Cambridgeshire Flood Risk Management Partnership St Neots Surface Water Management Plan

Transcription

1 Cambridgeshire Flood Risk Management Partnership St Neots Surface Water Management Plan Detailed Assessment and Options Appraisal Report Final

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3 Hyder Consulting (UK) Limited Aston Cross Business Village 50 Rocky Lane Aston Birmingham B6 5RQ United Kingdom Tel: +44 (0) Fax: +44 (0) Cambridgeshire Flood Risk Management Partnership St Neots Surface Water Management Plan Detailed Assessment and Options Appraisal Report Final Authors L A Foster/ A McNally Checkers L Foster Approver R Gunasekara Report No 5101-UA BMR-01 Date May 2012 This report has been prepared for the Cambridgeshire Flood Risk Management Partnership in accordance with the terms and conditions of appointment for the Surface Water Management Plan dated September Hyder Consulting (UK) Limited ( ) cannot accept any responsibility for any use of or reliance on the contents of this report by any third party. Front Cover: Drainage ditch in Eynesbury Manor area of St Neots. Picture taken by R Gunasekara during a site visit to St Neots in August Hyder Consulting (UK) Limited

4 CONTENTS 1 Introduction Terms of Reference Surface Water Management Plans Partnership Establishment Policy Framework Surface Water Flooding Sustainable Drainage Systems (SuDS) Scope of the SWMP Aims and Objectives Drivers for Change Geographic Extent Methodology Evidence Base Previous Studies Historical Flooding Sources of Flooding Potential Indicators of Surface Water Flood Risk Maintenance Regimes Model Development Model Evolution Hydraulic Modelling - Common Principles Stage 1 - Bare Earth/ Hydrological Analysis Modelling Wetspot Selection and Prioritisation Approach Stage 2 - Identification of Potential Wetspot Areas Prioritisation of Wetspot Areas Detailed Assessment Stage 3 - Detailed Model Development Model Verification Model Assumptions and Limitations Model Sensitivity Model Outputs Engineering Options Identification and Assessment Measures Identification St Neots Engineering Measures and Options Hyder Consulting (UK) Limited Page i

5 8 Economic Appraisal Introduction Damages Assessments - Assumptions Damages Assessment Exclusions Summary Key Surface Water Flooding Issues Preferred Options For Further Investigation Benefits of SWMP Next Steps Surface Water Management Action Plan Preparation, Implementation and Monitoring Engage with Stakeholders References Appendices Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Appendix H Appendix I Glossary Partnership Arrangements and Stakeholder Engagement Data Collection Process and Data Register Flood Incident Register Hydrological Parameters Engineering Options Details Detailed Modelling Results Options Modelling Results and Preferred Options Schematics Sensitivity Analysis Figures Hyder Consulting (UK) Limited Page ii

6 Report Version Control Schedule Version Date of Issue Document Reference Status /03/ _St Neots Detailed SWMP Report /05/ _St Neots Detailed SWMP Report Draft Final Hyder Consulting (UK) Limited Page ii

7 1 Introduction 1.1 Terms of Reference Hyder Consulting (UK) Limited (Hyder) was appointed to produce the Surface Water Management Plan (SWMP) for the entire county, which is to be completed by April The first phase of work was the Countywide Strategic Assessment report, which was completed in April Hyder were then asked to undertake a detailed surface water management assessment of St Neots, as this town was identified as being the highest priority wetspot within the Cambridgeshire County region. This St Neots SWMP is formed from the outputs of all the stages of the study, from a strategic assessment of the overall study area through to optioneering of the prioritised wetspots. The options assessed at this stage provide a theoretical assessment of how best to mitigate against flood risk in the wetspot. This provides an analysis of where investment could be directed in the future if finance is available and other key constraints can be overcome through investigation and stakeholder consultation. The St Neots Detailed SWMP was completed in May 2012, and the findings are detailed in this report. It is recommended that this report be read alongside the Countywide Strategic Assessment report mentioned above. 1.2 Surface Water Management Plans The wide scale flooding experienced during 2007 precipitated the publication of the Pitt Review 1 which contained a large number of recommendations for Government to consider. The key recommendation in the Pitt Review with respect to surface water management is Recommendation 18, reproduced below, which in turn refers to Planning Policy Statement 25 Development and Flood Risk (PPS25) 2. Recommendation 18: Local Surface Water Management Plans, as set out in PPS25 and coordinated by local authorities, should provide the basis for managing all local flood risk. Surface Water Management Plans (SWMPs) are referred to in Planning Policy Statement 25 (PPS25) as a tool to manage surface water flood risk on a local basis by improving and optimising coordination between relevant stakeholders. SWMPs will build on Strategic Flood Risk Assessments (SFRAs) and provide the vehicle for local organisations to develop a shared understanding of local flood risk, including setting out priorities for action, maintenance needs and links into local development frameworks and emergency plans. Guidance on the production of SWMPs was published in March informed by the Integrated Urban Drainage (IUD) Pilot Studies carried out under the Government s Making Space for Water (MSfW) 4 strategy. A SWMP outlines the preferred strategy for the management of surface water in a given location and the associated study is carried out in consultation with local partners having responsibility for surface water management and drainage in that area. The goal of a SWMP is to establish a long term action plan and to influence future strategy development for maintenance, investment, planning and engagement. Hyder Consulting (UK) Limited Page 3

8 The framework for undertaking a SWMP is illustrated using a wheel diagram, reproduced from the Defra Guidance³ as shown in Figure 1-1. Figure 1-1 SWMP Wheel (source Defra Guidance³) The SWMP process is formed of four principal phases; preparation, risk assessment, options, and implementation and review. This report contains the findings from the preparation stage and the strategic and intermediation elements of the risk assessment phase. Text boxes at the start of each chapter summarise the elements of the guidance addressed within the subsequent text. Hyder Consulting (UK) Limited Page 4

9 1.3 Partnership Establishment The formation of partnerships has an important role in the undertaking of a SWMP, and is required under Defra s SWMP guidance documentation. The SWMP guidance details the identification of those partners / organisations that should be involved and what their roles and responsibilities should be. It recommends the formation of an engagement plan, which should include objectives for the individual partners, and detail how and at what stages of the SWMP the engagement with stakeholders should take place. Appendix B describes the partners, their roles and responsibilities and their objectives as required by the SWMP guidance. 1.4 Policy Framework Flood Risk Regulations 2009 The Flood Risk Regulations 2009 (FRR) transpose the European Floods Directive 2007/60/EC into English and Welsh law and bring together key partners to manage flood risk from all sources and in doing so reduced the consequences of flooding on key receptors. Local authorities are assigned responsibility for management of surface water flooding. As part of the ongoing cycle of assessments, mapping and planning, the FRR requires the undertaking of a Preliminary Flood Risk Assessment (PFRA). National guidance was published by the Environment Agency (EA) in December The requirements of the FRR have also been used to shape this report and to inform the content of the Council s PFRA report to the Government produced by HCL. The PFRA for Cambridgeshire was produced in March Under Flood Risk Regulation 19-1 a Lead Local Flood Authority must prepare a flood hazard map and a flood risk map in relation to each relevant Flood Risk Area, if identified by the PFRA process. No significant Flood Risk Area has been identified by the EA nationally within Cambridgeshire, nor the first cycle of the Cambridgeshire PFRA at a local level. However, depth, velocity and hazard maps (Appendix G) have been prepared for the St Neots SWMP study area and they will inform Cambridgeshire s Local Flood Risk Management Strategy development (Section 2.1.3) and the second cycle of the PFRA process in six years time Flood and Water Management Act 2010 The Flood and Water Management Act places the responsibility for managing the risk of local floods on the Upper Tier or unitary authorities, as their role as Lead Local Flood Authorities (LLFAs), but allows for the delegation of flood risk management functions to other statutory authorities. The Act also seeks to encourage the uptake of Sustainable Drainage Systems (SuDS) by agreeing new approaches to the management of drainage systems and allowing, where delegated, for district councils and Internal Drainage Boards (IDBs) to adopt SuDS for new developments and redevelopments Planning Policy Statement 25 (to March 2012) Planning Policy Statement 25 (PPS25) required that new development should not increase flood risk, and requires developers to design, build and fund the maintenance of SuDS; a SWMP will support this by informing the Local Planning Authority (LPA) of areas at risk of Hyder Consulting (UK) Limited Page 5

10 surface water flooding and by providing an evidence base to aid the consideration of future development options National Planning Policy Framework (from April 2012) The National Planning Policy Framework (NPPF) has reviewed all existing planning policies and restructured the planning process 7. The aim of this new framework is to make planning more streamlined and transparent. The NPPF also aims to give local councils more control over local planning with more emphasis being placed on sustainable local growth. This new national Framework replaces PPS25 however retains the same attention to minimising flood risk and increasing the use of SuDS in new developments. The final version of the NPPF was published on 27 th March 2012 along with a Technical Guidance document for local planning authorities. 1.5 Surface Water Flooding In the context of SWMPs, the technical guidance 3 defines surface water flooding as: Surface water runoff; runoff as a result of high intensity rainfall when water is ponding or flowing over the ground surface before it enters the underground drainage network or watercourse, or cannot enter it because the network is full to capacity, thus causing flooding (known as pluvial flooding); Flooding from groundwater where groundwater is defined as all water which is below the surface of the ground and in direct contact with the ground or subsoil; Sewer flooding; flooding which occurs when the capacity of underground systems is exceeded due to heavy rainfall, resulting in flooding inside and outside of buildings. Note that the normal discharge of sewers and drains through outfalls may be impeded by high water levels in receiving waters as a result of wet weather or tidal conditions; Flooding from any watercourse not designated a Main River, including culverted watercourses which receive most of their flow from inside an urban area and perform an urban drainage function; Overland flows from the urban/rural fringe entering the built-up area; and Overland flows resulting from groundwater sources. This report aims to consider surface water flooding issues in St Neots as above but it does not address sewer flooding where it is occurring as a result of operational issues, i.e. blockages and equipment failure. It should also be noted that the compilation of all historical flooding within the county area does include some flooding due to main rivers, although further investigation of these occurrences is outside the remit of this report. Information on Main River Flooding is covered under other strategic planning documents such as Strategic Flood Risk Assessments, produced by district councils. 1.6 Sustainable Drainage Systems (SuDS) Sustainable drainage systems are used to manage rainfall run-off from impermeable surfaces. SuDS encompass a range of techniques which aim to mimic the natural processes of runoff and infiltration as closely as possible. These techniques can include green roofs, ponds, permeable paving and soakaways. Any SuDS scheme should integrate with existing drainage systems and be easily maintainable. Hyder Consulting (UK) Limited Page 6

11 SuDS schemes should be based on a hierarchy of methods termed the SuDS treatment train as illustrated in Figure 1-2. Figure 1-2 SuDS Treatment Train Guidance recommends that the management of surface water runoff should use a combination of site specific and strategic SuDS measures, encouraging source control where possible to reduce flood risk and improve water quality. Table 1-1 describes some of the SuDS techniques that will be considered in the development of the St Neots SWMP. SuDS techniques can be divided into two main groups; infiltration based or attenuation based. Infiltration based SuDS facilitate the discharge of water directly into the ground through soil and rocks; this is only possible where the underlying geology is permeable enough to allow the passage of water downwards. Attenuation based SuDS retain water on a site and allow it to discharge at a prescribed and controlled rate into a watercourse or sewer. The feasibility for the use of any SuDS technique should be investigated prior to their installation. Hyder Consulting (UK) Limited Page 7

12 Type Balancing Pond Brown Roof Description A pond designed to attenuate flows by storing runoff during the peak flow and releasing it at a controlled rate during and after the peak flow has passed. The pond always contains water. Also known as wet detention pond. A roof covered with a locally sourced material, its main aim is to partly mitigate any loss of habitat when new developments are constructed. Detention Basin A vegetated depression, normally dry except after storm events constructed to store water temporarily to attenuate flows. May allow infiltration of water to the ground Filter Strip Green Roof A vegetated area of gently sloping ground designed to drain water evenly off impermeable areas and filter out silt and other particulates. A roof with plants growing on its surface, which contributes to local biodiversity. The vegetated surface provides a degree of retention, attenuation and treatment of rainwater, and promotes evapotranspiration. Sometimes referred to as a living roof. Infiltration Basin A dry basin designed to promote infiltration of surface water to the ground. Road Side Rain Gardens Permeable Surface Rainwater Harvesting Swale Where space allows, these can be constructed alongside roads to allow run-off from roads or pavements to filter slowly through the root system of plants, rather than entering underground drainage systems. A surface formed of material that is itself impervious to water but, by virtue of voids formed through the surface, allows infiltration of water to the sub-base through the pattern of voids, e.g. concrete block paving. A system that collects rainwater from where it falls rather than allowing it to drain away. It includes water that is collected within the boundaries of a property, from roofs and surrounding surfaces. The harvested water is then re-used in applications where potable water is not essential. A shallow vegetated channel designed to conduct and retain water, but may also permit infiltration; the vegetation filters particulate matter. Table 1-1 SuDS Techniques (source Ciria 8 ) Hyder Consulting (UK) Limited Page 8

13 2 Scope of the SWMP Flood Risk Regulations 2009 Define the aims, objectives and purpose of the report Describe the overall approach and methodology applied 2.1 Aims and Objectives Study The final aim of the SWMP study is to produce a long term surface water management Action Plan for St Neots, which, once in place, will be reviewed every 6 years at a minimum. The objectives of this study are to: Map historical flood incident data Engage with partners and stakeholders Map surface water influenced flooding locations Identify surface water flooding wetspot areas Assess, compare and prioritise wetspot areas for detailed assessment Identify measures, assess options and confirm preferred options for the prioritised wetspots Make recommendations for next steps A wetspot is defined as being an area susceptible to Surface Water flooding following analysis of Modelled Surface Water outputs or historical records. These objectives will be met following the progression of a number of project stages. The first stage is data collection, involving contact with the varying partner organisations to obtain all Hyder Consulting (UK) Limited Page 9

14 relevant information. During this stage the collation of historical and future flooding along with information on flood receptors and flood consequences will take place. Once the data collection stage is complete, the surface water flooding information will be analysed to identify wetspots that have a history of flooding incidents or potentially could be at risk of future flooding. Those wetspots identified as being at higher risk or priority through agreed local assessment criteria will then progress forward to the next stages, detailed assessment and optioneering. Following the optioneering stage, recommendations for flood alleviation or mitigation will be considered Partnership Working The Cambridgeshire Flood Risk Management Partnership comprises all the flood risk authorities in Cambridgeshire, including Cambridgeshire County Council, Cambridge City Council, Huntingdonshire District Council, South Cambridgeshire District Council, the Environment Agency and Anglian Water. A SWMP Project Management Board was formed as a sub group of CFRMP to steer the production of SWMPs, and they are discussed in more detail in Section 3.1. The CFRMP has developed a Stakeholder Engagement Plan, which will aid in communicating the work of the partnership to the key stakeholders, and is discussed in further detail in Section 2.4 of the County Wide SWMP. It is of great importance that collaborative working of this nature is undertaken in order to share experience and expertise Context Alongside the legislative requirements discussed above, this SWMP will support the following initiatives. Local Flood Risk Management Strategies Local Flood Risk Management Strategies 9 came into force as part of the Flood and Water Management Act As LLFA, CCC must develop a strategy for local flood risk management. The strategy must be consistent with the National Flood and Coastal Erosion Risk Management Strategy for England, the regional CFMPs and River Basin Plans, and should be developed and maintained with consultation from other stakeholders, such as the public and other risk management authorities. The strategy must specify: the risk management authorities in the authority's area, the flood and coastal erosion risk management functions that may be exercised by those authorities in relation to the area, the objectives for managing local flood risk (including any objectives included in the authority's flood risk management plan prepared in accordance with the Flood Risk Regulations 2009), the measures proposed to achieve those objectives, how and when the measures are expected to be implemented, the costs and benefits of those measures, and how they are to be paid for, the assessment of local flood risk for the purpose of the strategy, how and when the strategy is to be reviewed, and Hyder Consulting (UK) Limited Page 10

