FORMER SHEEP PEN MILL LANE WELSHPOOL FLOOD CONSEQUENCES ASSESSMENT LIDL UK GMBH FOR. September Our Ref: RCEF R

Transcription

1 FORMER SHEEP PEN MILL LANE WELSHPOOL FLOOD CONSEQUENCES ASSESSMENT FOR LIDL UK GMBH Our Ref: RCEF R RPS Health, Safety & Environment Unit 12 Watersedge Business Park Modwen Rd, Salford Quays M5 3EZ Tel: rpsgroup.com

2 Report Status: Project Reference: Final RCEF32271 Name: Signature: Report Author: Technical Reviewer: Josh Rigby Environmental Consultant Neil Bagley Technical Director Date: This report has been prepared in the RPS Group Quality Management System to British Standard EN ISO 9001:2008 RPS Health, Safety & Environment is part of the RPS Group Plc with around 5,000 staff based at over 85 offices located throughout the UK, Ireland and the Netherlands and in the USA, Canada, the Russian Federation, Australia, Malaysia, Singapore and Abu Dhabi. RPS offers an unparalleled range of commercially focused services relating to property and land due-diligence, site development and geoenvironmental investigations (including liability reviews, planning feasibility, EIAs and flood risk, energy & sustainability assessments). rpsgroup.com

3 RPS HEALTH, SAFETY & ENVIRONMENT General Notes 1. The following notes should be read in conjunction with the report: 2. This report contains only that available factual data for the site, which was obtained from the sources, described in the text. These data were related to the site on the basis of the location information made available to RPS by the client. 3. The assessment of the site is based on information supplied by the client. Relevant information was also obtained from other sources. 4. The report reflects both the information provided to RPS in documents made available for review and the results of observations and consultations by RPS staff. 5. Where data have been supplied by the client or other sources, including that from previous site audits or investigations, it has been assumed that the information is correct but no warranty is given to that effect. While reasonable care and skill has been applied in review of this data no responsibility can be accepted by RPS for inaccuracies in the data supplied. 6. This report is prepared and written in the context of the proposals stated in the introduction to this report and its contents should not be used out of context. Furthermore new information, changed practices and changes in legislation may necessitate revised interpretation of the report after its original submission. 7. The copyright in the written materials shall remain the property of the RPS Company but with a royaltyfree perpetual licence to the client deemed to be granted on payment in full to the RPS Company by the client of the outstanding amounts. 8. This report contains Natural Resources Wales information Natural Resources Wales and database right. rpsgroup.com

4 CONTENTS PAGE 1 INTRODUCTION PLANNING POLICY CONTEXT CONSULTATION SITE DESCRIPTION PROPOSED DEVELOPMENT HYDROLOGICAL SETTING HYDROGEOLOGICAL SETTING EXISTING DRAINAGE / WATER MAINS FLOOD RISK AND MITIGATION SURFACE WATER MANAGEMENT SEQUENTIAL TEST AND EXCEPTION TEST SUMMARY AND CONCLUSIONS FIGURES 1 SITE LOCATION PLAN 2 DEVELOPMENT ADVICE MAP 3 NATURAL RESOURCES WALES FLOOD MAP APPENDICES A B C D E F NATURAL RESOURCES WALES CONSULTATION RESPONSE POWYS COUNTY COUNCIL CONSULTATION RESPONSE SEVERN TRENT WATER SEWER PLANS TOPOGRAPHIC SURVEY DEVELOPMENT PLAN MICRO-DRAINAGE WINDES MODELLING ASSESSMENT rpsgroup.com

