North Northamptonshire Flood Risk Management Study

Transcription

1 North Northamptonshire Flood Risk Management Study Environment Agency June 2007 Final Report 9S1244

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3 Rightwell House Bretton Peterborough PE3 8DW United Kingdom +44 (0) Telephone Fax Internet Document title North Northamptonshire Flood Risk Management Study Document short title Status Project name Project number Final Report Date June 2007 Client Reference North Northamptonshire Flood Risk Management Study 9S1244 Environment Agency Drafted by R Gunasekara Checked by Date/initials check.. Approved by Date/initials approval..

4 - i - Final Report June 2007

5 CONTENTS Page 1 BACKGROUND General Overview Scope and Report Format Objectives and Benefits 1 2 DATA COLLECTION 3 3 STUDY AREA AND DEVELOPMENT PROPOSALS General Kettering Wellingborough East Northamptonshire Corby 10 4 REVIEW OF SFRAS Overview Kettering and Wellingborough SFRA Introduction Data Assessment Historic Flooding Drainage and Hydrology Strategic Assessment of Flood Risk Compliance with PPG25/PPS Implications of recent studies and development proposals East Northamptonshire SFRA Introduction Data Assessment Historical Flooding Drainage and Hydrology Strategic Assessment of Flood Risk Compliance with PPG25/PPS Implications of recent studies and development proposals Corby SFRA Introduction Data assessment Historical Flooding Drainage and Hydrology Strategic Assessment of Flood Risk Compliance with PPG25/ PPS Implications of recent studies and development proposals 31 5 OUTLINE FLOOD RISK MANAGEMENT STRATEGY Overview Policies Planning Conditions Physical measures Residual Risk Management 42 - ii - Final Report June 2007

6 5.5.1 General Flood warning and emergency planning Guidance 44 6 FUNDING AND PLANNING ISSUES 45 7 CONCLUSIONS 47 8 RECOMMENDATIONS 49 9 REFERENCES 51 FIGURES 1 Study Area 2 Flood Map for North Northamptonshire 3 Kettering Flood Map and Development Sites 4 Wellingborough Flood Map and Development Sites 5 East Northants Flood Map and Development Sites 6 Corby Flood Map and Development Sites 7 LiDAR coverage for North Northamptonshire 8 DG5 Sewer Flooding Records within North Northamptonshire - iii - Final Report June 2007

7 1 BACKGROUND 1.1 General Overview North Northamptonshire is within the Government s growth plans for the Milton Keynes and South Midland Sub Region. Corby, Kettering, Wellingborough and East Northamptonshire Councils, together with Northamptonshire County Council, are working through a Joint Planning Unit (JPU) to create an overall town planning strategy for the area which then forms a key part of the Local Development Framework (LDF) for each Council. Each of the four Councils within North Northamptonshire has completed some form of a Strategic Flood Risk Assessment (SFRA) for their administrative areas within the last couple of years. However, an overall assessment for the whole of North Northamptonshire has not been undertaken to inform the emerging LDFs. In addition, the information contained in the completed SFRAs may be out of date due to new information on flood risk and development sites. Also, the recently enforced Planning Policy Statement (PPS) 25 is likely to affect some of the previous findings of the SFRAs. Therefore, the Environment Agency appointed Royal Haskoning in September 2006 to carry out a study which will produce an overall flood risk management strategy for North Northamptonshire. This report presents the finding of the study. 1.2 Scope and Report Format The four existing SFRAs have been analysed and information gleaned from them used to create an overall flood risk management strategy within North Northamptonshire. The key sections of the report include: Section 1 Background to the study; Section 2 Data collection; Section 3 General description of the study area and the development proposals within each Council area; Section 4 Findings of the review of existing SFRAs; Section 5 Details of the proposed outline flood risk management strategy; Section 6 Funding and planning issues; Section 7 Conclusions; and Section 8 Recommendations. 1.3 Objectives and Benefits The key objective of this desktop study is to ensure timely feeding of flood risk management needs of the growth proposals in North Northamptonshire into the Water Cycle Strategy (WCS) undertaken by the Environment Agency and the upcoming LDFs for each council. The anticipated key benefits of the study will include: avoidance of damage to the environment; avoidance of increasing flood risk to downstream areas from inappropriate major development proposals; creation of safe and sustainable development that account for future climate change impacts; Final Report June 2007

8 compliance with high level national and regional planning policies; avoidance of repeated ad-hoc extensions to the existing flood alleviation measures or new schemes at Government s expenditure; and enhancement of environmental opportunities through effective integration of Sustainable Drainage Systems (SUDS), green corridors and public amenity space. Final Report June 2007

9 2 DATA COLLECTION The study was carried out in close consultation with the Environment Agency, the Councils, Anglian Water and also Halcrow, who recently completed the Outline Water Cycle Strategy (WCS) for North Northamptonshire. The relevant information was gathered from the above parties and the data sources are noted under the references at the end of this report. The key data sources include: Existing SFRAs; Emerging LDF documents; o North Northamptonshire Core Spatial Strategy o Issues and Options for North Northamptonshire o Preferred options for North Northamptonshire Core Spatial Strategy Information on committed and proposed development sites; Recent Studies; o North Northamptonshire Outline WCS o River Nene Catchment Flood Management Plan o Hydrological Assessment of the Slade Brook Detention Reservoir o Corby Phase 2 WCS o Pre-feasibility Studies on Willow Brook and Harpers Brook Historic sewer flooding information from Anglian Water; Planning Policy Statement (PPS) 25: Development and Flood Risk; and Development and Flood Risk: A Practice Guide Companion to PPS25 Living Draft. The data gathered was reviewed as described in the following sections of the report to identify the key flooding issues and then to produce the proposed Flood Risk Management Study to accommodate the anticipated major growth plans in North Northamptonshire. Final Report June 2007