15 how the strategy contributes to the achievement of wider environmental objectives. Catchment Flood Management Plan (CFMP) The St Neots study area falls within the area covered by the Great Ouse CFMP, as discussed in Section The Action Plan associated with the Great Ouse CFMP, in conjunction with district wide SFRA s and this SWMP, will assist in informing the Local Plan process and future flood risk management. Anglian River Basin Management Plan (RBMP) The St Neots study area falls within the Anglian RBMP. The plan has been prepared under the Water Framework Directive and is designed to protect, improve and ensure the sustainable use of the water environment within the Anglian Basin. 2.2 Drivers for Change The CFRMP are undertaking this SWMP in order to: Better understand the risks and consequences of surface water flooding in St Neots; To meet or significantly assist in meeting some of the requirements on CCC as Lead Local Flood Authority under the Flood Risk Regulations 2009; To meet a number of the requirements of the Flood and Water Management Act specifically in terms of developing an asset register and producing a local flood risk management strategy. At this point it is worth noting that the developed area of St Neots is steadily increasing due to a number of large residential developments already constructed, and further large-scale developments are planned to the east of the study area. These will have had significant impacts on the natural environment, as greener rural areas have been replaced in part by housing and commercial developments, roads and other forms of community infrastructure. The SWMP process allows the opportunity to enhance the condition of these urbanised catchments helping to improve the water quality. Additionally, the implementation of the SWMP and Action Plan can help to provide significant economic and environmental benefits to the community through better preparation against these potential extreme rainfall events, which to a large extent has not occurred since this development has occurred. 2.3 Geographic Extent Flood Risk Regulations 2009 Define the geographic extent of the report and relate to the relevant river basin district and relevant maps This SWMP has been undertaken for St Neots which is the largest town in Cambridgeshire County with a population of approximately 28,000. The location of the study area is shown in Figure 2-1. St Neots is situated within the Anglian River Basin District and the River Great Ouse catchment. It sits on the border of Cambridgeshire and Bedfordshire County Council areas and within Huntingdonshire District Council. Hyder Consulting (UK) Limited Page 11

16 Figure 2-1 St Neots Location Plan The study area boundary has been defined by the urban extent of the town and is approximately 9.5 km 2 (Figure 2-2). Hyder Consulting (UK) Limited Page 12

17 Figure 2-2 St Neots SWMP Study Area (black arrows highlight direction of flow) Crown Copyright and database right All rights reserved. Ordnance Survey licence number St Neots is bisected by River Great Ouse with approximately 45% of the study area on the lower lying floodplain between 10m 15 maod. The remaining 55% of the study area is located on higher ridges of ground that range from maod. The land surrounding St Neots is predominantly arable agricultural land. 2.4 Methodology The methodology used to carry out this SWMP follows the advice set out in the Defra SWMP guidance3 for the preparation stage and the strategic risk assessment phase. Figure 2-3 illustrates the process carried out to inform this detailed assessment and options appraisal report, a key output of St Neots SWMP. It should be noted that this figure only shows the steps subsequent to the formal identification of the St Neots settlement as a priority wetspot during the Countywide Strategic phase. Hyder Consulting (UK) Limited Page 13

18 Further details on the methodology are discussed throughout the report in the relevant sections. The work undertaken for the study is also informed by the EA s PFRA guidance 5 in order to assist in meeting the obligations of CCC as the Lead Local Flood Authority (LLFA). Information on the methodology for subsequent phases of the SWMP is set out in Section 10 of this report. County wide SWMP EA Surface Water Map and Areas susceptible to Surface Water Flooding Maps BGS Groundwater Vulnerability Maps Collate and map Historic Flood Incident Data Map surface water influenced historic and future flood locations, mechanisms and consequences National and Local Information on flood receptors and consequence Undertake Strategic Assessment Produce List of Initial Wetspot Areas Consult stakeholders for local knowldge and to obtain missing data and prioritise wetspots Recommend Priority Wetspot Areas for Detailed Assessment Undertake Detailed Assessment and Options Appraisal Confirm Preferred Options Figure 2-3 Overall Approach to Study Methodology The specific methodology adapted for the St Neots study is further explained in Sections 4 to 8. Hyder Consulting (UK) Limited Page 14

19 3 Evidence Base 3.1 Previous Studies As part of this study, it has been critical to identify the links to other local and regional delivery plans which may influence or be influenced by the SWMP. The SWMP will seek to integrate and align these plans and processes to provide a clear and robust path to delivering flood risk management objectives throughout St Neots. These studies listed below have already been completed, however the information from the SWMP and future Local Flood Risk Management Strategy can be used to inform any updates to these studies. Appendix C provides further information on data collection and review process Great Ouse CFMP 10 The Great Ouse Catchment Flood Management Plan (CFMP) was published by the Environment Agency (EA) in July The catchment covers approximately 8,600 km 2, and is predominantly rural, with the larger population centres of Milton Keynes, Cambridge, Bedford and King s Lynn. For St Neots, the main sources of flood risk were identified as: river flooding from the River Great Ouse; surface water flooding A number of flood risk management policy options were identified across the whole catchment, and the policy option covering St Neots was Policy Option 5 - Areas of moderate to high flood risk where the EA can generally take further action to reduce flood risk Huntingdonshire SFRA 11 The final Level 1 SFRA for Huntingdonshire was completed by Mott MacDonald and Posford Haskoning in The aim of the study was to provide an assessment of the nature and extent of flood risk and its application to development planning. The study investigated flooding from Main Rivers, Ordinary Watercourses, Middle Level Commissioners high level carriers and the IDB s low level systems, and for each individual river reach, those properties predicted to be at risk of flooding were identified. This SFRA was subsequently updated in to take account of PPS25, and to take advantage of improved topography mapping and further flood risk mapping studies undertaken by the EA. The main findings of the 2010 report were that surface water flooding is a recurring problem in several locations, some of which are not shown to be at risk on the EA maps. This was suggested to be due to either the resolution of available topographic maps, or due to features that can divert overland flows, such as kerbs, that are not taken account of on the EA maps. St Neots was highlighted as one area where a number of surface water flooding events had occurred. Hyder Consulting (UK) Limited Page 15

20 3.1.3 Huntingdonshire Outline Water Cycle Strategy 13 In 2008, Faber Maunsell AECOM completed the water cycle strategy for Huntingdonshire. Its aim was to provide evidence to support the documents submitted to the Government as part of Huntingdonshire s Local Development Framework. A Detailed Water Cycle Strategy is currently being produced by Huntingdonshire District Council in partnership with the key stakeholders St Neots Drainage Area Plan 14 A drainage area plan was undertaken by Atkins in 2002 on behalf of Anglian Water. The main drivers for the Drainage Area Plan (DAP) were to assess the impact of significant growth within the sewer network and river quality compliance. As such, the model extent was limited to that required to solely address these drivers, rather than being a full catchment DAP model. Manhole, CCTV and a flow survey were carried out, and dry weather, storm and historical verification undertaken Anglian River Basin Management Plan 15 The Anglian River Basin Management Plan focuses on the protection, improvement and sustainable use of the water environment. It aims to help European Union Country s meet Water Framework Directive objectives such as promoting sustainable use of water as a natural resource, contributing to mitigating the effects of floods and droughts and conserve habitats and species that depend directly on water. The Ouse catchment supports a number of nationally and internationally important water-related sites that are of exceptional value Countywide SWMP The Countywide SWMP report summarises the strategic level assessment of surface water flood risk across Cambridgeshire County as per Defra SWMP guidance. The report identifies broad areas where surface water should be considered in more detail in a site based Flood Risk Assessment. The report aids the development of more general planning policies to help minimise the risk of flooding by surface water, such as policies promoting SuDS. The SWMP identified the top ten wetspots within the county, St Neots was first. The wetspots were identified by reviewing the historical incidents and the EA Flood Maps for Surface Water and using this information to identify susceptibility of Cambridgeshire settlements to surface water flooding through Multi-Criteria Analysis. The number of properties and critical infrastructure that were shown to be at risk from a number of flood sources were the key reasons that St Neots was identified as a priority wetspot for further investigation/work Flood Memories The Cambridgeshire Flood Risk Management Partnership carried out a programme of collecting flooding information focusing predominantly on small to medium localised flooding events. Members of the public were asked to complete a questionnaire on their memories of flooding incidents, either via a paper or online questionnaire, or via five road shows across the county. Over 250 responses were received. Hyder Consulting (UK) Limited Page 16

21 Four flooding events within St Neots were identified from this programme of works. Three records relate to fluvial flood events (Hen Brook, Fox Brook and Colmworth Brook) and one surface water event near Montagu Street. 3.2 Historical Flooding Flood Risk Regulations 2009 Introduce the local sources of flood risk being considered for past floods and possible future floods. Assess past floods which had significant harmful consequences for human health, economic activity, cultural heritage and the environment. The following sections outline the historical flooding recorded within the St Neots study area specifically within the context of the definition given in Section 1.3. This text should be read in conjunction with the Countywide Flood Incident Register shown in Appendix D. It is highlighted that this report is based on the information supplied by partners up to January 2011; the occurrence of flooding is not static and thus this represents an understanding of the situation as of this date. Major flooding events were recorded in St Neots in March 1947 and Easter 1998, when the River Great Ouse and many of its tributaries overtopped their banks. Further, smaller or localised flooding events have been recorded in: May 1978; July 1982; October 1993; April 2000; October 2001; January 2003; August 2004; July 2007; It should be noted that many of the historic records do not provide a date when the flooding occurred, and for the majority, there is no identified cause of flooding (pluvial, fluvial, sewer etc). It is also likely that flooding within St Neots is under-reported. In general, the historical information associated with flooding in St Neots is comparatively poor with few records in relation to the spatial extent of flooding and the frequency of inundation to properties. As discussed this is possibly due to under reporting of problems with flooding by the general public to the Local Authority / Environment Agency. Figure 3-1 shows the location of historical flood events in St Neots based upon the above data along with the available fluvial flood outlines. Hyder Consulting (UK) Limited Page 17

22 Figure 3-1 Historical Flooding in St Neots Whilst every effort has been made to analyse the data there is a high probability that there are deficiencies in quantity and the attribution of historical information. It is considered that the majority of the information pertinent to the SWMP falls within the Low to Medium Confidence categories (Table 3-1). Caution has therefore been exercised within this section of the report in interpreting the historical record. DG5 Flooding Register AWS maintains a register of confirmed internal and external sewer flooding locations due to hydraulic overloading. The Register only contains properties and areas at risk of internal and external flooding if they have suffered flooding from public sewers due to overloading of the system, the register however excludes any incident event that has a storm return period above a 1 in 22 (4.5% AEP) and is therefore classified as a severe weather event.. A sewer is overloaded when the flow from a storm is unable to pass through it due to permanent problem (e.g. small pipe, flat gradient). The Register does not include properties or areas flooded due to temporary operational problems e.g. blockage, siltation, collapse, equipment failure or operational failure. The Register does not contain properties or areas that have been subject to a flood alleviation scheme (to a satisfactory level of protection) or if new information reveals that the property or area does not meet the criteria to be on the register. AWS has provided its DG5 database for the study area to a four/five digit postcode so that individual properties were not identifiable. Hyder Consulting (UK) Limited Page 18

23 As of February 2011, there were 16 entries on the DG5 register within the study area. Properties must be recorded on the DG5 register before a scheme to reduce risk is considered. It should be noted that previous DG5 listings have been removed from the register as a result of remedial work, or the implementation of system improvements. AWS are required to undertake capacity improvements to alleviate some of the most severe sewer flooding problems on the DG5 register during the current 5 Year Asset Management Period ( ). The DG5 incidents are prioritised using a risk and value process to identify cost beneficial properties that meet targets set for reducing the number of properties on the register. The capacity improvement schemes are created to a design standard 1 in 30 for internal flooding schemes and 1 in 20 for external. Historical Flooding Record Highway and Pluvial Flooding The historical record, particularly the CCC Customer Reports of Flooding, includes a significant number of descriptive records of flooding tagged with general descriptions such as: blocked drains, foul sewer emissions, road drainage, foul sewer backup and drains silted up. The records clearly demonstrate that there are problems with sewer flooding. However, and in general, there is insufficient information to identify whether flooding is attributable to highway, foul or storm sewer flooding. Moreover, it is frequently not possible to determine the frequency and precise cause of the flooding. It is considered that the quality of historical flooding information falls within the low to medium confidence categories Flood Incident Register A sub task within the data assimilation stage, as part of the countywide SWMP, was the development of a flood incident register to show all the historical surface water flooding incidents in Cambridgeshire, which included those occurring within the St Neots study area. For each event the location of each flood incident was registered and approximate co-ordinates for the incident was also recorded where this was readily available or could be estimated within the available project timescale and resources. Each flooding incident was assigned a unique flood incident reference number. The flood incident register for the St Neots area is given in Appendix D. This has been updated to include additional locations that were identified during stakeholder consultations as part of this detailed SWMP. For many incidents the exact location of flooding was not reported, for example flooding occurred on High Street. Where the exact location was not known, an indicative location was picked at a central point on the street. Where known, the house number, incident date and time of incident was recorded. It should be noted therefore, that the flood incident register contains approximate grid co-ordinate locations that may not be the exact location of the historical flooding incident. A crucial component of the incident register is recording the confidence in the source of the information. Some flood events were well reported, with a high level of detail regarding the source, pathway and receptor and other reports did not provide such details. The criteria in Table 3-1 were used to assess the confidence in the flood source. It is recommended that this practice is continued for all new flooding incidents added to the register along with more accurate information on incident location and flood consequence. Hyder Consulting (UK) Limited Page 19

