial Sources Polygon, Polyline & point data GIS layer showing Water features such as lakes are shown that may From a review of mastermap and Source Identification locations of artificial sources and responsible Y act as flood sources and have been overlooked in records provided by stakeholders authority the fluvial section.
Mitigation From information provided by the Indicates which areas are covered by the flood Flood Warning areas Polygon layer showing areas benefiting from flood Y Environment Agency warning system, warning and emergency plans with query details Emergency Planning From information provided by the Local presenting what is involved in each. N Limited data areas Authority or County Council There are no areas that benefits from the defences in the study area. It does Point & Polyline layer showing NFCDD entries Shows where there are existing defences, heights, NFCDD Data base provided by the EA Y not give the depth of the culverts, the date of the last inspection or the next within the study area protecting from all flood type and design standard. inspection. sources and unofficial defences, providing details Indicates where natural landforms or engineered of the type of structure, operating/responsible This can only provide a broad assessment of unofficial defences and may Unofficial defences From a review of topographic data Y structures may act to provide an unofficial defence authority miss smaller features that could look to mitigate flood risk. from flooding
Planning Polygon layer showing LPA administrative area LPA/study area Boundary Provided by Local Authority Y Clearly identifies the study boundary on 1:50,000 or 1:10,000 base mapping Polygon layer showing development site locations Allocations Provided by Local Authority N & boundaries Other land use pressures From records provided by stakeholders Polygon GIS layer showing other land use Clearly shows what other land use pressures must Y (AONB, SSSIs) (English Nature, LPA etc) pressures on Flood Zone 1. be considered when allocating development sites.
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6 Guidance on Applying the PPS25 Sequential Test
6.1 What is the Sequential Test?
The PPS25 Sequential Test is a risk based approach to determine the suitability of development according to flood risk from fluvial and tidal flood sources. PPS25 requires LPAs to apply the Sequential Test at all stages of the planning process to ensure that where possible developments are removed from areas with a high probability of flooding. Through application of the Sequential Test LPAs are encouraged to guide new development towards areas of the lowest flood probability. Allied to the Sequential Test, PPS25 also assigns different vulnerabilities to different types of development. If when applying the Sequential Test development in the floodplain is necessary the LPA should also bear in mind the vulnerability classification of their proposed development to assess if it is appropriate in an area of flood risk. In exceptional circumstances the LPA may be required to undertake the Exceptions Test to justify development in the floodplain (discussed further in Section 7). Table D2 of PPS25 (Reference 2) presents 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 development vulnerability is appropriate to the flooding probability. Table 6-1 : Flood Risk Vulnerability Classification (from PPS25, Appendix D, Table D2) • Essential transport infrastructure (including mass evacuation ESSENTIAL routes), 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. LESS • Land and buildings used for agriculture and forestry. VULNERABLE • Waste treatment (except landfill and hazardous waste facilities). • 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. • MOD defence installations. WATER - • Ship building, repairing and dismantling, dockside fish processing COMPATIBLE and refrigeration and compatible activities requiring a waterside DEVELOPMENT 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.
Paragraph 4.64 of PPS25 Practice Guide (Reference 4) notes that ‘where a land use is not specifically referred to in Table D.2 it should be allocated to the most appropriate vulnerability classification based on comparison with the characteristics of other uses in the table, informed by the consideration of the risks from flooding.’
PPS25 acknowledges that some areas will (also) be at risk of flooding from flood sources other than fluvial and tidal systems. Consequently all sources of flooding must be considered when looking to locate development. Other sources of flooding requiring consideration when situating new development allocations in the WHBC administrative area include: • Pluvial; • Groundwater; • Sewers; and • Artificial Sources. These 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
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there is no guidance on suitable return periods to associate with floods arising from these sources. However, high, medium and low classifications have been defined for the different flood sources where possible, and details have been provided in Section 5 of when the flood risk to a site should be investigated further as part of a site specific flood risk assessment.
6.2 How should the SFRA be used to apply the Sequential Test?
The LPA should use the information presented and mapped (Figure 20 and Figure 21) in this Level 1 SFRA (and accompanying GIS layers) to undertake the Sequential Test. The Sequential Test should be accurately documented to ensure that the decision processes followed for the locating of a development are consistent and transparent. The Sequential Test should be carried out on all development sites and seek to guide development to the lowest flood risk areas (Flood Zone 1). Where there are no reasonably available alternative sites in Flood Zone 1 to accommodate development, sites in Flood Zones 2 or 3 may be considered but must balance the flood probability and development vulnerability of sites. This should be based on the Flood Zone and Flood Risk Vulnerability Compatibility which is summarised in Table 6-2. The Level 1 SFRA mapping provides the tools by which a Local Planning Authority can undertake the Sequential Test. This is achieved by presenting information to identify the variation in flood risk across the local authority administrative area, allowing an area-wide comparison of future development sites with respect to flood risk considerations. Table 6-2: Flood Risk Vulnerability and Flood Zone ‘Compatibility’ from PPS25, Appendix D, Table D.3
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
The following flow diagram (Diagram 2), taken from the Practice Guide to PPS25 (Reference 4) and the Flood Risk Matrix (Reference 17) illustrate how the Sequential Test should be undertaken. The full process is described fully in PPS25, A Practice Guide 2008 (Reference 4).
Additional guidance to assist WHBC strategically undertake the Sequential Test are detailed in Section 6.3.
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Diagram 2 : Application of the Sequential Test (from Figure 4.1 of PPS25: Practice Guide, (Reference 4)
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6.3 Additional Guidance
The sequence of steps presented below in tandem with Diagram 2 is designed to provide WHBC and developers additional guidance on how to apply the Sequential Test strategically. The steps are designed to ensure land allocations are allocated in line with the principles of the Sequential Test or, failing this, that the requirement for application of the Exception Test is clearly identified.
Figure 20 and Figure 21 should be used by the LPA in conjunction with the recommended stages for application of the Sequential Test.:
1. The strategic developments (i.e. housing, hospitals, industrial etc) that need to be accommodated in the WHBC administrative area within the lifetime of the LDF should be assigned a vulnerability classification in accordance with Table D.2 “Flood Risk Vulnerability Classification” in PPS25 (Reference 2); 2. The Flood Zone classification of all development sites identified by WHBC should be determined based on a review of the Sequential Test Maps (Figure 20 and Figure 21). 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- year design life (up to 2070) for commercial / industrial developments (however this should be reviewed on a site by site basis and agreed with the Environment Agency; and • 100 year design life (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 (described in Section 7). 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 (in that order). Less vulnerable development types are not appropriate in Flood Zone 3b – Functional Floodplain. 6. ‘Essential infrastructure’ developments should also be preferentially located in the lowest flood risk zone, however this type of development can be located in Flood Zones 3a and 3b, where necessary, through application of the Exception Test.
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7. Finally, water compatible development, (as defined in Table 6-1 above), typically has the least flood risk constraints and therefore it is considered appropriate to consider them last when allocating development sites. 8. For decisions made using steps 4 to 7 above, it will also be necessary to consider the risks posed to the site from other flood sources e.g. groundwater and pluvial. Comparable development sites in the same flood zone may be more suitable for development when considering the following factors: • flood risk management measures, • the rate of flooding, • flood water depth, or, • flood water velocity. Table 1 (Appendix D) is provided as a suggested pro-forma for WHBC to follow when undertaking the Sequential Test. The table has been prepared to assist WHBC provide a transparent and structured reporting system and to assist in identifying where developments/development sites may require application of the Exception Test.
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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 has not been possible for a development to be located in a low risk flood zone, or a flood zone where the development vulnerability is appropriate then it may be necessary and appropriate to apply the Exception Test to the allocation, providing the development is consistent with the wider sustainability objectives of the area. Table 6-1 (Reference 2) provides guidance on the vulnerability of types of development and in conjunction with Table D3 (Table 6-2) 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 of flood risk development may be required to ensure social or economic, blight does not occur, thus ensuring continued sustainable development or constraints on land elsewhere (i.e. areas protected by nature conservation designations [Figure 22]) preclude the identification of additional lower risk areas.
7.3 What is Required to Pass the Exception Test?
The Exception Test consists of three parts which are detailed below. All three parts must be satisfied before development in a flood risk area can be justified.
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’ (Reference 2). The developer is required to demonstrate evidence that will allow the LPA to assess whether wider sustainability benefits will be provided that will outweigh the flood risk implications of the site. PPS25 Practice Guide (Reference 4) notes that the following steps should be taken: • The planning application should be scored against the sustainability criteria of the Sustainability Appraisal (SA). In the absence of a SA the checklists should reflect the Government’s sustainability strategy (Reference 4). • Where a development fails to score positively against the SA the LPA could consider planning conditions or Section 106 Agreements (Reference 4). • The Practice Guide for PPS25 (Reference 4) suggests that where ‘ in the absence of a SA the developer / LPA will have to provide reasoned justification detailing how the planning
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application provides wider sustainability benefits to the community that outweigh flood risk. LPAs may consider the use of a sustainability checklist for this purpose.’ For development sites to successfully apply the Exception Test it must satisfy the above criteria in addition to the criteria provided under Parts B and C.
7.3.2 Part B – Brownfield Land
‘The development must be on developable previously developed land or, if it is not on previously developed land, that there are no reasonable alternative sites on developable previously developed land’ (Reference 2). Planning Policy Statement 3: Housing (Reference 18) defines brownfield land as: ‘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. For development sites to successfully apply the Exception Test it must satisfy the above criteria in addition to the criteria provided under Parts A and C.
7.3.3 Part C – Flood Risk
‘A FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall’ (Reference 2). The Practice Guide to PPS25 (Reference 4) notes that it is the responsibility of the developer to prepare a comprehensive flood risk management strategy for the site and this should cover the following points: • The design of any flood defence infrastructure; • Access and egress; • Operation and maintenance; • Design of development to manage and reduce flood risk wherever possible; • Resident awareness; • Flood warning; and
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• Evacuation procedures and funding arrangements. The PPS25 Practice Guide (Reference 4) also provides details on the definition of ‘safe’ in Chapter 4 – The Sequential and Exception Tests. In addition the Environment Agency has defined the minimum requirement for the definition of ‘safe’ as: • Dry access for more and highly vulnerable uses; • 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 between the Local Planning Authority and local Environment Agency officers and may require additional considerations depending on the precise nature of the proposed development and flood risk. Details of mitigation measures that can be used to alleviate flooding and contribute to making a development safe are provided in Table 11-1. Before any development in a floodplain is approved the Environment Agency must be consulted on the proposed mitigation measures. To ensure that part C of the Exception Test is satisfied with regard to preventing the increase of flood risk elsewhere, the EA require the following methods and techniques to be considered: • Floodplain compensation • Reduction in runoff rates • Reduction of building footprints • Replacement of solid buildings with buildings on stilts • Provision of community benefit for existing properties • Meeting the objectives of the Thames CFMP It must be recognised that this list is not prescriptive and the Environment Agency should be contacted on a site by site basis to determine what measures will be necessary to suit the scale and nature of the development and flood risk. Table 2 (Appendix D) is provided to assist WHBC ensure they have robust justification against parts A and B of the Exception Test for any developments requiring application of the test. Where necessary part C of the Exception Test will be addressed in a Level 2 SFRA.
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8 Flood Risk Management
Flood risk management measures are those measures put in place to reduce the risk to people and property from the hazards of flooding. These management measures can be divided into three principal types: • Flood Warning; • Flood Defences; and, • Flood Risk Operational and Water Level Management.
8.1 Flood Warning Areas
The first device in the flood risk management arsenal is flood warning. Ensuring people in areas of flood risk are aware of potential flooding is key to ensuring they are prepared, facilitating the protection of property and evacuation where necessary.
The Environment Agency operates a flood warning service in all areas at risk of flooding from fluvial or tidal sources. It consists of four flood warning codes from ‘All Clear’ to ‘Severe Flood Warning’ that indicate the level of danger. The flood warnings are disseminated through a variety of mediums that include TV, radio, an automated voice messaging service direct to a phone/fax/pager, the Internet and/or loudhailer. There is also an emergency Floodline number (0845 988 1188) and a quick dial number for individual rivers.
There are 3 flood warning areas covering the borough and surrounding areas (Figure 24) (GIS layer: Flood Warning), these are:
• The River Lee to Hertingfordbury, west of Hertford and other rivers and streams in the Harpenden and Wheathampstead areas. • The Rivers Colne, Ver, Gade, Bulbourne,Tykeswater, Hartsbourne and Chess, the Radlett, Hilfield and Mimmshall Brook, and other streams in the Dacorum, St. Albans, Hertsmere, Watford and Three Rivers districts and in the North Mymms and Hatfield areas. • The River Mimram and other rivers and streams in the Welwyn and Whitwell (north of Kimpton) areas.
8.2 Flood Defences
Flood defences are typically engineered structures designed to limit the impact of flooding. Flood defences can take several forms and be designed to alleviate flooding to individual properties or to several over a large area. Strategic flood defences include structures such as bunds/embankments, canalised channels, culverts and flood storage areas among others, property specific defences include demountable barriers, pressure sealing doors/windows and non return valves on sewers to prevent backflow. Flood defences are typically designed and constructed to protect people and property from a given magnitude of flood. This is referred to as the design standard and may vary depending on the value attributed to the people and property is it designed to serve and the scale of works necessary to
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construct the defence. In addition a standard of defence may alter over the lifetime of the structure for example due to the effects of climate change. For new defences, these issues and others are balanced through a cost benefit analysis to determine if investment in defence schemes can be justified.
8.2.1 Current Strategic Flood Defence Measures in WHBC
Currently the flood risk management measures employed in WHBC involve in channel maintenance and removal of blockages. Information on defence structures within the study area has been provided by the Environment Agency from their National Flood and Coastal Defence Database (NFCDD). Information from the NFCDD has been used to generate the flood defence layer introduced in Figure 8. From a review of the information presented in the NFCDD it is clear there are several defences in the study area (Figure 8(GIS layer: Flood Defence)). On the River Lee there are stretches of raised man- made defences and flood defence structures around Warren Farm, to the north of Hatfield between Stanborough and Mill Green and around Essendonbury Farm. On the Mimmshall Brook there is a stretch of raised man-made defence and flood defence structures to the west of the Royal Veterinary College. On the River Mimram there is a stretch of raised man-made defence in the Danesbury area and also a number of flood defence structures and culverts along its course. On the Upper Colne there is a stretch of defence in the Ellenbrook area which consists of stretches of culverted channel, maintained channel and flood defence structures. These defences are stated to have a design standard commensurate with a return period of 5 years with the Mimmshall Brook having a design standard of 50 years. The Environment Agency has also confirmed that there is no information to define areas benefiting from defences in the WHBC study area. The Lake Embankment flood defence near Burnt Farm, is located predominantly within the Broxbourne Borough Council administrative area, however some of this flood defence structure does lie in the study area. The draft Broxbourne SFRA (Reference 19) identifies that the standard of protection is 5 years and is of good condition and the embankment protects Burnt Farm and the surrounding land from flooding.