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11 3 STUDY AREA AND DEVELOPMENT PROPOSALS 3.1 General North Northamptonshire is generally formed by the main towns of Corby, Kettering and Wellingborough, along with Rushden and a number of other smaller towns and many villages. Figure 1 shows the extent of the study area and the key locations. The principal watercourses in the study area include: River Nene and its main tributaries (River Ise, Harpers Brook and Brook Willow); and River Welland and its main tributaries (River Jordan). As illustrated in Figure 1, the majority of the study area is within the River Nene catchment. However, the River Welland flows along the northwest boundary of the study area and therefore small parts of Corby, Kettering and East Northamptonshire Councils are within the River Welland Catchment. The River Nene rises on the mainly clay soils of the Northampton uplands and crosses gently sloping rural country to the flat plains of Peterborough see Figure 2. Upstream of Peterborough, the River Nene is joined by the principal tributaries of Wotton Brook, the Kislingbury Branch, the Brampton Branch, the River Ise, Harpers Brook and Willow Brook. The River Nene catchment is largely rural and the major land use is agriculture. There are some key urban areas within the catchment including Kettering, Wellingborough, and Corby that fall within the study area. The potential sources of flooding that may need further consideration include: overflowing of watercourses and existing flood defences including water retention facilities such as flood storage areas and storm water balancing ponds; blockages of existing bridges, culverts and other structures in particular during large rainfall events; mechanical, structural or other operational failure of existing hydraulic structures and pumps; breaching of existing flood defences(including flood storage areas that hold water above surrounding ground levels); localised surface water flooding (including sewer flooding and overland flooding); and groundwater flooding. The East Midlands Regional Spatial Strategy (RSS) is currently under review and the existing Milton Keynes and South Midlands Regional Spatial Strategy (MKSM RSS) housing projections will be included within the revised overall RSS. Also, the RSS will now look up to 2026 whereas the MKSM RSS has set a target of 99,500 homes to be completed in Northamptonshire between 2001 and Of this figure 52,100 homes are within the study area as shown in Table 1 overleaf. Final Report June 2007

12 Table 1: Homes constructed between 2001 and 2006 against MKSM targets Local Authority RSS Target Completed %of RSS completed Corby 16,800 2,500 15% Kettering 13,100 2,353 18% Wellingborough 12,800 1,521 12% East Northants 9,400 2,825 30% Total 52,100 9,199 18% (Source: North Northamptonshire Outline Water Cycle Strategy, January 2007 Reference 1) Committed or planned development within each of the Councils within North Northamptonshire is shown in Table 2. Committed development is that the ones granted with full planning permission but not yet fully completed. Table 2: Homes currently committed or planned in order to meet the RSS targets Local Authority Committed Development (April 2006) Local Plan allocation without planning permission Windfall Estimate Total future development identified Corby 3, ,489 7,222 Kettering 2, ,138 5,537 Wellingborough 1,419 4,170 2,037 7,626 East Northants 1, ,731 5,263 Total 9,834 5,419 10,395 25,648 (Source: North Northamptonshire Outline Water Cycle Strategy, January 2007 Reference 1) In addition to the above, Table 3 and Table 4 show the preferred distribution for housing development and employment land between 2001 and Table 3: Distribution of Housing Development in North Northamptonshire Settlement GROWTH TOWNS Preferred Option For Distributing Housing Growth Corby Min 15,510 8,350 Kettering Min 7,300 4,120 Wellingborough Min 11,590 4,990 SMALLER TOWNS Burton Latimer Desborough 1, Rothwell 1, Higham Ferrers 1, Irthlingborough 1, Rushden 2,370 0 RURAL SERVICE CENTRES Oundle Raunds 1, Thrapston 1, NETWORK VILLAGES Corby rural Max 1,290 0 Additional Growth Implied On Greenfield Extensions Final Report June 2007

13 Settlement Preferred Option For Distributing Housing Growth Kettering rural Max 1,640 0 Wellingborough rural Max 1,210 0 East Northants rural Max 1,460 0 Additional Growth Implied On Greenfield Extensions (Source: Preferred options for North Northamptonshire, November 2005, Reference 2) Table 4: Distribution of Employment Land and Projected jobs Growth for North Northamptonshire to 2021 Location Total Land Required (Ha) Indicative Existing Supply (Ha) Indicative Required Allocation (Ha) Job Growth CORBY 14,400 General Industrial Strategic Distribution Offices EAST NORTHAMPTONSHIRE 3,000 General Industrial Strategic Distribution Offices KETTERING 16,200 General Industrial Strategic Distribution Offices WELLINGBOROUGH 13,800 General Industrial Strategic Distribution Offices NORTH NORTHAMPTONSHIRE 47,400 General Industrial Strategic Distribution Offices (Source: Preferred options for North Northamptonshire, November 2005, Reference 2) Also, the preferred locations for the proposed sustainable urban extensions are East Kettering, Wellingborough East and Priors Hall on the northeast of Corby. Further potential extensions have been identified for development before 2021 at the western edge of Corby and the northwest edge of Wellingborough. It should be noted however that the recently submitted North Northamptonshire Core Spatial Strategy (Reference 3) may have some more up to date information on the distribution of housing and jobs than presented above and therefore it is recommended to make cross-reference with the Core Strategy for the latest information. Figure 2 shows the Flood Map for the entire North Northamptonshire together with the locations of the committed and proposed developments. Sections 3.2 to 3.5 give further details of the study area, watercourses and proposed development in each council area. Final Report June 2007