24 Flood Source Little or no evidence to support flood source in incident report Flood source provided by residents or non technical experts with high level of detail in the incident report Flood source provided by technical experts e.g. IDB staff or residents with compelling evidence i.e. photos Confidence in Flood Source Low - Source assumed Medium - Some evidence High - Compelling evidence Table 3-1 Confidence in flood report sources 3.3 Sources of Flooding The following sections summarise the various sources of flooding Surface Water Runoff / Pluvial Flooding Surface water runoff occurs as a result of high intensity rainfall causing water to pond or flow over the ground surface before entering the underground drainage network or watercourse, or when water cannot enter the network due to insufficient capacity. Pluvial flooding is defined as flooding that result from rainfall-generated overland flow. The historical records include a significant number of descriptive records of flooding which imply that there are issues with pluvial flooding. The records clearly demonstrate that there are problems with pluvial flooding but it should also be recognised that flooding will be the result of numerous factors rather than solely rainfall intensity or duration. The rainfall event Annual Event Probabilities assessed by the SWMP are shown in Table 3-2 below. The modelling methodology is discussed in Section 4. Annual Event Probability (%) (chance of occurring in any given year) Approximate equivalent Return Period 0.5% 1 in 200 1% 1 in % 1 in % 1 in 30 5% 1 in 20 Table 3-2 Annual Event Probabilities Assessed and Equivalent Return Periods Fluvial Flooding The watercourses in the St Neots study area are shown in Figure 2-2. Further details on their categorisation and those responsible for their upkeep are given in the following sections. Main Rivers Under the Water Resources Act 1991, the EA has powers to maintain and improve designated main rivers for the efficient passage of flood flow and the management of water levels for flood defence purposes. These powers are permissive only and there is no obligation on the Agency to carry out such works. The current maintenance regime for designated main rivers uses a risk Hyder Consulting (UK) Limited Page 20

25 based approach and government funding via Defra. The ultimate responsibility for maintaining the bed and banks of any watercourse, including its vegetation, rests with the riparian owner(s). The EA offers a flood warning service to areas covered by main rivers and some ordinary watercourse tributaries. We also provide protection to certain areas at risk from Main River flooding in the form of strategic flood defences. The main rivers in the St Neots study area are the River Great Ouse, Fox Brook and Hen Brook, shown in Figure 2-2. Information on the main rivers in the county area was provided by CCC and the EA. It is noted that the River Kym is located close to the study area. Awarded Watercourses Awarded watercourses are any watercourses for which responsibility has been transferred to the District Council under Enclosure Acts. In St Neots, they include Colmworth Brook and Duloe Brook, shown in Figure 2-2. Details of awarded watercourses were provided by the local authorities. Ordinary Watercourses Ordinary watercourses are all rivers, streams, ditches and drains that have not been designated as main rivers. The main responsibility for all watercourses lies with the riparian owners. Local authorities are responsible for any ordinary watercourses that fall within areas where they are the land owner or if the watercourse is awarded. If the watercourse falls within an IDB area then the IDB are responsible for the watercourse. Details of ordinary watercourses were provided by the local authorities Underground Systems Sewers Sewer flooding occurs when the capacity of underground sewerage system is exceeded due to heavy rainfall, resulting in flooding inside and outside of buildings. However, sewer flooding can be caused by excess surface water, blockages collapses or plant failure. Water companies, in this case Anglian Water Services Ltd (AWS), are obliged under the Water Industry Act 16 to maintain and operate systems of public sewers and works to ensure effective drainage within their area. There is no universal level of service associated with the sewer network. The water companies are also responsible for private drains and sewers, following the recent transfer of private sewers and lateral drains, that are connected to the public sewerage system, on the 1 st October Table 3-3 details the main public sewer asset types in urban areas. Hyder Consulting (UK) Limited Page 21

26 Asset Type Public foul sewer Description Maintained and operated by AWS, these should carry only foul sewage but, through misconnections, often also carry surface water. Public surface water sewer Public combined sewer Maintained and operated by AWS. They should carry only surface water.. Public combined sewers are maintained and operated by AWS. They carry both foul sewage and surface water.. Table 3-3 Public Sewerage Systems Since the first publication of Sewers for Adoption in 1980, this document has become the standard for the design and construction of sewers to adoptable standards in England and Wales although it has been updated a few times and the current version is the 6 th edition. It is also expected that the 7 th edition will be launched in the near future. Sewers for Adoption currently requires public surface water sewers to accommodate flows up to a 3.33% AEP (1 in 30 year chance) design storm. It is highlighted however that this level of service will change if ever increasing areas are connected to the sewers over time. The design standard also does not account for the capacity of connections such as gutters, gullies, highway drains and private drains which may limit the flow discharging to the sewer. The sewerage system within St Neots comprises a combined sewer system in the historic town centre, with suburban areas comprised of separate foul and storm water sewer systems. The following figure shows the complete sewer network and assets as initially provided by Anglian Water. Hyder Consulting (UK) Limited Page 22

27 Figure 3-2 St Neots AWS Sewer Network St Neots Wastewater Treatment Works (WwTW) is situated to the north of St Neots. All flows arrive at the works via rising mains from St Neots Road, Huntingdon Road, Cemetery Road and Loves Farm pumping stations. The works also receives foul flows from Little Paxton via Wantage Gardens Pumping Station. AWS has prepared an InfoWorks hydraulic model of the foul sewerage system as indicated in Table 3-3. Asset Type Foul sewer Surface water sewers Public combined sewer Description The foul system is maintained and operated by AWS, who also maintains an Infoworks model of the network which contains asset data, sewer network data and manhole locations The storm water system is predominantly maintained and operated by AWS; however no model exists of the system. They maintain a GIS database, which lists the locations of the manholes and the sewer network. There are also numerous areas of development where AWS have no records of the storm network. This is mainly due to these sewers not yet being adopted by AWS. The combined system operates only in the town centre and is owned and operated by AWS. A model of this system is integrated within the foul network model. Table 3-3 Hydraulic Modelling of St Neots Sewer System Hyder Consulting (UK) Limited Page 23

28 Additional Storm Water Network Data Due to the knowledge gaps within the storm water network, a number of consultations were carried out in order to try and complete, as far as was possible, the storm water records for St Neots. Initially, CCC s Highways Team provided a number of plans showing highways up for adoption under Section 38 of the Highways Act. These plans included the drainage systems, and in some cases provided ground and invert levels for the storm water network. Details were also obtained from developers Taylor Wimpey and Gallagher Estates for the new developments of Eynesbury Manor and Loves Farm, respectively. Further information came from connectivity checks and manhole surveys carried out by Global Surveys through Anglian Water. The Figure 3-3 shows the before and after storm water network following these consultations. Figure 3-3 Enhancement of existing AWS storm water network (blue nodes indicate storm water manholes) 3.4 Potential Indicators of Surface Water Flood Risk EA Areas Susceptible to Surface Water Flooding (AStSWF) Maps The Environment Agency has produced outputs of a simple surface water flood modelling at a national scale. The modelling did not take into account underground sewerage and drainage systems or smaller over ground drainage systems. No buildings were included and a single rainfall event was applied. The model parameters used to produce the maps were: 0.5% AEP event 240 minute storm duration 1km 2 resolution No allowance for underground pipe network No allowance for infiltration Hyder Consulting (UK) Limited Page 24

29 The AStSWF map gives three bandings indicating areas which are less, intermediate and more susceptible to surface water flooding. The map is not suitable for identifying individual properties at risk of surface water flooding. These maps were updated and republished in January The figure below shows the AStSWF across the St Neots study area. Figure 3-4 Areas Susceptible to Surface Water Flooding in St Neots EA Flood Maps for Surface Water (FMfSW) Following on from the release of the Areas Susceptible to Surface Water Flooding, the EA updated the original mapping in order to produce the Flood Maps for Surface Water (FMfSW), which were released in October The existing maps were updated to take account of buildings and the underground drainage system, and more storm events were analysed. It should be noted that these maps do not take into account artificial drainage regimes. The model parameters used to create these new maps were: External Publication Scale 1:25, % and 0.5% AEP events 66 minute storm duration 5m 2 resolution with country split into 5km squares In rural areas, rainfall was reduced to 39% to represent infiltration In urban areas, rainfall was reduced to 70% to represent infiltration Hyder Consulting (UK) Limited Page 25

30 Global use of Manning s n of 0.1 for rural and 0.03 urban areas The new maps have two bandings of deep or shallow and are produced for both 3.33% and 0.5% AEP events. The figure below shows the FMfSW for the 1 in 200 year return period across the St Neots study area. Figure % AEP Event Flood Maps for Surface Water in St Neots British Geological Survey Groundwater Flooding Susceptibility Maps Groundwater flood risk has been assessed by the British Geological Survey (BGS) for the whole country via national flood hazard maps. The groundwater flooding susceptibility data shows the degree to which areas of England, Scotland and Wales are susceptible to groundwater flooding on the basis of geological and hydro-geological conditions. The dataset provided does not show the likelihood of groundwater flooding occurring, i.e. it is a hazard not risk-based dataset. The risks have been derived using set rules in order to identify areas based on geological considerations, where groundwater flooding could not occur, i.e. areas where non-aquifers are present at the ground surface (BGS). Areas susceptible to groundwater accumulation are passed through a second set of rules in order to create a groundwater level surface (this was taken from groundwater contours, inferred river levels, borehole data and other BGS datasets). The final groundwater level was then compared to a DTM, and the resulting modelled depths of groundwater level above the surface were translated into associated risk categories Very High, High, Moderate, Low and Very Low. Hyder Consulting (UK) Limited Page 26

31 BGS note that The susceptibility data is suitable to establish relative, but not absolute, risk of groundwater flooding at a resolution of greater than a few hundred metres. In all cases it is strongly recommended that the confidence data is used in conjunction with the groundwater flooding susceptibility data. In addition, the susceptibility data should not be used on its own to make planning decisions at any scale, and, in particular, should not be used to inform planning decisions at the site scale. The susceptibility data cannot be used on its own to indicate risk of groundwater flooding. At this stage of the SWMP, these maps have been used only in a limited capacity, however, it is expected that during future stages, these maps will be used more extensively (in conjunction with site investigation data) to inform the optioneering and implementation process. The mapping shows that the fluvial floodplains in St Neots are susceptible to very high groundwater flooding. The more urbanised areas of St Neots are susceptible to moderate to low groundwater flooding. 3.5 Maintenance Regimes Maintenance regimes are critical to ensuring the continued and effective functioning of assets to manage surface water flood risk. Existing maintenance tasks/ responsibilities have been reviewed as part of the SWMP where information is currently available and these are listed below. The SWMP will also assist in identifying and focussing needs in terms of future maintenance and it is recommended that all partners and stakeholders provide the relevant information for inclusion in the final version of this report as appropriate. Cambridgeshire County Council Highways Authority The CCC Highways Authority has the over-riding responsibility for all highways and highway structures throughout the council area (with the exception of motorways and some major trunk roads, such as the A11), and operates programmes of inspection and maintenance for bridges and gullies within the county area. Huntingdonshire District Council Huntingdonshire District Council has responsibility for maintaining 62 miles of Awarded Watercourses within the Council boundary. In St Neots, they include Colmworth Brook and Duloe Brook. Anglian Water Maintenance regimes are critical to ensuring the continued and effective functioning of assets. For the surface water network, Anglian Water assumes that the self-cleansing velocity design standard is sufficient to clear any blockages. This is done in conjunction with periodic CCTV of the network and incident investigations. As a result they do not have any listed expenditure for maintenance of surface water systems. For foul and combined systems, Anglian Water follow both a proactive maintenance plan and a reactive approach to maintenance. Therefore maintenance costs vary between years dependent on any reported flooding incidents. AW assumptions of self cleansing, together with periodic cctv of the infrastructure and incident investigations allows the promotion of pollution and process monitoring or rehabilitation to be completed. Hyder Consulting (UK) Limited Page 27

32 Environment Agency The Environment Agency carries out maintenance on those rivers or streams designated as main rivers. The Environment Agency's annual maintenance programme can be viewed by using their website 18. Hyder Consulting (UK) Limited Page 28

33 4 Model Development 4.1 Model Evolution There are a number of factors influencing surface water flooding as a result of a localised heavy rainfall event in St Neots. These include: A relatively flat topography in places providing an increased array of flow paths Highway and building surface water runoff The capacity of the sewer network Infiltration and depression storage Interaction between the Main River and local watercourses Recent advances in hydrological and hydraulic modelling techniques have allowed for a gradual improvement in assessing sources of flooding and flood risks. Of particular note for this study, advances in direct rainfall modelling allow representation of storms that are not purely fluvial. This technique allows analysis of surface water runoff, infiltration, depression storage and rainfall distribution and its effects on flooding. This method of raining on the model domain allows sites at risk of surface water flooding to be identified and also illustrates the main flood pathways by which flooding occurs. In doing so, the model represents a means of identifying areas at risk of flooding, from which multi-criteria analysis scores and financial damages can be calculated. Once the baseline flood risk has been identified, the model then provides a useful tool to assess the viability of potential flood alleviation measures. The use of 2D surface terrain modelling is designed to ensure that the flooding mechanisms are appropriately represented by the model. This approach enables the effect of the topography on overland flood routes to be simulated by direct application of a rainfall profile to a 2D hydraulic model domain. For this study, InfoWorks ICM software was used. This package utilises standard GIS practices to manage, manipulate and present input and output data. In order to model surface flows, ICM requires terrain data. This can be from any source (GPS, LiDAR, photogrammetry etc.) but the more detailed and accurate the source of the data, the more accurate and reliable the solution is likely to be. For this study, the terrain used by ICM has been generated from 1 and 2 metre resolution LiDAR data provided by the EA. In order to address the specific issues relating to the St Neots SWMP, a three stage modelling strategy was developed for this study. Stage 1 - Hydrological Analysis and development of broad scale, bare earth model of St Neots (see Section 4.3). Stage 2 - Identification and evaluation of wetspots using the Stage 1 model results and discussions with the CCC and key stakeholders (see Section 5). Stage 3 - Development of a detailed model to represent an integrated terrain, river and sewer model (see Section 6). Development and testing of engineering options and economic analysis (see Sections 7 and 8). Hyder Consulting (UK) Limited Page 29

34 4.2 Hydraulic Modelling - Common Principles D Terrain In InfoWorks ICM, the 2D model domain is represented using a triangular mesh that covers the extent of the study area. This mesh is created using LiDAR data, with each triangle being set at a ground level equal to the average of the ground levels at each of its three corners. The mesh can be made more detailed by adjusting the size of the triangles comprising the mesh. Further definition can be added to areas within the mesh via the lowering or raising of levels as required, for instance to represent drainage ditches or embankments. Figure 4-1 2D Mesh Representation Crown Copyright and database right All rights reserved. Ordnance Survey licence number Roughness A single Manning s n roughness figure of has been applied to the 2D mesh area. The creation and use of catchment-wide roughness polygons based on the MasterMap data was initially considered but after sensitivity testing (see Section 6.4 and Appendix I) was discarded due to its noticeable effect on increased meshing and simulation times of the detailed ICM model that covers the entire St Neots study area. Following the sensitivity analysis a lower roughness value of 0.02, which is representative of harder urban surfaces, was also discarded as applying this value had a minimal impact on the modelled flood extent. Therefore, to achieve a balance between study programme and other objectives the modelling process was simplified accordingly. Under storm conditions, surface water flows are routed down steep roads and either accumulate in topographic depressions or discharge to the main river network within the catchment. The effect of surface topography is a far more significant routing factor than surface roughness. Further discussion of the sensitivity analysis is made in Section 6.4 and Appendix I. Hyder Consulting (UK) Limited Page 30