5 1 INTRODUCTION 1.1 RPS was commissioned by Lidl to undertake a Flood Consequences Assessment of former sheep pen off Mill Lane in Welshpool in relation to the proposed development of a Lidl food store, along with car parking, access road, and associated soft landscaping. 1.2 The aim of the Flood Consequences Assessment is to outline the potential for the site to be impacted by flooding, the impacts of the proposed development on flooding in the vicinity of the site, and the proposed measures which could be incorporated into the development to mitigate the identified risk. The report has been produced in accordance with the guidance detailed in Planning Policy Wales (PPW) and Technical Advice Note 15 (TAN 15): Development and Flood Risk. Reference has also been made to the CIRIA SuDS manual (C697), BRE Digest 365 Soakaway Design, the Powys County Council (PCC) Strategic Flood Consequences Assessment (SFCA), the PCC Flooding Project Appraisal Report (FPAR), the Preliminary Flood Risk Assessment (PFRA) and the River Severn Catchment Flood Management Plan. 1.3 This report has been produced in consultation with Natural Resources Wales (NRW) (formerly Environment Agency Wales) and Powys County Council. 1.4 This report is not intended to provide formal details of the final drainage design for the development. It provides information regarding the capabilities of the conceptual surface water drainage strategy to meet the requirements of TAN The desk study was undertaken by reference to information provided / published by the following bodies: Natural Resources Wales (NRW) Powys County Council British Geological Survey Ordnance Survey Severn Trent Water 1.6 A site walkover was conducted on the Monday 8th September. RCEF R Mill Lane, Welshpool 5

6 2 PLANNING POLICY CONTEXT Regional Planning Policy 2.1 Planning Policy Wales sets out the land use planning policies of the Welsh Government. Chapter 13 Minimising and Managing Environmental Risk and Pollution outlines the Welsh Government s objectives in terms of addressing flood risk. 2.2 Paragraph 13.4 of Planning Policy Wales states: Development proposals in areas defined as being of high flood hazard should only be considered where: new development can be justified in that location, even though it is likely to be at risk from flooding; and the development proposal would not result in the intensification of existing development which may itself be at risk; and new development would not increase the potential adverse impacts of a flood event 2.3 Planning Policy Wales is supplemented by a series of Technical Advice Notes (TAN). TAN 15 provides technical guidance on development and flood risk. Local Planning Policy 2.4 The Powys Local Development Plan (LDP) will replace the Powys Unitary Development Plan (UDP), identifying land that will be used to meet the County s development needs and policies that will be used to determine planning applications in the County. The following policies contained within the UDP highlight the councils strategy for the management of flood risk; 2.5 POLICY DC13 - SURFACE WATER DRAINAGE A. Development proposals will be permitted where: 1. They make adequate provision for land drainage and surface water disposal. 2. They would not adversely affect flood management or maintenance schemes. 3. They would not give rise to unacceptable on or off site flooding. B. If mitigation measures are required to permit a development to proceed, a condition will be attached to any planning permission, or a planning obligation by agreement sought, to ensure that the necessary works are completed prior to the commencement of the development. RCEF R Mill Lane, Welshpool 6

7 C. Where proposals include satisfactory alleviation measures to overcome any problem of drainage these will only be acceptable where they will not result in: 1. The loss of wetland habitats. 2. Unacceptable adverse impacts on surface or groundwaters. 3. The loss or impediment of access to watercourses and public rights of way. 2.6 POLICY DC14 - FLOOD PREVENTION MEASURES Flood prevention and alleviation measures will be permitted where practicable and reasonable, subject to their being in accordance with the environment and conservation policies and proposals of the plan. If flood defence measures are required to permit a development to proceed, a condition will be attached to any planning permission, or a planning obligation by agreement sought, to ensure that the necessary works are completed prior to the commencement of the development. Where proposals include satisfactory alleviation measures to overcome any problem of flooding risk including those to third parties, these will only be acceptable where they will comply with policy dc13 criteria c1, c2 and c The PCC Stage 1 and Stage 2 Strategic Flood Consequences Assessment (SFCA) identifies and maps flood risk from all sources at a borough-wide scale as well as providing guidance on producing site specific FCAs. Relevant information from the SFCA has been referenced throughout this Flood Consequences Assessment report. RCEF R Mill Lane, Welshpool 7

