FLOOD RISK ASSESSMENT

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1 Firth Buildings, Leeds Road, Dewsbury, WF12 7BU t f e trevor.haigh@haighhuddleston.co.uk FLOOD RISK ASSESSMENT ON LAND AT BUTT LANE SNAITH WEST YORKSHIRE FOR HARRON HOMES Ltd. E15/6486/FR01 SEPTEMBER 2015 T. Haigh. B.Sc., C.Eng., M.I.C.E. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

2 1.0 INTRODUCTION 1.1 This report is commissioned to investigate and report on the Flood Risk for this site in accordance National Planning Policy Framework (2012). The proposals are for a residential development on the site which is currently open agricultural land. The report is based on information supplied by the client and from relevant authorities in both written and verbal format. Some of this information is in verbal form only. No liability can be accepted for information supplied by third parties which is subsequently found to be inaccurate or incorrect. 2.0 THE SITE 2.1 The site is located to the east of Butt Lane, Snaith and is situated around Ordnance Survey grid reference , There are residential developments to the north and south of the site and open field to the west and east. A site location plan is included in Appendix A. 2.2 The site is irregularly shaped, and situated in a mixed arable/urban area. Butt Lane forms the westernmost boundary of the site, Cowick Road runs alongside the northern boundary and Mill Lane is adjacent the eastern boundary. There are residential developments to the north and south of the site and open field to the west and east. To the south east is Top House Farm. The overall site area to be developed is approximately 4.0ha. 2.3 The site consists of two separate fields that had been recently harvested at the time of the investigation. Access to the site was from Butt Lane on the western boundary, with access to the eastern field made through a dividing hedge 2.4 An adopted sewer runs across the western field from the residential development to the north of the field to Butt Lane in the west. An overhead electricity cable runs along the hedge dividing the two fields, becoming an underground cable adjacent the residential development as it heads north to Cowick Road. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

3 2.5 The majority of the site boundary is formed by hedges, with some area of fencing to the residential developments to the north west and south of the site. Uniform tree growth was noted on the boundary with Top House Farm to the south east and there was sporadic tree growth to the north western boundary. 2.6 A site highpoint of approximately 10.6m AOD is located at the south end of the hedge diving the two fields. From the highpoint the site falls towards Cowick Lane in the north east at an average grade of 1 in 115 and to the west at an average grade of 1 in 45 to a low point of approximately 6.6m AOD on Butt Lane. 2.7 The nearest main river is the river Aire which runs west to east approx. 0.7mile north of the site. 3.0 PROPOSED DEVELOPMENT AND CONSTRINTS 3.1 It is understood that the proposed development is for a series of detached, semi-detached and terraced town houses with associated car parking and access roads. A preliminary site layout is attached in the appendices. 3.2 The BGS Digital Geological Map of Great Britain at 1:50,000 scale has been consulted and we would report as follows:- 3.3 The majority of the site is shown to be overlain by superficial Lacustrine Beach Deposits consisting of sands and gravels. The south west corner of the site is overlain by the Hemingbrough Glaciolacustrine formation consisting of silty clay. 3.4 The site is underlain by the Sherwood Sandstone Group consisting of Sandstone. 3.5 There are no fault lines shown within 500m of the site. 3.6 The south west corner of the site is noted to have a moderate risk of compressible deposits and the remainder of the site is noted to have a low risk of running sands. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

4 3.7 There are no records of flooding affecting this site although there have been problems with the surrounding area in the past during major events. 3.8 The site does lie over a major aquifer but does not lie within a groundwater protection zone. 4.0 FLOOD RISK 4.1 On reviewing the Environmental Agency websites flood risk maps, the vast majority of the site currently falls within flood zone 1 and outside flood zones 2 and 3. There is a small area in the south west area of the site that appears to fall within flood zone three. This part of the site however is not to be developed and forms an area of public open space. The developable area of the site is therefore considered not to be at risk from fluvial flooding from rivers or sea for the 1 in 100 or 1 in 1000 year flood event. The proposed use of the site would be classified as More Vulnerable in Table 2 of the Technical Guidance to the National Planning Policy Framework March In accordance with NPPF the proposed development would be considered in appropriate for the area falling within flood zone The site does not fall within an area subject to surface water flooding according to the EA maps. The EA Flood Risk map for flooding from fluvial sources does show flood waters encroaching onto the lower south west area of the site. As stated above, this area is not to be developed for building and forms a public open space area. The area is low lying relative to the development site and the developable area stands approx. 1m above the flood levels indicated on the flood risk maps. The highest recorded flood levels at Carlton Bridge on the River Aire to the north of the site, is 6.15m AOD which is still below the lowest level on the site. 4.3 The site does appear to fall with an area subject to flooding from reservoirs according to the EA maps. Nor does it fall within a flood warning zone but the surrounding areas to the village do fall within such a zone. 4.4 Due to the size of the development over 1Ha it would be necessary to prepare a site Specific Flood risk Assessment for the site. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