14 3.2 Kettering Figure 3 illustrates the principal watercourses, flood extents and development sites within the Borough of Kettering. The watercourses within Kettering include: Tributaries of River Nene; and - River Ise - Slade Brook - Thorpe Malsor Arm - Alledge Brook - Harpers Brook - East Brook River Welland and it s main tributary, River Jordan (at northwest edge of the Borough). Kettering is planned to accommodate 13,100 new homes by Only a limited amount of this growth can be accommodated within the existing urban areas and it is proposed that the town needs to grow eastwards, as indicated in Figure 3. East Kettering development is currently planned in two phases as follows: 3.3 Wellingborough Phase 1 (up to 2021) - 5,500 new homes - A district centre with shops, cafes, bars and restaurants, office and studio space, community facilities health clinic and park and ride. - Three housing neighbourhoods each with a primary school facility at their centre. - An extensive park system to support nature conservation and to create a network of open space for all types of recreation. Phase 2 (up to 2026) - Extending the site for the full capacity of 8,900 houses with a full range of appropriate supporting facilities. Figure 4 illustrates the principal watercourses, flood extents and development sites within the Borough of Wellingborough. The watercourses within Wellingborough include: River Nene; and Tributaries of the River Nene - River Ise - Harrowden Brook - Swanspool Brook - Barton Brook - Denington Brook - Grendon Brooks - Wollaston Brook - Knuston Brook Final Report June 2007

15 Wellingborough is planned to accommodate further 12,800 new homes by It is anticipated that the majority of this will be provided at Wellingborough East (WEAST) and at the Castleridge development of north west of Wellingborough. The current development plans for Castleridge include health and education facilities, formal and informal public open space, leisure and district facilities, as well as up to 5,000 houses. 3.4 East Northamptonshire Figure 5 illustrates the principal watercourses, flood extents and development sites within East Northamptonshire District. The watercourses within East Northamptonshire include: River Nene; Tributaries of the River Nene; and - Harpers Brook - Willow Brook - Gretton Brook - Alledge Brook - Southwick Brook - Stonepit Brook - Skew Bridge Dyke - Chelveston Brook - Hog Dyke - Barnwell Brook - Polebrook Brook - Warmington Dyke River Welland (at the north western edge of East Northamptonshire). East Northamptonshire is planned to accommodate 9,400 new homes by Table 5 summarises the distribution of housing according to the preferred options documents from the East Northamptonshire District Council. Table 5: Preferred Housing Distribution within East Northants up to 2016 Settlement Rushden 2,433 Irthlingborough 1,541 Higham Ferrers 1,116 Oundle 238 Thrapston 490 King s Cliffe 70 Total No. of Houses (Source: Preferred Options for Three Towns Rushden, Higham Ferrers and Irthlingborough, September 2006 Reference 4; Preferred Options for Rural North, Oundle and Thrapston, January 2006 Reference 5) The preferred options for Raunds Area were later published in January Table 6 demonstrates the Council s intentions for providing the required 1,100 new houses within East Northamptonshire. Final Report June 2007

16 Table 6: Preferred Housing Distribution for Raunds up to 2021 Settlement 3.5 Corby Completions Under Construction and Extant Planning Permissions Likely Large Urban Potential Sites (Some to be allocated) New Greenfield Allocations Total Raunds ,100 (Source: Raunds Area Preferred Options, January 2007 Reference 6 Figure 6 illustrates the principal watercourses, flood extents and development sites within Corby Borough. The watercourses within Corby include: Tributaries of River Nene; and - Harpers Brook - Willow Brook - Gretton Brook River Welland (at the north western edge of Corby). An additional 28,000 houses and 30,000 new jobs will be required in Corby by 2031 to meet the Government s growth plans. Out of this 16,800 new houses will be required by Figure 6 shows the locations of committed and proposed development sites in accordance with the information obtained from the Corby Phase 2 Water Cycle Strategy (Reference 7). Final Report June 2007