8 3 CONSULTATION 3.1 The Flood Consequences Assessment has been produced in consultation with the Partnership and Strategic Overview Team at NRW. NRW (Peter Evans, 28/07/2014) has advised that the site will benefit from the proposed PCC flood alleviation proposals although, in an extreme event, a level of risk may remain because of the proximity of the site to the canal. Any proposed, less vulnerable retail development should therefore be designed to be compliant with Tables A1.14 and A1.15 of TAN15. Modelled flood level data has been requested from NRW, as well as details of historic flooding and flood defences within the vicinity of the site. The information provided and the consultation with the NRW is included as Appendix A. 3.2 PCC were contacted in relation to the proposed development, Robert Gilman (senior engineer for Powys County Council) advised that the site is set to benefit from a series of flood alleviation measures to be developed by PCC, with works programmed to commence in September of PCC also reiterated that the works have been designed to reduce the risk of flooding for all events up to and including the 1 in 100 year return period, with a 20% allowance for climate change. The information provided and the consultation with the PCC is included as Appendix B. 3.3 The site is not located within an Internal Drainage Board District. 3.4 The public sewer network within the vicinity of the site is operated by Severn Trent Water. A copy of the public sewer system records is provided in Appendix C. RCEF R Mill Lane, Welshpool 8

9 4 SITE DESCRIPTION Site Description 4.1 The site is located immediately to the east of Welshpool Town Centre at National Grid Reference , It is rectangular in shape, occupying an area of approximately 0.69 hectares. A site location plan is provided in Figure The site is currently an area of disused land. It was previously occupied by a car park which served a nearby livestock market. The entire site comprises the former car park which has a bitumen surface in a significant state of disrepair. This was partially broken out during the site investigation to expose the underlying sub base. 4.3 Vehicular and pedestrian access to the site is gained via Mill Lane along the south-western site boundary and a spur road along the south-eastern site boundary. Surrounding Land Uses 4.4 The site is in an area of mixed land uses. The site is bound by Mill Lane to the south-west, with a Tesco store and associated petrol filling station and car park beyond; the Marston s public house and associated car park are situated to the south-east; residential properties to the north-east; and the Shropshire Union Canal to the north-west, with further residential properties beyond. 4.5 Information provided by the Department for Environment Food & Rural Affairs (Defra) online mapping indicates that the site is situated within close proximity to both a Sites of Special Scientific Interest (SSSIs) and a Special Area of Conservation (SAC) in relation to the Montgomery Canal which hosts several rare species of aquatic plant life. The site is not identified as being within close proximity to a Special Protection Area (SPA). Topography 4.6 A topographic survey has been undertaken for the site by EDI Surveys Limited, drawing number 14238/T/01-01, dated June 2013 and is located in Appendix D. The topographic survey indicates that ground levels fall by approximately 1.4 m, from 76.4 m AOD to the west of the site to 75.0 m AOD to the east. Ground levels locally fall steeply to the north-east along part of the south-western site boundary due to the rapid change in elevation associated with The Clerk s Bridge. RCEF R Mill Lane, Welshpool 9

10 5 PROPOSED DEVELOPMENT 5.1 The proposed development comprises the construction of a new Lidl food store and warehouse to the north western end of the site with car parking (65 spaces) situated towards the south eastern end. Soft landscaping areas will be located within the peripheral areas of the site. Development plans are shown in Appendix E. 5.2 Vehicular and pedestrian access to the site will remain via Mill Lane along the south-western site boundary and a spur road along the south-eastern site boundary. Further pedestrian access to the site will be provided directly from Mill Lane and a new pedestrian link to the canal tow path will be constructed towards the northern end of the site. 5.3 The proposed development will result in a substantial increase in soft landscaped areas and an overall decrease in hardstanding (buildings, access road and car parking). Hardstanding will comprise 4,765 m 2 or 86% of the total site area. The remaining permeable, soft landscaped areas will occupy 764 m 2 or 14% of the total site area. 5.4 Under TAN 15, retail development is classified as less vulnerable. 5.5 At this stage, an indicative drainage layout has not been designed for the site. However, it is anticipated that surface water runoff will either pass straight to ground via infiltration techniques and or pass to the sewer situated within Mill Lane. The acceptable discharge rate to the existing mains sewer will need to be agreed with Severn Trent Water in consultation with Natural Resources Wales. 5.6 The potential to provide surface water attenuation, including the use of SuDS, has been considered as part of the preliminary design process (see Section 10 Surface Water Management). RCEF R Mill Lane, Welshpool 10