5 4.5 There are a number of potential flooding mechanisms that NPPF now requires are evaluated for each proposed development site. Each method of flooding requires an assessment to be made on its probability relative to the site development. The normal requirement of the document is for no flooding of properties for storms up to a 1% probability or a once in a 100 years storm. The risk assessment also includes for flooding both on site and off site, and the effects of the development on the downstream catchment or the flow regime of the watercourse. NPPF also requires that the effects of severe storms above the normal 1% probability are reviewed together with the effects of climatic change relating to the design life of the development. 4.6 It also requires that the effects of climate change are taken into account together with the impacts of extreme events and flood defence failures. Prior to this the Sequential Test or the Exceptions test as outlined in NPPF, must also be applied to each development site. These are not covered in this report. 4.7 NPPF requires that each flooding mechanism is addressed and levels of risk evaluated. We consider there are three main risks of flooding to the site the alternative mechanisms are not applicable to this site Inundation from floodwaters leaving watercourses or rivers entering the site. This can include the effects on culverted watercourses and where the risk of blockage can occur and from breach scenarios Rainwater falling on the site and not being able to leave the site at sufficient rate to prevent flooding on the site Overland flows from adjacent land sites due to surcharging of sewerage systems or other watercourses The impact of the developed site on the existing drainage systems and off-site surface water systems must also be assessed as part of this flood risk assessment. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

6 5.0 Discussion of Flood risks 5.1 Flood Risk from Watercourses, River & Tidal The proposed development area does fall within the 1% probability Flood Risk Maps (Zone 3) as published by the Environment Agency. The site is therefore considered not to be at risk from fluvial flooding for the once in 100 year flood or the once in 1000year flood event. We therefore consider the risk of flooding of the site from River and sea is acceptable for this type of development. 5.2 Risk of Flooding from overland flows from adjacent land The site lies on a very slightly sloping site with residential development to the north,. There is open fields to the south, east and west but these are at a lower level than the site and any overland flows would be away from the site. The surrounding area to the north is all served by public drainage systems and as such the level of risk of flooding from surcharged sewers or drains is considered to be less than 3%. There is always the risk of these systems blocking or being surcharged in extreme events. We would therefore recommend that an overland flood route is provided through the site to cater for such events as is normal under the sewers for adoption criteria. 5.3 Risk of Flooding from Rainwater Falling on Site The risk of flooding from water falling on site and not being able to leave the site is relatively high. The impermeable area of the site will increase due to the development and this would increase the run off from the site. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

7 5.3.2 The normal hierarchy for surface water discharge is primarily the use of infiltration systems, in accordance with current policies of the EA and East Riding of Yorkshire MDC. There has been a preliminary site investigation for this site which indicates there are saturated sands beneath the upper clays on the site and that infiltration systems would not be suitable for the whole of the site. The possibility that a suitable geological formation to allow soakaways for the site may exist is considered to be very low If infiltration systems are utilised then the risk of overland flows from the site are considered to be very low and as such the risk of flooding on site is considered to be acceptable If Infiltration systems are not feasible then storm water attenuation systems should be utilised to ensure the flows from the site to suitable watercourse or surface water sewer are reduced to agricultural run rates to avoid increasing the flood risk off site. These can take the form of open swales, ponds, or detention basins but these do tend to take a substantial area of developable land. The systems can also utilise underground tanks and over-sized pipes to store stormwater before discharge at agricultural rates to a suitable outfall These system would all have to limit flows down to an acceptable level. At present there is a substantial area of permeable surfaces on the site and the majority of the site is greenfield. The discharge rate for the site would have to reduce to agricultural rates of discharge. The provision of open space on the site would probably allow the use of open swales and pons for such a purpose subject to suitable adoption arrangements. If these cannot be achieved then a storage facility would have to be provided. This could be in the form of a concrete tank or a series of oversized pipes. These may not provide a suitable level of treatment of the run off from the site and biological system at source may be needed to ensure suitable discharge contaminants are dealt with. The use of open swales and ponds would allow the use of reed beds and other organic systems to be employed so should be considered in the final designs. Primary treatment for the roads would be the use of trapped gullies for all hard standings. Car-park areas over 12 number cars may require the use of an interceptor to remove contaminants. The use of filter drains adjacent to private drives would also provide a first stage treatment of run off from drives. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

