Lidl, Chippenham. Flood Risk Assessment & Drainage Strategy. September 2017

Transcription

1 September 2017

2 DOCUMENT VERIFICATION RECORD CLIENT: SCHEME: INSTRUCTION: Lidl UK GmbH Proposed Lidl Food store at the former Middlefield Adult Training Centre, Chippenham SN14 0JP The instruction to carry out this was received from Ian O Gorman of Lidl UK GmbH DOCUMENT REVIEW & APPROVAL AUTHOR: CHECKER: APPROVER: Ifan Dafydd Jones BSc (Hons) MSc Joshua Rigby BSc (Hons) Victoria Griffin BSc (Hons) MSc MIEnvSc CEnv ISSUE HISTORY ISSUE DATE COMMENTS 16/06/2017 First issue 30/06/2017 Second issue 28/06/2017 Third issue 03/08/2017 Fourth issue 18/09/2017 Fifth issue w fra & Drainage Strategy

3 Contents Introduction... 1 Existing Conditions... 1 Development Proposals... 4 Flood Zone Classification and Policy Context... 4 Consultation... 6 Sources of Flooding and Probability... 7 Surface Water Management Maintenance Foul Drainage Water Efficiency Conclusions Recommendations Appendices Appendix A Location Plan & Aerial Image Appendix B Development Plan & Topographical Data Appendix C Infiltration Tests Appendix D Sewer Plan and Correspondence Appendix E Environment Agency Flood Maps Appendix F SFRA / PRFA Maps Appendix G Runoff Rates and MicroDrainage Storage Volumes Appendix H Concept Drainage Sketch Appendix I Maintenance Schedules w fra & Drainage Strategy

4 Tables Table 1 Existing Runoff Rates Table 2 Pollution Hazard Indices Table 3 SuDS Mitigation Indices w fra & Drainage Strategy

5 Introduction Waterco Consultants have been commissioned to undertake a Flood Risk Assessment and Drainage Strategy in relation to a proposed commercial redevelopment at the former Middlefield Adult Training Centre, Chippenham, SN14 0JP. The purpose of this report is to outline the potential flood risk to the site, the impact of the proposed redevelopment on flood risk elsewhere, and the proposed measures which could be incorporated to mitigate the identified flood risk. This report has been prepared in accordance with National Planning Practice Guidance (NPPG). From April 2015, Wiltshire Council as a Lead Local Flood Authority (LLFA) is a statutory consultee for major planning applications in relation to surface water drainage, requiring that all planning applications are accompanied by a Sustainable Drainage Strategy. The aim of the Sustainable Drainage Strategy is to identify water management measures, including Sustainable Drainage Systems (SuDS), to provide surface water runoff reduction and treatment. Existing Conditions The 1.14ha development site is located at National Grid reference: E N. A location plan and an aerial image are included in Appendix A. The site comprises the former Middlefield Adult Training Facility Centre which has recently been demolished. The site is bordered by Bristol Road to the north, a grassed area with trees and Hungerdown Lane to the east, private gardens and residential properties to the south, the demolished Middlefield Adult Training Facility Centre to the south-west, and undeveloped greenfield land to the north-west. Access to the site is provided from Allington Way by a private road south of the site. The site currently comprises approximately 4,000m 2 of hardstanding occupying approximately 35% of the site area in the form of the remnant concrete slabs of the demolished Middlefield Adult Training Centre, car parking and access road. The remaining 7,400m² (approximately 65%) is permeable in the form of peripheral soft landscaping. w fra & Drainage Strategy 1