11 6 HYDROLOGICAL SETTING Nearby Watercourses 6.1 Reference to Ordnance Survey Mapping indicates that the nearest surface water feature is the Montgomeryshire Branch of the Shropshire Union Canal, which is situated approximately 5-10 m from and runs parallel to the north-western boundary of the site. 6.2 Lledan Brook is identified approximately 90m to the south-eastern end of the site at its closest point. The Lledlan brook is identified as flowing through Welshpool town centre in a southeasterly direction collecting inflows from the Ceunant Brook, Bron-y-Buckley and Bull Dingle Brook. The majority of Lledan Brook is culverted with only 200 m of open watercourse remaining, before discharging to the River Severn approximately 700 m to the east of the site. 6.3 Lledan Brook is shown to be culverted below the Montgomery Canal at the Montgomery Canal Aqueduct and presents a significant restriction to flow in the Lledan Brook. The Lledan Brook cross sectional area at the aqueduct was measured in 1972 and 2003 and found to be in the order of 0.6 m Llyn Du, a lake situated within the grounds of Powis Castle is identified as being fed by surface water drains and a spring before discharging via Llyn-du Ditch. Lyn-du Ditch is identified as flowing north-eastwards towards the western end of Welshpool before entering a culvert which likely discharges to the culverted Lledan Brook. 6.5 No other artificial watercourses / features (e.g. canals, reservoirs) other than those already detailed have been identified within 1km of the site. Current Flood Risk Classification TAN15 Flood Zones 6.6 The Welsh Assembly Government produces Development Advice Maps (DAM) to accompany TAN15. These maps show the degree of flood risk which is to be applied to the site for the planning process and thus establish the suitability of the site for development. These maps are based upon the Environment Agency Wales flood risk maps and similarly they can be modified through presentation of data (i.e. hydraulic modelling) to illustrate that a site is within a different Flood Zone. 6.7 The TAN15 Development Advice Map (2009) indicates that the majority of the application site is located within Zone C2. A small area of Zone A is identified to the south-western boundary of the site. RCEF R Mill Lane, Welshpool 11

12 6.8 Zone C2 is described in TAN15 as those areas, areas without significant flood defence infrastructure. Zone A is described as those areas considered to be at little to no risk of fluvial or coastal/tidal flooding. The TAN15 Development Advice Map is shown as Figure 2. Natural Resources Wales Flood Map 6.9 The NRW floodplain map closely resembles the TAN15 Development Advice Map and indicates that the majority of the site is located within Flood Zone 3, which is classified as having a high annual probability of flooding (greater than 1 in 100 annual probability of flooding) in the absence of any defences. The south-western end is identified as being situated with Flood Zone 2, which is classified as having a moderate annual probability of flooding (between a 1 in 100 and 1 in 1,000 annual probability of flooding) in the absence of any defences. Mill Lane which forms the western boundary is identified as being situated within Flood Zone 1 which is identified as having a low annual probability of flooding (less than 1 in 1000 annual probability of flooding) in the absence of any defences. The NRW Flood Map is included as Figure 3. Historic Flooding 6.10 The PCC FPAR included eye witness statements and information obtained from PCC operatives who confirmed that in July 1968 and October 2000 water overtopped from the Lledan Brook into the canal. This led to some flooding on the Smithfield livestock market (development site) in July 1968 but fortunately no homes or properties were flooded Details of historical flood events recorded within the vicinity of the site have been provided by NRW. The map identifies that the River Severn and the eastern extent of Lledan Brook have experienced flooding in October 1998 and February The site is not shown to have been affected by the flood events and is situated 340 m from the affected area. Modelled Flood Levels 6.12 NRW has provided modelled flood levels for the subject site as shown in table 1 below. These levels are provided in full in Appendix A. Undefended 1 in in 1000 Elevation, max (m AOD) Elevation, mean (m AOD) Velocity, max (m/s) Table 1: Natural Resources Wales Modelled Flood Levels RCEF R Mill Lane, Welshpool 12