17 4 REVIEW OF SFRAS 4.1 Overview In July 2001 Central Government produced Planning Policy Guidance Note 25 (PPG25) Development and Flood Risk to give guidance to Local Planning Authorities (LPAs) how to consider flood risk in land-use planning process and development control decisions. This document gave the Government s policy at that time to the management and reduction of flood risk in the planning process by acting on a precautionary basis and also by taking account of climate change. It also stated that flood risk is a material planning consideration. PPG25 was replaced by Planning Policy Statement 25 (PPS25) Development and Flood Risk in December PPG25 stated that LPAs should adopt a risk-based approach to proposals for development in or affecting flood risk areas (Paragraph 27) and apply a risk-based approach to the preparation of development plans and their decision on development control through a sequential test (Paragraph 30). Therefore, in drawing up or revising policies in development plans and in considering applications for development, LPAs should have given priority in allocation or permitting sites for development, in descending order to the Flood Zones set out in Table 1 of PPG25. In paragraph 31, it also stated that LPAs should take account of the resulting level of actual risk. This process typically redefined and subdivided the Flood Zones initially established by the Environment Agency s high level Flood Map that ignores the effect of existing defences. Allocating development sites without this information could have led to inappropriate decisions or to challenge by others, as the application of sequential test required by PPG25 may have been flawed. Some local authorities (including the ones in North Northamptonshire) have, with guidance from the Environment Agency, undertaken SFRAs to deal with this potential issue. Many of the authorities have taken the opportunity to investigate and assess specific strategic sites and make recommendations for measures that would be needed both on site and at strategic locations and also to provide general development control guidance. The process of flood risk assessment is evolving both strategically (as national guidance is developed) and locally (as more detailed knowledge is gained about the location, nature and concept of the site proposals). Some types of development will be more vulnerable to flooding than others and therefore need special consideration in the assessment. The stronger emphasis on these issues is a particular aspect of the new Planning Policy Statement (PPS) 25: Development and Flood Risk, which was published in December This PPS sets out the Government s latest national policy on dealing with development and flood risk within the land-use planning system. PPS25 replaced the PPG25, published in Compared with PPG25, the new PPS25 imposes requirements for regional planning bodies and LPAs to prepare Regional Flood Risk Appraisals and SFRAs respectively. It strengthens the risk-based approach and guides new development away from the highest risk areas. However, it introduces the concept of an exception test that complements the sequential approach in exceptional circumstances where necessary to meet the wider aims of sustainable development for example where areas of high flood risk are extensive and there are no sites available in lower risk areas. Final Report June 2007

18 The following sections of this report discuss the key findings of the review for the existing SFRAs within North Northamptonshire. Based on the currently available information to Royal Haskoning, this review is largely focussed on the suitability of the SFRAs in terms of their Fitness for Purpose for the inclusion within the emerging LDFs and Water Cycle Strategy (WCS) for North Northamptonshire. The findings are then fed into the development of proposal Flood Risk Management Strategy described in Section 5.0. The key considerations to be reviewed are summarised in Box 1 below. Figure 7 also shows the Environment Agency s LiDAR coverage within the study area illustrating the current topographic survey availability for modelling and mapping purposes. The LiDAR spatial resolution in this area is 2m. Taken together with the generally accepted vertical accuracy of ±11cm to 25cm, this indicates that in the areas covered by the LiDAR data would provide a good representation of ground surface for the required flood risk mapping where further improvements to the existing SFRA outputs are required. Data assessment Historic Flooding Drainage and Hydrology Strategic Assessment of Flood Risk Compliance with PPG25/ PPS25 Implications of recent studies and development proposals Quality of data used and rigor of review carried out in the strategic assessment The record of past flood events and any factors that may have affected the risk of flooding, including development or land-use change within the catchment or implemented river flood management works Strategic understanding of the hydrology and drainage of river catchments Identification of land most likely to flood (from all sources) whilst taking into account the effect of climate change and any existing defences. This will include the residual risk of flooding arising from any flood defences Compliance of sequential test approach to flood risk management within the planning process (For this purpose, the latest development proposals from the emerging LDF documents has been used) and impact of new information and legislation such as PPS25 which can challenge the conclusions of the SFRAs already completed Impact of major development proposals on flood risk due to runoff issues including the use of strategic and local SUDS measures Box 1: Key considerations for the SFRA review Final Report June 2007

19 4.2 Kettering and Wellingborough SFRA Introduction Royal Haskoning completed this SFRA in May 2005 and the Environment Agency and the Councils subsequently approved the report (Reference 10). We have assessed the SFRA against the above criteria and our findings are summarized below Data Assessment The SFRA uses the best available data following a sufficient level of quality check to satisfy the needs of a strategic assessment of this nature. Since the completion of the SFRA there is no valuable new data or studies in addition to the information discussed in Section Historic Flooding Comprehensive analysis of fluvial flooding events is available in the SFRA, however it lacks records of possible sewer flooding. In particular, the SFRA highlights the potential flood risk on the following watercourses within each Council area: Kettering Borough Council Slade Brook; East Brook; River Ise; and River Jordon. Borough Council of Wellingborough Harrowden Brook; Swanspool Brook; Denington Brook; River Ise; and River Nene. The lower reaches of Grendon, Wollaston and Knuston Brooks are also subject to flooding. Therefore, the opportunities to alleviate the current flood risk at vulnerable locations (e.g. Venture Park and Pytchley Lodge Industrial areas in Kettering Borough and Finedon Road and Denington Industrial Estate areas within the Borough of Wellingborough) should be explored as part of the proposal Flood Risk Management Strategy (FRMS) for North Northamptonshire. Table 7 and Table 8 summarize the records of known flooding within the Kettering Borough and the Borough of Wellingborough respectively. Final Report June 2007