6 Local Topography A Topographical Survey has been undertaken by EDI Surveys in December 2016 and is included in Appendix B. The Topographical Survey shows that the site slopes from 71.11metres Above Ordnance Datum (m AOD) in the north-west to 69.60m AOD in the south-east. A number of rubble spoil heaps are identified on the topographic survey. Topographic levels to m AOD have been derived from a 1m resolution Environment Agency (EA) composite Light Detecting and Ranging (LiDAR) Digital Terrain Model (DTM). The LiDAR plan (Appendix B) indicates that the topography of the wider area falls to the east. Ground Conditions Reference to the British Geological Survey online mapping (1:50,000 scale) indicates that the site is underlain by the Cornbrash Formation consisting of Limestone. No superficial deposits have been identified. Infiltration testing has been undertaken by Geotechnical Engineering Limited, the Trial Pit Logs and infiltration testing results are included in Appendix C. The intrusive works were carried out on the 8 th June 2015, and comprised four pits, excavated to depths of between 1.5 meters and 2 meters below ground level (m.bgl). The investigations observed strata consistent with those predicted by the British Geological Survey online mapping. The trial pits indicate that the superficial deposits at the site generally comprise soft Sands, Gravels and Clays, underlain by bedrock consisting of weathered Limestone. A soakaway test was carried out in accordance with BRE365 (2007). However, over a period of 250 minutes, there was an insufficient fall in water level to calculate infiltration rates. Seepage was recorded between 1.5m.bgl and 1.8m.bgl. According to the EA s online Groundwater Vulnerability Mapping the Cornbrash Formation is classified as a Secondary A Aquifer defined as permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. These are generally aquifers formerly classified as minor aquifers. The Environment Agency s online Groundwater Source Protection Zones map indicates that the site is located within Outer Zone (2C) defined by a 400 day travel time from a point below the water table. w fra & Drainage Strategy 2

7 The National Soil Resources Institute Soilscapes online mapping describes the underlying soil on the site as Shallow lime-rich soils over chalk or limestone. Local Drainage Public sewer records obtained from Wessex Water included in Appendix D, show that there are public sewers located within the site boundary and immediately adjacent to the site. There is a 375mm public surface water sewer located within the northern boundary of the site, manhole 5103 has a cover level of 69.96m AOD and in invert level of 68.40m AOD. The sewer flows south-east before converging with a 225mm public surface water sewer at manhole 6100, which flows north within Hungerdown Lane and upsizes to 820 x 200mm north-east of the site. A further public surface water sewer, which flows east within Bristol Road, north of the site, converges with the aforementioned sewer within Bristol Road at manhole There is a 225mm public foul water sewer located within the northern boundary of the site which flows south-east. Manhole 5104 has a cover level of 69.97m AOD and an invert level of 67.17m AOD. There is a 225mm public foul water sewer located within Hungerdown Lane which flows north, before converging with the 225mm public foul sewer aforementioned which flows east. There is a public foul water sewer south of the site, within the gardens of the neighbouring residential properties which borders the site. The sewer flows east; however it is unclear where the sewer discharges to. Records indicate that there is a 375mm highway drain which crosses the south-eastern boundary of the site before flowing north on Hungerdown Lane. It is not clear where the drain discharges to. The Topographic Survey (Appendix B) identifies the existing surface water drainage features on site. The drawing indicates that surface water from the site appears to discharge into the public surface water sewer system unrestricted via a 150mm private surface water sewer. It is assumed that the private surface water sewer discharges into the 300mm public surface water sewer within Hungerdown Lane. Invert levels have been provided for several of the manholes on site, the furthest downstream manhole has a cover level of 69.76m AOD and an invert level of 67.54m AOD. The manhole is located within the south-eastern extent of the site. The drawing also indicates that foul flows from the site appear to discharge into the public foul water sewer system unrestricted via a 150mm private foul water sewer. It appears that the private foul sewer discharges into the 225mm public foul water sewer within Hungerdown Lane. w fra & Drainage Strategy 3