8.2.2 Future Strategic Flood Defence Measures in WHBC
There are no flood defence schemes planned to alleviate flood risks to existing or future development in WHBC however, the Thames CFMP (Reference 13) advocates that: • In Welwyn Hatfield the floodplain needs to be seen as the most important asset for reducing flood risk. As the extent of the floodplain in this borough is small and generally confined to existing green space, avoiding development in the floodplain should therefore be possible, this is the most effective manner to reduce future flood risk. • Reducing flood risk in the Middle Lee catchment needs to look to reduce the consequences of flooding, this will involve a spectrum of activity, from spatial planning policies at a borough level, through to flood resilience at the individual property level. The aim of this is to enable sustainable development throughout the catchment that is increasingly resilient to flooding.
The CFMP (Reference 13) also advocates that any proposed flood alleviation schemes are required to:
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• Define the functional floodplain and protect greenfield functional floodplain from future development. They must also develop appropriate flood risk management policies for the brownfield functional floodplain, focusing on risk reduction. • Protect remaining greenfield floodplain from future development. • Identify sites where developer contributions could be used to fund future flood risk management schemes. • Look at opportunities to make space for water to accommodate climate change.
The draft Broxbourne SFRA (Reference 19) has identified several potential Flood Storage Area (FSA) sites. Those located close to or across the WHBC boundary include: • Woolens Brook FSA; • Spittal Brook FSA; • Wormley Brook FSA; and, • Cuffley Brook FSAs (there are potentially two FSAs on the Cuffley Brook) (Figure 8). These sites may need to be safeguarded from development for future flood alleviation works. The East Hertfordshire SFRA (Reference 20) does not directly identify potential FSAs, but rather refers to the Lower Lee Flood Risk Management Strategy (FRMS), which is currently being drafted and was not available at the time of writing this SFRA.
8.3 Flood Risk Operational and Water Level Management
8.3.1 Operational Management
In addition to the flood defence and warning services provided by the Environment Agency, there is also a substantial amount of ongoing maintenance of defences, structures and on watercourses within the study area, see Figure 8.
8.3.2 Water Level Management
The Environment Agency is also involved in the management of water levels and watercourses throughout the study area. The Environment Agency maintenance and operations department carry out channel clearances, to ensure that water levels are regulated and maintained. These activities form an important part of the overall flood risk management of the area and ensure that flood defences and flood warning assets operate as designed.
8.4 Flood Proof Construction
Flood proof construction can be incorporated into the design of future developments to help safe guard properties against the risk of flooding. It is also possible to retro fit some forms of flood proof construction techniques into existing properties. Flood proof construction can be split into two techniques, resistance and resilience, which are described below, and further information on flood
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proof construction techniques can be found in CIRIA and DCLG documents (Reference 21 and Reference 22).
8.4.1 Flood Resistance
Flood resistant construction involves the use of technologies and building materials to prevent the passage of flood waters into a building and the use of building materials or form to allow easy cleaning and drying. This is also sometimes known as ‘dry proofing’. In general, permanent flood resistant measures (e.g. solid floors) are preferable to temporary measures (door guards) as the permanent measures are always in place and don’t require fitting by the occupier. Flood resistance is the favoured approach in areas where low flood depths (<0.6m) are predicted. Flood resistance is split into two measures: • Active measures – involve the provision of temporary measures to prevent ingress of water; and • Passive measures – involve incorporation of fixed permanent measures into the design of the building. Flood resistant construction measures include: • Implementation of demountable flood barriers to prevent the ingress of water through doors, windows and other external openings in walls, such as airbricks. • Construction of external walls using low permeability materials to limit water penetration. • Construction of internal walls should avoid the use of gypsum plaster and plasterboard. More flood resistant linings such as hydraulic lime and ceramic tiles should be used. • Construction of solid floors with continuous damp proof membrane and damp proof course to prevent ingress of water.
8.4.2 Flood Resilience
Flood Resilience involves the use of building methods that whilst allowing flood waters to enter a building, ensure that the impact of the flood waters is minimised This method is also sometimes referred to as ‘wet proofing’. Flood resilience ensures that no permanent damage is caused and the structural integrity of the building is maintained. This method is favoured in areas where the flood depth could be significant (>0.6m). At such depths if water is allowed to build up on the outside of the building structural damage could result from the pressure difference. Flood resilient construction measures include: Fixtures, fittings and services should be located above design flood level. Use removable plastic fittings. Avoid fitted carpets. Locate electrical, gas and telephone equipment above design flood level. • Coating external walls in a water resistant coating to prevent permeation of water and reduce drying time. • Place plasterboard sheets horizontally rather than vertically to reduce exposure to water damage
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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 discharge 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 and inconvenience associated with up grading sewer systems it is uncommon for sewer and drainage systems to keep pace with the rate of development/re-development. As development progresses and/or urban areas expand, the drainage 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 required of new conventional sewer systems (typically with a design standard of 1 in 30 year return period [Reference 9]) and the typical design standard flood (1 in 100 year). This results in drainage inadequacies for the flood return period developments need to consider (typically the 1% annual probability), 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. The Government’s preferred method to achieve this is through the use of Sustainable Drainage Systems (SuDS). PPS25 notes that regional planning bodies and local authorities should promote the use of SuDS for the management of runoff. The PPS25 Practice Guide notes that flood risk management is an important consideration in all of the steps of the flood risk management hierarchy: These four steps are: Assess risk associated with surface water through the RFRA, SFRA and FRA; Avoid risks from surface water by controlling water at source (SuDS) and locating development away from risk areas; Substitute apply the sequential approach to locate more vulnerable development in lowest risk areas; and Control use SuDS and implement Surface Water Management Plans to manage and reduce risk.
9.2 What are SuDS?
SuDS seek to manage surface water from new developments as close to its source as possible, mimicking the surface water flow regime present on a site prior to development. Typically this approach involves a move away from conventional piped systems to softer engineering solutions inspired by natural drainage processes.
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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 to account for the predicted effects of climate change. Wherever possible, a SuDS technique should seek to contribute to each of the three goals of sustainability identified below with the favored system contributing significantly to each objective. The three goals of sustainable drainage systems are: 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 train (see Table 9-3 below) incorporating a chain of techniques throughout a development, (as outlined in CIRIA C697 [Reference 23]), 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 management from a number of sites (e.g. into a wetland).
In keeping with the guidance of PPS25, local authorities should encourage the application of SuDS techniques. This chapter presents a summary of a selection of the SuDS techniques available and a review of the soils and geology of the study area, enabling WHBC to identify what type of SuDS techniques could be employed in development schemes. Detailed guidance for the design of SuDS is available in the CIRIA SuDS Manual C697 (Reference 23), and the associated document ‘Site Handbook for the Construction of SuDS, C698’ (Reference 24). These publications provide best practice guidance on the planning, design, construction, operation and maintenance of SuDS, to ensure effective implementation within developments.
9.3 SuDS Policies
In addition to the policies presented in PPS25 there are a number of other policies and planning documents that promote the implementation of SuDS in new developments.
9.3.1 PPS25 (Reference 2)
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.”
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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 powers of direction, developments that do not incorporate SuDS without sound reasons 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.
9.3.2 Building Regulations 2002 H3 Rainwater Drainage (Reference 25)
The Building Regulations 2002 enable the principles of PPS25 to be enforced during construction by stipulating that: 1. Adequate provision shall be made for rainwater to be carried from the roof of the building; 2. Paved areas around the building shall be so constructed as to be adequately drained; 3. Rainwater from a system provided pursuant to sub-paragraphs 1) or 2) shall discharge to one of the following, listed in order of priority: • an adequate soakaway or some other adequate infiltration system; or, where that is not reasonably practicable, • a watercourse; or, where that is not reasonably practicable, • a sewer.
9.3.3 Code for Sustainable Homes (Reference 26)
The Code for Sustainable Homes has become a mandatory requirement for all new homes as of April 2008. It means that potable water consumption and surface water drainage techniques in new homes will be assessed against the six level rating system of the Code. There are three levels of mandatory standards for potable water consumption and one level for surface water runoff. Proposed developments are assessed against a number of sustainability criteria that include ‘water’ and ‘surface water run-off’. For each category points are awarded depending on the sustainability of the management technique proposed (i.e the more sustainable the more points are awarded). The points for each category are collated and the development is given a code level from 1 – 6. Table 9-1 and Table 9-2 summarises the measurement criteria used for both potable water consumption and surface water runoff in the Code for sustainable homes. Table 9-1: Summary of the measurement criteria for potable water consumption
CATEGORY 2 WATER ISSUE MEASUREMENT CRITERIA POINTS AWARDED Where predicted water consumption (calculated using One of the the Code water calculator) accords with the following following point levels: scores Internal ≤ 120 l/p/d 1.5 potable water ≤ 110 l/p/d 3 consumption ≤ 105 l/p/d 4.5 ≤ 90 l/p/d 6 ≤ 80 l/p/d 7.5 External For providing a system to collect rain water for use in 1.5
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CATEGORY 2 WATER ISSUE MEASUREMENT CRITERIA POINTS AWARDED potable water external irrigation/watering e.g.water butts. consumption
Table 9-2 : Summary of the measurement criteria for surface water runoff
CATEGORY 4 SURFACE WATER RUNOFF ISSUE MEASUREMENT CRITERIA POINTS AWARDED Where rainwater holding facilities/sustainable drainage (SuD) is used to provide attenuation of water run-off to either natural watercourses or municipal systems. Points 0.5 for attenuation covering: • Hard surfaces Reduction of AND OPTIONALLY surface water
run-off from • Roofs site AND The percentage peak time attenuation should be OPTIONALLY provided as follows 0.5 • 50% in low flooding risk areas; • 75% in medium flooding risk areas; and, • 100% in high flooding risk areas. Where evidence is provided to demonstrate that the EITHER assessed development is located in an area of EITHER 1 • Low annual probability of flooding OR • Medium/high annual probability of flooding (subject to plans being approved by the relevant statutory Flood Risk bodies) and where; • The ground level of buildings, car parks and access OR routes are above the flood level; 0.5 • An appropriate assessment of how the building will react to flooding (including the use of resilient construction where necessary) to mitigate residual risk.
9.3.4 Environment Agency Policies
The Thames Catchment Flood Management Plan (Reference 13) also advocates policies relating to Sustainable Drainage Systems, these are: • The use of SuDS on all new development sites. • All sites greater than 1 hectare in size require the following: • SuDS, • Greenfield discharge rates, • 1 in 100 year on-site attenuation taking into account climate change. • Allocated land should set-aside space for SuDS.
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These are more arduous that those presented in PPS25 but may deliver significant benefit in some areas.
9.3.5 Other Policies
Section 3 outlines the policies that govern development and flood risk management in WHBC. SuDS are also promoted in ‘Making Space for Water’; RPG9 (INF2); Draft Regional Spatial Strategy for the East of England; and the Welwyn Hatfield District Plan (R10).
9.4 SuDS Methods
SuDS techniques can be used to reduce the rate and volume and improve the quality of surface water discharges from sites to the receiving environment (i.e. a natural watercourse, public sewer or groundwater). Various SuDS techniques are available, however the techniques operate on two main principles: • Infiltration • Attenuation 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 a 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, and Thames 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 9-3 provides a summary of SuDS techniques and their suitability to meet the three goals of sustainable drainage systems. This table is based on the SuDS philosophy of a management train for surface water, where ideally surface water runoff is prevented or retained as close to the source as possible. In some cases this is not possible and therefore Table 9-3 provides a range of SuDS techniques available for different scales of management.
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Table 9-3: Summary of SuDS Techniques and their Suitability to meet the three goals of sustainable drainage systems
MANAGEMENT WATER WATER AMENITY COMPONENT DESCRIPTION TRAIN QUANTITY QUALITY 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 perforated Filter drains X pipe along the base. ● ● Similar to filter drains but allows infiltration through Source Infiltration trenches X sides and base. ● ● Soakaways Underground structure used for store and infiltration. ● ● X 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 X media consisting of sand. ● ● Dry depressions outside of storm periods, provides Basins temporary attenuation, treatment and possibly ● ● ○ Site Site infiltration. Designed to accommodate water at all times, provides
Ponds attenuation, treatment and enhances site amenity ● ● ● Regional value. Similar to ponds, but are designed to provide Wetland continuous flow through vegetation. ● ● ●
Key: ● – highly suitable, ○ - suitable depending on design, X – unsuitable
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9.5 Where can SuDS be utilised?
The underlying ground conditions of a development site will often determine the type of SuDS approach to be used at an individual site. 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, SuDS techniques that are compatible with the underlying geology can be identified: • The Soil Survey of England and Wales 1993 – 1:250,000 Soils Maps (Sheet 6); • The Geological Survey of Great Britain (England and Wales) 1:50,000 Series Drift Edition Sheets 239 (1978); and • The Soils Map Legend. Through examination of the maps an indication of the areas in which SuDS techniques may be appropriate can be identified and are presented in Appendix E. Tables presented in Appendix E provide an indication of where SuDS may be suitable and are provided as a guide alone and should not be used to accept or refuse SuDS techniques. The geology and pedology of the study area is also shown in Figure 3, Figure 4 and Figure 25 (GIS layer: Solid Geology / Drift Geology / Pedology). Overall the suitability and design of a SuDS system should be determined on a site by site basis through consultation with the authority responsible for the receiving waterbody.
9.6 SuDS Constraints
If, after, geotechnical analysis and permeability testing of the strata underlying a site infiltration is considered appropriate, the allocation site must also be categorised in terms of proximity to a groundwater abstraction source to ensure appropriate treatment measures are put in place to preserve the groundwater resource.
The use of sustainable drainage systems throughout the study area can be limited based on a number of issues, which include: • Groundwater vulnerability and potential contamination of an aquifer; • The presence of groundwater source protection zones and potential contamination of the potable water source; • Restrictions on infiltration on contaminated land to prevent the spread of contamination; and, • Restrictions on space on development sites where housing densities are high.