13 Flooding Mechanism 6.13 The PCC FPAR identified that during periods of high flow, water levels in the Lledan Brook upstream of the Montgomery canal aqueduct rise and water overtops into the Montgomery canal. The canal pound extends from Welshpool to Poole Quay which gives a significant reservoir for spillage. However, the length of the canal close to the north-western boundary of the site is at a lower level in relation to neighbouring embankments allowing flood water from the canal to spill on to the site. This flooding mechanism has been confirmed by modelling; eye witness accounts of flooding in 1968 and internal reports from PCC in Welshpool Flood Alleviation Scheme 6.14 Powys County Council has advised that works to implement a flood alleviation scheme are programmed to start in September of The Flood alleviation works will consist of raising the canal towpath at a couple of locations; including the area directly adjacent to the site, and constructing a new sluice discharge to the Lledan Brook. The PCC advised that The works have been designed to reduce the risk of flooding for all events up to and including the 1 in 100 year return period, with a 20% allowance for climate change. The consultation with PCC is included as Appendix B Powys County Council have provided a schematic (drawing number; H1316_T_101) showing the proposed improvements to the towpath immediately adjacent to the north-western end of the site. The plans indicate that the tow path will be raised to a consistent height of m AOD. The towpath will act as barrier to flood waters which have entered the Montgomery canal, by comparing the maximum flood elevation provided by NRW with the raised towpath flood water from the 1 in 100 year flood event (76.53 m AOD) and 1 in 1000 year flood event (76.61 m AOD) will be retained within the canal effectively removing the site from the floodplain. The PCC towpath schematic is included as Appendix B Following the implementation of the flood alleviation scheme the sites current flood classification will be revised. The flood alleviation scheme will provide at least a 1 in 100 plus climate change standard of protection therefore removing the site from NRW s Flood Zone 3, and as the site is not situated within the natural floodplain of the Lledlan Brook the site will no longer be classified as being within the TAN15 Development Advice Map Zone C2. The extent of flooding during a 1 in 1000 year event would be reduced and the whole site may be removed from the flood zone altogether. RCEF R Mill Lane, Welshpool 13

14 7 HYDROGEOLOGICAL SETTING 7.1 Reference to the British Geological Survey online mapping (1:50,000 scale) indicates that the site is underlain by superficial Glaciofluvial Fan Deposits and bedrock consisting of the Nantglyn Flags Formation. 7.2 According to the Environment Agency s online Groundwater Vulnerability Mapping, the site is underlain by a Secondary A aquifer in relation to the underlying superficial Glaciofluvial Fan Deposits. Secondary A aquifers are 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. The bedrock beneath the site consisting of the Nantglyn Flags Formation is identified as a Secondary B aquifer, Secondary B aquifers are predominantly lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering. 7.3 Opus International Consultants (UK) Ltd, Geo-Environmental Investigation was produced in July 2014 (Ref. CAB/J-B (R01)). The investigation included the advancement of six window sample boreholes and five trial pits over the site. The investigation observed strata consistent with those predicted by the British Geological Survey online mapping. The ground conditions encountered in the exploratory holes advanced into the ground generally comprised bitumen surfacing and/or limestone sub-base underlain by thin horizons of gravels and clays and further underlain by natural Sands and Gravels to the maximum investigation depth of 6.0 m bgl. 7.4 Groundwater strikes were not encountered in the window sample boreholes. The strata was however recorded as being damp at depths from between 2.30 m bgl and 3.80 m bgl to the base of the window sample boreholes. 7.5 Soilscapes online mapping describes the underlying soil on the site as, Freely draining slightly acid loamy soils. RCEF R Mill Lane, Welshpool 14

15 8 EXISTING DRAINAGE / WATER MAINS 8.1 Reference to Severn Trent Water plans of public sewers (shown in Appendix C) indicates that there are no public sewers within the immediate vicinity of the site. The closest public sewer is a 450 mm diameter combined sewer situated within Greenfields to the south east of the site. The sewer is identified as flowing southward to manhole 9508 before discharging to a 300 mm diameter combined sewer passing beneath the Lledan Brook to manhole node 9406 and away to the south. 8.2 Two 300 mm diameter Foul sewers (one of which appears not to be adopted) are situated on the western end of the neighbouring Marston s public house development and are shown to discharge to manhole node The topographic survey produced by EDI Surveys Limited, drawing number 14238/T/01-01, dated June 2013 (included as Appendix D) identifies that there is currently limited surface water drainage features on the site. The survey identifies a 100 mm diameter sewer leading from a concrete structure located towards the northern end of the site. The Opus International Consultants (UK) Ltd, Geo-Environmental Investigation suggests that the concrete structure may be a soakaway therefore the sewer which flows from it may be an overflow. The sewer is identified as following the eastern boundary but there is no indication of where it flows to. 8.4 The topographic survey also identifies two concrete channel runs which run in a south easterly direction however there is no indication if these connect to a wider drainage system. Two 150 mm diameter surface water sewers are identified as discharging to a 300 mm diameter surface water sewer within the internal access road which forms the southern boundary of the site. The sewer is shown to flow in a northerly direction however again it is not shown where this sewer discharges. 8.5 Reference to water network plans provided by Severn Trent Water indicates that the site is not currently served by public water mains. The closest water main is a 100 mm diameter main identified within Greenfields to the south. east of the site. RCEF R Mill Lane, Welshpool 15