8 Development Proposals The proposed development is for the construction of a Lidl food store with associated car parking, access road and peripheral landscaping. Development plans are included in Appendix B. Access to the site will be provided by a newly formed access from Hungerdown Lane, east of the site. The proposed redevelopment will result in an increase in hardstanding area on the site. Hardstanding will occupy approximately 9,488m² or 84% of the total site area. The remaining permeable soft landscaped areas will occupy 1,812m² or 16% of the total site area. Flood Zone Classification and Policy Context The Environment Agency (EA) Flood Map for Planning Rivers and Sea included in Appendix E shows that the site is located within Flood Zone 1 - an area outside of the extreme flood extent, considered to have a less than 0.1% annual probability of flooding from rivers or the sea. In accordance with Table 2 of the NPPG: Flood Risk and Coastal Change, commercial development is classified as less vulnerable. Table 3 of the NPPG states that less vulnerable development is considered appropriate within Flood Zone 1. The development therefore passes the flood risk Sequential Test and the Exception Test do not need to be applied. The Wiltshire Core Strategy Development Plan Document ( the plan ) was formally adopted on the 20 th of January The plan provides a positive and flexible overarching planning policy framework for Wiltshire for the period up to The Core Strategy Development Plan includes the following policies relating to flood risk and drainage; Core Policy 67 - Flood Risk Development proposed in Flood Zones 2 and 3 as identified within the Strategic Flood Risk Assessment will need to refer to the Strategic Housing Land Availability Assessment when providing evidence to the local planning authority in order to apply the sequential test in line with the requirements of national policy and established best practice. All new development will include measures to reduce the rate of rainwater run-off and improve rainwater infiltration to soil and ground (sustainable urban drainage) unless site or environmental conditions make these measures unsuitable. w fra & Drainage Strategy 4

9 Core Policy 68 - Water resources Development must not prejudice the delivery of the actions and targets of the relevant River Basin or Catchment Management Plan, and should contribute towards their delivery where possible. Non-residential development will be required to incorporate water efficiency measures. Developers will be expected to submit details of how water efficiency has been taken into account during the design of proposals. Development proposals within a Source Protection Zone, Safeguard Zone or Water Protection Zone must assess any risk to groundwater resources and groundwater quality and demonstrate that these would be protected throughout the construction and operational phases of development. The West Wiltshire District Plan 1st Alteration (2004) is the current local development plan for the west Wiltshire area of the county. In 2004, the Planning and Compulsory Purchase Act introduced a new system of plan making. The old system of county Structure Plans and district Local Plans was replaced with the new Local Development Framework process. Under transitional arrangements, implemented in September 2004, existing policies within Structure and Local Plans were to be "saved" (i.e. kept) for a period of at least 3 years. The following policy is the only saved policy relating to flood risk and drainage; U1a - Foul Water Disposal Development will only be permitted where adequate foul drainage, sewerage and sewage treatment facilities are available or where suitable arrangements are made for their provision. In sewered areas new development will be expected to connect to mains drainage. New sewers will be expected to be constructed to a standard adoptable by Wessex Water. The Wiltshire Core Strategy Development Plan Document ( the plan ) was formally adopted on 20th January The Wiltshire Core Strategy replaces the South Wiltshire Core Strategy as well as a number of policies from the former district council's local plans. Core Policy 67 - Flood Risk Development proposed in Flood Zones 2 and 3 as identified within the Strategic Flood Risk Assessment will need to refer to the Strategic Housing Land Availability Assessment when providing evidence to the local planning authority in order to apply the sequential test in line with the requirements of national policy and established best practice. w fra & Drainage Strategy 5

10 All new development will include measures to reduce the rate of rainwater run-off and improve rainwater infiltration to soil and ground (sustainable urban drainage) unless site or environmental conditions make these measures unsuitable. Consultation Wiltshire County Council were contacted in May 2017 to request comments on flood risk and surface water drainage on the site. In their response (Appendix D), the council state that; Infiltration is unlikely to work area is served by public storm sewer; We would expect to see details of existing disposal methods and flow rates; As government guidelines brownfield sites should aspire to greenfield run off rates where ever possible; Likely that WW would seek you proving existing connections and require no increase in flow rate; Council will look at 20% reduction in flow rate if greenfield cannot be achieved; 40% climate change to be used; Any submission will need to detail attenuation, ownership and maintenance regime; and Mapping shows site lies in an area affected by surface water flood risk for 1 in 30/100 event. Wessex Water were contacted in May 2017 to request comments on flood risk and surface water drainage on the site. In their response (Appendix D), Wessex Water stated: Proposed Surface Water comments Surface water to be discharged to local land drainage systems with agreed flood risk measures approved by the Lead Local Flood Authority. Surface water connections to the public foul sewer system will not be permitted, as surface water discharges to the public sewer will lead to sewer flooding. A more suitable approach would be to discuss any run off rates & any necessary Flood risk measures with the Lead Local Flood authority. Once these rates have been agreed, we will then be able to determine surface water capacity for the proposed flow rates. This should be based on the size of the site, impervious area etc. w fra & Drainage Strategy 6