9.6.1 Groundwater Vulnerability
Groundwater resources are vulnerable to contamination from both direct sources (e.g. into groundwater) or indirect sources (e.g. infiltration of discharges onto land). Groundwater vulnerability within the study area has been determined by the Environment Agency, based on a review of aquifer characteristics, local geology and the leachability of soils.
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The vulnerability of the groundwater is important when advising on the suitability of SuDS. Through examination of groundwater vulnerability maps the aquifer classifications which can be found within the study area are detailed in Appendix E. The following map was examined: • NRA (1986) ‘Groundwater Vulnerability of West London: Sheet 39, HMSO: London. Appropriate SuDS based on groundwater vulnerability are summarised in Appendix E and shown in Figure 12.
9.6.2 Source Protection Zones
In addition to groundwater vulnerability, the Environment Agency also defines groundwater Source Protection Zones around groundwater abstraction points. Source Protection Zones (SPZ) are defined to protect areas of groundwater that are used for potable supply, including public/private potable supply, (including mineral and bottled water) or for use in the production of commercial food and drinks. SPZs are defined based on the time it takes for pollutants to reach an abstraction point. This transmission time enables the Environment Agency to define 3 zones around a groundwater abstraction point. The majority of the study area has not been classified, but the four zones and their locations are shown in Figure 12: • Zone 1 (Inner Protection Zone) – This is defined as ‘any pollution that can travel to the borehole within 50 days from any point within the zone is classified as being inside zone 1’. • Zone 2 (Outer Protection Zone) – This is defined as the area that ‘covers pollution that takes up to 400 days to travel to the abstraction borehole, or 25% of the total catchment area – whichever area is the biggest’. • Zone 3 (Total Catchment) - The total catchment is the total area needed to support removal of water from the borehole, and to support any discharge from the borehole. • Zone 4 (Zone of special interest) – In the study area a fourth zone has been defined. ‘This is usually where local conditions mean that industrial sites and other polluters could affect the groundwater source even though they are outside the normal catchment area’. Depending on the nature of the proposed development and the location of the development site with regards to the SPZs, restrictions may be placed on the types of SuDS appropriate to certain areas. Any restrictions imposed on the discharge of site generated runoff by the Environment Agency will be determined on a site by site basis using a risk based approach.
9.6.3 Planning Considerations for SuDS
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 to the site and surrounding area. 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. Attenuation cannot be “traded” between developments. The design of SuDS measures should be undertaken as part of a drainage strategy proposed during the design of a development sites. A ground investigation will be required to access the suitability of using infiltration SuDS, with this information also being used to assess the required volume of on-site
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storage. Hydrological analysis should be undertaken using industry-approved procedures; to ensure a robust design storage volume is obtained. All relevant organisations should meet at an early stage of the drainage design process to agree on the most appropriate drainage system for the particular development. These organisations may include the Local Authority, the sewage undertaker, Highway Agency, and the Environment Agency. Liaison with these organisations should focus on establishing a suitable design methodology, any restrictions and provision for the long-term maintenance of the feature. There are, at present, no legally binding obligations relating to the provision and maintenance of SuDS. However, PPS25 (Reference 2) states that: “where the surface water system is provided solely to serve any particular development, the construction and ongoing maintenance costs should be fully funded by the developer.” The most convenient vehicle for agreeing long-term management responsibilities is through an agreement reached under Section 106 of the Town and Country Planning Act. Under this, agreement for SuDS maintenance can be a requirement of the planning application, forcing the issue to be addressed. Table 3 (Appendix D) is provided to assist WHBC develop an understanding of the types of SuDS features that could be incorporated into development sites they may bring forward through their LDF. The same approach can also be used when assessing the feasibility of SuDS systems for windfall sites.
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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 Thames CFMP (Reference 13). From a review of these existing policies and the information on flood risk collated through this Level 1 SFRA the following catchment wide and specific area policy recommendations have been developed. The policy recommendations are proposed under the headings Flood Risk, Sustainable Drainage, Flood Mitigation and the Water Environment. Integration of these suggested policy considerations into the LDF / LDDs should ensure that the objectives and aspirations of the Environment Agency, national and regional policy are met whilst strengthening the position of the Local Planning Authority with regard to Flood Risk. The policies below in italics are those required by the Environment Agency in the study area. The policies in bold are those that have been developed through a review of the data as part of this SFRA.
10.1 Flood Risk
Planning Policy Statement 25 (PPS25) (Reference 2) aims 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 highest risk’.
10.1.1 Study Area Wide Strategies
To achieve the aim of PPS25 the following policy considerations are recommended: • Abide by the principles of PPS25 and liaise closely with the Environment Agency to ensure that all flood risk concerns and issues are dealt with. • Have regard to the cumulative impact of development on flood risk. • Define areas where windfall development would be appropriate across the study area, and assessing what type of development would be appropriate in each area, based on the flood risk. Where a windfall site has not been sequentially tested by WHBC, the developer will need to provide evidence to WHBC that they have considered other reasonably available sites, through comparing windfall sites against allocated sites in the Local Development Plans. • Consider flood risk as one of a number of policies that in tandem can provide mechanisms to deliver sustainable developments with multiple benefits. • Engage with developers and local regulators such as the Environment Agency, Thames Water, and Hertfordshire Highways, throughout the development/planning process to develop and instigate initiatives for the reduction of flood risk. • Prepare Flood Risk Assessments for all scenarios identified in Table 11-1 that would not automatically be picked up by the Environment Agency in their role as a Statutory Consultee. • The Environment Agency are a statutory consultee for: • development over 1 ha in Flood Zones 2 and 3, • development in Flood Zone 1 where there are critical drainage problems,
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• any development exceeding one hectare in extent, • development within 20 metres of the bank top of a Main River, and • Any culverting operation or development which controls the flow of any river or stream • PPS25 (Reference 2) defines a major development as: • For residential development, the number of dwellings to be provided in 10 or more, or the site area is 0.5 hectares or more, or • for non-residential development, the new floorspace to be provided is 1,000 square metres or more, or the site area is 1 hectare or more. Due to the criteria under which the Environment Agency are consulted, the Local Authority are required to request, review and approve Flood Risk Assessments for developments at risk from other flood sources (groundwater, surface water, drainage and infrastructure failure) or developments less than 1 hectare is size in flood zone 1. In such circumstances the LPA should; • Ensure flood risk assessments prepared for developments conform to national policy and the additional elements identified in this SFRA, where required by the planning authority. • Have regard to the role development sites could have to alleviate flood risk elsewhere .
10.1.2 Area Specific Strategies
Groundwater flooding has been noted as occurring in the areas of Cuffley, East Hatfield and Brookmans Park.
The Council should ensure new developments in these areas have a Flood Risk Assessment prepared for them to determine the risks from groundwater flooding and incorporate mitigation measures into the design of any buildings to prevent flood damage from this source.
Highway flooding has occurred in a number of locations throughout the study area.
The Council should ensure that developers whose sites are bounded by roads with a medium or high probability of flooding prepare a Flood Risk Assessment to determine the risks from highway flooding and incorporate mitigation measures into the design of any buildings to prevent flood damage from this source. The determining factor for the requirement for a Flood Risk Assessment should be development vulnerability, as defined under PPS25.
The Council should ensure the cumulative impact of new developments in the area do not increase the burden on the existing drainage system either though restricting site discharge rates and/or through capital contributions to improvements works of the existing drainage infrastructure.
10.2 Flood Risk Management
It is envisaged that with the impact of climate change, flooding is expected to increase as a result of poor capacity of structures. To mitigate for this impact the council should consider the following policy recommendations.
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10.2.1 Study Area Wide Strategies
• Surface Water Management Plans provide co-ordination between the fragmented mechanisms and responsibilities for managing surface water drainage, which is currently split between local authorities, highway authorities, water companies, the Environment Agency and Internal Drainage Boards (in drainage districts). A SWMP should be prepared depending on the nature of the flood sources and size/scale of development. • Further culverting of rivers should be discouraged and opportunities should be sought to deculvert rivers, 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; • Where this is not possible seek opportunities to facilitate the investigation and where necessary and suitable the upgrade of, bridges, culverts, drainage systems etc in line with current climate change considerations, through local schemes. • Review the condition of existing local defences, the dependence of proposed and existing development on them for flood mitigation and where necessary the Council should ensure defences which come under their jurisdiction are maintained and or improved to be commensurate with the lifetime of the development. • Safeguard floodplains from development, ensuring the maximum possible capacity is available to attenuate floodwater and thereby safeguard existing property. Where development in the floodplain is unavoidable and floodplain storage is removed, the development should provide compensatory storage on a level for level basis to ensure that there is no loss in flood storage capacity. • Seek opportunities to restore natural river forms and floodplains (through managed retreat where possible) and in so doing restore river corridors and floodplains as areas for biodiversity or increasing their amenity value. The Environment Agency require that: • Main rivers will require an 8 metre wide undeveloped buffer strip alongside the river channel • Culverted main rivers will require a 8 metre wide undeveloped buffer strip alongside the channel • Ordinary Watercourses will require an 8 metre wide undeveloped buffer strip alongside the river channel • Where a development identified as located within Flood Zone 3b, 3a or 2 is applying for a change of use flood evacuation plans should be developed through liaison with the emergency services. • Adopt a policy for the routine maintenance of all watercourses ensuring they are clear of debris that could affect flood flow conveyance and water quality.
10.3 Sustainable Drainage Systems
Due to expansion of developed areas, the drainage systems designed to serve the original settlements can become overloaded as development/expansion takes place on their periphery, leading to flooding of old centres. In addition the design standard of modern sewer systems is typically to accommodate the 30 year storm, with events in excess of this expected to result in
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flooding. With the impacts of climate change the effective design standard of the sewer system is expected to decrease potentially leading to more frequent flooding. In addition, conventional drainage systems typically discharge storm water to nearby watercourses. As urbanisation and intensification of development in catchments increases, storm water inputs have the potential to impact on water quality. With the incorporation of the Water Framework Directive into UK law the Council should seek opportunities to contribute to the goal of improving the quality of local watercourses.
10.3.1 Study Area Wide Policies
• Require sustainable drainage design to take account of the impacts of climate change for the lifetime of the development at the site and downstream. • Consider the potential benefits an appropriately designed Sustainable Drainage System could have for the biodiversity, amenity value, water quality and resource value of a development and/or surrounding area. • Consider the vulnerability and importance of local water resources and key infrastructure when determining the suitability of drainage strategies/SuDS. The use of SuDS should be considered on all development sites, unless the underlying geology or other conditions precludes their use. • Developments that are greater than 1 hectare in area should restrict discharge rates to greenfield rates up to and including the 1 in 100 year rainfall event, accounting for climate change, through the use of SUDS. • On sites which are less that 1 hectare, SUDS should also be incorporated. A reduction in runoff rates during the 1 in 100 year storm event plus climate change to greenfield rates is the ideal, however, where space does not allow for greenfield rates to be attenuated, the development must prove that a betterment has been achieved in the reduction of runoff rates, with SUDS being utilised where possible. • Seek opportunities to contribute to the goal of improving the quality of local watercourses in line with the Water Framework Directive through improving the quality of storm water discharges from developments to watercourses. • Seek opportunities to utilise SuDS in areas shown to be potentially at risk of overland flow flooding.
10.4 Flood Risk & Environment
As the population increases and climate change leads to hotter drier summers, the prospect of droughts will increase. New development can tackle this by incorporating water efficiency measures, such as greywater recycling, rainwater harvesting and water use minimisation technologies in accordance with mandatory standards (potable water consumption) within the Code for Sustainable Homes. In doing so, knock-on benefits could be felt by the sewer system which will receive less wastewater from properties, potentially freeing up capacity during flood events. In addition, increasing people’s awareness of the water environment around them together with its importance and its hazards, will contribute to their understanding of where floods come from and what they can do to limit the consequences of flooding and resource shortages in line with the governments objectives which are set out in Making Space for Water (Reference 11).
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10.4.1 Study Area Wide Strategy
• Ensure that proposed developments can be accommodated by the existing resource provision/strategic infrastructure. Where a development cannot be accommodated by current resources, ensure that the phasing of development is in tandem with resource infrastructure investment. • Seek opportunities to implement flood defence schemes that will contribute towards ‘Good Ecological Status’ which is required under the Water Framework Directive.
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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, and 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 in the WHBC SFRA study area is not limited to just rivers, in fact flooding is proven 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 elsewhere. Early identification of flood risk constraints can ensure developments maximise development potential whilst achieving the principles of sustainability. This Level 1 Strategic Flood Risk Assessment presents sufficient information to assist WHBC to apply the ‘Sequential Test’ and identify where the Exception Test may be required. However, the scale of assessment undertaken for a Level 1 Strategic Flood Risk Assessment is typically inadequate to accurately assess the risks faced by a particular discrete development at a location within the study area. The Level 1 SFRA has attempted to identify all sources of flood risk at the catchment and borough scale using the best available information. More local and site specific sources of flooding may become apparent during preparation of the Level 2 SFRA or during the course of a site specific FRA. Therefore, as part of planning applications for both allocated and non-allocated sites, site specific FRA will be required in areas at risk of flooding, 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. This section presents the recommendations for site specific FRAs; the circumstances under which they should be prepared and their requirements for submission with planning applications to WHBC. The site specific FRA guidance presented in the following sections has been developed based on: • The recommendations presented in Planning Policy Statement 25 and the accompanying June 2008 Practice Guide; • The Environment Agency’s Piper Networking Standing Advice; • A review of the policies contained within the existing Local Plan for WHBC; and • The information and findings gathered and developed during preparation of this Level 1 SFRA.
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11.1 When are 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 accordance with PPS25 and the General Development Procedure Order (GDPO) (Reference 27) FRAs should always be provided with planning applications in the following situations: • The development site is located in Flood Zone 2 or 3; • Where a residential proposed development comprises 10 or more residential dwellings and/or the site areas is greater than 0.5 hectares (even if the site is located in Flood Zone 1. This is to ensure storm water generated by the site is managed sustainably and does not increase the burden on existing infrastructure and/or flood risk to neighbouring property); • Where a non-residential proposed development has a floor area greater than 1000m² or the site areas is greater than 1 hectare; • The development site is located in an area known to have experienced flooding problems from any flood source; and, • The development is located within 20m of any watercourse regardless of Flood Zone classification. In these situations planning applications situated in fluvial flood zones and for development sites greater than 1 hectare will be referred to the Environment Agency for comment. However for flooding from other sources or development scenarios within the study area, the LPA must establish the requirements for FRAs and assess their suitability as part of the planning application. Such situations, established through a review of the information collected for this Level 1 SFRA and review of national policy, include sites: • of operational development less than 1 hectare within Flood Zone 1; • At risk from sewer flooding, groundwater flooding, artificial sources or in an area identified as at risk from overland flow flooding. Table 11.1 presents the flood risk guidance table that can be used to determine if an FRA is required for a range of development types..