20 Table 7: Historic Flooding Records within Kettering Borough Town/Village/Location Watercourse and Dates Source Comments Cause Geddington River Ise exceedance of channel and structure capacity. 1947, 1973, 1980, 1981, 1998 EA 1998 River Ise Flood Investigation Preliminary Study (Draft Report); EA Improvement scheme implemented since historical flood map for 1947/1998 Burton Mill Finedon Mill River Ise exceedance of channel and structure capacity. 1947, 1998 EA (AWA) 1985 detailed appraisal on alleviation of flooding River Ise Improvement scheme implemented since Northamptonshire; EA Kettering d/s of Pytchley Lane Slade Brook Exceedance of 1998 EA historical flood map for 1947/1998 channel and structure capacity Braybrooke properties flooded River Jordan exceedance of channel and structure capacity, 1968,1980 (2 times), 1981, 1983 EA (AWA) 1984 River Jordan Braybrooke Flood Alleviation Scheme Improvement scheme implemented since flood duration is less than 12 hours Agricultural fields along the River Welland. North of Weston by Welland and Ashley and west of Sutton Bassett River Welland Exceedance of channel and Structural capacity 1947, 1998 EA historical flood map for 1947/1998 (Source: Kettering and Wellingborough SFRA Stage 2 Report, May 2005) Note This is not an exhaustive list of flooding records. Table 8: Historic Flooding Records within Borough of Wellingborough Town/Village/Location Watercourse and Dates Source Comments Cause Wellingborough (mainly agricultural fields at the eastern parts). River Ise Exceedance of channel and structure capacity 1998 EA 1998 River Ise Flood Investigation Preliminary Study Draft Report + EA historic flood Map for 1947/1998. Wilby houses and gardens on the south side of Brook Vale Swanspool Brook exceedance of channel and structure capacity 1981, 1980 EA (AWA) 1983 letter entitled Flood Damage at Brook Vale, Wilby, Northants. Improvement scheme implemented since Final Report June 2007

21 Town/Village/Location Wellingborough Swimming pool Croyland Park changing rooms Croyland Park Agricultural land London Road and Doddington Road with adjacent properties Coryland Road Kingsway pedestrian subway Houses at Brook Vale, Wilby Doddington Road, Wilby Watercourse and Cause Swanspool Brook exceedance of channel and structure capacity Dates Source Comments 1981, 1978, EA (AWA) project Improvement Serious Flooding appraisal report scheme in 1978 (17 Swanspool Brook: implemented houses and a pub Wellingborough to since flooded) and 1980 Wilby (44 houses, a improvements; pub, a EA (AWA) 1981 supermarket, a Swanspool Brook shoe factory and a Improvement veterinary surgery Scheme flooded) Wellingborough Section Harrowden Brook 1947 EA historic flood The 1947 flood of the Harrowden Brook exceedance of map for 1947/1998 outline should be d/s of Kettering road to channel and considered very confluence with Ise. structure capacity carefully as the Finedon Road flood outline Industrial Estate doesn t include the river bed d/s of Finedon Road Industrial Estate. The floodplain also stops at the southern boundary OF sp875e OS map tile. Agricultural fields along Grendon Brook 1947, 1981 EA (AWA) 1:2,500 The 1947 flood the lower and middle exceedance of map (Drawing outline should be reaches of Grendon Brook channel and 32/9/860/1079); considered very at north and east of structure capacity EA historic flood carefully as the Grendon map for 1947/1998 floodplain stops at the southern boundary of the SP865E OS Map tile. Agricultural land along Wollaston Brook 1947 EA historic flood The 1947 flood lower and middle reaches exceedance of map for 1947/1998; outline should be of Wollaston Brook at channel and EA (AWA) 1984 considered very north west of Wollaston structure capacity Wollaston Brook carefully as the Improvement floodplain stops at Scheme the southern boundary of the Final Report June 2007

22 Town/Village/Location Watercourse and Cause Dates Source Comments SP865E OS Map tile. Agricultural land along the lower and middle reaches Knuston Brook exceedance of 1947 EA historic flood map for 1947/1998 of Knuston Brook at north east of Irchester channel and structure capacity Wellingborough lower Denington Brook Not known BCW map reaches of Denington Brook near Denington Industrial Estate due to inlet blockage (frequent occurrence) Wellingborough north of A45 including Victoria Mills Area and the agricultural fields along the river valley south of Wellingborough River Nene exceedance of channel and structure capacity. 1947, 1998 EA historic flood map for 1947/1998 (Source: Kettering and Wellingborough SFRA Stage 2 Report, May 2005) Note This is not an exhaustive list of flooding records in particular note that the entire River Nene valley has been subjected to significant flooding during 1947 and Drainage and Hydrology FEH hydrology has been used for all studied major watercourses in the catchment but using limited model calibration due to the absence of gauged flow and water level data in the catchment. Therefore the model calibration has been limited to the Environment Agency s River Nene Strategic Model, which also includes its principal tributaries of River Ise, Willow Brook, Harpers Brook and Slade Brook. No further hydrological assessment has been undertaken for other watercourses as part of the SFRA although basic FEH hydrology using catchment descriptors has been used in the Environment Agency s national Flood Zones for all watercourses that have a catchment area greater than 3km 2. However, this assessment may not reflect the hydraulic effects imposed by existing flood defences and other structures on the water levels and river flows Strategic Assessment of Flood Risk The SFRA covers the council areas of Kettering and Wellingborough. It gives particular attention to the locations where development sites are committed or proposed at that time and also to the places where known to have historic flooding problems. Therefore within these areas the results from various hydraulic models, which also consider the impact of any flood defences or storage reservoirs, have been used in the assessment. The key flood defences the Boroughs include: Kettering Town Short sections of raised flood defences along Northfield Avenue along the Slade Brook; Geddington Village new flood relief channel implemented as part of the recent scheme; and Final Report June 2007