11 If surface water discharge is required to the existing public surface water sewer, the applicant will need to prove no increase in existing positively connected flows (30 % betterment for climate change). The LLFA may also have further requirements under flood risk measures and SuDs hierarchy. 1. Existing site possibly drained to our existing 375 mm SWS; therefore, we would want to see that the new surface water discharge rates over all critical return events no greater than the existing runoff based on the existing hardstanding area. I.e. - the developer calculates existing discharge rate for the 1, 2, 5, 10, 30 and 100yr + 30 % CC in accordance with any planning stipulations and NPPF framework and matches these rates on the same events on the proposed SW flows and provided the required volume of storage behind the control. I.e. no increase on SW flows over the existing rates discharging into our sewer. 2. Obvious point of connection is to our existing 375 mm diameter SWS that runs through the North of site. Developer will probably need to go over our existing FWS with it being deeper than the SW with no determent if they were to connect at this location in accordance with the above. 3. A sterilisation zone will be implemented and adhered to of 3.0 meters from our existing sewers to the North with no new dwellings closer than this. As previously advised, the site will have had connections to our networks, which can be re-used as appropriate or sealed at the point of connection with agreement required from Wessex Water. Local guidance documents including the Wiltshire Council Level 1 Strategic Flood Risk Assessment (SFRA) Update (July 2013), the West Wiltshire District Council Level 1 SFRA (August 2008), the Wiltshire Council Surface Water Management Plan (SWMP) (November 2011) and the Wiltshire Council Flood Risk and Surface Water Management Interim Report (December 2014) have been reviewed to inform this report. Sources of Flooding and Probability Fluvial The nearest watercourse is the Hardenhuish Brook which is located approximately 100m north of the site and flows east in this location. There is an unnamed land drain located approximately 211m west of the site. w fra & Drainage Strategy 7

12 Fluvial flooding could occur if the Hardenhuish Brook overtopped its banks during or following an extreme rainfall event. The EA Historic Flood Map (Appendix E) indicates that historic flooding was recorded approximately 290m east of the side, which was associated with Hardenhuish Brook. The map also identifies another recorded flood event located approximately 470m south of the site, which was associated with Ladyfield Brook. The SFRA North Wiltshire Other Sources of Flooding Map (Appendix F) indicates that there have been no recorded fluvial flooding records at or near the site. A review of the LiDAR data indicates that the site is approximately 7m above Hardenhuish Brook, and is therefore not considered at flood risk from this watercourse. The site is located within Flood Zone 1 on the EA Flood Map for Planning Rivers and Sea an area outside of the extreme flood extent, considered to have a less than 0.1% annual probability of flooding from rivers or the sea. It can therefore be concluded that the site is at low risk of fluvial flooding. Tidal The site is situated at a minimum of approximately 69.6m AOD, and is significantly above sea level. Therefore, there is no risk from tidal flooding. Surface Water Surface water flooding occurs when rainwater does not drain away through the normal drainage system or soak into the ground. It is usually associated with high intensity rainfall events, but can also occur with lower intensity rainfall or melting snow where the ground is saturated, frozen or developed, resulting in overland flow and ponding in depressions in topography. Surface water flooding can occur anywhere without warning. However, flow paths can be determined by consideration of contours and relative levels. The EA Risk of Flooding from Surface Water map (Appendix E) indicates that the majority of the site is at very low risk of surface water flooding meaning it has a less than 0.1% annual probability of flooding. A proportion of the northern extent of the site is shown at low risk of surface water flooding, meaning it has between a 1% and 0.1% annual probability of flooding. The access road and car park is shown at high risk of surface water flooding, meaning it has a greater than 3.3% annual probability of flooding. There appears to be a distinctive flow route generating on-site, flowing from the central to the south-eastern section of the site. w fra & Drainage Strategy 8