11.2 FRA Requirements
In general for all planning applications where a FRA is required by PPS25 or the GDPO, it will be necessary to prepare a document to the satisfaction of the Environment Agency. Table 11-1 presents an indication of the requirements for FRAs meeting these criteria, however the precise requirements should be agreed and established with the Environment Agency at the outset of the project. In scenarios where the LPA are the consultee the requirements of the FRA should be based on the guidance presented in Table 11-1 and discussed with the LPA at the outset of the assessment. The Practice Guide to PPS25 advocates a staged approach to site specific FRA with the findings from each stage informing the next and site master plans, iteratively throughout the development process.
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The staged approach comprises: • Level 1 FRA - Screening Study • Level 2 FRA - Scoping Study • Level 3 FRA - Detailed Study However it will not always be necessary to prepare each of the documents, in some cases where a site is known to flood it may be appropriate to prepare a Level 2 or 3 assessment directly. Annex E of PPS25 presents the minimum requirements for FRA at each stage in the process. These include:
• Considering the risk of flooding arising from the development in addition to the risk of flooding to the development; • Identifying and quantifying the vulnerability of the development to flooding from different sources and identify potential flood risk reduction measures; • Assessments of the remaining ‘residual’ risk after risk reduction measures have been taken into account and demonstrate that this is acceptable for the particular development; • 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; • Considering 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. • Fully account for current climate change scenarios and their effect on flood zoning and risk. Where a particular element of the FRA cannot be achieved to the satisfaction of the EA or LPA it will be necessary to advance the next level of FRA.
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 this Level 1 SFRA. Therefore this type of study could 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 could then be used by the developer as a basis to further their understanding of how the flood risks could potentially affect their development.
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 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 site master plans raising a developer’s awareness of the flood management elements the proposed development may need to consider.
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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 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 required to ensure details of flood flow velocity, onset of flooding and depth of flood water is fully understood and that the proposed development incorporates appropriate mitigation measures. Further details of the elements a Level 2 and/or a Level 3 site specific FRA should consider are presented in Table 11-1. Flood Risk Assessment Guidance Table The Flood Risk Assessment Guidance Table (Table 11-1) is intended to provide guidance to developers and the LPA on the requirements of site specific FRA for flooding from all sources present in the study area and for the common range of development scenarios.
11.3 Specific Requirements within the Flood Risk Guidance Table
The Flood Risk Assessment Guidance Table (Table 11-1) is intended to provide guidance to developers and Local Authorities on the requirements of site specific FRA for flooding from fluvial sources or major developments where the Environment Agency is a statutory consultee, and for those areas or flood sources for which the Environment Agency is not a statutory consultee. The table provides a framework with which Local Authorities and Developers will be able to assess the requirements of each individual development with regard to flood risk. The table is intended to be used as a matrix working from the column on the left and the row at the top establish when a FRA is required and what the FRA should be comprised of: 1. The initial column provides the details of different development scenarios; 2. The initial row sets out the different sources of flood risk or development locations which would require an FRA to be submitted with a planning application 3. The matrix provides a colour coded guide to indicate which party will be responsible for reviewing the submitted FRAs. Scenarios colour coded green the will be assessed by the Environment Agency and scenarios colour coded yellow will need to be assessed by the Local Authority 4. Each scenario provides guidance on what should be included in any FRA submitted, details on a number of potential mitigation measures and sources of further information. The lists provided are not meant to be exhaustive but provide guidance which can be built on and added to as policy dictates.
11.3.1 Flood Evacuation Plan
Where Flood Evacuation Plans (FEP) are required as part of a planned development, these should be prepared in conjunction with the guidance presented in PPS25 (Reference 2). The FEP should define the principles for dealing with the flood risk to a specific development. The following should be included within the FEP:
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• Emergency Route Plan. • Checklist of Do’s and Don’ts formulated taking into account the specific building design. • Poster design to be displayed in Communal Areas setting out the flood risks and emergency procedures for particular building. The fundamental points of the flood evacuation strategy are as follows: • Education and training - increase awareness regarding the risk of flooding, ensure that all people are familiar with the flood evacuation strategy and the emergency route plan provided for each building. • Practical flood advice – Checklists, flood ‘dos and don’ts’ will make reference to the how floodwaters enter a building, the risks they pose, how to react, as well as describe other potential hazards. • Flood warning systems and Flood Wardens - Each building should be linked to the Environment Agency’s flood warning system if available. This system issues warnings of flood events for areas at risk of fluvial flooding. If appropriate to the nature of development a flood warden should be nominated to ensure an appropriate system within the building for receiving the warnings is set up and that appropriate action is taken. • Emergency Route Plan - The Health and Safety Plan for each building should include an emergency route plan showing recommended routes offsite to nominated reception centres or, where evacuation is not possible, routes to the upper floors and possible rescue or resupply locations.
11.3.2 Drainage Strategy
A drainage strategy should be designed to demonstrate to the Environment Agency/LPA that a site can be drained in a sustainable manner, and that through redevelopment flood risk to properties downstream of the site will not be exacerbated. A drainage strategy should include the following information: • Permitted discharge rates; • Storm water runoff calculations from parcels of land on the site; • Attenuation required on each parcel of land to restrict runoff to permitted discharge rates; • Proposed means of attenuation (SuDS techniques); • Distribution of storm water attenuation across the site; and, • Design standards and parameters of the proposed storm drainage techniques. This list should not be considered as exhaustive and may require additional elements depending on the nature and scale of the proposed development and mitigation required.
11.3.3 Storm water Runoff Calculations
Storm water runoff calculations should be undertaken for the following development scenarios: • greenfield • current development
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• post development A study undertaken by Defra and the Environment Agency (Reference 29) identifies the preferred methods for calculating runoff rates from the different development scenarios.
11.3.4 Preservation of Flood Flow Routes
Ensure that on redevelopment known flood routes are not blocked by buildings. This could be prevented through opening up green corridors adjacent to the river or orientating buildings in such away not to obstruct flood flows.
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11.4 Consequences of Failing to Adequately Consider Flood Risk
Developing in flood risk areas can result in significant risk to a development, site users and neighbouring property. Following the advice provided in Table 11-1 should enable flood risks to be identified and mitigated. In the event flood risks are not assessed and/or mitigated for the following issues should be considered:
• Failure to consider wider plans prepared by the Environment Agency or other operating authorities may result in a proposed scheme being objected to by the Environment Agency. • Failure to identify flood risk issues early in a development project could result in failure of a development proposal, requiring redesign of the site to mitigate flood risk. • Failure to adequately assess all flood risk sources and construct a development that is safe over its lifetime could increase the number of people at risk from flooding and/or increase the risk to existing populations. • Failure to mitigate the risk arising from development may lead to claims against the developer if an adverse effect can be demonstrated (i.e. flooding didn’t occur prior to development) by neighbouring properties/residents. • Properties may be uninsurable and therefore unsalable if flood risk management is not adequately provided for the lifetime of the development. • By installing SuDS without arranging for their adoption or maintenance the SuDS will eventually cease to operate as designed and may present a flood risk to the development and/or neighbouring property.
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Table 11-1: Flood Risk Assessment Guidance Table
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Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment
Table 11 – 1: Flood Risk Guidance Table
Development Includes (including boundary culverting or Sewer Flooding Groundwater Flooding Overland Flow Artificial Sources Development walls etc.) within 20 control of flow Within Flood Zone 3b Within Flood Zone 3a Within Flood Zone 2 Within Flood Zone 1 Category For areas defined as at medium or high risk of flooding from this metres of the top of a of any river or For areas defined as at medium of high risk or flooding from this For areas defined as within or adjacent to ‘Source For areas located 00m from this source the source and from historical Hertfordshire Highways flooding data the source the following issues must be considered Areas of Potential Overland Flow’ following issues must be considered bank of a main river. stream. following issues must be considered
Consult EA Consult EA with Residential development Planning application must include details of flood mitigation measures, either: Planning application must include details of flood mitigation measures, Rainwater to be discharged to one of following in In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Home owner to ensure that they are registered with FRA showing should not be permitted. • Set floor levels no lower than existing levels AND include flood proofing where either: order of priority: • Raising finished floor levels – to be considered on site by site basis. • Assessment of risk from Groundwater flooding • Assessment of risk posed by overland flow, the LPA’s flood warning service. design details of appropriate, • Set floor levels no lower than existing levels AND, include flood • Adequate soakaway or infiltration system • Construct buildings with solid floors • Flood resilient and flood resistant measures should be considered in any culvert or flow • OR, set floor levels 300mm above the 1 in 100 year flood level plus proofing where appropriate. • a watercourse; or, where not reasonably practicable, • Flood resilient and flood resistant measures should be considered line with Building Regulations. control structure climate change. • Or, set floor levels 300mm above the 1 in 100 year flood • a sewer • For developments in Cuffley, East Hatfield and Brookmans Park the Possible mitigation may include: Home owner to be aware of emergency plans. proposed. • Details of flood resilience or resistant techniques to be included. level plus climate change. LPA should ensure the FRA determines the risks from groundwater • Raising finished floor levels – to be considered on
• If the site is considered defended, residual risk must be identified in the FRA in • If the site is considered defended, residual risk must be identified in flooding and incorporate mitigation measures into the design of any site by site basis
the event of a failure of the defences, either through overtopping or breach the FRA in the event of a failure of the defences, either through buildings to prevent flood damage from this source. • Construct buildings with solid floors LPA emergency planning service must obtain flood
EA Flood Defence (depending on the nature of the defences). Flow paths to the site should be overtopping or breach (depending on the nature of the defences). • Orientate buildings to prevent impediment of plans for the residential area and incorporate them.
Householder Consent required. identified to determine the level of residual flood risk to the site and potential Flow paths to the site should be identified to determine the level of overland flow routes.
development egress/access routes. residual flood risk to the site and potential egress/access routes. Possible mitigation may include: • Flood resilient and flood resistant measures should
• If the site is considered within the undefended floodplain, any increases in • Details of flood resilience or resistant techniques to be included. • Raising finished floor levels – to be considered on site by site basis be considered in line with Building Regulations. More specifically areas in close proximity to the and alterations building footprint may require flood compensation storage on a level for level • If the site is considered within the undefended floodplain, any • Constructing buildings with solid floors following should consider the residual risk as part of a
and volume for volume basis. increases in building footprint may require flood compensation • Providing raised walkways to ensure safe access and egress during a FRA:
• Exception Test may be required depending on PPS25 vulnerability storage on a level for level and volume for volume basis. flood event. classification • Ensure that building foundations and piling do not disrupt groundwater • FURTHER INFORMATION FURTHER INFORMATION flow routes Brocket Hall Lake, Lemsford, Nr Welwyn Garden FURTHER FURTHER INFORMATION • Code for Sustainable Homes 2008 FURTHER INFORMATION • DCLG Improving the Flood Performance of New Buildings – Flood City; FURTHER INFORMATION INFORMATION • • Code for Sustainable Homes 2008 • Building Regulations 2000 Approved Document H3 Rainwater • Code for Sustainable Homes 2008 Resilient Construction 2007 FURTHER INFORMATION The Broadwater, Welwyn Garden Cuty; • http://www.pipernetworking • http://www.piper • • Building Regulations 2000 Approved Document H3 Rainwater Drainage Drainage • Building Regulations 2000 Approved Document H3 • Building Regulations 2000 Approved Document H3 Rainwater Drainage FURTHER INFORMATION • Building Regulations 2000 Approved Document H3 Stanborough Lake; and, .com networking.com • • http://www.pipernetworking.com • http://www.pipernetworking.com Rainwater Drainage • Code for Sustainable Homes 2008 • Building Regulations 2000 Approved Document H3 Rainwater Drainage Rainwater Drainage Gravel pits near Coopers Green.
Consult EA Consult EA with Extension to non-residential Planning application must include details of flood mitigation measures, either: Planning application must include details of flood mitigation measures, Rainwater to be discharged to one of following in order In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: LPA to set up flood warning service for development at FRA showing developments in Flood Zone • Set floor levels no lower than existing levels AND, include flood proofing where either: of priority: • Stormwater runoff calculations in relation to the capacity of the sewer • Assessment of risk from GW flooding • Assessment of risk posed by overland flow, risk. design details of 3b should not be permitted. appropriate. Details of Flood Resilience or Resistant techniques to be • Set floor levels no lower than existing levels AND, include flood • Adequate soakaway or infiltration system system • GW monitoring should be considered to assess GW levels in relation to including velocities, pathways, flood depths, any culvert or flow included. proofing where appropriate. • a watercourse; or, where not reasonably practicable, • Analysis of surcharged flood levels topographic levels of the site. ponding etc. control structure A flood evacuation plan • If the site is considered defended, residual risk must be identified in the FRA in • OR, set floor levels 300mm above the 1 in 100 year flood • a sewer • Drainage strategy to ensure sewer flooding will not be exacerbated. • Flood resilient and flood resistant measures should be considered in • Flood evacuation plan Owner/user to have a flood evacuation plan in place proposed. should be in place. the event of a failure of the defences, either through overtopping or breach level plus climate change. • Flood evacuation plan line with Building Regulations. (depending on the nature of the defences). Flow paths to the site should be • Details of flood resilience or resistant techniques to be included. • Flood resilient and flood resistant measures should be considered identified to determine the level of residual flood risk to the site and potential • Flood Evacuation plan required Possible mitigation may include: More specifically areas in close proximity to the EA Flood Defence egress/access routes. • Exception Test may be required depending on PPS25 vulnerability Possible mitigation may include: • Raising finished floor levels following should consider the residual risk as part of a Consent required. • Flood Evacuation plan required classification Possible mitigation may include: • Raising finished floor levels • Construct buildings with solid floors FRA: Non- residential • Access to higher floors is desirable (essential for higher vulnerability • If the site is considered within the undefended floodplain, any • Raising finished floor levels • Constructing buildings with solid floors • Incorporate SuDS to limit runoff classifications such as schools). increases in building footprint may require flood compensation • Construct buildings with solid floors • Providing raised walkways to ensure safe access and egress during a • Flood resilient and flood resistant measures should extensions with • • If the site is considered within the undefended floodplain, any increases in storage on a level for level and volume for volume basis. • Provide raised walkways to ensure safe access and egress during a flood event. be considered in line with Building Regulations. Brocket Hall Lake, Lemsford, Nr Welwyn Garden a footprint of building footprint may require flood compensation storage on a level for level flood event • Ensure that building foundations and piling do not disrupt groundwater • Orientate buildings to prevent impediment of City; • less than 250m² and volume for volume basis. • Incorporate SuDS to limit runoff flow routes. overland flow routes. The Broadwater, Welwyn Garden Cuty; • • Exception Test may be required depending on PPS25 vulnerability Stanborough Lake; and, • classification Gravel pits near Coopers Green.