35 4.2.3 Infiltration An infiltration value of 2.5mm/hr has been applied across the whole of the 2D mesh area. Appendix E provides justification for using this infiltration values based on the available local information. Initially 2D infiltration zones were used to represent the varying surfaces in St Neots. Mastermap was used to identify the different surfaces and an appropriate rate was applied to each zone. Several issues were encountered in the meshing process once these zones were incorporated into the model. Following discussions with Innovyze, the software manufacturer of InfoWorks ICM, the infiltration zones were removed as the latest version of ICM available in December 2011 (v2.0.2) was unable to deal with the large number of multi-part polygons that were required in St Neots. At the end of February 2012 a new version of ICM (v2.5) was released which is now capable of dealing with multipart infiltration polygons. Due to the programme requirements for St Neots it was not feasible to wait for the release of this latest version. As illustrated in Section 6.4 and Appendix I, sensitivity analysis were undertaken using an increased catchment wide infiltration value of 5 mm/hr, as well as with varying infiltration rates, allowing the potential impact of underlying gravels within the Great Ouse flood plain to be modelled using a much higher infiltration value of 10,000mm/hr as indicated by the Hydrology Note in Appendix E. The results of this sensitivity analysis are summarised in Appendix I Representation of Buildings Buildings have been represented within ICM as voids. This has been achieved by incorporating the building footprints from Mastermap into the 2D mesh. This method forces runoff to flow around the building, representing a more realistic routing of surface water flows. Other impermeable structures such as boundary walls or fences have not been added to the large catchment wide model due to the lack of available and specific information on these items Representation of Roads MasterMap base data was used to extract all roads within the study area. This separate road polygon dataset was stamped onto the underlying DTM with a 100mm drop applied. The 100mm height difference is designed to represent the kerb level. This method allows flow to run along the lower road network before spilling over the kerb and affecting other areas. This represents a more realistic routing of surface water flows in urban environments Representation of Foul Flows As the majority of the town centre is serviced by combined sewerage, it was necessary to include a baseline foul flow component along with the surface water inflows. The DAP study undertaken in 2002 by AWS had a foul flow survey component but the catchment data may have changed since the DAP was completed. To facilitate this, Anglian Water provided a consumption rate of 144 litres/head/day and address point data to calculate the updated foul flows for the SWMP modelling purpose. County Council information regarding ward populations was obtained (see Table 4-1), and these figures were used to calculate a population per address, which was then applied to the modelled combined sewer network. It should be noted that the 2007 populations were used, as being higher; they represented a more conservative approach. Hyder Consulting (UK) Limited Page 31

36 Ward Code Ward Name Population 2007 Projected Population 2021 No. Of Properties Calculated Occupancy Rate 12UEHG Eaton Ford UEHH Eaton Socon UEHJ Eynesbury UEHK Priory Park Table 4-1 Cambridge County Council Ward Populations for St Neots 4.3 Stage 1 - Bare Earth/ Hydrological Analysis Modelling Bare Earth and River Model Construction The first stage of modelling was to create the Bare Earth and River Model. This model comprises a 2D model terrain mesh and the local river system of the Great Ouse, plus Fox, Hen, Duloe, Colmworth, Wintringham and Eynesbury Brooks. The overall extent of the model is shown in the figure below. Figure 4-2 St Neots Direct Rainfall Model - Extent of ICM Domain Hyder Consulting (UK) Limited Page 32

37 The St Neots ICM domain was established by drawing a polygon around the surface water catchment. This catchment is generally contained by the A1 to the west and railway line to the east. The downstream boundary for the model is the sluice gate on the Great Ouse at Mill Lane, in the north of the catchment. The initial bare earth model was run with a coarse mesh to ensure that all key flow paths affecting the St Neots urban area were captured within the 2D domain. The results for a 0.5% AEP 120 minute storm are shown in Figure 4-3. This model output shows a clear problem associated with Main River flooding from the Great Ouse, although this is for the most part contained with the existing floodplain. The results also show that surface water flooding is predicted at several locations across St Neots. The designed flood flows for the watercourses and rainfall parameters used for the pluvial modelling are further described below. Figure 4-3 Bare Earth and River Model Results for 0.5% AEP storm Hydrological Analysis As noted above, the purpose of developing an ICM model of St Neots was to analyse the effects of rainfall on the town by looking at flow paths, velocities, interaction with river flows and Hyder Consulting (UK) Limited Page 33

38 overall catchment response. This was achieved by applying Depth Duration Frequency (DDF) rainfall, derived from the FEH CD-ROM, over the model area. The application of direct rainfall to a 2D model domain is a fairly novel approach to assess flood risk. One advantage of the approach is that the model does not require an estimation of flow at discrete locations since flow is automatically generated from the incident rainfall according to the way in which it is channelled by the modelled topography. Whilst the direct rainfall model explicitly simulates the channelling and pooling of surface water, losses to the ground through infiltration are not immediately accounted for. Such a scenario in which no infiltration losses are represented could be assumed to be indicative of a frozen or highly saturated catchment response. However this is a very conservative assumption and hence it may be desirable to include a measure of infiltration losses in the model to make it more representative if local data supports this. Whilst it would be ideal to apply a varying infiltration rate across the study catchment as noted in Section 4.2.3, this was not feasible for this study as the version of ICM used (2.0.2) was unable to represent differing rates across the entire study area. Therefore one representative infiltration rate was identified for the study domain. The predominant soil type was established by using both Cranfield University s Soilscape 19 website and the BGS Borehole Scan 20 website. Based on this soil type a conservative infiltration rate (2.5 mm/hr) was chosen using the MicroDrainage infiltration coefficient guidance (Appendix E). As noted in Section 4.2.3, sensitivity runs also performed using higher infiltration rates. The hydrological review undertaken as part of this SWMP has recommended that 20% AEP event peak water levels/flows should be applied to the River Great Ouse as a representative downstream boundary condition for the local drainage system in St Neots. Due to the large upstream catchment area of the River Great Ouse and its tributaries the flood peak will take a much longer duration (up to 5 days) to fully respond through St Neots rather than the shorter duration events that are being analysed as part of this SWMP. Appendix E outlines the steps taken to derive the hydrology used for each of the watercourses within the study area. Hyder Consulting (UK) Limited Page 34

39 4.3.3 Design Rainfall Design rainfall for a variety of return periods and storm durations was generated using Depth Duration Frequency (DDF) rainfall catchment descriptors, derived from the FEH CD-ROM. These catchment descriptors are inputted directly into ICM, which automatically creates rainfall hyetographs that are then applied over the catchment area. The following figure shows a hyetograph used with the Bare Earth and River Model. The hyetograph defines point rainfall and duration and is applied over the entire extent of the model. Figure 4-4 Hyetograph for 0.5% AEP 120 minute duration storm Surface Features While, in general, MasterMaps show all stretches of open watercourse, St Neots has a number of roadside drainage ditches that are not included within this OS mapping. This was evident by a number of outfalls identified within the Anglian Water database that appeared to discharge into open spaces. These were investigated during a site visit, and a number were mapped and photographed. Further drainage ditches were then identified using aerial photography. An example of these roadside ditches is shown in Figure 4-5. Hyder Consulting (UK) Limited Page 35

40 Figure 4-5 Example of roadside drainage ditch on Potton Road These drainage ditches were included in the integrated model through the use of mesh polygons. These polygons allow a more detailed mesh to be utilised within the main 2D terrain mesh, and can be set at a particular level (maod) or raised or lowered from the existing level given in the LiDAR data. In a balance between cost and programme, the mesh polygons representing the drainage ditches were lowered by 1m to identify and include within the modelling the potential effect from the presence of these drainage assets. Hyder Consulting (UK) Limited Page 36

41 5 Wetspot Selection and Prioritisation Flood Risk Regulations The assessment of the possible harmful consequences of future floods from local sources of flood risk 5.1 Approach The principal purpose of a strategic assessment is to identify broad locations which are considered more or less vulnerable to surface water flooding. These are then taken through to an intermediate assessment. This chapter describes the selection and prioritisation of areas in line with the strategic and intermediate risk assessment phases. 5.2 Stage 2 - Identification of Potential Wetspot Areas A wetspot is an area deemed to be at significant risk of surface water flooding. This risk is identified using either historical flooding reports and / or the Environment Agency s Flood Maps and localised modelling. A number of principles were established in relation to identifying wetspot areas within the St Neots SWMP. These were: The extents of the wetspots were initially identified by depth using the Stage 1 bare earth modelling of St Neots, historical flooding information and discussions with town, parish and district council members at a progress workshop. The wetspots must consider the impact of all of the upstream contributing areas to ensure that surface water runoff to the area where water accumulates is considered by the detailed assessment. In order to meet this criterion, the velocity and flow outputs from the Stage 1 bare earth model were interrogated to delineate the wetspot areas. Figure 5-1 shows the results of the Stage 1 bare earth modelling for a 0.5% AEP event for St Neots. Areas of inundation shown in blue are equivalent to a flood depth of between 0.1m and 0.3m. Areas of inundation shown red are equivalent to flood depths greater than 0.3m. The figure allows a preliminary delineation of the wetspot catchments. Hyder Consulting (UK) Limited Page 37

42 Figure 5-1 Bare Earth Model Results for 0.5% AEP results (120 minutes storm duration) for St Neots with wetspot areas identified in green Crown Copyright and database right All rights reserved. Ordnance Survey licence number As can be seen in Figure 5-1 above, the most extensive areas of flood risk in St Neots fall within the River Great Ouse floodplain. This area is predominantly open space with minimal existing development. Using the bare earth modelling results and discussions with the Project Board and Local Councillors, four wetspots have been identified in St Neots based on the likely impact on flood receptors and previously known surface water flooding issues. These are listed in Table 5-1. Figure Ref Wetspot Meadowsweet Riverside Eynesbury Manor Town Centre Table 5-1 Stage 2 Wetspots for St Neots and their associated figure numbers Hyder Consulting (UK) Limited Page 38

43 Figure 5-1 shows several other areas at high risk of deep surface water flooding. These areas were discussed with the Project Board and Local Councillors; however the wetspots listed in Table 5-1 were highlighted as key areas of interest for this SWMP. Therefore this study focussed in on these four areas of interest. The other areas identified at high risk of surface water flooding should be re-visited in future studies, and considered in targeting maintenance activities. Hyder Consulting (UK) Limited Page 39

44 5.3 Prioritisation of Wetspot Areas Meadowsweet Wetspot Figure 5-2 shows the bare earth model results for a 1 in 200 year (0.5% AEP) return period for the Meadowsweet area of St Neots. Areas of inundation shown in blue are equivalent to a flood depth of between 0.1m and 0.3m. Areas of inundation shown in red are equivalent to flood depths greater than 0.3m. The figure shows a well defined accumulation area to the north of Meadowsweet in the open area of land. This flooded area is affecting property on Meadowsweet, Burwell Road and Langley Close. There are several other areas of smaller, shallower accumulation within this area however fewer properties are shown to be at risk with a majority of the flooding falling within residential road carriageways. The flood outline indicates that surface water partially follows the route of a culverted watercourse which runs between Burwell Road and Sambar Close. Information about this culverted watercourse was not available at the time modelling undertaken and therefore further investigation is required in the subsequent stages of this SWMP. Figure 5-2 Bare Earth Model Results for 0.5% AEP results (120 minutes storm duration) for St Neots Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 40

45 In order to establish the main flow paths in Meadowsweet the velocity vectors from the model run were interrogated. Figure 5-3 shows the principal flow paths. Figure 5-3 Delineation of Meadowsweet Wetspot (green outline) based on principal flow paths (red arrows) Crown Copyright and database right All rights reserved. Ordnance Survey licence number The arrows indicate that surface water flows drain towards the open area of land to the north of Meadowsweet. Figure 5-4 shows the topography of the Meadowsweet wetspot (note that some vertical exaggeration has been applied to this figure and subsequent Figures 5-7, 5-11 and 5-14 to emphasis the features in the terrain). Hyder Consulting (UK) Limited Page 41

46 Depression to north of Meadowsweet Figure 5-4 Meadowsweet Topography (green outline represents the wetspot) There is a clear topographical depression to the north of Meadowsweet. Therefore any surface water flow on the surrounding ground will naturally travel towards this depression. Flow that reaches this area is trapped in place by the embanked section of the A Riverside Wetspot Figure 5-5 shows the model results for a 0.5% AEP event for the Riverside wetspot. Areas of inundation shown in blue are equivalent to a flood depth of between 0.1m and 0.3m. Areas of inundation shown in red are equivalent to flood depths greater than 0.3m. The figure shows several areas of deep ponding that are affecting residential properties in the Riverside area. There are smaller isolated deep patches of ponding which are being caused by water being trapped against property boundaries and the raised Environment Agency river defence at the Paddocks. The bare earth model does not include the EA pumps behind the defence so the extent in this location is greater than the Do Minimum model outline with the surface water pumps in place. Hyder Consulting (UK) Limited Page 42

47 Figure 5-5 Bare Earth Model Results for 0.5% AEP results for St Neots (120 minutes storm duration) Crown Copyright and database right All rights reserved. Ordnance Survey licence number In order to establish the main flow paths contributing to the ponding in the Riverside area the model velocity vectors were assessed. Hyder Consulting (UK) Limited Page 43

48 Figure 5-6 Delineation of Riverside wetspot (green outline) based on principal flow paths (red arrows) Crown Copyright and database right All rights reserved. Ordnance Survey licence number The arrows indicate that this wetspot is divided by the Mill Hill Road with flows from the north draining to this location and flows from the south also converging on this location. Figure 5-7 shows the topography of the Riverside wetspot. Hyder Consulting (UK) Limited Page 44

49 Figure 5-7 Riverside Topography (green outline represents the wetspot) The topography of this wetspot illustrates that a majority of the area falls within the lower lying River Great Ouse floodplain. The following figure outlines spot levels within the Riverside wetspot. Hyder Consulting (UK) Limited Page 45

50 Figure 5-8 Riverside spot heights (maod) Crown Copyright and database right All rights reserved. Ordnance Survey licence number Although the difference in ground level in the lower lying floodplain is smaller in comparison to those on the higher ridge to the north west, there is a clear drop in elevation towards the centre of the wetspot and down towards the River Great Ouse to the east. This supports the velocity vector arrow locations which highlighted flow paths towards Mill Hill Road from the northern and southern ends of the wetspot Eynesbury Manor Wetspot Figure 5-9 shows the model results for a 0.5% AEP event for the Eynesbury Manor area of St Neots. Areas of inundation shown in blue are equivalent to a flood depth of between 0.1m and 0.3m. Areas of inundation shown in red are equivalent to flood depths greater than 0.3m. Hyder Consulting (UK) Limited Page 46

51 Figure 5-9 Bare Earth Model Results for 0.5% AEP results (120 minutes storm duration) for St Neots Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 47