23 Wellingborough Town Sections of raised flood defences along the Swanspool Brook. Flood storage reservoirs / water retention facilities also present at: Kettering leisure village (Slade Brook Flood Detention Reservoir); West of Wilby village on the Swanspool Brook; Northern part of Wellingborough along the Harrowden Brook; Cransley and Thorpe Malsor Reservoirs on the tributaries of Slade Brook; Sywell reservoir south of Sywell village on the upstream reach of the Barton Brook; and Numerous storm water balancing ponds including at Kettering North Business Park and Prologies Business Park at Wellingborough. In addition to the above, significant natural flood attenuation is available at some locations but most noticeably upstream of Glendon Railway culvert north of Kettering Town on the Slade Brook. The modelled watercourses as part of the SFRA include: River Nene; River Ise; Swanspool Brook; Harrowden Brook; Slade Brook; East Brook; Denington Brook; Ecton Brook; and Harpers Brook. For other watercourses, the Environment Agency s National Flood Zones that ignore the presence of existing flood defences have been used in the assessment and mapping. Although the SFRA identified storm sewer flooding as a possible cause of flooding no modelling was undertaken to map the areas that may be at direct risk of such flooding Compliance with PPG25/PPS25 The SFRA provides a suitable reference document in order to satisfy the LPAs responsibilities expected by the Government through PPG25 at the time of publication of the SFRA in May The SFRA also provides brief details of the existing flood warning and emergency evacuation procedures in the area; these can be further developed as part of the proposed Flood Risk Management Study. It should generally satisfy the needs of the new PPS25 and has been noted as a case study within the recently published Living Draft Practice Guide (Reference 9) that accompanies the PPS25. The SFRA recognises all sources of flooding and provides good quality flood mapping for the entire area (including for future climate change scenario). However, it does not provide detailed flood risk outlines neither for additional breach risk arising from flood storage areas nor for urban flood risk from urban storm sewers. Final Report June 2007

24 The SFRA provides good guidance on managing the impact of runoff from new developments but no modelling has been carried out to assess the potential impact. It also highlights the importance of whole water cycle management. The SFRA identifies the key flood risk issues associated with the committed and future growth sites and the sequential approach required by the PPG25/PPS25. It gives good guidance on the need for undertaking site specific Flood Risk Assessments (FRAs) and the importance of improving flood risk information including flood warning and emergency evacuation. The SFRA give clear guidance on how to carry out FRAs, the use of Sustainable Drainage Systems (SUDS) and details mechanisms for ensuring maintenance responsibilities for flood defence and SUDS. It identifies the need for further detailed studies in particular where watercourses are currently suffering from flooding problems or where sites are proposed within or closer to flood risk areas for all watercourses whether modelled or not modelled as part of the SFRA. The SFRA finally highlights its limitations and gives recommendations to address them and also to promote integrated flood management and environmental enhancement (encompassing whole water cycle management) within the Boroughs. The current SFRA will need updating when further flood risk information is available through the ongoing studies as described in Section including from large development proposals such as Kettering East and Castlegate. An update of some of the terminology used in the SFRA outputs, a further review of the latest site allocations from the emerging Development Planning Documents and LDFs and the risk of flooding from sources other than main rivers will be required if a full compliance to PPS25 is to be achieved immediately. If these becomes a need for application of the Exception Test according to Table D.3 of PPS25 (see Table 9 below) then further detailed modelling will be required to consider the detailed nature of the flood hazard taking account of the presence of flood risk management measures such as flood defences. This should include the assessment of flood probability, flood depth, flood velocity and rate of onset of flooding within each flood zones. Table 9: Flood Risk Vulnerability and Flood Zone Compatibility (Source: Table D.3, PPS25, Communities and Local Government, December 2006) Final Report June 2007

25 In addition, functional flood plain and climate change impacts will have to be further assessed and mapped in accordance with the PPS25 requirements. Section also discusses the likely impact of latest development proposals and the recent studies on the strategic flood risk within the boroughs based on the information gathered to date Implications of recent studies and development proposals The only key recent study that may have some impact on the findings of the SFRA is the hydrological assessment of the Slade Brook Detention Reservoir at the Leisure Village, commissioned by Kettering Borough Council. The draft report was produced in July 2006 by W A Fairhurst and Partners (Reference 11) but no further work has taken place since. It provides the necessary background information to the Council for addressing the concerns of the Environment Agency (and the Reservoir Panel Engineer) regarding the public safety of the balancing reservoir under the terms of the Reservoir Act The Slade Brook Detention Reservoir is an online storage reservoir and is formally maintained by Kettering Borough Council. It has three spillway sections (central spillway is 300mm lower than the other two). Downstream of the spillway and the embankment of this reservoir, the golf course access road provides a secondary flood embankment where the Slade Brook flows through two 1500mmX 1500mm concrete box culvert sections. Also, potential overtopping of the reservoir can lead to a breach of the embankments of the reservoir and the golf course access road causing significant flooding downstream where several residential developments, commercial and industrial units are located (including the Pytchley Lodge Industrial Estate and the Kettering Venture Park). Therefore, it is essential to review this recent study and incorporate its findings into the proposed Flood Risk Management Strategy (FRMS) for North Northamptonshire. The hydrological study confirms that the golf course access road will be overtopped during 1 in 10,000 year flood event which can lead into a possible breach of the road embankment causing major flooding downstream. It also confirms that the Detention Reservoir is providing only very limited attenuation of the inflows from the Slade Brook even for 1 in 5 year event. This is mainly due to the high outflows over the three spillway sections at this online reservoir. For 1 in 50 year event there is hardly any flow attenuation within the Detention Reservoir. The report also clearly indicates the benefit of natural flood attenuation upstream of the reservoir (e.g. at Glendon Railway, Brooklands Farm, Rothwell Cemetry, Kettering Golf Club and Nunnery Farm) and the inevitable flood threat to Kettering Town if this storage is lost in the future. The inability of the Slade Brook Detention Reservoir to act as a flood mitigation feature up to 100 year event now is a significant drawback. This reservoir was originally constructed in 1990s under the constraints of the overall development plan and design standards at that time. Despite the above findings, the SFRA mapping and the reports are still valid as they consider the current state of the Slade Brook and River Ise Catchment using an ISIS hydraulic model - compared to the recent study by the Council which uses an ISIS hydrological routine model. The SFRA has already highlighted potential issues with the Slade Brook Detention Reservoir and the recent hydrological study by the Council Final Report June 2007