13 The SFRA North Wiltshire Other Sources of Flooding Map (Appendix F) indicates that there are two recorded flooding incidents classified as Highways Historic Flooding located within Hungerdown Lane, east of the site. Surface water generated on site would be accommodated within the proposed surface water drainage system discussed below. Any potential surface water flooding arising at or near to the site would be directed north and then east, away from the site, following the local topography. It can therefore be concluded that the site is at moderate risk of surface water flooding. Sewer Flooding Flooding from sewers can occur when a sewer is overwhelmed by heavy rainfall, becomes blocked, is damaged, or is of inadequate capacity. Flooding is mostly applicable to combined and surface water sewers. The SFRA Properties at Risk North Wiltshire (Appendix F) indicates that there have been no recorded sewer flooding incidents at or near the site. Any potential flooding arising from the sewer along the northern boundary would be directed south-east onto Hungerdown Lane away from the site, following the local topography. It can therefore be concluded that the risk of sewer flooding is low. Groundwater Flooding Groundwater flooding occurs when water levels beneath the ground rise above normal levels. Prolonged heavy rainfall soaks into the ground and can cause the ground to become saturated. This results in rising groundwater levels which leads to flooding above ground. The Areas Susceptible to Groundwater Flooding Map (Appendix F) obtained from the Wiltshire Council Flood Risk and Surface Water Management Interim Report indicates that the site is classified as being <25% susceptible to groundwater flooding. No records of groundwater flooding are located within the vicinity of the site. It can therefore be concluded that the risk of groundwater flooding is low. Artificial Sources of Flooding There are no canals within the vicinity of the site. The EA Risk of Flooding from Reservoirs map (Appendix E) shows that the site is not at risk of flooding from reservoirs. w fra & Drainage Strategy 9

14 It can therefore be concluded that there is very low of flooding from artificial sources. Summary of Potential Flooding It can be concluded that the site is at low risk of flooding, with the exception of surface water flooding. The majority of the site is at very low risk from surface water flooding (<0.1%), however there is a distinctive flow route flowing from the central to the south-eastern section of the site; categorised as high flood risk (3.3% or greater). The finished floor level of the proposed building should be set a minimum of 150mm above surrounding ground levels in accordance with Building Regulations. Surface Water Management The proposed redevelopment is for a Lidl food store with associated car parking, access road and peripheral landscaping. The proposed food store will result in an increase in hardstanding area on the site. Hardstanding will occupy approximately 9,488m² or 84% of the total site area in comparison to approximately 4,000m² or 35% of hardstanding pre-development. Discharge Rate Greenfield As stated in the Council Correspondence (Appendix D), in the first instance, developers should aim to achieve a greenfield discharge rate. Existing greenfield runoff rates have been estimated using the Revitalised Flood Hydrograph Model (ReFH2) method, provided as Appendix G. The existing 1 in 2 year storm event (QMED equivalent) greenfield rate for the 11,400m² development site is 5.5 l/s. Discharge Rate Brownfield Brownfield runoff rates from the site have been estimated using the Revitalised Flood Hydrograph Model (ReFH2) method (see Appendix G) and are provided in Table 1 overleaf: w fra & Drainage Strategy 10

15 Table 1 Existing Runoff Rates Return Period (yr) Runoff Rate (l/s) 1 in in in The existing runoff rate for the 1 in 2 year event is 9.26 l/s. In accordance with Wiltshire County Council requirements, discharge rates will be limited to the existing runoff rate for a 1 in 2 year rate plus 20% betterment. This rate equates to 7.4 l/s. Attenuation Storage Greenfield Discharge Rate In order to achieve a discharge rate of 5.5 l/s, attenuation storage will be required. A storage estimate is included in Appendix G. An estimated storage volume of 312m³ will be required to accommodate the 1 in 30 year event. An estimated storage volume of 727m 3 will be required to accommodate the 1 in 100 year plus 40% Climate Change (CC) event. The storage estimates are based on storage within a tank or pond structure, an impermeable drainage area of 9,488m 2, a design head of 1m and hydro-brake flow control. Due to the large volume of attenuation (727m³) required to achieve the greenfield rate of 5.5 l/s, it is not considered feasible for the site. A tank of this size would accommodate a large percentage of the car park area. Therefore, Attenuation storage for the Brownfield rate with 20% betterment equating to 7.4 l/s should be considered for the site. Attenuation Storage Brownfield Discharge Rate In order to achieve a discharge rate of 7.4 l/s, attenuation storage will be required. A storage estimate is included in Appendix G. An estimated storage volume of 275m³ will be required to accommodate the 1 in 30 year event. An estimated storage volume of 657m 3 will be required to accommodate the 1 in 100 year plus 40% Climate Change (CC) event. The storage estimates are based on storage within a tank or pond structure, an impermeable drainage area of 9,488m 2, a design head of 1m and hydro-brake flow control. w fra & Drainage Strategy 11