52 In order to establish the main flow paths in Eynesbury the velocity vectors from the model run were interrogated. Figure 5-10 shows the principal flow paths. Figure 5-10 Delineation of Eynesbury Wetspot (green outline) based on principal flow paths (red arrows) Crown Copyright and database right All rights reserved. Ordnance Survey licence number The arrows indicate that the predominant flow path for surface water is from south to north-west through this wetspot. With surface water flows from the south draining towards St Neots Community College to the west of the wetspot. Surface water from the east of the wetspot flows across Cromwell Road to the west of the wetspot towards the River Great Ouse floodplain. The arrows also indicate flow towards Barford Road and the open ditch to the west that runs along the edge of the new development near the marina. Figure 5-11 shows the topography of the Eynesbury wetspot. Hyder Consulting (UK) Limited Page 48

53 St Neots Community College Figure 5-11 Eynesbury Topography (green outline represents the wetspot) There is a clear topographical depression at St Neots College. Therefore any surface water flow on the surrounding ground will naturally travel towards this depression. There are a series of drainage ditches currently in place along the more major roads within this wetspot which are intercepting some of the overland flow Town Centre Wetspot Figure 5-12 shows the model results for a 0.5% AEP event for St Neots Town Centre. Areas of inundation shown in blue are equivalent to a flood depth of between 0.1m and 0.3m. Areas of inundation shown in red are equivalent to flood depths greater than 0.3m. Hyder Consulting (UK) Limited Page 49

54 Figure 5-12 Bare Earth Model Results for 0.5% AEP results (120 minutes storm duration) for St Neots Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 50

55 In order to establish the main flow paths around the Town Centre the velocity vectors from the model run were interrogated. Figure 5-13 shows the principal flow paths. Figure 5-13 Delineation of Town Centre Wetspot (green outline) with principal flow paths (red arrows) Crown Copyright and database right All rights reserved. Ordnance Survey licence number The arrows indicate that surface water runoff drains from right to left towards Fox Brook and the River Great Ouse floodplains. Figure 5-14 shows the topography of the Town Centre wetspot. Hyder Consulting (UK) Limited Page 51

56 Figure 5-14 Town Centre Topography (green outline represents the wetspot) There is a clear topographical depression along Fox Brook which runs through the southern section of this wetspot. Therefore any surface water to the south and east of this wetspot will naturally travel towards the lower lying river valley. To the north of the wetspot there is not a significant change in ground level however there is a slight drop in level from right to left toward the River Great Ouse floodplain. This supports the velocity vector arrow directions in Figure Hyder Consulting (UK) Limited Page 52

57 6 Detailed Assessment 6.1 Stage 3 - Detailed Model Development This chapter describes development of the detailed model, which includes the underground drainage network and river network, to enable a detailed assessment whilst focussing on the options development to mitigate against flooding within the prioritised wetspots in St Neots Underground Drainage Network Anglian Water provided their current sewer network in GIS format, including combined, foul and storm water sewers, as well as ancillary assets such as outfalls and pumping stations. The St Neots network is a mixture of combined and separate sewerage systems, with the combined system predominant in the older town centre, and newer development featuring separate sewerage. The Anglian Water network showed only a limited storm water system, and so several consultations and surveys were undertaken to increase the coverage of the storm water network. The following figure shows the varying sources of data used to inform the sewer network model used for this study. Figure 6-1 Sources for Sewer Network Once a fuller extent of surface water sewers had been identified, an assessment of the data gaps within the asset data was then undertaken. Further surveys and consultations were carried out to complete, as far as practically possible, the asset information, such as ground levels for manholes and invert levels for pipes and outfalls. Hyder Consulting (UK) Limited Page 53

58 Remaining missing information was interpolated or inferred using automated routines built into the ICM software Coupling of River and Sewer Models Once the sewer model was fully populated, it was incorporated into the existing bare earth model to create a fully integrated network. Care was taken to ensure that all sewer outfalls discharging into the river system were fully connected to the modelled river network Model Parameters A summary of the key model parameters are outlined in Table 6-1. For the broad scale investigation that is required under Stage 1, a mesh with a maximum triangle size of 100 m 2 and a minimum of 25 m 2 was chosen for the 2D mesh, as noted in Table 4-2. This modelling was undertaken to provide an initial assessment of areas where significant ponding was predicted to occur. Model Parameters Grid Size 25 m m 2 Time Step Storm Duration Return Periods Total Simulation Length (per run) 1 second 30, 60, 120, 240, 480, 960 minutes 5%, 3.33%, 2.5%, 1% and 0.5% AEP 1 day Table 6-1 Model Parameters The 2D model grid was set up with a maximum triangle size of 100 m 2 and a minimum of 25 m 2. This grid resolution provided a sufficiently detailed grid to pick up key flow routes through the study area. The total simulation length is longer than the duration of the storm to allow for an accurate assessment of flow paths following a storm event. The critical storm duration was found to be 120 minutes for St Neots. The shorter duration events particularly in the larger return periods resulted in large volumes of water trying to enter the pipe network in a short space of time. This overwhelmed the system resulting in a larger modelled flood extent. The enhanced bare earth model incorporating the pipe network was run again using the above same detailed parameters applied to generate the Do Nothing scenario results for economic analysis purposes (see Section 7.2 and 7.3). The Do Nothing scenario represents a situation where the St Neots sewer network is no longer maintained. The detailed model that also includes the sewer network was used to create the Do Minimum model which assumes a basic level of maintenance to the existing system Model Results Analysis The Do Nothing results have been discussed in Section 5 including the key overland flow routes across the study area and the evidence for selection of the four prioritised wetspots. The results for the Do Minimum 0.5% AEP 120 minute storm for the full modelled area are shown in Figure 6-2. This model output shows the mapped extent across the whole of St Neots. Hyder Consulting (UK) Limited Page 54

59 Figure 6-2 Detailed Model Results (Do Minimum) for 0.5% AEP storm (120mins duration) Crown Copyright and database right All rights reserved. Ordnance Survey licence number Figure 6-2 shows the main areas of flood risk within St Neots. As described in Section 6.5 further detailed mapping is presented in Appendix G. The areas at highest risk fall within the lower lying fluvial floodplains. There are clear flow paths of shallower risk to both the east and west of the watercourse. The predominant flow paths seem to follow the road network and connect into the main river. Based on a visual inspection the model results the most significant differences between the Stage 1 bare earth and the Stage 3 detailed model are in the urban areas of St Neots indicating the effectiveness of existing sewer system. The modelled extents in the fluvial reaches are almost identical. The Stage 3 Do Minimum model results show a consistent reduction in flood extent across the rest of St Neots but some residual flood risk is still noticeable for all the modelled storm events (partly due to capacity issues of the existing network). 6.2 Model Verification Unfortunately, there is currently no information in the form of detailed historical evidence or flow monitoring to undertake full verification of the ICM model for St Neots. The draft model outputs have been reviewed and discussed with the Project Board and the Local Councillors during the preparation of SWMP. Following these discussions and a comparison with all available historical flooding records the modelled outputs were deemed representative as best available Hyder Consulting (UK) Limited Page 55

60 locally agreed surface water data to inform SWMP recommendations accordingly. Section 6.3 and 6.4 below highlight the key model assumptions/limitations and sensitivity testing completed. 6.3 Model Assumptions and Limitations A number of assumptions have been made within the St Neots model. These have been summarised below along with limitations. - Where gaps existed in the AWS foul network dataset an assumed cover level, invert level and/or pipe diameter was required. These gaps have been filled using automated inference tools in ICM. - It was not possible to obtain a full record for the public or highway surface water network in St Neots. Therefore, it is likely that there are gaps in our knowledge and as such, not the entire surface water network has been incorporated into the model. Survey data and site visit information provided as much information as possible within the programme and budget available. - The model does not have any representation of the highway gully system. This information was not readily available and would have been difficult to incorporate into ICM. Therefore there may be some surface water shown in the model extents which would in fact be drained away through the gully system for the lower return period events subject to the capacity of the pipe system. - The model assumes that 100% of runoff from properties connected to soakaways will be lost to the soakaway in all events. In reality this may not be the case however it would increase the level of uncertainty and inconsistencies associated with the model if estimates were made as to the capacity of each property level soakway. - Foul flows have been calculated using the latest available data from AWS and CCC. However this estimate is not as accurate as a full flow survey. - Uniform infiltration rate of 2.5mm/hr and Manning s n roughness value of have been assumed across the study area. - The additional impact of boundary walls, fences, ancillary buildings and street/ garden furniture on the surface water flow routes and extents have not been modelled. - Accuracy of LiDAR data and resolution of ICM model mesh sizes. 6.4 Model Sensitivity To assess the sensitivity of the results to changes in model parameters a series of test runs were set up. Five sensitivity tests were undertaken on the St Neots model: - Decreased roughness from to Increased infiltration from 2.5mm/hr to 5mm/hr - Increased infiltration rates in the alluvial floodplain and areas identified with the same soil type. Infiltration increased from 2.5mm/hr in these areas to 10,000mm/hr - Blockage analysis at four locations Hyder Consulting (UK) Limited Page 56

61 - Historic inflow from Easter 1998 event applied to the River Great Ouse The results of the sensitivity analysis are summarised in Appendix I. The tests showed that the model was not sensitive to small changes in roughness or infiltration as the first two tests only showed small scale localised differences in the model results. The third test involved applying a higher infiltration rate along the main river floodplains. The increased infiltration rate was derived from the infiltration analysis as summarised in Appendix E which highlighted that the soil type in these areas has a significantly higher infiltration capacity than the rest of St Neots. Although this third test resulted in a significant difference in model extent the difference was localised to the floodplains and had a minimal impact on the surrounding areas. A majority of the water lost in these areas is likely to be from river overtopping rather than from the pipe network or surface water runoff. The fourth test involved applying a 50% blockage to four structures within the catchment. The locations and results of this test are outlined in Appendix I. The blockage runs had a minimal impact on the modelled flood extent and only had a small scale localised impact. A further sensitivity was undertaken using the Easter 1998 recorded flows on the River Great Ouse at Roxton. This inflow information was applied to the model rather than the 20% AEP event flows to investigate the impact of higher river levels on the flood risk from other local sources modelled.the model shows that the impact of the higher river levels is fairly localised to the floodplain. The river levels have a minimal impact on flood risk beyond this area. The results of this sensitivity test are shown in Appendix I Model Outputs Flood Depth, Velocity and Hazard Maps Flood depth, velocity and flood hazard mapping has been produced from the ICM models for St Neots for the following return period events 3.33% AEP and 0.5% AEP using a critical storm duration of 120 minutes. The mapping is included within Appendix G. Appendix H includes maps associated with the various options discussed in subsequent sections. Flood hazard are important factors in the assessment of flood risk and evacuation of the general public. Three categories of flood hazard have been identified in the DEFRA / Environment Agency Documents: Flood Risk Assessment Guidance for New Development 21, (DEFRA Report FD2320) and Flood Risks to People Methodology 22 (DEFRA Report FD2321). These are Danger for All, Danger for Most and Danger to Some. The equation below gives the relationship between hazard, depth, velocity and debris: H = (v+0.5) x d +Df Where H = hazard v = velocity d = depth Df = debris factor Df = 0.5 for d < 0.25m Df = 1.0 for d > 0.25m Hyder Consulting (UK) Limited Page 57

62 The mapping presented in the SWMP has been based upon the following thresholds, taken from DEFRA Report FD2320. However it should be noted that DEFRA Report FD2321 places a different hazard rating of the transition to Category 3. The FD2320 indicates that the change occurs at 2.0 whereas the FD2321 report indicates that this happens at 2.5. This has a significant impact on the interpretation of the results for the SWMP which are discussed below but it should be noted that the results are presented conservatively as set out below (based on FD2320 rating). Danger to Some Category 1 H > 0.75 Danger to Most Category 2 H > 1.25 Danger to All Category 3 H > 2.00 The colouring of the flood hazard mapping is commensurate with the hazard categorisation given in Figure 6-3. Areas coloured red are considered dangerous for all; areas in dark yellow are dangerous to most; light yellow is dangerous to some and blue areas are inundated areas mainly on the margins of the flood plain which are considered to hold little hazard. The time series graphs show the depth (left axis) and hazard category (right axis) for specific control point locations as discussed above. Velocity of Flooding (m/s) Depth of Flooding (m) Figure 6-3 Danger for Some Danger for Most Danger for All Hazard Categorisation Hyder Consulting (UK) Limited Page 58

63 7 Engineering Options Identification and Assessment 7.1 Measures Identification As noted above the engineering elements evaluated in this section are based upon employing the most appropriate techniques for the various sites. The engineering elements proposed within this section fall into a range of categories as shown in Figure 7-1 and where possible and economical the use of sustainable drainage systems (SuDS) and surface water reduction strategies has been promoted over hard infrastructure alternatives such as the upgrading of existing sewers. Accordingly, the engineering options proposed within the report have been designed to be accommodated within the urban environment where possible although further investigation and consultation with the key stakeholders and land owners will be required to fully confirm such solutions. It should be noted that the engineering options proposed are potential solutions to current issues and priorities that have been identified during this study. During the course of the SWMP time frame, it is possible that these issues or priorities may change and new constraints and priorities may present themselves. The options may, therefore, be difficult to implement, and it should be borne in mind that the implementation of engineering works for some of the suggested options may be needed over a long period. In all four wetspots, there are several open spaces which can be potentially utilised for attenuation as part of multi-functional open sapaces. However, in general the surface area is dominated by roads and sub-urban housing. Nevertheless attenuation has been explored at several locations with the introduction of attenuation basins, wetlands and ponds and there has been consideration of the use of swales where possible. In Eynesbury Manor, for example, open spaces at school have been investigated as potential sites for attenuation structures. It should be noted however that other pressures such as the need to expand and improve existing school sites may be contrary to using school open spaces Hyder Consulting (UK) Limited Page 59

64 in flood mitigation works. New developments may however offer alternative opportunities for partnership working, such as utilising green roofs in new school developments. The street environment is also a significant constraint in the installation of drainage infrastructure. Within these areas techniques including permeable paving, filter drains, Road side rain gardens are discussed in detail in the following sections. Installation of new sewers Underground Attenuation Upgrading Sewer Infrastructure Separation of Foul & Surface Up-grading of Existing Sewers Wetland / Basin Attenuation Installation of Source Control Measures Peripheral Protection Retrofitting of SuDS Systems Overland Flood Routing Figure 7-1 Surface Water Flood Mitigation Options The key constraints (see Figure 7-2) associated with the implementation of all of the options are space and cost. Hyder Consulting (UK) Limited Page 60