26 further confirms them. Therefore, the flood risk impact of new development in Kettering Borough upstream of the Detention Reservoir (e.g. Desborough, Rothwell and northern and western parts of Kettering) and the possible changes to the natural flood attenuation on the Slade Brook and the River Ise catchments should be carefully considered when making future development planning decisions. Future developments will need a robust approach to surface water management using source control SUDS and strategic flow attenuation. Opportunities should be explored to enhance flow attenuation at the existing Detention Reservoir and at the places where natural attenuation is already available although these opportunities are currently overlooked by the emerging LDF documents. In addition, it should be noted that the emerging LDF documents suggest that Kettering East development is much more extensive than originally planned at the time of SFRA preparation. It now extends considerably into the hydrological catchment of the Alledge Brook which was not studied in detail during the SFRA as there was no development proposal or flood risk concerns on this watercourse at that time. The upper parts of the Alledge Brook are within Kettering Borough but the lower parts (i.e. downstream of A14) are within East Northamptonshire District. The middle and lower reaches of the Alledge Brook were improved in early 1990s during the construction of A14 but it now needs a detailed hydrological and hydraulic review due to the major development proposals of Kettering East Development. In particular, the ability of this watercourse to take additional flows from the proposed development upstream of A14 crossing should be sufficiently investigated. Note that Cranford St John Village is located in this stretch of Alledge Brook which may limit further improvements to this watercourse but the flood risk to the existing community may be increased unless surface water from the Kettering East development is not adequately managed. Castlegate development at North West of Wellingborough is also more extensive than originally thought at the time of SFRA. Additional runoff from this large development into the Harrowden Brook needs careful consideration as this watercourse has a history of flooding. Two flood storage reservoirs have already been built to alleviate the flooding problems on this constrained watercourse that almost runs through the built up areas along its entire length. The limited hydraulic capacity of the railway culvert at the downstream limit of the Harrowden Brook is a key constraint to flood risk management on this urban watercourse. Sewer flooding information presented in the North Northamptonshire Outline Water Cycle Strategy (WCS) report (Reference 1) has been reviewed to identify any important issues that SFRA might have overlooked. The Outline WCS also reports about some sewer capacity problems at Severn Way in Kettering where the sewer carries flow from Rothwell and Desborough and on the Swanspool Brook sewer serving the west of Wellingborough. During this Flood Risk Management Study we have consulted Anglian Water to obtain the sewer flooding records from the DG5 register so that this information can be assessed within the review of SFRAs. The DG5 register is updated annually and it lists the areas and properties which have previously experienced an internal or external sewer flooding incident caused by lack of capacity of a sewer (i.e. from a public sewer, whether foul, combined or surface water). Temporary problems such as blockages, siltation, sewer collapses, and equipment or operational failures are excluded from the register. An entry upon this register will not be removed until the problem has been solved. Final Report June 2007

27 Anglian Water's position on the use of sewer flooding records within the SFRAs is outlined below: Occurrences of sewer flooding within the study area are recorded by Anglian Water under its license obligations. All flooding instances will be prioritized within their capital investment process, and solutions implemented through the regulatory funding mechanism. Sewer flooding is an indication of localized under-capacity, not as a constraint to development. It should be recognized that reporting is not necessarily complete as some property owners do not report sewer flooding events. In addition, spillages from sewerage systems in remote areas are unlikely to be reported. Nevertheless, Anglian Water has subsequently supplied postcodes of places that have been subject to sewer flooding. The listing gives the number of properties which suffered internal flooding, and the number of places subject to external flooding. External flooding therefore includes highways, public open space, open land, parkland, as well as private gardens. Table 10 lists all of the postcodes where sewer flooding has occurred within the Boroughs of Kettering and Wellingborough. These locations are also shown in Figure 8. Table 10: DG5 Sewer Flooding Records within the Boroughs of Kettering and Wellingborough Council Kettering Wellingborough Postcode Total flooding External flooding Internal Flooding Both internal and areas by only only external flooding postcode NN142LF NN146AR NN146DN NN146JJ NN156NY NN168EF NN168QL NN168LD NN169HN NN8 1DJ NN8 INB NN8 2BY NN8 2DP NN8 3PN NN8 4HH NN297AB NN297LY Finally, it can be concluded that the SFRA together with the additional information presented in this FRMS report should be considered when assessing flood risk needs within the Boroughs. These should form the basis for preparing flood risk management policies for the area and provide sufficient information to carry out a Sustainability Final Report June 2007