16 Discharge Method Paragraph 080 of the NPPG: Flood Risk and Coastal Change sets out the following hierarchy of drainage options: into the ground (infiltration); to a surface water body; to a surface water sewer, highway drain or another drainage system; to a combined sewer. Infiltration The first consideration for the disposal of surface water is infiltration (soakaways and permeable surfaces). During the site investigation, the site was shown to be underlain by superficial deposits comprising a mixture of soft Sands, Clays and Gravels over weathered Limestone. The LLFA s drainage officer (Peter Weston) stated within the Council s correspondence (Appendix D) that Infiltration is unlikely to work. Infiltration tests have been undertaken in accordance with the BRE365 specification to determine the suitability of soakaways (Appendix C). The results of the infiltration test indicated that over a period of 250 minutes, there was an insufficient fall in water level to calculate infiltration rates. The use of infiltration as a method of surface water disposal is therefore not considered feasible for the site. Watercourse Where soakaways are not suitable a connection to watercourse is the next consideration. The nearest watercourse is Hardenhuish Brook which is located approximately 100m north of the site. The site is separated from the watercourse by third party land therefore a direct connection is not considered to be an appropriate discharge methodology for the site. Sewer Where disposal of surface water to watercourse is not possible, a connection to the public sewer system is the final consideration. As discussed, the site is currently drained by a 150mm private surface water sewer at an unrestricted rate. It is assumed that this sewer discharges into the 300mm public surface water sewer within Hungerdown Lane, east of the site. A connection to this sewer appears to be a feasible option, utilising the existing connection. The invert level of the private surface water sewer, within the south-eastern extent of the site is 67.64m AOD and as such a gravity connection should be achievable. As agreed with Wiltshire County Council, surface water discharge should be limited to the brownfield rate with 20% betterment of equating to 7.4 l/s. w fra & Drainage Strategy 12

17 Sustainable Drainage Systems Attenuation storage should be provided in the form of Sustainable Drainage Systems (SuDS) where practical. The following SuDS options have been considered: Soakaways As described above, the results of the infiltration test indicate that soakaways will not be a feasible method of surface water disposal for the site. Swales, detention basins and ponds The proposed building, car park and access roads take up the majority of the site area. As such there is insufficient space to incorporate a swale, detention basin or pond on site. Furthermore, an open water attenuation features such as a pond or a swale in an urban area presents a significant safety risk. Rainwater Harvesting The attenuation benefits provided through the use of rainwater harvesting are considered to be limited, and would only be realised when the tanks were not full. However, rainwater harvesting techniques could be incorporated within the final design. Green Roofs Green roofs are not identified on development plans. Given the nature of the proposed development, the significant additional cost involved in installing and maintaining green roofs and the additional works required to allow for the additional loading on the building, green roofs are not considered a practical option. The benefits achieved through installing a green roof would be disproportionate to the significant ongoing maintenance and construction costs involved. Porous / Permeable Paving The internal central access road leading to the southern extent of the building will be trafficked with frequent HGV movements. The surfacing material required to support such HGV activity is not considered suitable for accommodating the use of permeable / porous surfacing. Restricting porous paving to the car parking bays provides a potential porous paved area of approximately 2,552m 2. Based on a sub-grade depth of 0.3m with a void ratio of 30%, there is potential to accommodate up to 223m 3 of attenuation within the permeable paving sub-grade. Further investigation into the use of porous paving will be undertaken at the detailed design stage. w fra & Drainage Strategy 13