65 Disruption during Construction Capital Cost of Major Infrastructure Capital Cost of Major Infrastructure Timing of Investment Space within the Urban Environment Maintenance Costs Installation of Source Control Measures Space within the Urban Environment Maintenance of Green Infrastructure Adoption by Operating Authorities Figure 7-2 Engineering Options Constraints 7.2 St Neots Engineering Measures and Options Engineering Measures Modelled This section of the report considers the engineering elements and the option combinations for the mitigation of surface water flooding in the four prioritised wetspots. The engineering elements and option combinations considered in this document have been developed from work undertaken during the course of the project. The hydrological, hydraulic and risk analyses has allowed the options to be developed further in order to compare the schemes in terms of cost and technical suitability. The modelled 0.5% AEP outlines were initially used to visualise the worst case scenario in each wetspot and identify potential intervention measures. However the options developed were targeted to address the risk posed by a 3.33% AEP event whilst aiming to reduce the risk for larger events where possible. The options are focussed on intercepting, transfering or storing water in the urban environment to remove properties from flood risk; specific attention was given where known flood risk issues exist and/or significant risk is predicted by the modelling undertaken. The full design criteria will be established at the next stage of the SWMP based on economic viability, risk and buildability of the identified options during this study. Cambridgeshire County Council, the Lead Local Flood Authority under the Flood and Water Management Act, has powers to carry out works for the management of surface water run-off, ordinary watercourses and groundwater. Table 7-1 gives a summary description of the engineering elements assessed as part of the St Neots SWMP. Each of the engineering elements are mainly based on the Source Control and Site Control categories of the SuDS train (Figure 1-2) whilst taking into account the site constraints present in an existing builtup urban envrironment. The nature, feasibility and benefits associated with each of the engineering elements are discussed in Section 8. The engineering elements have been combined to form option combinations which have been modelled and analysed to evaluate their technical suitability and economic benefits to inform a preferred Hyder Consulting (UK) Limited Page 61

66 engineering option. The Option Combinations and a description of each Engineering Option are included in Section 8. Engineering Element Engineering Element Name Description Wetspot Figure No STN-A Attenuation Basin - Meadowsweet 5200m 2 attenuation basin in open area of land to the north of Meadowsweet and to the south of Burwell Road. Meadowsweet 7-3 STN -B Raised Kerb Meadowsweet 200m raised kerb along the northern half of Meadowsweet. Meadowsweet 7-3 STN -C Raised Kerb Sundew Close 140m raised kerb extended along Sundew Close. Meadowsweet 7-3 STN -D Swale Meadowsweet 35m swale added to section of car park to the north-east of Meadowsweet with a connection to STN-A. Meadowsweet 7-3 STN -E Swale - Silverweed 52m swale added in open area of land to the east of Silverweed. Meadowsweet 7-3 STN -F Swale Sundew Close/Burwell Road 160m swale to north towards Burwell Road and south towards Sundew Close. Meadowsweet 7-3 STN -G Attenuation Basin Sundew Close/Teasel Close 1100m 2 attenuation basin added to area of open land at the end of Sundew Close and Teasel Close Meadowsweet 7-3 STN - H Additional Pipe Meadowsweet 55m of new SW pipe added to channel SW from Meadowsweet into STN-A basin. Meadowsweet 7-3 STN -I Raised bund/wall Meadowsweet 70m raised bund/wall added to surround property to north of Meadowsweet to prevent flow through property. Divert main flow path around this property to open area of land. Meadowsweet 7-3 STN -J Attenuation Basin Foxglove Close 170m attenuation basin added in open area of land at the end of Foxglove Close. Meadowsweet 7-3 STN -K Outfall from pipe network Meadowsweet Outfall added from SW pipe network at upstream end of Meadowsweet. Outfall connects into STN-F swale. Meadowsweet 7-3 STN-L Additional SW Pipe Orchard Close 230m new SW pipe added to Orchard Close to intercept surface water flow. Riverside 7-5 STN-M Additional SW Manhole St Neots Road New node added at junction of Orchard Road and St Neots Road. Riverside 7-5 STN-N Attenuation Basin Orchard Close 560m 2 attenuation basin added in open area of land to west of Orchard Close. Riverside 7-5 STN-O Swale Great North Road & Orchard Close 170m swale added in open area of land between the Great North Road and Orchard Close. Riverside 7-5 STN-P Attenuation Basin River Road 4500m 2 attenuation basin with outfall from SW network. Riverside 7-5 Hyder Consulting (UK) Limited Page 62

67 Engineering Element Engineering Element Name Description Wetspot Figure No STN-Q Swale Mill Hill Road 210m swale added to Mill Hill Road. Riverside 7-4 STN-R Attenuation Basin Reynolds Court/Constable Avenue Three attenuation basins (1770m 2 ) added in open area of land between Reynolds Court and Constable Avenue. Riverside 7-4 STN-S Swale Crosshall Road 130m extension of existing swale to the north along Crosshall Road Riverside 7-4 STN-T New outfall Crosshall Road New outfall added from pipe running along Milton Avenue into new section of swale. Section of connecting pipe to the south along Crosshall Road has been removed from the model. Riverside 7-4 STN-U Attenuation Basin Crosshall Road 1200m 2 attenuation basin added on Crosshall Road to the north of the new outfall Riverside 7-4 STN-V Pipe Upsizing St Neots Road Upsizing of 270m pipe along St Neots Road from 300mm pipe to 600mm. Riverside 7-5 STN-W Attenuation Basin St Neots Road Three small attenuation basins (300m 2 ) have been added at the junction of St Neots & Orchard Road Riverside 7-5 STN-X Attenuation Basin & Swale Trafalgar Road 250m 2 attenuation Basin & series of swales (220m) incorporated into the open area of land between Trafalgar Road and Great North Road. Riverside 7-4 STN-Y Attenuation Basin Weston Court Two small attenuation basins (250m 2 ) in open areas of land near to Weston Court Riverside 7-5 STN-Z Attenuation Basin Ivel Close Two small attenuation basins (500m 2 ) in open area of land at the end of Ivel Close. Riverside 7-5 STN-AA Attenuation Basin Almein Court Two storage areas (460m 2 ) located at opposite ends of Almein Court in areas of open land. Riverside 7-4 STN-AB Swale Almein Court 50m swale along section of Almein Court Riverside 7-4 STN-AC Swale Amhem Close to Almein Court 125m swale in open area of land between ends of Arnhem Close and Almein Court Riverside 7-4 STN-AD Swale Orchard Close Two sections of swale (140m) on either side of Orchard Close Riverside 7-5 STN-AE Swale Orchard Road Two sections of swale (220m) on either side of Orchard Road at junction with St Neots Road Riverside 7-5 STN-AF New Pipe Almein 150m of new surface water pipe along length of road with an outfall into storage RIverside 7-4 Hyder Consulting (UK) Limited Page 63

68 Engineering Element Engineering Element Name Description Wetspot Figure No Court area (STN-AA) in open area of land to east. STN-AG New Pipe Jutland Rise 120m of new surface water pipe along length of road with an outfall into STN-AC swale in open area of land at the end of Jutland Rise. Riverside 7-4 STN-AH Attenuation Basin Eynesbury Drain 1900m 2 attenuation basin between railway line and A428 to provide upstream storage for Eynesbury Drain. Eynesbury 7-7 STN-AI Pipe Upsizing Ridgeway 465m pipe upsizing from 150mm to 750mm. Eynesbury 7-6 STN-AJ Swale Middlefield Community Primary School 1700m swale system along edge of primary school grounds. Eynesbury 7-7 STN-AK Attenuation Basin Balmoral Way 1900m 2 attenuation basin in open area of land behind Balmoral Way. Eynesbury 7-7 STN-AL Attenuation Basin Richmond Close 4000m 2 attenuation basin added into open area of land to the south of Richmond Close. Eynesbury 7-7 STN-AM Attenuation Basin William Drive 200m 2 attenuation basin added to open area of land on William Drive. Eynesbury 7-7 STN-AN Attenuation Basin Middlefield Community Primary School 280m 2 attenuation basin put in place to the north-east of the school grounds. Eynesbury 7-7 STN-AO Attenuation Basin Ridgeway Several small attenuation basins (460m 2 ) have been added to Ridgeway to intercept overland flow. Eynesbury 7-6 STN-AP Attenuation Basin the Broad Walk Two small attenuation basins (550m 2 ) have been added to the open area of land in the Broad Walk. Eynesbury 7-6 STN-AQ Swale Hardwick Road 60m swale added along the southern end of Hardwick Road. Eynesbury 7-6 STN-AR Attenuation Basin Eynesbury Green 440m 2 attenuation basin added at Eynesbury Green. Eynesbury 7-6 STN-AS Attenuation Basin Criccieth Way 770m 2 attenuation basin added in open area of land on Criccieth Way. Eynesbury 7-7 STN-AT Speed Hump Ridgeway Speed hump half way along Ridgeway Eynesbury 7-6 STN-AU Pipe upsizing Huntingdon Street 700m increase of pipe along Huntingdon Street from 300mm to 750mm. Town Centre 7-8 STN-AV Attenuation Basin Tebbutts Road Two small attenuation basins (320m 2 ) have been incorporated into the car park Town Centre 7-8 Hyder Consulting (UK) Limited Page 64

69 Engineering Element Engineering Element Name Description Wetspot Figure No in Tebbutts Road. STN-AW Attenuation Basin Almond Road/Huntingdon Street 1550m 2 attenuation basin in open area of land behind properties on Almond Road/Huntingdon Street. Town Centre 7-8 STN-AX Swale Dovehouse Close 150m swale added on Dovehouse Close. Town Centre 7-8 STN-AY Attenuation Basin Cambridge Street/East Street 750m 2 attenuation basin in the open area of ground in between Cambridge Street and East Street. Town Centre 7-8 STN-AZ Attenuation Basin Specialist School 3800m 2 attenuation basin to the north of the Specialist School in open area of land Town Centre 7-8 STN-BA Underground Storage Tebbutts Road 1950m 3 underground storage under car park on Tebutts Road Town Centre 7-8 STN-BB Storage Area Dovehouse Close 550m 2 attenuation basin in open area of land on Dovehouse Close at junction with Huntingdon Street Town Centre 7-8 Table 7-1 St Neots engineering elements Figure 7-3 to 7-8 shows the Engineering Elements modelled to reduce surface water flood risk in the four St Neots wetspots. Hyder Consulting (UK) Limited Page 65

70 Figure 7-3 Meadowsweet Engineering Elements Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 66

71 Figure 7-4 Riverside North Engineering Elements Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 67

72 Figure 7-5 Riverside South Engineering Elements (green nodes indicate manhole existing locations) Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 68

73 Figure 7-6 Eynesbury North Engineering Elements Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 69

74 Figure 7-7 Eynesbury South Engineering Elements Crown Copyright and database right All rights reserved. Ordnance Survey licence number Hyder Consulting (UK) Limited Page 70

75 Figure 7-8 Town Centre Engineering Elements Crown Copyright and database right All rights reserved. Ordnance Survey licence number St Neots Engineering Option Combinations In order to address flooding within the St Neots wetspots and for the purposes of the SWMP, several options that have a combination of engineering elements described above have been developed. These have been tested for their effectiveness of reducing flooding within each of the wetspots. The engineering elements described in Section have been combined into option combinations. These combinations reflect the flooding mechanisms described above with the objective of determining their technical suitability and whether there is an economic case for the mitigation of surface water flooding within the wetspots. The option combination elements were combined into Do Something, which includes Do Minimum and other Option Combinations C1, to C3, as shown in Table 7-2. The option combinations are listed below:- Do Nothing The Do Nothing option assumes that no maintenance, clearance or other intervention is made to interfere with the natural fluvial processes or sewer network. The evaluation of the Do Nothing option is a technical requirement required by the Treasury Hyder Consulting (UK) Limited Page 71

76 in order to enable comparisons to be made between the Do Minimum and Do Something options. Do Minimum Maintenance of the existing storm sewer, ordinary watercourse and highway drainage including, gully cleaning, jetting, removal of debris / vegetation; treeworks and periodic removal of deposition and sediments. Option Combination C1 This comprises the larger scale engineering options to intercept overland flows identified in the Do Nothing & Do Minimum runs. Option Combination C2 This option assesses the benefits of creating smaller scale engineering options in key areas within each wetspot. Option Combination C3 This option represents a combination of C1 and C2 with network upgrades incorporated in key areas of each wetspot. Option Combination C4 This option represents a refined option combination that is most effective at mitigating flood risk within each wetspot. Table 7-2 summarises the combined modelled engineering options applied to the St Neots Wetspot. The nature, feasibility and benefits associated with each of the engineering elements are discussed in Section 8. Engineering Element C1 C2 C3 C4 Wetspot STN-A Meadowsweet STN -B Meadowsweet STN -C Meadowsweet STN -D Meadowsweet STN -E Meadowsweet STN -F Meadowsweet STN -G Meadowsweet STN - H Meadowsweet STN -I Meadowsweet STN -J Meadowsweet STN -K Meadowsweet STN-L Riverside STN-M Riverside STN-N Riverside STN-O Riverside STN-P Riverside STN-Q Riverside STN-R Riverside STN-S Riverside STN-T Riverside Hyder Consulting (UK) Limited Page 72

77 Engineering Element C1 C2 C3 C4 Wetspot STN-U Riverside STN-V Riverside STN-W Riverside STN-X Riverside STN-Y Riverside STN-Z Riverside STN-AA Riverside STN-AB Riverside STN-AC Riverside STN-AD Riverside STN-AE Riverside STN-AF RIverside STN-AG Riverside STN-AH Eynesbury STN-AI Eynesbury STN-AJ Eynesbury STN-AK Eynesbury STN-AL Eynesbury STN-AM Eynesbury STN-AN Eynesbury STN-AO Eynesbury STN-AP Eynesbury STN-AQ Eynesbury STN-AR Eynesbury STN-AS Eynesbury STN-AT Eynesbury STN-AU Town Centre STN-AV Town Centre STN-AW Town Centre STN-AX Town Centre STN-AY Town Centre STN-AZ Town Centre STN-BA Town Centre Hyder Consulting (UK) Limited Page 73

78 Engineering Element C1 C2 C3 C4 Wetspot STN-BB Town Centre Table 7-2 Option Combinations St Neots Other Engineering Elements Considered Other engineering elements such as underground storage, extensive pipe upsizing and permeable paving were also considered as part of this stage. However these were not appropriate or cost effective mitigation measures within the four wetspots and so were not taken any further Non Engineering Measures The engineering elements considered above do not completely remove the flood risk within the St Neots. Therefore other measures were explored to assess their feasibility within each of the wetspots. Increasing targeted maintenance on assets throughout St Neots would provide a quick win in areas where there are existing issues. For example the open ditch that runs from east to west from Barford Road to the River Great Ouse is very overgrown. Clearance of this ditch would increase conveyance of surface water from Eynesbury. Increased maintenance would also be of benefit in areas such as the Town Centre where the implementation of engineering options is costly. As engineering elements are not completely effective at mitigating flood risk in this area resistance and resilience measures should be considered at a property level. There are a wide range of resistance and resilience measures outlined in Appendix F these vary in price and effectiveness. A range of policy lead measures have also been considered in Appendix F in addition to the engineering and resistance/resilience options above. Hyder Consulting (UK) Limited Page 74