28 Appraisal of the Local Development Documents and the Sequential Test of development sites. 4.3 East Northamptonshire SFRA Introduction Faber Maunsell completed this SFRA in September 2006 (Reference 12) and formal approval of the Environment Agency was given in November We have assessed the SFRA against the criteria described in Section 4.1 and our findings are summarized below Data Assessment The SFRA uses the best available data following a sufficient level of quality check to satisfy the needs of a strategic assessment of this nature. Since the completion of the SFRA there is no valuable new data or studies in addition to the information discussed in Section Historical Flooding Details of fluvial flooding events are available including some incidents of sewer flooding. In particular, the SFRA highlights the potential flood risk on the following watercourses: River Nene; Skew Bridge Dyke; Harpers Brook; Hog Dyke; and Unnamed stream at Polopit near Titchmarsh. It also highlights some sewer and overland flood risk at Rushden, Irthlingborough, Glapthorn and Oundle. Table 11 summarises the records of known flooding within the East Northamptonshire District. Table 11: Historic Flooding Records within East Northamptonshire District Town/Village/Location Watercourse and Cause Rushden Duck Street Skew Bridge Dyke Oundle Glapthorn Road Oundle Glapthorn Road Oundle Prince William School at Hern Road Blocked culvert in Hall Park Runoff from new development and adjoining fields Sewer flooding but not clear whether due to surface water or foul water Sewer flooding and runoff from adjacent fields (blocked drains and Dates Oct 02 Mar 98 and Aug 04 Source ENDC/Parish Council ENDC/Parish Council Nov 2000 Anglian Water Frequent Environment Agency Final Report June 2007

29 Town/Village/Location Watercourse and Dates Source Cause inadequate drainage) Titchmarsh Polopit Flooding from stream 1992, 1998 and others ENDC/Parish Council Nassington Not given 1998 and others ENDC/Parish Council Elmington Main Road (A605) Aug 04 ENDC/Parish Council Glapthorn Surface water runoff from fields Aug 04 ENDC/Parish Council Sudborough Harpers Brook Regular ENDC/Parish Council Thorpe Waterville Local drainage problems Not known ENDC/Parish Council Fotheringhay Tansor Road Not given probably River 1998 and others ENDC/Parish Council Nene Raunds Brook Street (1952 and 1998), Midland Road Hog Dyke lack of capacity in lengthy culverts and short lengths of open 1952, 1998 and others Raunds Drainage Study, July 2001 by Bullen Consultants Ltd. channel (1 in 10 yr standard) Brigstock Not given as map is not attached to the SFRA report Not given except for stating the RP of the event is 1 in Typed report without its provenance years possibly Harpers Brook Rushden Paddocks Road Sewer flooding but Nov 2000 Anglian Water not clear whether due to surface water or foul water Irthlingborough Portland Sewer flooding but Nov 2000 Anglian Water Road not clear whether due to surface water or foul water Barnwell Barnwell Brook 1980s Not known. (Source: East Northamptonshire SFRA Stage 2 Report, September 2006 and Environment Agency) Note This is not an exhaustive list of flooding records in particular note that the entire River Nene valley has been subjected to significant flooding during 1947 and Drainage and Hydrology FEH hydrology has been used for River Nene and its two principal tributaries within East Northamptonshire District (i.e. Willow Brook and Harpers Brook). This was done by the Environment Agency at the time of development of River Nene Strategic Model. Final Report June 2007

30 No further hydrological assessment has not been undertaken for other watercourses as part of the SFRA although basic FEH hydrology using catchment descriptors has been used in the Environment Agency s national Flood Zones for all watercourses that have a catchment area greater than 3km Strategic Assessment of Flood Risk The SFRA covers entire East Northamptonshire District mainly using a combination of modelled flood levels from the Environment Agency s River Nene model (including Willow Brook and Harpers Brook), national Flood Zones and Section 105 mapping. Other information such as historic flooding, flood defence information and topographic data has been used in the assessment. Within East Northamptonshire District, the flood defences or flood storage structures are present at: Thrapston upstream of Thapston (Nine Arch) Bridge; Sudburough upstream of Slipton Road Bridge; Brigstock upstream of Grafton Road Bridge; Titchmarsh a private flood defence near No.27 Polopit; Barnwell Flood Storage Reservoir; and Several small balancing facilities. It gives particular attention to the locations where development sites are proposed at that time and therefore providing a slightly more detailed flood risk assessment for each study area than for the remainder of the District. Although the SFRA identified storm sewer flooding as a significant issue in the District (especially where large development sites are proposed in urban areas without sufficient capacity in existing sewers e.g. Rushden, Raunds and Irthlingborough) no modeling was undertaken to map the areas that at direct risk of such flooding. Where modelled information is not available from the River Nene Strategic Model the Flood Zones have been generally used in the assessment although at places they have been amended according to the SFRA report. However, it is not very clear the detail of the exact locations where the Flood Zones have been changed due to their anomalies. The lack of documentation within the maps and the report where Flood Zones have been amended leads some difficulty in following the audit trail Compliance with PPG25/PPS25 The SFRA provides a suitable reference document in order to satisfy the LPA s responsibilities expected by the Government through the PPG25 at the time of publication of the SFRA in September It provides useful information (including the existing arrangements for operational and emergency planning) for the proposed flood risk management strategy but further work to clarify its contents may be needed to fully comply with the new PPS25. Especially, it is not clear how the assessment has addressed the impact of climate change now this is a key objective of the PPS25. Final Report June 2007