18 Underground Attenuation Tanks Storage could be provided within an underground attenuation tank or oversized pipes. Sufficient space for an underground tank or oversized pipes is provided within the car parking area. Concept Surface Water Drainage Scheme Surface water runoff generated from the site will be discharged to the 300mm public surface water sewer within Hungerdown Lane, utilising the existing connection via the 150mm private surface water sewer on site at a rate of 7.4 l/s. A total attenuation volume of 657m³ is required to accommodate the 1 in 100 plus 40% CC event. There is potential to provide the full balance of the required surface water attenuation within an underground attenuation tank (modular storage) measuring 63m x 11m x 1m deep within the car parking area in the northern extent of the site. A tank of this size including a 95% void ratio provides a total attenuation of m³. A plan indicating the potential location of the underground 1m deep tank is included in Appendix H. The plan illustrates a sketch proposal only and as such is subject to detailed site investigation. Permeable paving could be incorporated within the access road and car parking areas within the development. The proposed surface water drainage scheme will ensure no increase in runoff over the lifetime of the development and will create betterment over the existing situation. Exceedance Event Storage will be provided for the 1 in 100 year plus 40% CC event. Storm events in excess of the 1 in 100 year plus 40% CC event should be permitted to produce temporary shallow depth flooding within the car park / access road. Finished floor levels will be set at a minimum of 150mm above surrounding ground levels ensuring exceedance flooding will not affect the buildings. w fra & Drainage Strategy 14

19 Surface Water Treatment In accordance with the CIRIA C753 publication The SuDS Manual (2015), other roofs (applicable to commercial premises) have a low pollution hazard level. Commercial yards, delivery areas, all roads except low traffic roads and non-residential parking have a medium pollution hazard level. Table 2 below shows the pollution hazard indices for each land use. Table 2 Pollution Hazard Indices Land Use Pollution Hazard Level Total Suspended Solids (TSS) Metals Hydrocarbons Other roofs Low 0.3* Commercial yard and delivery areas, nonresidential car parking with frequent change (e.g. retail), all roads except low traffic roads Medium Table extract taken from the CIRIA C753 publication The SuDS Manual Table 26.2 * Indices values range from 0-1. Where practical, runoff from roofs and roads will be directed to permeable paving or an underground tank located within the car parking area. Table 3 below demonstrates that permeable paving provides sufficient treatment. Table 3 SuDS Mitigation Indices Mitigation Indices Type of SuDS Permeable Pavement Total Suspended Solids (TSS) Metals Hydrocarbons Table extract taken from the CIRIA C753 publication The SuDS Manual Table 26.3 It can be concluded that the inclusion of permeable paving will provide sufficient treatment. Where attenuation is provided in a below ground system (tank storage), treatment will need to be provided by a suitably sized separator. w fra & Drainage Strategy 15

20 Maintenance Maintenance of drainage features such as an attenuation tank accommodating runoff during storm events up to and including the 1 in 100 year plus 40% climate change event will be the responsibility of the site owner. Maintenance schedules for an attenuation tank and permeable paving are included in Appendix I. Foul Drainage Foul flows from the development should continue to discharge via the existing 150mm diameter foul sewer south of the site. Given this is an existing sewer a gravity connection can be achieved. Water Efficiency Lidl are committed to constructing stores that have a minimal impact on their environment. The major water consuming appliances in the building will be taps, water closets and urinals. Water consumption is carefully monitored and on average is limited to 13 cubic metres per month, and therefore approximately 156 cubic metres per year. The store will be fitted with automated tap controls and a reduced water urinal system that uses 90% less water than the standard system. The following additional sustainable water systems will be considered for inclusion within the development and will be incorporated should they be feasible; Flow restrictors; Spray taps; Dual flush WCs; Eco showerheads; Low water use washing machines and dishwashers; Leak detection. w fra & Drainage Strategy 16