FURTHER INFORMATION FURTHER INFORMATION FURTHER INFORMATION • Code for Sustainable Homes 2008 FURTHER INFORMATION FURTHER • • Code for Sustainable Homes 2008 • Building Regulations 2000 Approved Document H3 Rainwater Code for Sustainable Homes 2008 • DCLG Improving the Flood Performance of New Buildings – Flood FURTHER INFORMATION FURTHER INFORMATION INFORMATION • • Building Regulations 2000 Approved Document H3 Rainwater Drainage Drainage Building Regulations 2000 Approved Document H3 Resilient Construction 2007 FURTHER INFORMATION • Building Regulations 2000 Approved Document H3 • http://www.pipernetworking • http://www.piper • http://www.pipernetworking.com • http://www.pipernetworking.com Rainwater Drainage • • Building Regulations 2000 Approved Document H3 Rainwater Drainage Rainwater Drainage .com networking.com Building Regulations 2000 Approved Document H3 Rainwater Drainage
Only Consult EA if site falls Consult EA under Less vulnerable uses are not Consult EA with FRA Water treatment plants and waste treatment plants need to include Rainwater to be discharged to one of following in order In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Possible mitigation measures may include: within Flood Zone 3a or 3b part IVV, Section permitted in Flood Zone 3b. flood resistant measures. of priority: • Stormwater runoff calculations in relation to the capacity of the sewer • Assessment of risk from GW flooding • Assessment of risk posed by overland flow, • Raising finished floor levels 109 (1) of the • Adequate soakaway or infiltration system system • GW monitoring should be considered to assess GW levels in relation to including velocities, pathways, flood depths, • Construct buildings with solid floors Water Resources All other uses to include flood resilient measures. • a watercourse; or, where not reasonably practicable, • Analysis of surcharged flood levels topographic levels of the site. ponding etc. Flood Risk Assessment Act 1991 to gain • a sewer • Drainage strategy to ensure sewer flooding will not be exacerbated. • Flood resilient and flood resistant measures should be considered in • Drainage strategy to ensure overland flow will not Required consent when Flood evacuation plan required. • Flood evacuation plan line with Building Regulations. be exacerbated. LPA to set up flood warning service for development at plans include • Flood resilient and flood resistant measures should be considered • Flood evacuation plan risk. erection of any Change of use structure in, over or Possible mitigation may include: FROM ‘water under a Possible mitigation may include: • Raising finished floor levels Possible mitigation may include: Owner/user to have a flood evacuation plan in place compatible’ TO watercourse • Raising finished floor levels • Constructing buildings with solid floors • Raising finished floor levels ‘less vulnerable’ • Construct buildings with solid floors • Providing raised walkways to ensure safe access and egress during a • The use of roads as flood channels
• Provide raised walkways to ensure safe access and egress during a flood event. • Construct buildings with solid floors More specifically areas in close proximity to the development EA Flood Defence flood event • Ensure that building foundations and piling do not disrupt groundwater • Incorporate SuDS to limit runoff following should consider the residual risk as part of a Consent required. • Incorporate SuDS to limit runoff flow routes. • Flood resilient and flood resistant measures should FRA:
be considered in line with Building Regulations. • Brocket Hall Lake, Lemsford, Nr Welwyn Garden City; FURTHER INFORMATION FURTHER • FURTHER INFORMATION • DCLG Improving the Flood Performance of New Buildings – Flood FURTHER INFORMATION The Broadwater, Welwyn Garden Cuty; FURTHER INFORMATION INFORMATION FURTHER INFORMATION • • Code for Sustainable Homes 2008 Resilient Construction 2007 FURTHER INFORMATION • Building Regulations 2000 Approved Document H3 Stanborough Lake; and, • http://www.pipernetworking • http://www.piper • http://www.pipernetworking FURTHER INFORMATION • • Building Regulations 2000 Approved Document H3 • Building Regulations 2000 Approved Document H3 Rainwater Drainage • Building Regulations 2000 Approved Document H3 Rainwater Drainage Rainwater Drainage Gravel pits near Coopers Green. .com networking.com .com • http://www.pipernetworking.com Rainwater Drainage
Only Consult EA if site falls Consult EA under Highly and more vulnerable Highly vulnerable uses are not permitted in Flood Zone 3a Consult EA with FRA Rainwater to be discharged to one of following in order In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Possible mitigation measures may include: within Flood Zone 3a or 3b part IVV, Section uses are not permitted in of priority: • Stormwater runoff calculations in relation to the capacity of the sewer • Assessment of risk from GW flooding • Assessment of risk posed by overland flow, • Raising finished floor levels 109 (1) of the Flood Zone 3b. Consult EA with FRA • Adequate soakaway or infiltration system system • GW monitoring should be considered to assess GW levels in relation to including velocities, pathways, flood depths, • Construct buildings with solid floors Water Resources • a watercourse; or, where not reasonably practicable, • Analysis of surcharged flood levels topographic levels of the site. ponding etc. Flood Risk Assessment Act 1991 to gain • a sewer • Drainage strategy to ensure sewer flooding will not be exacerbated. • Flood evacuation plan • Drainage strategy to ensure overland flow will not Required consent when • Flood evacuation plan • Flood resilient and flood resistant measures should be considered in be exacerbated. LPA to set up flood warning service for development at Change of use plans include • Flood resilient and flood resistant measures should be considered line with Building Regulations. • Flood evacuation plan risk. RESULTING IN erection of any structure in, over or ‘highly under a Possible mitigation may include: Possible mitigation may include: Possible mitigation may include: Owner/user to have a flood evacuation plan in place vulnerable’ or watercourse • Raising finished floor levels • Raising finished floor levels • Raising finished floor levels ‘more • Construct buildings with solid floors • Constructing buildings with solid floors • The use of roads as flood channels
• Provide raised walkways to ensure safe access and egress during a • Providing raised walkways to ensure safe access and egress during a • Construct buildings with solid floors More specifically areas in close proximity to the vulnerable’ EA Flood Defence flood event flood event. • Incorporate SuDSto limit runoff following should consider the residual risk as part of a development Consent required. • Incorporate SuDS to limit runoff • Ensure that building foundations and piling do not disrupt groundwater • Flood resilient and flood resistant measures should FRA:
flow routes. be considered in line with Building Regulations. • Brocket Hall Lake, Lemsford, Nr Welwyn Garden FURTHER FURTHER INFORMATION City; FURTHER INFORMATION FURTHER INFORMATION FURTHER INFORMATION INFORMATION FURTHER INFORMATION • • Code for Sustainable Homes 2008 • DCLG Improving the Flood Performance of New Buildings – Flood • Building Regulations 2000 Approved Document H3 The Broadwater, Welwyn Garden Cuty; • http://www.pipernetworking • http://www.piper • http://www.pipernetworking FURTHER INFORMATION FURTHER INFORMATION • Building Regulations 2000 Approved Document H3 • Stanborough Lake; and, • • Resilient Construction 2007 FURTHER INFORMATION Rainwater Drainage .com networking.com .com http://www.pipernetworking.com http://www.pipernetworking.com Rainwater Drainage • • Gravel pits near Coopers Green. • Building Regulations 2000 Approved Document H3 Rainwater Drainage Building Regulations 2000 Approved Document H3 Rainwater Drainage
Consult EA Consult EA with Consult EA with FRA and Consult EA with FRA and Sequential Test evidence (and where required confirm Consult EA with FRA and Sequential Test evidence (and where Rainwater to be discharged to one of following in order In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Possible mitigation measures may include: FRA showing Sequential Test evidence Exception Test has been applied) required confirm Exception Test has been applied) of priority: • Stormwater runoff calculations in relation to the capacity of the sewer • Using a sequential approach, areas at high risk of GW flooding should • Using a sequential approach, development should • Raising finished floor levels design details of (and where required confirm • Adequate soakaway or infiltration system system be avoided when allocating development GW monitoring should be be avoided within overland flow routes. • Construct buildings with solid floors any culvert or flow Exception Test has been • a watercourse; or, where not reasonably practicable, • Analysis of surcharged flood levels considered to assess GW levels in relation to topographic levels of the • Drainage strategy to ensure overland flow will not control structure applied). • a sewer • Drainage strategy to ensure sewer flooding will not be exacerbated. site. be exacerbated. proposed. • Flood evacuation plan • Flood evacuation plan • Flood evacuation plan LPA to set up flood warning service for development at • Flood resilient and flood resistant measures should be considered. • Flood resilient and flood resistant measures should be considered in risk. Only water compatible uses line with Building Regulations. Operational EA Flood Defence are permitted in this flood Possible mitigation may include: Consent required. zone and ‘essential • development of Possible mitigation may include: Raising finished floor levels Owner/user to have a flood evacuation plan in place infrastructure’ through the • Raising finished floor levels Possible mitigation may include: • The use of roads as flood channels less than 1 process of the Exceptions • Construct buildings with solid floors • Raising finished floor levels • Construct buildings with solid floors Test. hectare • Provide raised walkways to ensure safe access and egress during a • Constructing buildings with solid floors • Incorporate SuDS to limit runoff More specifically areas in close proximity to the
flood event • Providing raised walkways to ensure safe access and egress during a • Flood resilient and flood resistant measures should following should consider the residual risk as part of a
• Incorporate SuDS to limit runoff flood event. be considered in line with Building Regulations. FRA:
• Ensure that building foundations and piling do not disrupt groundwater • Orientate buildings to prevent impediment of
flow routes. overland flow routes. FURTHER INFORMATION • Brocket Hall Lake, Lemsford, Nr Welwyn Garden FURTHER INFORMATION FURTHER • Code for Sustainable Homes 2008 • DCLG Improving the Flood Performance of New Buildings – Flood City; FURTHER INFORMATION INFORMATION FURTHER INFORMATION • • Building Regulations 2000 Approved Document H3 Resilient Construction 2007 FURTHER INFORMATION FURTHER INFORMATION The Broadwater, Welwyn Garden Cuty; • http://www.pipernetworking • http://www.piper • http://www.pipernetworking FURTHER INFORMATION FURTHER INFORMATION • Rainwater Drainage • • Building Regulations 2000 Approved Document H3 Rainwater Drainage • Building Regulations 2000 Approved Document H3 Stanborough Lake; and, .com networking.com .com • http://www.pipernetworking.com • http://www.pipernetworking.com Building Regulations 2000 Approved Document H3 Rainwater Drainage • http://www.pipernetworking.com Rainwater Drainage • Gravel pits near Coopers Green.
Consult EA Consult EA with Consult EA with FRA and Consult EA with FRA and Sequential Test evidence (and where required confirm Consult EA with FRA and Sequential Test evidence (and where Consult EA with FRA. In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Possible mitigation measures may include: FRA showing Sequential Test evidence Exception Test has been applied) required confirm Exception Test has been applied) • Stormwater runoff calculations in relation to the capacity of the sewer • Using a sequential approach, areas at high risk of GW flooding should • Using a sequential approach, development should • Raising finished floor levels design details of (and where required confirm system be avoided when allocating development GW monitoring should be be avoided within overland flow routes. Drainage • Construct buildings with solid floors A FRA is required for all sites any culvert or flow Exception Test has been A Flood Risk Assessment would need to be undertaken • Analysis of surcharged flood levels considered to assess GW levels in relation to topographic levels of the strategy to ensure overland flow will not be over 1ha. control structure applied). on all sites greater than 1Ha in size in compliance with • Drainage strategy to ensure sewer flooding will not be exacerbated. site. exacerbated. proposed. PPS25. • Flood evacuation plan if non residential development • Flood evacuation plan • Flood evacuation plan LPA to set up flood warning service for development at • Flood resilient and flood resistant measures should be considered • Flood resilient and flood resistant measures should be considered in risk. Only water compatible uses line with Building Regulations. A FRA is required are permitted in this flood Possible mitigation may include: Operational for all sites over zone and ‘essential Possible mitigation may include: • Raising finished floor levels Owner/user to have a flood evacuation plan in place development of 1ha. infrastructure’ through the • Raising finished floor levels Possible mitigation may include: • The use of roads as flood channels process of the Exceptions • Construct buildings with solid floors • Raising finished floor levels • Construct buildings with solid floors 1ha or greater Test. • Provide raised walkways to ensure safe access and egress during a • Constructing buildings with solid floors • Incorporate SuDSto limit runoff More specifically areas in close proximity to the EA Flood Defence flood event • Providing raised walkways to ensure safe access and egress during a • Flood resilient and flood resistant measures should following should consider the residual risk as part of a Consent required. • Incorporate SuDS to limit runoff flood event. be considered in line with Building Regulations. FRA:
• Ensure that building foundations and piling do not disrupt groundwater • Orientate buildings to prevent impediment of
flow routes. overland flow routes. • Brocket Hall Lake, Lemsford, Nr Welwyn Garden FURTHER FURTHER INFORMATION City; FURTHER INFORMATION INFORMATION FURTHER INFORMATION • • DCLG Improving the Flood Performance of New Buildings – Flood FURTHER INFORMATION The Broadwater, Welwyn Garden Cuty; • http://www.pipernetworking • http://www.piper • http://www.pipernetworking FURTHER INFORMATION FURTHER INFORMATION FURTHER INFORMATION • Resilient Construction 2007 FURTHER INFORMATION • Building Regulations 2000 Approved Document H3 Stanborough Lake; and, .com networking.com .com • http://www.pipernetworking.com • http://www.pipernetworking.com • http://www.pipernetworking.com • • Building Regulations 2000 Approved Document H3 Rainwater Drainage • Building Regulations 2000 Approved Document H3 Rainwater Drainage Rainwater Drainage Gravel pits near Coopers Green. Key: Scenarios colour coded Green the will be assessed by the Environment Agency as they are a statutory consultee Scenarios colour coded Yellow will need to be assessed by the Local Authority
Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment
12 Emergency Planning
In Sir Michael Pitts Review of the 2007 floods he recognises the’ dedicated and quick response’ of emergency services which prevented the worsening of many situations. However he also identified a number of failings and opportunities to improve our preparedness for future flood events. In particular he advises that with ‘stronger local leadership of flood risk management, clarification of roles, more effective co-operation between responsible organisations, better protection of infrastructure and wider and deeper public engagement’ the impact of flooding on communities could be significantly reduced. If many of these opportunities identified by Sir Michael Pitt are to be achieved, the role local authorities have in planning and responding to flood events must be clearly defined. To assist local authorities in understanding their role it is essential to have a technically sound emergency plan in place to provide clear procedural instructions to the organisations, companies and individuals involved and affected. The mobilisation and organisation of the emergency services and supporting agencies, for example Hertfordshire County Council and WHBC can be integral in the coordinated rescue, treatment and transport of potentially large numbers of displaced residents or casualties. Similarly during and after a flood event the role of the local authority can include providing transport for the evacuees and safe rest centres in the event of homes being flooded. Further health and welfare issues are inevitable as a result of serious flood events, which may impact on the ability of people to return to their homes or places of business. Whilst this SFRA is not designed to fulfil that role it does contain useful information for WHBC and other key organisations to assist them in understanding their risks and begin the process of developing an appropriate co-ordinated response.