16 9 FLOOD RISK AND MITIGATION 9.1 The key sources of flooding that could potentially impact the site are discussed below: Fluvial / Tidal Flooding 9.2 As detailed in Section 6 of this report, the Development Advice Maps indicate that the site is currently, mainly located in Zone C2 (without significant flood defence infrastructure). A small area of Zone A is identified to the southern boundary of the site. 9.3 The NRW flood maps (available on the Environment Agency website) indicate that the site is mainly located within Flood Zone 3, which is classified as having a high annual probability of flooding (greater than 1 in 200 annual probability from tidal sources) in the absence of any defences. 9.4 The data provided by the NRW identifies that the north-western majority of the site currently is predicted to experience flooding between 0.0 and 0.3 m during the 1 in 100 and 1 in 1000 year fluvial flood event. 9.5 Flood modelling carried out as part of the PCC FPAR identified that the flooding mechanism associated with the site occurs during periods of high flow, water levels in the Lledan Brook upstream of the Montgomery canal aqueduct rise and water overtops into the Montgomery canal. The water level subsequently rises within the canal and overtops adjacent to the site where the level of the towpath is supressed compared to surrounding levels. 9.6 The site is set to benefit from a series of flood alleviation measures to be developed by PCC, with works programmed to commence in September of The plans include raising a section of the towpath adjacent to the north-western end of the site which is topographically lower than the surrounding towpath to a height of m AOD, resulting in a consistent elevation along the length of the canal as well as constructing new trash screens to prevent blockages of the Montgomery canal aqueduct. The flood alleviation scheme will provide at least a 1 in 100 plus climate change standard of protection therefore removing the site from NRW s Flood Zone 3, and as the site is not situated within the natural floodplain of the Lledlan Brook the site will no longer be classified as being within the TAN15 Development Advice Map Zone C2. The extent of flooding during a 1 in 1000 year event would be reduced and the whole site may be removed from the flood zone altogether. In addition as the height of the tow path adjacent to the site will be raised so that the height is consistent along the length of the canal within Welshpool, the probability of flood waters overtopping the canal towpath will be consistent along the length of the towpath reducing the overall probability of flooding at the site during a greater than 1 in 100 year plus climate change event. RCEF R Mill Lane, Welshpool 16

17 9.7 Appendix A.14 of TAN 15 states that retail development should remain flood free in up to the 1 in 100 year fluvial event. Beyond this threshold frequency, development would be expected to flood under extreme conditions. Appendix A1.15 of TAN 15 provides indicative guidance (which should not be regarded as prescriptive) on what are considered tolerable conditions for different types of developments. For retail development, a maximum flood depth of 600 mm is considered tolerable. 9.8 Mill Lane which forms the south-western boundary of the site is identified as being situated wholly within Flood Zone 1 and DAM Zone A, adjacent to the south and to the north. Therefore dry/safe access is available via this route. 9.9 Once the flood alleviation scheme is completed the site will be located outside the 1 in 100 year plus climate change event floodplain and as such no floodplain storage compensation is considered necessary. This is confirmed by correspondence with the NRW included as Appendix A. Proposed Mitigation 9.10 Ground floor levels should be set should be set 300 mm above surrounding ground levels which will result in the building remaining above the existing 1 in 100 year and 1 in 1000 year modelled flood level which complies with the guidance set out within Appendix A1.15 of TAN 15. Flooding from sewers 9.11 Sewer flooding can occur during periods of heavy rainfall when a sewer becomes blocked or is of inadequate capacity. The site is not currently served by public sewers. The closest public sewer is a 450 mm diameter combined sewer situated within Greenfields to the south east of the site Should sewer surcharging occur in the vicinity of the site, it is likely that water would be retained within the road by the roadside kerbs, and would follow the topographic gradient along Greenfields and Smithfield Road to the south and away from the site Should it be determined that discharging to the public sewer is required the discharge rate will be agreed with Severn Trent Water to ensure that there is capacity to receive discharge from the site without significantly increasing flood risk. Proposed Mitigation 9.14 No mitigation is considered necessary. Surface water flooding (overland flow) 9.15 This can occur during intense rainfall events, when water cannot soak into the ground or enter drainage systems. RCEF R Mill Lane, Welshpool 17