79 8 Economic Appraisal 8.1 Introduction The engineering elements described in Section 7 have been combined to form flood risk mitigation option combinations shown in Table 7-2. These option combinations have been assessed in relation to whole life costs, flood damages and residual damages in accordance with the methodology contained in the following documents. Flood and Coastal Erosion Project Appraisal Guidance (FCERM-AG Manual) 23. The Benefits of Flood and Coastal Risk Management: A Handbook of Assessment Techniques 24 The latter document is also known as the Multi-Coloured Handbook, and is based on the Multi- Coloured Manual (MCM) originally published in The evaluation of the residual risk of flooding has been discussed in the relevant sections associated with the option combinations. The residual flood risk damages relating to the Do Something options have been discounted annually to enable a direct comparison with other options. A variable discount rate, beginning at 3.5%, has been adopted for the cost benefit analysis, in line with HM Treasury guidance. A value engineering exercise was undertaken to evaluate the Option Combinations which indicated that a significant factor in the costs of works was associated with the excavation and disposal to landfill of materials for the formation of attenuation basins and other flood mitigation infrastructure. Accordingly the economic analysis assumes that all excavated materials will be re-used on site to avoid the cost of disposal of the material. This could include the formation of embankments and other landscaping features, positioned appropriately so as to avoid exacerbating or blocking identified surface water flow paths. This avoids costs associated with disposal including land fill tax. It also promotes the sustainable credentials of the project by reducing the carbon footprint associated with the transportation of materials for disposal. Whilst an optimism bias of 60% has been applied to all of the cost estimates (as per the current guidance applicable for a strategy of this nature) there are a number of economic risks or uncertainties associated with the development of the cost estimates. The principal economic risks associated with the preferred option are: Cost associated with dealing with utilities which have not been itemised in the cost estimates The cost of land negotiations and compensation for disruption Agreement with local residents to implement/contribute towards the costs of property level measures It is therefore considered that significant effort should be placed into obtaining agreements with landowners and stakeholders to undertake the proposed works. In order to mitigate this risk it is recommended that CCC and the Cambridgeshire Flood Risk Management Partnership enter into discussion with all landowners and stakeholders at the earliest opportunity during the design process to ensure their collaboration. A review of the service/utilities locations during prefeasibility into the scheme would help to identify their impact on scheme costs Methodology Damages Assessment The assessment of cost associated with flood damage of properties in St Neots has been assessed using the DEFRA and Environment Agency approved approach outlined in the Multi- Coloured Handbook. This method for assessing damages uses depth/damage curves based on Hyder Consulting (UK) Limited Page 75

80 property type, age and social class of the dwellings occupants, in order to evaluate the overall damage avoided in a flood risk area. For the preferred option, flood depth results for each return period were extracted for all properties within the modelled region. With respect to the flood depth, damages result from the physical contact of flood water with damageable property. National Property Dataset (NPD) has been used to form the basis of the damages assessment. In order to calculate flood damages at a property level, the following information is required for each property includes: A property type A floor area (for non-residential property) A property threshold The NPD dataset used for this study does not provide the property threshold level therefore LiDAR inclusive of an additional 0.1m was used to determine the threshold level of each property. By applying flood depths to each property (adjusted to account for threshold), the flood damages were calculated using the depth-damage curves for each individual event. Annual Average Damages (AAD) were then calculated for each scenario: Do Nothing, Do Minimum and Do Something preferred option. Depending on the size or severity of each individual flood event of a given annual probability, each flood event will cause a different amount of flood damage. The Average Annual Damage (AAD) is the average damage per year in monetary terms that would occur at each specific address point, within the modelled domain, from flooding over a 100 year period, assuming that present-day conditions (in terms of frequency of extreme rainfall) are maintained. In many years there may be no flood damage, in some years there will be minor damage (caused by small, relatively frequent floods) and, in a few years, there may be major flood damage (caused by large, rare flood events). Estimation of the AAD provides a basis for comparing the effectiveness of different flood alleviation and management measures (i.e. through measuring the reduction in AAD). The methodology for assessing the benefits of flood alleviation combines: An assessment of risk, in terms of the probability or likelihood of future floods to be averted, and A vulnerability assessment in terms of the damage that would be caused by those floods and therefore the economic saving to be gained by their reduction. Through assessment of the associated damage values and the benefits incurred through Engineering Options, proposed schemes are compared against each other using their benefitcost ratio (BCR). To identify a preferred option, a comparison was undertaken between the consequences of Do Something Engineering Options against the baseline option, Do Minimum. The SWMP Project Board requested that the Do Minimum scenario was used as the baseline for the economic analysis as it assumes a continuance of existing maintenance of the sewer and local drainage network. The Board felt that the Do Nothing scenario was unrealistic as the water companies have a statutory right to maintain public sewers. The cost of each option combination and the relative damages incurred are combined to create a benefit cost ratio. This ratio is used to assess the viability of the preferred option and also the Hyder Consulting (UK) Limited Page 76

81 levels of effectiveness for how capital can be spent to protect against the effects of flooding. The BCR is the ratio of benefits produced through introduction of flood alleviation options, expressed in monetary terms, relative to its cost, identifying the greatest value for money. The Multi-Coloured Handbook 24 states that; Projects are only viable if the benefits exceed the costs (i.e. the ratio of benefits to costs is greater than 1.0). Where benefits marginally exceed costs, there is often high uncertainty as to whether an option is justified, because only a small change or error in either the benefits or costs would tilt the balance the other way. So when comparing a Do Something option to the baseline option, confidence is needed that a Do Something option is clearly preferable. In this regard, the decision process explored whether the best value for money is provided while achieving the most appropriate standard of risk management defence. This is undertaken by assessing the incremental benefit-cost ratio of each economically viable option. Key Parameters For reference, key parameters which have guided the economic assessment process, in line with FCERM-AG/MCM techniques, are repeated below: Property Values: Properties were assigned a market value in order that present value damages (PVd) were capped if they exceeded a property s market value over the appraisal period. These capping values were derived according to Environment Agency best practice. Distributional impacts (DI) were considered, in order to remove social class bias from the property value estimates. A DI factor was calculated using Approximate Social Grade (UV50) data for the relevant Cambridgeshire authority, available from neighbourhood.statistics.gov.uk. Emergency services costs: These were incorporated in the assessment by adding 5.6% to all calculated property damages. This is as stated in the Multi-coloured Handbook and is based on data from the 2007 floods, a revision downwards from the previous values of approximately 10%, reflecting economies of scale of providing emergency services in urban areas during flood events. Temporary accommodation costs: These were incorporated in the assessment by allowing for an average rental cost, post-flood, of 5.7k per property flooded, determined in an Environment Agency review of the summer 2007 floods. Section 8.2 highlights the key assumptions made in damage assessment (including limitations and further recommendations) and specific attention should be given to these prior to using the currently published SWMP economic assessment outputs. Section 8.3 also summarises key exclusions from the damage assessment Economic Risks The principal economic risks are associated with the construction of all Engineering Options are:- Cost of possible diversion of utilities; Cost of land negotiations Compensation for disruption Buildability Hyder Consulting (UK) Limited Page 77

82 Ecolological and other environmental risks and associated costs Sensitivity of flood damage assessment and model limitations (e.g. actual property threshold levels and flood levels see section 8.2 and 6.3) It is recommended that as part of Business Case Justification/ Project Appraisal Reporting process and the subsequent detailed design of the flood mitigation proposals the above risks are addressed accordingly. The project lead should approach utility companies to obtain agreements for the relocation of services as necessary. In addition the project lead should engage with all landowners and stakeholders at the earliest opportunity during the design process to ensure their collaboration Cost Estimates Each of the proposed Option elements has been costed in accordance with information on maintenance expenditure obtained from Cambridge City Council and Cambridgeshire County Council and SPON s Civil Engineering and Highways Price Book 25. In terms of developing the discounted capital costs of construction works each Option Element was assigned into an investment profile to simulate the implementation of expenditure, spread over a number of years. The final capital costs for each Option Combination are therefore subject to a specific pattern of spending. Hyder Consulting (UK) Limited Page 78

83 8.1.4 Benefit Cost Analysis Table 8-1 summarises the Present Value Damages associated with the Do Nothing, Do Minimum, Option C1, C2, C3 and C4 for each of the wetspots. Meadowsweet Riverside Eynesbury Manor Town Centre Option Combination Present Value Damages ( ) Do Nothing 3,724,000 12,705,000 15,022,000 11,149,000 Do Minimum 3,076,000 9,938,000 9,773,000 10,127,000 Option C1 2,207,000 8,263,000 4,645,000 9,770,000 Option C2 2,821,000 9,429,000 5,296,000 9,951,000 Option C3 2,320,000 7,672,000 4,520,000 8,589,000 Option C4 1,996,000 7,807,000 4,636,000 8,585,000 Table 8-1 Flood and Residual Flood Damages Based upon the assessment of damages and the cost estimates given for each option combination, the present value damages have been combined with the whole life cost estimates within Table 8-2 to 8-5. These tables summarises the costs, benefits and residual damages associated with each option. Standard routine costs for maintenance of the existing drainage assets have not been included in the appraisal since they are not funded via capital funding scheme grants and would be commensurate for all options. However, maintenance costs over and above ongoing maintenance associated with the proposed engineering elements are included in the option costs where applicable. Hyder Consulting (UK) Limited Page 79

84 A B C D E F Do Do Option Option Option Option Costs and benefits Nothing Minimum C1 C2 C3 C4 1 PV costs from estimate 356,000 76, , , Optimism bias adjustment (C1*0.6) 213,600 45, , ,200 Total PV Costs from appraisal (PVc) (C1+C2) 569, , , ,200 4 PV damage (PVd *) 3,724,000 3,076,000 2,207,000 2,821,000 2,320,000 1,996, PV damage avoided (B4-C4) 869, , ,000 1,080,000 Total PV benefits (PVb) (=C5) 869, , ,000 1,080,000 Net Present Value (NPV) (C6-C3) 299, , , ,800 Average benefit cost ratio (C6/C3) Table 8-2 Summary of Costs and Damages for Meadowsweet Based on the benefit cost ratios calculated above Option C2, the smaller scale engineering option is shown to be the most economically viable solution for Meadowsweet. By looking specifically at the wetspot location the benefit and cost of each engineering option combination can be seen more clearly. Hyder Consulting (UK) Limited Page 80

85 A B C D E F Do Do Option Option Option Option Costs and benefits Nothing Minimum C1 C2 C3 C4 1 PV costs from estimate 841, , , ,000 Optimism bias 2 adjustment (C1*0.6) 504, , , ,600 Total PV Costs 3 from appraisal (PVc) (C1+C2) 1,345, ,200 1,436,800 1,121,600 4 PV damage (PVd *) 12,705,000 9,938,000 8,263,000 9,429,000 7,672,000 7,807,000 PV damage 5 avoided (B4-C4) 1,675, ,000 2,266,000 2,131,000 Total PV benefits 6 (PVb) (=C5) 1,675, ,000 2,266,000 2,131,000 Net Present Value 7 (NPV) (C6-C3) 329,400 49, ,200 1,009,400 Average benefit 8 cost ratio (C6/C3) Table 8-3 Summary of Costs and Damages for Riverside Based on the benefit cost ratios calculated above Option C4, the combined small and large scale engineering element option is shown to be the most economically viable solution for Riverside. Hyder Consulting (UK) Limited Page 81

86 A B C D E F Do Do Option Option Option Option Costs and benefits Nothing Minimum C1 C2 C3 C4 1 PV costs from estimate 917, ,000 1,042,000 1,114,000 Optimism bias 2 adjustment (C1*0.6) 550, , , ,400 Total PV Costs 3 from appraisal (PVc) (C1+C2) 1,467, ,000 1,667,200 1,782,400 4 PV damage (PVd *) 15,022,000 9,773,000 4,645,000 5,296,000 4,520,000 4,636,000 PV damage 5 avoided (B4-C4) 5,128,000 4,477,000 5,253,000 5,137,000 Total PV benefits 6 (PVb) (=C5) 5,128,000 4,477,000 5,253,000 5,137,000 Net Present Value 7 (NPV) (C6-C3) 3,660,800 3,997,000 3,585,800 3,354,600 Average benefit 8 cost ratio (C6/C3) Table 8-4 Summary of Costs and Damages for Eynesbury Based on the benefit cost ratios calculated above Option C2, the small scale engineering element option, is shown to be the most economically viable solution for Eynesbury. However the other option combinations considered for Eynesbury also have good benefit cost ratios therefore further consideration will be needed in this area as one option combination may provide increased flood protection but may be a more expensive option. One of the key surface water drainage outfall routes within this wetspot is via an un-named open ditch that runs from East to West from Barford Road to the River Great Ouse. Following HCLs site visit in August it was noted that this drain was poorly maintained. This has not been incorporated into the options considered as part of this SWMP as it should be treated as a priority action for this area in consultation with the relevant housing developer. Hyder Consulting (UK) Limited Page 82

87 A B C D E F Do Do Option Option Option Option Costs and benefits Nothing Minimum C1 C2 C3 C4 1 PV costs from estimate 585, ,000 2,851,000 2,582,000 Optimism bias 2 adjustment (C1*0.6) 351, ,800 1,710,600 1,549,200 Total PV Costs 3 from appraisal (PVc) (C1+C2) 936, ,800 4,561,600 4,131,200 4 PV damage (PVd *) 11,149,000 10,127,000 9,770,000 9,951,000 8,589,000 8,585,000 PV damage 5 avoided (B4-C4) 357, ,000 1,538,000 1,542,000 Total PV benefits 6 (PVb) (=C5) 357, ,000 1,538,000 1,542,000 Net Present Value 7 (NPV) (C6-C3) -579, ,800-3,023,600-2,589,200 Average benefit 8 cost ratio (C6/C3) Table 8-6 Summary of Costs and Damages for Town Centre Based on the benefit cost ratios calculated above Option C2, the small scale engineering element option, is have the highest benefit-cost ratio for the Town Centre. However all of the schemes in this area have a benefit cost below one which suggests that the benefit of the schemes are not sufficient to justify the cost expenditure. Therefore, increased targeted maintenance in conjunction with Option C2 is the currently preferred option for this wetspot. Option C3 and C4 are significantly more expensive as a result of the underground storage solution in Tebbutts Road car park. With this option in place the model showed the most significant improvement in flood risk however the cost of putting this option in place is greater than the likely benefits in terms of reduced flood damages over the appraisal period. Additionally, this wetspot could consider and include for the potential for traffic and business disruption associated with localised surface water flooding and factor these additional damages into a future more detailed Project Appraisal study, to arrive at a more specific and beneficial cost benefit analysis, should any of the identified options be considered locally to be viable schemes. Hyder Consulting (UK) Limited Page 83