8 5.3.6 The sizes of the storm water storage facilities would need to be determined accurately in the final detailed designs. These should be all in accordance with the National Planning policy Framework Technical guidance issued in March The volumes of storage can include flooding to roads and designated areas such as carpark areas or public open space, but must ensure that no buildings are flooded. The most sustainable and economical way of providing this would be in above surface systems such as detention basins, open swales and ponds but these do take up significant areas of land and can be problematical in terms of future maintenance and adoption by the relevant authorities Utilising the ADAS systems and Windes source control modelling the estimated agricultural discharge rates for the site are 11.9lit/s/ha for the 1 in 2 year storm, 22.3lit/s/ha for the 1in 30 year event and 26.3 lit/s/ha for the 100 year event. At present there are no outfalls from the site other than an existing surface water sewer that crosses the land close to the northern boundary. It is therefore proposed to collect these flows from the site and attenuate these flows to an agreed level and provide storage on site The estimated volumes of storage required for each of the storm profile based on not exceeding the current modelled discharge rate from the site suggests a discharge rate of 2.5 lit/s/ha of new impermeable area of the site. Based on these estimates the volumes of storage required are 916 cu.m for the 1 in 30 year storm, 1217cu.m for the 1 in 100 year storm which rises to 1692cu.m when 30% increase due to climatic effects, is taken into account The proposed discharge point, and discharge rates from the site needs to be agreed with Yorkshire Water and the Local Land Drainage Authority prior to detail design being undertaken. It is possible that an offsite requisition of a surface water sewer may be required which may entail negotiations with riparian land owners adjacent to the nearest water course which is to the south west of the site, in order to secure a right to discharge in perpetuity from the site to a watercourse. This would be required in order to secure a requisitioned sewer. It may be possible to discharge to the Y.W Sewer crossing the site but this needs to be agreed with them. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

9 5.4 Impact on existing drainage systems If the site is drained to soakaways then the impact on the existing drainage systems would be negligible If the site is developed with attenuation systems and an agreed the discharge rate reduced to agricultural rates of discharge, there would be no increase in the flood risk to properties off site or in the drainage networks downstream of the site. In real terms there would be slight reduction in flood risk to adjacent land due to the attenuation provided for the 1 in 100 year storms with the 1 in 2 year discharge rate The site does not lie within an flood warning area but the village of Snaith could be cut off by flood waters in an extreme event and it would be prudent to ensure householders are aware of the possible risk which covers the whole of the Snaith area. 6.0 CONCLUSIONS 6.1 The area of the site to be developed currently falls within Flood Zones 1 as defined by the EA Flood maps. The area of the site to be developed is not at risk of flooding from river or tidal water up to a 1% return period. The flood risk is considered to be acceptable for residential development. 6.2 The development of the site utilising infiltration techniques will not affect the down-stream drainage systems but this is considered to be unlikely. The use of attenuation systems to reduce the run-off from the site to agricultural discharge rates would be required in the final site designs to ensure there is no increase in flood risk to the downstream catchment. 6.3 The risk of overland flows entering the site is considered to be very low due to the topography of the area around the site and the drainage systems to the previously developed areas to the north. The risk can be further minimised by providing a flood water route through the site to ensure flood water flows are directed away from the existing and proposed housing. Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

10 6.4 The floor levels of the proposed house should be a minimum of 200mm above the existing ground level. Trevor Haigh B.Sc., C.Eng., M.I.C.E. Copyright: Haigh Huddleston & Associates Haigh Huddleston Associates is the trading name of Haigh Huddleston Associates Limited Registered Office: Firth Buildings, Leeds Road, Dewsbury, WF12 7BU Registered in England Number

11 APPENDIX A LOCATION PLAN

14 APPENDIX B PROPOSED SITE LAYOUT

16 APPENDIX C FLOOD RISK MAPS

26 APPENDIX D YORKSHIRE WATER DRAINAGE RECORDS

27 7 32 PU 300 VC S PU 300 VC S VC F PU 1200 CO S PU 1200 CO S PU 1200 CO S 375 CO S 8 U 150 VC F PU 300 VC S PU 900 CO S MORE NS PU 225 VC S PU 225 VC S 12 PU 100 SI F RMN 32 El Sub Sta CLOSE BUTT LANE PU 100 U F PU 100 U F 1 PU 225 VC S a 17 PU 225 VC S 22 MILL LANE Medical Centre Path (um) PU 300 U S PU 300 U S PU 225 VC S Top House Farm 3 Littlestone Dia-Mal THE HOLL PU 225 VC S PU 225 VC S 1 21b a PU 150 CI F RMN Metres b 35 21a : Map Name : SE6421NW Yorkshire Water, PO Box 500, Halifax Road, Bradford BD6 2LZ Contact Name : Cropper Contact Tel : Title Notes (Ody) COPYRIGHT STATEMENTS: Reproduced by permission of Ordnance Survey on behalf of HMSO Crown copyright and database All rights reserved Ordnance Survey Licence number Partial Key Foul Sewer = F Combined Sewer = C Surface Water Sewer = SW Trade Sewer = TD Partially Separate = PS Source : Sewer Network Enquiry This plan is furnished as a general guide only and no warranty as to its correctness is given or implied. This plan must not be relied upon in the event of excavations or other works made in the vicinity of public sewers. No house or property connections are shown. Date Req : 23/03/2015, 09:48:54 Date Gen : 23/03/2015, 09:48:55 UPN: Undefined Originator: Cropper,,

28 APPENDIX E STORMWATER STORAGE CALCULATIONS

Rising Sun. Scaffold Hill. Flood Risk Assessment

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