21 Conclusions The proposal is for the redevelopment of a Lidl food store at the former Middlefield Adult Training Centre. The site is located within Flood Zone 1 on the Environment Agency (EA) Flood Map for Planning (Rivers and Sea) an area considered to have the lowest probability of fluvial flooding. The site is shown to be located outside of the extreme 0.1% annual probability flood extent. The risk of flooding from all sources has been assessed and the flood risk to the site is considered to be low, while the main potential source of flooding to the site is surface water flooding. The identified flood risk will be mitigated by raising ground floor levels 150mm above surrounding ground levels. The proposed redevelopment will introduce impermeable drainage area in the form of the Lidl store access road and car parking areas. This will result in an increase in surface water runoff. In order to ensure the increase in surface water runoff will not increase flood risk elsewhere, flow control will be used and attenuation provided on site to accommodate storm events up to and including the 1 in 100 year plus 40% climate change event. All methods of surface water discharge have been assessed. Where soakaways are not possible, discharge of surface water will be discharged to the 300mm public surface water sewer within Hungerdown Lane, utilising the existing connection via the 150mm private surface water sewer on site at a rate of 7.4 l/s. Attenuation can be provided within the sub-grade of permeable paving or in the form of am attenuation tank located within the car parking areas of the development. A gravity connection appears feasible. The site owner will be responsible for the maintenance of the surface water drainage system. Foul flows from the development should continue to discharge via the existing 150mm diameter foul sewer south of the site. w fra & Drainage Strategy 17

22 Recommendations 1. Submit this Flood Risk Assessment and Drainage Strategy to the Planning Authority in support of the Planning Application; 2. Verify the attenuation volumes included in this report when undertaking detailed drainage design. w fra & Drainage Strategy 18

23 Appendix A Location Plan & Aerial Image w fra & Drainage Strategy

24 ± NOTES: 1) ALL DIMENSIONS ARE IN METRES AND ALL LEVELS IN METRES ABOVE ORDNANCE DATUM UNLESS STATED OTHERWISE LEGEND Site Boundary Watercourses / Water Bodies CLIENT: LIDL UK GMBH SCHEME: LIDL, CHIPPENHAM PLOT TITLE: LOCATION PLAN PLOT STATUS: FINAL DATE: 16/05/2017 DRAWN: CHECKED: APPROVED: PLOT A3: m IJ PLOT NAME: JR VG w10375-location_plan 1:5,000 (UNLESS STATED OTHERWISE) REV: - CONTAINS OS DATA CROWN COPYRIGHT (2017)

25 ± NOTES: 1) ALL DIMENSIONS ARE IN METRES AND ALL LEVELS IN METRES ABOVE ORDNANCE DATUM UNLESS STATED OTHERWISE LEGEND Site Boundary CLIENT: LIDL UK GMBH SCHEME: LIDL, CHIPPENHAM PLOT TITLE: AERIAL PLAN PLOT STATUS: FINAL DATE: 16/05/2017 DRAWN: CHECKED: APPROVED: PLOT A3: m IJ PLOT NAME: JR VG w10375-aerial_plan 1:5,000 (UNLESS STATED OTHERWISE) REV: - CONTAINS OS DATA CROWN COPYRIGHT (2017) BASEMAP: WORLD IMAGERY. SOURCES: ESRI, DIGITALGLOBE, GEOEYE, I-CUBED, EARTHSTAR GEOGRAPHICS, CNES/AIRBUS DS, USDA, USGS, AEX, GETMAPPING, AEROGRID, IGN, IGP, SWISSTOPO, GIS USER COMMUNITY

26 Appendix B Development Plan & Topographical Data w fra & Drainage Strategy

27

28 NORTH THE SURVEY ASSOCIATION ISO 9001

29 NORTH THE SURVEY ASSOCIATION ISO 9001

30 ± 66 NOTES: 1) ALL DIMENSIONS ARE IN METRES AND ALL LEVELS IN METRES ABOVE ORDNANCE DATUM UNLESS STATED OTHERWISE LEGEND 72 E Site Boundary Site Levels Ground Elevation (m AOD) < m m > m m m CLIENT: LIDL UK GMBH m 68 SCHEME: LIDL, CHIPPENHAM PLOT TITLE: LIDAR ELEVATIONS 1m RESOLUTION 70 PLOT STATUS: FINAL DATE: 16/05/ m DRAWN: IJ PLOT NAME: CHECKED: APPROVED: JR VG w10375-lidar_plan PLOT A3: 1:1,500 (UNLESS STATED OTHERWISE) REV: - CONTAINS OS DATA CROWN COPYRIGHT (2017) ENVIRONMENT AGENCY COPYRIGHT AND/OR DATABASE RIGHT (2017). ALL RIGHTS RESERVED.