12.1 Developing an Emergency Flood Plan
To assist the local authority to develop an emergency plan it is recommended that a staged approach is followed. This may consist of the following stages: 1. Understanding of how a flood might impact on the WHBC area by identifying key infrastructure, buildings and organisations that may be at risk of flooding, 2. Liaise with the organisations responsible for at risk infrastructure and buildings institutions to establish what the consequences of a flood would be on them and individuals who rely on them and what emergency procedures they have in place for dealing with a flood, 3. Liaise with the same organisations to establish a coordinated strategy for dealing with flood events. The Development of an Emergency Flood Plan (EFP) should be a multi-agency process as it is too complex for a single agency approach to be appropriate. The Local Emergency Planning Forum (LEPF), established under the Civil Contingencies Act 2004, would be an appropriate forum for the development of an EFP.
12.1.1 Identifying at Risk Installations
Using the maps provided as part of this Level 1 SFRA, WHBC can identify (map) key installations located in flood risk areas. In achieving this WHBC may find it advantageous to adopt a tiered approach to identify those installations that are at primary flood risk (i.e. at risk themselves) and those
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areas at risk as a consequence of an installation being un-operational as a result of a flood. For example a flooded fire station would be a primary impact of flooding, the area served by that fire station would be at a secondary impact of the flooding, due to the potentially limited ability of the fire service to provide adequate coverage in the area. It is recommended that the following installations are considered (but not necessarily limited to) for mapping of primary and secondary impacts: • Police stations; • Ambulance stations; • Fire stations; • Hospitals • Command centres; • Telecommunications installations; • Emergency dispersal points; • Water treatment works; and • Electricity sub stations It is also recommended that installations that may be relied on to house displaced residents or from where vulnerable residents may need to be evacuated swiftly are also identified. These may include: • Residential institutions such as residential care homes, children’s homes, social services homes, prisons and hostels; • Student halls of residence; and, • Non-residential uses for health service, nurseries and educational establishments. • Leisure centres; • Churches; • Schools; and, • Community Centres. This list of suggested installations/institutions presented above should not be considered exhaustive. Discussions should be held within WHBC and with the emergency services to establish those installations and organisations that should be mapped to assist in developing a robust emergency plan.
12.1.2 Individual Flood Responses/Mitigation Measures
Once at risk installations have been identified, WHBC should liaise with the relevant organisations and the LEPF to establish if they are aware of their risk to flooding and if so if they are also aware of the primary and secondary impacts of their flood risk. Discussions should also be held to establish any flood event procedures they maintain and/or mitigation measures. Using this information it may be necessary to return to the mapping exercise to redefine impact areas, to either account for mitigation measures already in place (i.e. transfer of fire personnel to a secondary
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facility suitable to maintain an adequate level of cover in the event of an emergency) or to extend secondary impact areas.
12.1.3 Coordinated Flood Emergency Planning
The LEPF, as discussed above, should use the information collected through stages two and three to establish the development of a coordinated flood emergency plan with the emergency services and key local stakeholders. This should build on the findings of the first two phases identifying where the emergency response of an installation/organisation may be at conflict with the procedures of another or local hazards. For example the evacuation of a school by sending pupils home, would not be suitable for those pupils whose homes are flooded, consequently an intermediary step may be required to safeguard pupils.
Similarly understanding how key infrastructure (evacuation routes, telecommunications) may be affected by a flood should be understood to ensure emergency services can access an area before flood hazards become too great. Telecommunications companies are not represented on the LEPF, however there is a specific group for these issues as a sub group of Hertfordshire Resilience, who should also provide input to the coordinated flood emergency plan. The involvement of Hertfordshire Resilience would also allow for the involvement of all Hertfordshire Local Authorities and the production of a fully coordinated county-wide emergency plan.
The aim of this stage should be to identify how key organisations should respond during a flood event to ensure the smooth and effective protection of local residents and key infrastructure.
In addition the process may also identify installations and or organisations where their risks are currently too great given their vulnerability or importance wither economically or in responding to flood events. In such circumstances the information collected may be of use in justifying strategic flood alleviation measures.
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13 Recommendations
13.1 Further Work
1. WHBC should apply the Sequential Test to the development site allocations and identify those sites that they consider it will be necessary to apply the Exception Test. 2. Following completion of the Sequential Test and parts ‘a’ and ‘b’ of the Exception Test a meeting should be sought with the Environment Agency to confirm their acceptance of WHBC’s arguments and justification for progressing with sites requiring the Exception Test. This meeting should also be used to confirm the information the Environment Agency will require to demonstrate a site is safe in line with part ‘c’ of the Exception Test. 3. Once WHBC and the Environment Agency are in agreement regarding those sites for application of the Exception Test, WHBC’s consultant will confirm the works necessary as part of the Level 2 SFRA to demonstrate a site is safe in line with the Exception Test. 4. WHBC should incorporate the policies recommended within Section 10 into the emerging LDF. 5. WHBC should ensure that all future planning applications are reviewed against the recommendations of Table 11-1 to identify the requirement for and content of site specific flood risk assessments. 6. Following the completion of the SFRA, WHBC should receive training on the review of Flood Risk Assessments. 7. WHBC should use the information mapped as part of this SFRA and the recommendations in Section 12 – Emergency Planning to begin a process of developing a robust and coordinated flood emergency plan. 8. WHBC should develop a database for the coordinated capture and storage of flooding records for the borough. Recording all flooding incidents that WHBC are aware of/notified about in the borough would enhance future revisions of the SFRA.
As populations grow and the effects of climate change make our weather more unpredictable, flooding will be just one of a number of water related issues that WHBC will need to consider in the future. As well as increasing pressure on floodplains, the projected growth in housing stocks and population will have an impact on the availability of water resources. This could be especially significant in areas of low water yield during the drier summers forecasted due to the effects of climate change. Several key pieces of legislation and planning policy, currently in force or due to be implemented will have an impact on the management of the water environment now and in the future. For example: • The Water Framework Directive; • The Groundwater Daughter Directive; • The EU Flooding Directive; • Planning Policy Statement 25: Development & Flood Risk; • Planning Policy Statement 23 Planning and Pollution Control; • Planning Policy Statement: Planning and Climate Change - Supplement to Planning Policy Statement 1; and,
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• BREEAM Guidelines on the Code for Sustainable Homes. Failure to fully grasp the implications of these documents could result in WHBC’s best interests being marginalized. Creating a role within WHBC to oversee and coordinate the council’s interests under these directives and documents will ensure water resource issues are proactively managed, minimizing the impact they could potentially have in achieving growth and the sustainability aspirations of the Borough.
13.2 How and when the SFRA should be updated?
PPS25 and the Environment Agency intend for SFRAs to be living documents, updated as new data is available. New sources of data become available all the time and as such WHBC should liaise with the Environment Agency to determine a suitable period for review and update of the SFRA that is acceptable to all parties. This may include consideration of: • New climate change updates; • Modelling result updates; • New model data; and • Development of all allocations.
13.2.1 EU Flooding Directive
In addition to the above considerations the integration of the EU Flooding Directive may also form a reason for revisiting the SFRA. The EU Flooding Directive was published in November 2007 and The Flooding Directive is a mandatory statutory framework for flood risk management, requiring Member States to prepare preliminary risk assessments, flood mapping, and the preparation of flood risk management plans. It applies to all types of flooding, although inclusion of sewerage floods will be optional. The Directive will be carried out in coordination with the Water Framework Directive through preparation of flood risk management plans and river basin management plans Member States have to designate competent authorities to implement the Directive; for England, this is the Environment Agency. The Environment Agency hope to achieve the requirements of the Directive through the use of existing published information. This may include reference or use of Strategic Flood Risk Assessments, Catchment Flood Management Plans and/or Strategic Flood Risk mapping projects. In some cases the assessments may require new information to be generated to inform the stages of assessment required by the Directive. The preparation or finalisation of Preliminary Risk Assessments, required by the Directive, may form a useful point in time to review the SFRA and assess its contribution to the Flooding Directives requirements or where an update to the SFRA may benefit from new data generated as part of assessments prepared to meet the requirements of the Flooding Directive.
13.3 Level 2 SFRA
From a wider review of the available data, and based on and Scott Wilson’s experience in producing flood risk assessments and SFRAs, we consider it unlikely that the available data will be sufficient to satisfy part ‘c’ of the Exception Test for any developments within the floodplain. To satisfy part ‘c’ of the Exception Test the Practice Guide companion to PPS25 requires the following minimum data to be derived for each development site:
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• Flood probability • Flood water depth • Flood water velocity, and, • Rate of Onset of flooding This data can only be determined through hydraulic modelling. The current lack of suitable data for some watercourses in the study area may prevent this from being achieved using the existing data set, depending on the location of the allocation sites. It is therefore likely that a Level 2 SFRA will require hydraulic modelling for some sites identified by WHBC as requiring the Exception Test.
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14 References
Reference 1 : HMSO (June 2004) ‘Planning and Compulsory Purchase Act’, (The Queens Printer of Acts of Parliament) http://www.opsi.gov.uk/acts/acts2004/20040005.htm . Reference 2 : HMSO Department for Communities and Local Government (December 2006) ‘Planning Policy Statement 25: Development and Flood Risk’ 2006, (The Stationary Office: Norwich). http://www.communities.gov.uk/index.asp?id=1504639 . Reference 3 : East of England Plan: The revision to the Regional Spatial Strategy (RSS) for the East of England, 2008. http://www.gos.gov.uk/goeast/planning/regional_planning/ Reference 4 : HMSO Department for Communities and Local Government (June 2008) ‘Planning Policy Statement 25: Development and Flood Risk: Practice Guide’ , (Communities and Local Government Publications: London). Reference 5 : Census (2001) http://www.statistics.gov.uk/census/default.asp Reference 6 : The Geological Survey of Great Britain (England and Wales) (1978), 1:50,000 Series Drift Edition Sheets 239 Reference 7 : NRA (1986) ‘Groundwater Vulnerability of West London: Sheet 39, (HMSO: London). Reference 8 : Natural England (1986) Water End Swallow Holes SSSI Notification, http://www.english- nature.org.uk/citation/citation_photo/1002507.pdf Reference 9 : WRc plc., (2006), Sewers for Adoption, 2006, (WRc publications) Reference 10 : DETR (March 2001) ‘Regional Planning Guidance for the South East (RPG9)’ London: The Stationary Office. Reference 11 : DEFRA (March 2005) ‘Making Space for Water,’ DEFRA Publications: London. Available at http://www.defra.gov.uk/environ/fcd/policy/strategy.htm . Reference 12 : Welwyn Hatfield District Plan, 2005 http://welhat.leadpartners.co.uk/document.aspx Reference 13 : Environment Agency (October 2007) ‘Thames Catchment Flood Management Plan:, Flood Risk Management Policy Messages Environment Agency: Hatfield Reference 14 : Environment Agency (March 2007) ‘Upper Lee Flood Risk Management Strategy, Phase 2 (Inception Report) Reference 15 : Environment Agency Thames Region, Upper Colne Flood Risk Management Strategy (4293), Position Statement, Halcrow Group Limited, June 2005 Reference 16 : Environment Agency (June 2006), The Upper Lee Catchment Abstraction Management Strategy Reference 17 : Environment Agency Standing Advice Development and Flood Risk – England (2007), Consultation Matrix, http://www.pipernetworking.com/floodrisk/matrix.html Reference 18 : HMSO Department for Communities and Local Government (December 2006) ‘Planning Policy Statement 3: Housing , (The Stationary Office: Norwich). Reference 19 : Broxbourne Borough Council, (2007), Draft Strategic Flood Risk Assessment Volume 1, Halcrow Group Limited.
Reference 20: East Hertfordshire District Council (2008) Level 1 Strategic Flood Risk Assessment,
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Reference 21 : CIRIA C624, Development and Flood Risk – guidance for the construction industry, 2004, Lancaster, J.W., Preene, M., Marshall, C.T. Arup Reference 22 : DCLG (2007) Improving the flood performance of new buildings, HMSO, London. Reference 23: Woods Ballard, B., Kellagher, R., (2007), The SuDS Manual (C697), (CIRIA; London) Reference 24 : Woods Ballard, B., Kellagher, R., (2007) Site Handbook for the Construction of SuDS (C698), (CIRIA; London) Reference 25 : HMSO Office of Public Sector Information (2002) Building Regulations, (HMSO, London) Reference 26 : HMSO Department for Communities and Local Government (2007) ‘Code for Sustainable Homes: Technical Guide’, Communities and Local Government Publications: London. Reference 27: The Town and Country Planning (General Development Procedure) (Amendment) (No 2) (England) Order 2006 (GDPO) Reference 28 : Development Control Technical Specialists - Environment Agency, Thames Region (2006), SuDS – A Practical Guide. Reference 29 : Defra & Environment Agency, (2004), Preliminary Rainfall Runoff Management for Developments, Technical Report W5-074A/TR/1 Revision B.