88 8.1.5 Funding Sources Flood Defence Grant in Aid From 2012/13, the amount of Flood Defence Grant in Aid (FDGiA) available from the Government (through Flood and Coastal Partnership Resilience Funding) to any surface water alleviation capital scheme will directly relate to the number of households protected, the damages avoided, and the wider benefits of a project. As part of implementing this new funding policy, revised Outcome Measure definitions have been agreed to replace the previous suite of measures that expired in March The Outcome Measures are detailed in the list below: Outcome Measure 1 Economic Benefits Outcome Measure 2 Households at risk Outcome Measure 2b Households at very significant and significant risk Outcome Measure 2c Deprived households at very significant and significant risk Outcome Measure 3 Households at risk from coastal erosion Outcome Measure 3b Households at risk from coastal erosion in 20 years Outcome Measure 3c Deprived households at risk from coastal erosion in 20 years Outcome Measure 4a Hectares of water dependent habitat created or improved Outcome Measure 4b Hectares of intertidal habitat created Outcome Measure 4c Kilometres of rivers protected under the EU Habitats/Birds The key data from the Engineering Options that will be required for completion of the Flood Defence Grant in Aid Calculator for each individual wetspot will be available to CCC through SWMP outputs. 8.2 Damages Assessments - Assumptions Assumption 1 Property thresholds across the study catchment are 0.1m and no flooding of properties will occur below this 0.1m threshold. Due to the number of properties across the study area it would not be possible to estimate threshold levels for each property. As such an assumption of a threshold level of 0.1m at all properties has been made. Furthermore it has been assumed that no damage occurs to property when the flood level at the property is between 0-0.1m (below the threshold). It is possible that flood water can still enter properties below the threshold level via airbricks but this is not considered in this damages assessment. The 0.1m property threshold level is a standard assumption that a property will have a small step on entry however this threshold may result in an overestimation in the number of properties at risk and the level of damages. Sensitivity analysis has been undertaken to assess the impact of raising the property threshold to 0.2m and 0.3m respectively in the economic analysis. By increasing the threshold level to 0.2m in the economic analysis there was a 75% average reduction in the number of flooded properties across the four wetspots. With a threshold level of 0.3m set there was a 90% average reduction in the number of flooded properties across the wetspots. The Eynesbury Manor wetspot showed the most significant difference in flooded Hyder Consulting (UK) Limited Page 84

89 properties with 99% of the properties within the wetspot being eliminated from analysis when the threshold level was increased to 0.3m. This variation in the damages is because of the type of flooding that is simulated (surface water flooding) and the fact that a number of properties experience flooding up to a few millimetres, which, in reality, is unlikely to enter a property unless its entrance is completely flush with the ground, which is extremely rare. Based on previous examinations of topographic survey of property threshold, a value between mm is a reasonable average, especially given the difficulties of surveying the thousands of properties assessed in this study. This sensitivity testing has highlighted the importance of establishing accurate property threshold levels to ensure that the most appropriate and effective options can be identified. Therefore, doorstep surveys of sample of impacted properties are strongly recommended so that a representative assumption can be taken about property thresholds as part of further detailed Business Case Justification/ Project Appraisal Reporting for each wetspot. Assumption 2 Damage to property does not occur at events lower than 5% AEP. The lowest return period modelled was the 5% AEP event. Whilst it is possible within the flood damages equations to interpolate flood damages for return periods below the lowest return period modelled, these damages are not based on any modelled outputs and as such are subject to significant uncertainty. Furthermore, since they occur more frequently within the appraisal process, they have a disproportionate impact on present value damages. As such, and in keeping with the approach set out in FCERM-AG, it has been assumed that no damages occur to property within the study area at flood events lower than the 5% AEP event. Assumption 3 Maximum depth extracted from model results is representative of flood depth at properties. The worst case depth at each property is captured the maximum depth of flooding at each property has been currently used in the economic analysis. However in some cases the maximum depth extracted may have been exaggerated by anomalies in the underlying LiDAR. In some cases this can lead to an overestimation in the level of damages recorded at a property. Also, the use of average flood depth at the property may have a significant impact on the calculated flood damages. A sense check has been undertaken at the top ten properties that contribute most to the damage assessment for each wetspot to ensure that the assumptions used in the economic analysis are not skewing results at a handful of key properties. An additional sensitivity test was undertaken using average flood depth at each property (available from the 2D model results). This test suggests that total PV Damages for each wetspot for each modelled scenario could be reduced by 13% on average. Assumption 4 Raw modelled outputs have been used to calculate flooding at properties. In this sensitivity test all of the model outputs have been cleaned to remove isolated wet islands less than 200m 2 from the flood extents. The flood extents generated are reliant on the quality of the underlying DTM. In some cases there may be anomalies in the surface which may cause the modelled outputs to pond in low spots which in reality do not exist or to create disconnected areas of flooding where there should be a continous flow path. An automated process was used in the sensitivity test to remove the small wet islands so each anomoulous small wet spot has not been analysed so areas where there is an actual flow path are not removed. Currently, the raw model outputs with these discontiguous wet islands provides the most conservative estimate of flood risk and ensures that all potential flow paths are taken into account. Hyder Consulting (UK) Limited Page 85

90 Sensitivity testing showed that removing discontiguous wet islands less than 200m 2 from the flood outline removed on average 36% of properties from all four of the wetspots from the assessment of flood risk for the 0.5% AEP Do Minimum event scenario. Hyder Consulting (UK) Limited Page 86

91 8.3 Damages Assessment Exclusions The following key items were excluded from the assessment: Risk to life: whilst all flooding poses a risk to life, it can be argued that the nature of the widespread surface water flooding such as is assessed in this study limits maximum depths and velocities such that overall risk to life is low. Furthermore, its calculation for a large study area could require appraisal time that would be disproportionate to the scale of benefits expected. Transport disruption: flooding in a populated urban area has the potential for significant impact of transport networks, which can add to the economic impact of flooding. Although surface water flooding is frequently associated with transport disruption, it is not practical to assess, on the scale of this study, the sort of alternative routes and diversions required. The damages would be highly variable for each wetspot location and dependent on level of traffic, infrastructure and connectivity, it is recommended that assessment of this is left until further more detailed project appraisal stages. Environmental benefits: no accounting has been made for the potential environmental/amenity improvements associated with any of the proposed options. Health and social benefits: these perceived benefits attributable to undertaking flood prevention works and increasing health and well-being were not included. This view was taken because it was considered unlikely that the local population would necessarily perceive any benefit from a form of flooding which does not result in a noticeable flood pathway or a great depth of flooding (as would be the case for river or sea flooding). Hyder Consulting (UK) Limited Page 87

92 9 Summary In order to address the specific issues relating to the St Neots SWMP, a three stage modelling strategy was developed and implemented: Stage 1 - Hydrological Analysis and development of broad scale, bare earth model of St Neots (see Section 4.3). Stage 2 - Identification and evaluation of wetspots using the Stage 1 model results and discussions with the CCC and key stakeholders (see Section 5). Stage 3 - Development of a detailed model to represent an integrated terrain, river and sewer model (see Section 6). Development and testing of engineering options and economic analysis (see Sections 7). The SWMP direct rainfall analysis and review of historical data have improved the understanding of future surface water flood risk within the St Neots wetspot at a strategic level. The detailed modelling has defined the surface water flood risk to St Neots but further improvement can be achieved by further enhancing and calibrating the existing model. The model results have refined the extent of surface water flooding from the Environment Agency AStSWF and FMfSW and been verified where possible by the available historical data. A range of potential engineering measures and options have been identified, modelled and costed for all four wetspots, which highlight the need and benefit of reducing the future flood risk. These engineering options should be considered along with non engineering policy measures in order to maximise benefits. Funding constraints and stakeholder buy-in are likely to be a key obstacle to implement catchment wide solutions at both wetspots, highlighting the need for further stakeholder consultation and prioritisation of viable measures. The engineering elements assessed in Section 7 were designed to deal with local flooding issues so the overall impact on St Neots is minimal. By looking at the whole of St Neots in the benefit cost analysis the localised benefits would not be visible. To assess the localised benefit cost each wetspot was analysed individually. The separate benefit cost analysis for Meadowsweet resulted in Option C2 showing the highest benefit cost ratio. This option incorporated small scale engineering options to channel surface water flow away from properties. In the Riverside wetspot Option C4 had the highest benefit cost ratio. This option combines small and large scale engineering elements to both store and convey flow away from property. In both Eynesbury and the Town Centre wetspots Option C2 had the highest benefit cost ratios. However for the Town Centre all four of the combined options has benefit cost ratios of less than one indicating that that they are not economically viable and therefore the importance of targeted increased maintenance and proactive flood preradeness/resilience measures. For some wetspots more than one combined option was economically beneficial based on the current analysis. Therefore further assessment is required to ensure that the most effective flood mitigation measures are taken forward for each wetspot in order to address the stakeholder concerns as these may not always be seen as the most economically viable or the practical option. Where a positive benefit cost has been identified, it is recommended to undertake a Project Appraisal Report (PAR) as soon as possible to engage key stakeholders and the SWMP Project Board. The PAR process will allow for the confirmation and development of a preferred Hyder Consulting (UK) Limited Page 88

93 option for each wetspot and will help to identify any constraints with each suggested option so that a Business Case can be justified for further work if appropriate. The sensitivity of the economic results was checked, which included raising the property flood threshold levels. This resulted in a significant reduction in the number of properties shown as flooding when the threshold was increased from 0.1m to 0.2m and 0.3m. This analysis clearly highlighted the importance of gathering accurate property threshold data to confirm the preferred flood mitigation options as it will have a significant impact on economic viability of potential flood mitigation schemes. 9.1 Key Surface Water Flooding Issues Detailed modelling of the four St Neots wetspots (see Appendix F) identified a number of potential issues in the study area: Overland flow paths and ponding of water in natural depressions results in noticeable flood depths and hazards. Limitations in the hydraulic capacity of the below ground surface water network causing surcharging during heavy rainfall events. Limitations in the entry capacity for below ground surface water networks causing localised ponding The interaction between the surface water pipe network, foul system, Main River and the ordinary watercourses running through St Neots. The culverted stretches of watercourse are susceptible to hydraulic throttling/ backing up due to size and shallow gradient in the pipe network. This results in surcharging storm water manholes during heavy rainfall. This slows down and adversely interferes with the movement of water within each wetspot, increasing flood risk to people and property. The culverts and bridges are also liable to blockage requiring regular maintenance. 9.2 Preferred Options For Further Investigation The currently identified Preferred Options through modelling and economic appraisal process for the St Neots wetpots (including the rest of the study area where applicable) that require further investigation and consideration are: 1. Increased maintenance of ordinary watercourses and surface water drains within the wetspots 2. Gather more information relating to culverted sections of ordinary/awarded watercourses within each wetspot to ensure all are being taken into account 3. Smaller scale Engineering Elements (Option C2) in specific locations except for the Riverside wetspot where combined small and large scale Engineering Elements (Option C4) 4. Property level resistance/resilience measures 5. Investigate the current flood resilience of schools and other critical infrastructure identified as being at risk from Surface Water Flooding within St Neots 6. Policy Recommendations Hyder Consulting (UK) Limited Page 89

94 Whilst an optimism bias of 60 % has also been applied to all of the cost estimates (as per the current guidance applicable for a strategy of this nature) there are a number of economic risks or uncertainties associated with the development of the cost estimates. The principal economic risks associated with all of the option combinations are: The availability of land to form the attenuation storage areas Cost associated with dealing with utilities which have not been itemised with the cost estimates. The cost of land negotiations and compensation for disruption Ecolological and other environmental risks and associated costs Sensitivity of flood damage assessment and model limitations (e.g. actual property threshold levels and flood levels see Section 8.2 and 6.3) It is recommended that as part of Business Case Justification/ Project Appraisal Reporting process of the wetspots the above risks are further assessed and the currently identified Preferred Options are further refined accordingly. As part of this and subsequent detailed design process, accuracy of the current modelling results can be further enhanced by adding further detail to the existing model to address some of the limitations highlighted. The project lead should approach utility companies to obtain agreements for the relocation of services as necessary. Significant effort should be placed into obtaining agreements with landowners and stakeholders to undertake the proposed works. In order to mitigate this risk it is recommended that Huntingdonshire District Council and the Cambridgeshire Flood Risk Management Partnership enter into discussion with all landowners and stakeholders at the earliest opportunity during the design process to ensure their collaboration. Additionally, it is recommended that further discussion and guidance is sought to determine the potential implications associated with areas being designated to serve multiple land use functions, such as using sports pitches or public open spaces as above ground attenuation structures. It is preferable, for instance to use recreational open space as temporary flood storage as opposed to potentially impacting people and property, however, it is noted that ongoing maintenance, ownership, liabilities and warning are key concerns for these facilities and that these will require attention as part of taking forward the conceptual options identified in this SWMP through to fruition. The implementation of the proposed preferred engineering option combination described in the report will have significant beneficial impact on dealing with predicted flooding risk but it should also be recognised that there will be additional benefits streaming from the implementation of flood mitigation strategies. This includes: Beneficial impacts on bio-diversity associated with wetland options. Beneficial impacts on bio-diversity associated with the implementation of greener highway source control measures which includes planting in verges. Improvements in the design of the urban realm through the shift from grey to green infrastructure. This includes retro-fitting and incorporation of green infrastructure with highways design and other areas of urban design. Potential benefits in integration of investment with targets associated with bio-diversity. Potential benefits in amenity function and connectivity across the St Neots study area. Hyder Consulting (UK) Limited Page 90

95 9.3 Benefits of SWMP The modelling results, assessments and maps created during this SWMP, with emphasis on the four identified wetspots, can be used as follows: Indication of potential development constraints and opportunities to reduce the predicted flood risk Identification of which stakeholders should be consulted with regard to new development Highlights broad scale risk and indication as to whether a developer is required to undertake further investigation Evidence (locally agreed surface water data) as to why Developers should undertake further investigation and develop appropriate surface water mitigation measures and development layouts The CCC Highways Team can see where highway flooding has occurred in the past and during times of high rainfall focus maintenance and emergency response efforts in these areas The Emergency Planning team can use historical flooding data, modelling outputs and impacted flood receptors, to identify more vulnerable areas and prepare suitable emergency planning measures Development of future planning policies and local flood risk management policies as part of Huntingdonshire District Council s future Local Development Documents and CCC s Local Flood Risk Management Strategy. In particular, with regard to the consideration of surface runoff from any infill development or other new development that can influence flood risk within the four prioritised wetspots. This SWMP should be used by Huntingdonshire DC as locally agreed surface water data to inform decision making as to possible allocation areas for future development whilst safeguarding green areas currently identified for potential mitigation solutions. It informs Huntingdonshire DC with the Local Plan that is currently being produced. The emerging SWMP planning guidance produced by CCC aims to further clarify this and decision making process. Hyder Consulting (UK) Limited Page 91

96 10 Next Steps 10.1 Surface Water Management Action Plan Preparation, Implementation and Monitoring The next stage of the SWMP will be the Implementation and Review Stage as illustrated above. It will involve the review of evidence and recommendations from the previous stages of St Neots SWMP and parallel countywide broad brush SWMP in order to prepare implement and monitor an appropriate Action Plan for the four St Neots detailed wetspots. Consideration could also be given for combining implementation of the engineering elements across the wetspots so that certain areas/ mitigation elements may be prioritised to formulate the preferred option or strategy where the greatest cost-benefit can be achieved within both areas in a combined Action Plan. This combined approach may potentially provide a greater justification for capital investment and stakeholder support; in particular, within the short to medium term period where the impact of current economic climate is even greater rather than trying to solve the predicted flooding issues in isolation. Other key considerations for detailed design to take in to account are: Limited extents of open land in the study area to create attenuation features require careful planning and negotiations with the impacted land owners. Location of attenuation structures should be carefully balanced against economic, social and environmental needs. Limitations associated with the surface water and foul network data in St Neots. Resolution and description of features in the urban realm should be improved within the hydraulic model for the purposes of the detailed design. For example there is limited Hyder Consulting (UK) Limited Page 92