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Appendices
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Appendix A – Figures
Figure 1: SFRA Study Area Figure 2: Watercourses and Catchment Areas Figure 3: Solid Geology Figure 4 : Drift Geology Figure 5: Environment Agency Flood Map Figure 6: Hydraulic Model Extents Figure 7: Historical Flood Record Figure 8: Flood Defences, In Channel Structure and Flood Storage Areas Figure 9: Topographic Data (LiDAR) Figure 10: Sources of Potential Overland Flow Figure 11: Groundwater flooding incidents and trends Figure 12 : Groundwater Vulnerability Zones Figure 13 : Source Protection Zones Figure 14: Sewer Flood Points Figure 15 : Hertfordshire Highways Road Flooding Frequency Data Figure 16: Artificial Sources Figure 17: Previous Flood Risk Assessments Undertaken Figure 18: Modelled Flood Plain 2007 Figure 19 : Modelled Flood Plain 2115 Figure 20 : Sequential Test Maps 2007 Figure 21 : Sequential Test Maps 2115 Figure 22: Important Environmental Sites for Planning Figure 23: Development Constraints Map Figure 24: Environment Agency Flood Warning Areas Figure 25: Pedology
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Appendix B – Historical Records of Flooding
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GIS CREATION DATE INCIDENT TYPE EASTING NORTHING WARD DISTRICT REFERENCE Welwyn Hatfield HF_FR_1 06/01/2006 FLOODING 522,591 206,734 Hatfield South Ward District Welwyn Hatfield HF_FR_2 09/01/2006 FLOODING 524,612 217,385 Welwyn North Ward District Welwyn Hatfield HF_FR_3 16/01/2006 FLOODING 522,648 209,806 Hatfield North Ward District Welwyn Hatfield HF_FR_4 27/01/2007 FLOODING 522,514 208,172 Hatfield Central Ward District Welwyn Hatfield HF_FR_5 01/02/2007 FLOODING 522,962 209,776 Hatfield North Ward District Welwyn Hatfield HF_FR_6 17/02/2006 FLOODING 524,766 212,712 Peartree Ward District Welwyn Hatfield HF_FR_7 18/02/2007 FLOODING 523,454 212,004 Handside Ward District Welwyn Hatfield HF_FR_8 22/02/2007 FLOODING 530,377 202,262 Northaw Ward District Welwyn Hatfield HF_FR_9 23/02/2007 FLOODING 523,897 214,368 Sherrards Ward District Welwyn Hatfield HF_FR_10 04/03/2005 FLOODING 523,130 209,139 Hatfield North Ward District Brookmans Park and Little Heath Welwyn Hatfield HF_FR_11 06/03/2007 FLOODING 526,129 205,674 Ward District Welwyn Hatfield HF_FR_12 17/03/2007 FLOODING 521,525 208,867 Hatfield North Ward District Welwyn Hatfield HF_FR_13 16/03/2004 FLOODING 524,048 211,279 Hollybush Ward District Welwyn Hatfield HF_FR_14 02/04/2006 FLOODING 524,766 212,712 Peartree Ward District Welwyn Hatfield HF_FR_15 21/04/2004 FLOODING 522,439 209,814 Hatfield North Ward District
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Welwyn Hatfield HF_FR_16 10/05/2004 FLOODING 526,226 212,533 Panshanger Ward District Welwyn Hatfield HF_FR_17 07/06/2006 FLOODING 522,097 209,033 Hatfield Central Ward District Welwyn Hatfield HF_FR_18 26/05/2005 FLOODING 525,004 214,395 Haldens Ward District Welwyn Hatfield HF_FR_19 01/07/2006 FLOODING 524,685 212,618 Peartree Ward District Welwyn Hatfield HF_FR_20 16/06/2004 FLOODING 523,718 205,418 Welham Green Ward District Welwyn Hatfield HF_FR_21 18/06/2005 FLOODING 524,849 212,432 Peartree Ward District Welwyn Hatfield HF_FR_22 09/08/2004 FLOODING 525,276 211,901 Peartree Ward District Welwyn Hatfield HF_FR_23 09/08/2004 FLOODING 525,106 217,905 Welwyn North Ward District Welwyn Hatfield HF_FR_24 09/08/2004 FLOODING 526,160 213,017 Panshanger Ward District Welwyn Hatfield HF_FR_25 10/08/2004 FLOODING 525,283 211,900 Peartree Ward District Welwyn Hatfield HF_FR_26 10/08/2005 FLOODING 524,725 212,670 Peartree Ward District Welwyn Hatfield HF_FR_27 12/08/2004 FLOODING 525,276 211,901 Peartree Ward District Welwyn Hatfield HF_FR_28 20/08/2004 FLOODING 525,283 211,900 Peartree Ward District Welwyn Hatfield HF_FR_29 20/08/2004 FLOODING 522,007 212,395 Hatfield North Ward District Welwyn Hatfield HF_FR_30 20/08/2004 FLOODING 525,295 211,934 Peartree Ward District Welwyn Hatfield HF_FR_31 19/08/2005 FLOODING 526,360 212,643 Panshanger Ward District Welwyn Hatfield HF_FR_32 29/08/2004 FLOODING 522,643 207,182 Hatfield West Ward District Welwyn Hatfield HF_FR_33 24/08/2005 FLOODING 521,897 208,995 Hatfield Central Ward District
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Welwyn Hatfield HF_FR_34 05/09/2004 FLOODING 524,688 211,445 Hollybush Ward District Welwyn Hatfield HF_FR_35 08/09/2004 FLOODING 524,833 214,527 Haldens Ward District Welwyn Hatfield HF_FR_36 07/09/2005 FLOODING 526,007 212,506 Panshanger Ward District Brookmans Park and Little Heath Welwyn Hatfield HF_FR_37 09/09/2005 FLOODING 525,547 205,297 Ward District Welwyn Hatfield HF_FR_38 01/10/2006 FLOODING 525,106 217,905 Welwyn North Ward District Welwyn Hatfield HF_FR_39 01/10/2006 FLOODING 522,993 216,118 Welwyn South Ward District Welwyn Hatfield HF_FR_40 01/10/2006 FLOODING 522,815 215,868 Welwyn South Ward District Welwyn Hatfield HF_FR_41 01/10/2006 FLOODING 523,075 216,094 Welwyn South Ward District Welwyn Hatfield HF_FR_42 01/10/2006 FLOODING 523,408 216,261 Welwyn South Ward District Welwyn Hatfield HF_FR_43 26/09/2004 FLOODING 524,380 211,707 Hollybush Ward District Welwyn Hatfield HF_FR_44 09/10/2006 FLOODING 523,075 205,750 Welham Green Ward District Welwyn Hatfield HF_FR_45 12/10/2006 FLOODING 523,298 216,229 Welwyn South Ward District Welwyn Hatfield HF_FR_46 14/10/2004 FLOODING 525,289 211,898 Peartree Ward District Welwyn Hatfield HF_FR_47 11/11/2004 FLOODING 524,524 214,500 Haldens Ward District Welwyn Hatfield HF_FR_48 05/11/2005 FLOODING 522,196 206,450 Hatfield South Ward District Welwyn Hatfield HF_FR_49 17/11/2006 FLOODING 522,848 204,235 Welham Green Ward District Brookmans Park and Little Heath Welwyn Hatfield HF_FR_50 18/11/2004 FLOODING 524,304 204,078 Ward District Welwyn Hatfield HF_FR_51 20/11/2006 FLOODING 522,753 208,587 Hatfield East Ward District
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Welwyn Hatfield HF_FR_52 25/11/2004 FLOODING 524,678 212,409 Peartree Ward District Welwyn Hatfield HF_FR_53 27/11/2006 FLOODING 521,553 208,896 Hatfield North Ward District Brookmans Park and Little Heath Welwyn Hatfield HF_FR_54 20/11/2005 FLOODING 525,463 203,998 Ward District Welwyn Hatfield HF_FR_55 21/11/2005 FLOODING 525,240 211,261 Howlands Ward District Welwyn Hatfield HF_FR_56 07/12/2006 FLOODING 530,478 202,709 Northaw Ward District Welwyn Hatfield HF_FR_57 04/12/2005 FLOODING 522,577 210,819 Hatfield North Ward District Welwyn Hatfield HF_FR_58 25/12/2006 FLOODING 523,613 211,111 Hollybush Ward District Welwyn Hatfield HF_FR_59 28/12/2004 FLOODING 521,737 208,409 Hatfield Central Ward District Welwyn Hatfield HF_FR_60 30/12/2004 FLOODING 525,208 211,086 Howlands Ward District Welwyn Hatfield HF_FR_61 30/12/2006 FLOODING 525,106 217,905 Welwyn North Ward District Welwyn Hatfield HF_FR_62 30/12/2006 FLOODING 523,407 216,294 Welwyn South Ward District Welwyn Hatfield HF_FR_63 31/12/2006 FLOODING 522,780 209,914 Hatfield North Ward District Welwyn Hatfield HF_FR_64 28/12/2005 FLOODING 530,601 202,737 Northaw Ward District
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Appendix C - Data/Document Registers
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TITLE SYNOPSIS
Defence (as lines) and Structures (as points) - asset ref, asset type, maintainer, asset protection type, asset comments, alternative asset ref, asset description, asset location, design type, consent number, asset length, asset height, asset width, invert level, no of pipes, pipe length, replacement cost, replacement cost assessment, natural, next EA GIS - Defences and inspection, Defence reference, subreach reference, area number, region structures number, subreach reference, reach reference, watercourse reference, frontage subunit reference, frontage unit reference, frontage reference, recommended action, recommended action date, recommended action description, year built, lead team, action date, downloaded, grid reference, Data owner, coastal, supporting, changed, for upload, spatial Data quality.
EA GIS -Flood Zones Flood Zones 2 and 3
EA GIS - Modelling Data - Upper Colne flood extents for 3 return periods (unknown). Has degree of Upper Colne confidence in browser table.
EA GIS - Modelling Data - River Lee modelled flood extents for 20yr, 100yr and 100yrCC, no Lee information in browser table.
Environment Agency Main Rivers - browser table shows - region EA GIS - Main Rivers number, watercourse ref, watercourse name, length.
EA GIS - Historic Events Historic flood event outlines, flood event name, start date, end date,
EA GIS- Flood Warning Digital outline of flood warning areas Areas
EA GIS- Catchments Catchments of Lower Lee and Upper Colne
EA GIS- Groundwater Groundwater Vulnerability outline for Lower Lee and Upper Colne Vulnerability catchments
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TITLE SYNOPSIS
EA GIS- Geology Solid and drift geology for lower Lee and Upper Colne catchments
EA GIS- Landfill sites Digitised landfill sites
EA GIS- NVZ Nitrate vulnerability zones
EA GIS- soil types Location of soil types
EA GIS- SPZ source protection zone
EA GIS - Boundary Welwyn Hatfield Borough Council Boundary - area
Postcode data - Includes postcode; number of properties flooded from overloaded sewer in the last 10 years; number of properties flooded by Thames Water Flooding surface water from overloaded sewers in the past 10 years; number of Database properties flooded by foul water from overloaded sewers in the last 10 years; number of properties flooded by combined overloaded sewers in the last 10 years.
All of flooding incidents from 2004 that they have been called to. Herts Fire and Rescue Includes - time; date; address; grid ref; ward; district; number or times Service (count).
Drainage Improvement States road name, location details (eg. Whole length); town/village; Works Planned 24.09.07 district (eg.WH).
Highways Maintenance Programme - Ranking A form that each district fills in when they are applying for drainage System - Drainage: improvements ie. If they have experienced flooding problems. Manual of Guidance
Lee Mapping Report Methods and outputs of the River Lee modelling.
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TITLE SYNOPSIS
Outlines for 1 in 1000 year (defended); 1 in 1000 year (undefended); Lee Modelling Nodes for 1 in 1000 year (defended); 1 in 1000 year (undefended) with flows and levels.
Lee Modelling Outlines for 1 in 20 year, 1 in 100 year and 1 in 100 year + CC.
Upper Colne Modelling 1 in 20 year, 1 in 100 year, 1 in 200 year flood outlines. Flood Warning Areas Flood Warning Areas for the WHBC study area Has a list of FRA's in the study area, states that the EA do no hold any Ea Letter Dated Emergency Plans. States there are no WLMP within the study area. 22/10/2007 States that the Upper Lee Flood Risk Management Strategy is underway.
Upper Colne Flood Risk Summary Management Strategy
River Catchments Broad shows Mimram, Lee-Source/Mimram, Colne, Lee-Stort/Thames
Solid and Drift Geology Covering whole study area
GVZ Covering whole study area
Landfill Sites Only one in the west, states the name and address.
Main Rivers EA Main Rivers including names and length
Nitrate Vulnerable Zones Whole area appears to be in NVZ
SPZ Covering whole study area
Soils Very broad only two soil types and description
Li ght Detection and Ranging data covering the majority of the study LiDAR Data area.
.odf files on 'applications from 1974 to 1999, presumed to be planning Applications applications but unable to open them
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TITLE SYNOPSIS Areas of Archaeological No attribute data Significance Areas of Special Restraint No attribute data Shows main rivers. States which proposals map (eg. 3), policy number Main Rivers and chapter number.
Points showing wildlife sites. Has which proposals map its from eg. 5, Wildlife Site Points reference, ID number, policy and chapter.
Public Transport Route Has one transport route marked west of Hatfield.
Areas that are designated wildlife sites. Attribute table contains policy Wildlife Site Areas number, chapter number, reference, and proposals map number.
Watling Chase Outline of Watling Chase Community Forest. Attribute table contains Community Forest policy number, chapter number and proposals map number.
Polygons showing areas of urban open land. Attribute table contains Urban Open Land proposals map sheet number, policy number, chapter number and ID.
Outline of University of Hertfordshire. Attribute table contains proposal University map number, reference, ID, policy number and chapter number.
Outline of University playing fields. Attribute table contains map number, University Playing Fields ID, policy and chapter.
Town Centre Inset Outline of town centres of Welwyn Garden City and Hatfield.
Structural Landscape Outlined Structural Landscape Area near Panshanger Airfield. Attribute Area table contains proposals map number, ID, policy and chapter.
Outline of Stanborough Park. Attributes contain proposals map number, Stanborough Park reference, ID, policy number and chapter.
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TITLE SYNOPSIS
Outlines former splashland complex near River Lee in Welwyn Garden Splashland Complex City.
Outline SAC in Wormley's Wood and Derrys Wood. Attribute table Special Area of contains proposals map sheet number, reference, ID, policy number and Conservation chapter.
Outlines SSSIs in the study area. Attribute table contains proposals map SSSI sheet number, reference, ID, policy number and chapter.
One new road has been marked Hatfield and Hatfield Garden Village. Road Attribute table contains map reference, ID, policy number and chapter.
Outlines retail frontages in WGC and Hatfield. Attribute table shows map Retail frontages reference, ID, policy number and chapter.
Outlines new housing developments. Attribute table shows map Residential reference, reference, ID, policy and chapter.
Outlines primary retail core. Attribute table shows ID, policy number and Primary Retail Core chapter.
Outline of mixed use frontages. Attribute table has map reference, ID, Mixed Use Frontages policy number and chapter number.
Major Developed Site in Outline of major developed site in the Green Belt near 'The Fryth'. the Green Belt Attribute table contains map reference, ID, policy number and chapter.
Outlines LNRs. Attribute table contains map reference, reference, ID, Local Nature Reserves policy number and chapter.
Outlines extensive areas around WGC and Hatfield as Landscape Landscape Character Character Areas. Attribute table contains LCA number, policy number Areas and chapter number.
Highlights one site northwest of Hatfield. Attribute table contains map Hotel Site reference, ID, policy number and chapter.
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TITLE SYNOPSIS
Historic Parks and Outlines historic parks and gardens. Attribute table contains map Gardens reference, reference, ID, policy number and chapter number.
Outlines the extent of the aerodrome. Attributes table contains map Hatfield Aerodrome Inset reference, ID and chapter.
Delineates Green Belt. Attributes show proposal s map sheet reference, Green Belt ID, policy number and chapter.
Outlines floodplains of River Lee, Mimmshall Brook and Mimram. Floodplains Attribute table contains proposals map reference and ID. No indication on where data came from.
Large areas have been highlighted as employment areas. Attribute table Employment Areas shows ID, map reference, reference, policy number and chapter.
Outline EMP3 area. Attribute table shows proposals map reference, EMP3 reference, policy and chapter.
One education area been outlined. Attribute table shows map reference, Education ID, policy and chapter.
Outlines undeveloped area west of Hatfield. Attribute table shows map District Centre reference, ID, Policy number and chapter.
Borough Boundary GIS layer of the Welwyn Hatfield Borough Boundary
Outlines development sites. Attribute table has map reference, Development Sites reference, ID, policy and chapter.
Highlights culvert west of Hatfield. Attribute table has map reference, Culverting reference, ID, policy and chapter.
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TITLE SYNOPSIS
Outlines Conservation Areas. Attribute table has proposals map Conservation Areas reference, reference, ID, policy and chapter.
On A1001 in Hatfield. Attribute table shows map reference, ID, policy Aviation Heritage Centre and chapter.
Mastermap Mastermap data for the study area
Monthly groundwater monitoring data from 20 boreholes. Has grid Groundwater Data reference, name, date/time, water level (mAOD) and quality flag.
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Appendix D - Tables
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Appendix D Table 1:Sequential Test Table
DEVELOPMENT EXCEPTION TEST FLUVIAL FLOOD ZONE TIDAL FLOOD ZONE GROUNDWATER DRAINAGE PLUVIAL VULNERABILITY CANDIDATE (Y/N) Compare Flood Zone Essential SITE EASTING NORTHING and Development 3 Infrastructure / Water 1 2 3a 3b 1 2 3a (Y/N) (Y/N) (Y/N) Vulnerability within b Compatible Table 6-2Table 6-2 / Highly / More / Less
Exampl ###### ###### Residential - More Exception Test e Vulnerable
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Those sites considered necessary for application of the Exception Test should be presented in Appendix D Table 2. The table should be completed to include the LPAs justification under parts ‘a’ and ‘b’ of the Exception Test for discussion and review with the Environment Agency before commencing with the Level 2 SFRA. Appendix D Table 2: Sites for Application of the Exception Test (copy as necessary)
EXCEPTION TEST DEVELOPMENT VULNERABILITY PART A PART B PART C SITE FLOOD ZONE Essential Infrastructure / To be addressed in the Water Compatible Wider Sustainability Brownfield Land (Y/N) Level 2 SFRA / Highly / More / Less Example Flood Zone More Vulnerable • Close proximity to Development of brownfield 3a transport infrastructure site assists LPA to satisfy • Gentrification government targets • Intensification to reduce pressure for Green Belt review
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Appendix Table 3 : Sustainable Drainage Systems Summary for Allocation Sites
GENERAL SITE GENERAL AQUIFER GROUNDWATER SITE NOTES DRAINAGE APPROPRIATE SUDS FRA REQUIREMENTS NAME GEOLOGY TYPE VULNERABILITY AREA ASSESSMENT
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Appendix E - SuDS Suitability based on Geology, Hydrogeology and Soils in the SFRA Study Area
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Appendix E Table 4 : Table Suitable SuDS Techniques Dependent on Geology
SUDS GEOLOGY DESCRIPTION PERMEABILITY LOCATION TECHNIQUE Lemsford, Between Glacial Undifferentiated sands and High Hatfield and Welwyn Gravel gravels Garden City Generally consist of poorly River Terrace High to
bedded sands and flint gravels South East of Hatfield Deposits Moderate with some silt and clay beds
Welwyn, Digswell, NFILTRATION AND I Oaklands, Ayot, East Upper Chalk, Permeability dependent on the High Hatfield TTENUATION
NFILTRATION undivided level of the water table. I A Underlies majority of study area OMBINED
C Low to Reading
OR OR Sands silts and clays Moderate South East of Hatfield Beds
Permeability low if high River Lee and River Alluvium Low proportion of silt and clay Mimram
Generally comprised yellow- West Hatfield, Welwyn brown to blue-grey sandy and Very Low Boulder Clay Garden City, Welwyn silty clay with chalk fragments, Garden flints and scattered erratics. TTENUATION A South East of Hatfield, London Clay Silty Clay Very Low Sherradspark Wood
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Appendix E Table 5 : Suitable SuDS Techniques Dependant on Soil Type
SUDS SOIL DESCRIPTION PERMEABILITY LOCATION TECHNIQUE
Well drained silty soils affected by Lemsford, Hatfield Hamble 2 groundwater, over gravel, usually High Garden Village, 571z on flat land Welham Green
Ludford Well drained fine loamy, sandy TTENUATION OMBINED High River Lee Valley
A 571x soil, slight risk of water erosion C
/ Well drained fine loam over clay Marlow and clayey soils, parts with slowly Medium Digswell Water 581e permeable subsoils and slight seasonal waterlogging Deep fine loam over clay and clay NFILTRATION I Hornbeam 3 soils with slowly permeable Low Ayot, Oaklands NFILTRATION AND
I 582d subsoils and slight seasonal water logging Calcareous in places Slowly permeable seasonally Windsor water logged clayey soils with Low Brookmans Park 712c brown subsoils
Slowly permeable seasonally Essendon South East of waterlogged coarse loam over Low 714d Hatfield clayey soils
Gresham Slowly permeable seasonally Hatfield Airfield, Low
TTENUATION 711v waterlogged coarse loam over clay Coopers Green A
Slowly permeable, seasonally Oak 2 waterlogged fine loam over clay Home Park, Low 714c and fine silt over clay. Some Millward’s Park clayey soils with chalky subsoil
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Appendix E Table 6 SuDS Suitability based on groundwater vulnerability
FULL VULNERABILITY APPROPRIATE SUDS VULNERABILITY LOCATION DESCRIPTION TECHNIQUES CLASSIFICATION River Lea Valley Infiltration and Combined Major High (H) 1 Highly Vulnerable River Mimram Valley Infiltration/Attenuation Major Intermediate Moderately Lemsford, Cromer Hyde, Infiltration and Combined (I) 1 Vulnerable Digswell,Ayot Infiltration/Attenuation Hatfield Airfield, Infiltration and Combined Major Low (L) Low Vulnerability Hatfield Garden Village Infiltration/Attenuation Minor Intermediate Moderately Ayot, Attenuation techniques (I) 1 Vulnerable Sherradspark Wood Welham Green, Essendon, Minor Low (L) Low Vulnerability Attenuation techniques East Hatfield Brookmans Park, Great Non-Aquifer Low Vulnerability Wood, Wild Hill, Cold Attenuation techniques Harbour, West End
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Appendix F: Policy Text
East of England Plan – The revision to the Regional Spatial Strategy (RSS) for the East of England
Policy WAT1: Water Efficiency The Government will work with the Environment Agency, water companies, OFWAT, and regional stakeholders to ensure that development provided for in the Spatial Strategy is matched with improvements in water efficiency, which will be delivered through a progressive, year on year, reduction in per capita consumption rates. Savings should be monitored against a per capita per day consumption target in the Regional Assembly's monitoring framework.
Policy WAT2: Water Resource and Waste Water Infrastructure Development The Environment Agency and water companies should work with OFWAT, EERA and the neighbouring regional assemblies, local authorities, delivery agencies and others to ensure timely provision of the appropriate additional infrastructure for both water supply and waste water treatment to cater for the levels of development provided through this plan, meeting agreed surface and ground water standards. A co-ordinated approach to plan making should be developed through a programme of water cycle studies to address water supply, water quality, wastewater treatment and flood risk issues in receiving watercourses relating to development proposed in this RSS. Complementing this approach, local development documents should plan to site new development so as to maximise the potential of existing water/waste water treatment infrastructure, minimise the need for new/improved infrastructure.
Policy WAT3: Integrated Water Management Local planning authorities should work with other partners to ensure that their plans, policies, programmes and proposals take account of the environmental consequences of river basin management plans, catchment abstraction management strategies, groundwater vulnerability maps, groundwater source protection zone maps, and proposals for water abstraction and storage. The Environment Agency and water industry should work with local authorities and other partners to develop an integrated approach to the management of the water environment.
Policy WAT4: Flood Risk Management Coastal and river flooding is a significant factor in parts of the East of the England. The priorities are to defend existing properties from flooding and locate new development in locations with little or no risk of flooding.
Local development documents should: • use strategic flood risk assessments to guide development away from floodplains, other areas at medium or high risk (or likely to be at future risk) from flooding, and areas where development would increase the risk of flooding elsewhere; • include policies which identify and protect flood plains and land liable to tidal or coastal flooding from development, based on the Environment Agency's flood maps, supplemented by historical and modelled flood data, Catchment Flood Management Plans and emerging policies in Shoreline Management Plans and Flood Management Strategies, including 'managed re-alignment' where appropriate; • only propose departures from the above principles in exceptional cases where suitable land at lower risk of flooding is not available, the benefits of development outweigh the risks from flooding, and appropriate mitigation measures are incorporated; and • require that sustainable drainage systems are employed in all appropriate developments.
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Areas of functional floodplain needed for strategic flood storage in the Thames Estuary should be identified and safeguarded by local authorities in their LDDs. Policy SS14 has been edited and placed in policy chapter WAT4
Policy SS1: Achieving Sustainable Development The strategy seeks to bring about sustainable development by applying: 1. The guiding principles of the UK Sustainable Development Strategy 2005; • living within environmental limits; • ensuring a strong, healthy and just society; • achieving a sustainable economy; • promoting good governance; and • using sound science responsibly. 2. The elements contributing to the creation of sustainable communities in Sustainable Communities: Homes for All, which are: • active, inclusive and safe (in terms of community identity and cohesion, social inclusion and leisure opportunities); • well run (in terms of effective participation, representation and leadership); • environmentally sensitive; • well designed and built; • well connected (in terms of good transport services); • thriving (in terms of a flourishing and diverse economy); • well served (in terms of public, private, community and voluntary services); and • fair for everyone.
Local development documents and other statutory and non-statutory strategies relevant to spatial planning within the region should: a) assist the achievement of carbon emissions; and b) adopt a precautionary approach to climate change by avoiding or minimising potential contributions to adverse change and incorporating measures which adapt as far as possible to unavoidable change.
In particular the spatial strategy seeks to ensure that development: • maximises the potential for people to form more sustainable relationships between their homes, workplaces, and other concentrations of regularly used services and facilities, and their means of travel between them; and • respect environmental limits by seeking net environmental gains wherever possible, or at least avoiding harm, or (where harm is justified within an integrated approach to the guiding principles set out above) minimising mitigating and/or compensating for that harm'
LA3: Welwyn Garden City and Hatfield Key Centre for Development and Change Welwyn Garden City and Hatfield will jointly be a Key Centre for Development and Change within the London Arc. The strategy for these towns involves the following main elements: 1. Overall housing growth of 10,000 in Welwyn Hatfield District by 2021, focused mainly at these towns. Brownfield redevelopment opportunities will be maximised but sustainable urban extensions will also be required, to be planned through the LDF process.
2. Provision for substantial employment growth over the period to 2021 by: • capitalising on strategic links to Stevenage and Central London; • developing new employment sites and regenerating existing ones; • making the most of opportunities associated with the University of Hertfordshire and the new health campus; • reinforcing the town centre of Welwyn Garden City; and
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• creating a more vital Hatfield town centre. 3. Focused and coordinated action to raise opportunities and expectations and make better provision for local residents in terms of health, education, employment, transport and quality of life. 4. Retention/ reinforcement of the best qualities of Welwyn Garden City and substantial improvements to the image and quality of the two towns' built fabric and public realm, including provision of multi-functional green space and enhanced green infrastructure between Hatfield and St Albans. 5. Additional waste water treatment capacity, planned and delivered working with the industry and its regulators.
Welwyn Hartfield District Plan 2005 Saved policies: Policy SD1 - Sustainable Development 'Development proposals will be permitted where it can be demonstrated that the principles of sustainable development are satisfied and that they accord with the objectives and policies of this plan. To assist the Council in determining this, applicants will be expected to submit a statement with their planning application demonstrating how their proposals address the sustainability criteria in the checklist contained in the Supplementary Design Guidance.'
Policy R6 - River Corridors 'Initiatives to protect and enhance the river environment for biodiversity including proposals for deculverting and naturalisation of the river channel, will be supported. Suitable public access and informal water based or waterside recreation within main river corridors will also be supported where it is appropriate, provided that there is no conflict with the biodiversity of the site. Development will not be permitted which would involve the culverting or diverting of any watercourse, and/or the siting of buildings in close proximity to the river channel, unless the Council is satisfied that there would be no detriment to the river corridor'.
Policy R7 - Protection of Ground and Surface Water 'Planning permission will not be granted for development which poses a threat to the quality of both surface and/or groundwater. Where proposals are acceptable the use of sustainable drainage systems will be encouraged, dependent on local site and underlying groundwater considerations.'
Policy R10 - Water Conservation Measures 'New development will be expected to incorporate water conservation measures wherever applicable, including sustainable drainage systems, water storage systems, soft landscaping and permeable surfaces to help reduce surface water run-off.'
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