18 9.16 Reference to the topographic survey identifies that site is located within a wider area where levels are relatively flat, however ground to the northwest of the site is generally higher than the site and ground to the southeast is generally lower. The Montgomery Canal will act as a physical barrier stopping surface water overland flow migrating onto the site from the northwest The SFCA, assessed Surface water flood risk using the Environment Agency Wales Flood Map for Surface Water data, the Environment Agency online surface water flood risk map does not identify the site as at risk of surface water flooding Surface water flooding from on-site sources is considered in Section 10 of this report. Proposed Mitigation 9.19 Raising floor slabs 300 mm above surrounding ground level or grading levels at building entrances will mitigate against overland flows entering buildings. Groundwater flooding 9.20 This can occur in low-lying areas when groundwater levels rise above surface levels, or within underground structures. BGS mapping and the subsequent Phase 2 Site Investigation indicates that the site is underlain by superficial Glaciofluvial Fan Deposits and bedrock consisting of the Nantglyn Flags Formation. The site investigation identified groundwater at a depth of 2.3 m.bgl The SFCA states; There is no local information on historic groundwater flooding, which suggests that the risk of groundwater flooding in Powys is low No underground structures or basement levels are proposed. Proposed Mitigation 9.23 No mitigation is considered necessary. Other Sources 9.24 There is a limited risk of flooding occurring as a result of a break in a water main. The locations of the water mains in the immediate vicinity of the site are described in Section 8. It is likely that water would be retained within the road by the roadside kerbs, and would flow south eastward following the topographic gradient of Mill Lane which forms the south-western boundary and away from the site along Smithfield Road Reference to the Natural Resources Wales (NRW) Reservoir Flood Map accessed from the Environment Agency s website, indicates that the site is not situated within an area that would be affected in the event of reservoir failure. RCEF R Mill Lane, Welshpool 18

19 Proposed Mitigation 9.26 No mitigation is considered necessary. Event Exceedance and Residual Risk 9.27 The mitigation measures proposed as part of the development scheme are considered appropriate to mitigate against any residual risks or event exceedance scenarios. RCEF R Mill Lane, Welshpool 19

20 10 SURFACE WATER MANAGEMENT Introduction 10.1 The proposed redevelopment of the site from a disused car park to a retail store will result in a substantial increase in soft landscaped areas and an overall decrease in hardstanding (buildings, access road and car parking). Hardstanding will comprise approximately 86% of the total site whilst the remaining permeable, soft landscaped areas will occupy 14% of the total site area TAN 15 states that the aim should be for new development not to create additional runoff when compared with the undeveloped situation, and for redevelopment to reduce runoff where possible The proposed development comprises the construction of a new Lidl food store (retail). Generally, this type of development is considered to have a design life of 50 years. For the purposes of this assessment, taking into account the recommended national precautionary sensitivity ranges for peak rainfall intensity, a 20% increase in peak rainfall intensity has been included as climate change allowance Micro drainage WinDes calculations have been undertaken to determine the attenuation requirements for the site based on the betterment requirements and considering rainfall for the 1 in 100 year event plus an allowance for climate change. The drainage infrastructure that once served the site appears to have been removed during the demolition works. It is therefore likely that due to the topography of the site, surface water will follow the topographic gradient to the south. The Modified Rational method was utilised to estimate the existing brownfield runoff rates; (2.78 x i (Rainfall intensity) x Ap (impermeable area)) 10.5 Therefore the existing brownfield runoff rates were derived assuming that 100% of the existing catchment areas and are impermeable (Ap). Rainfall intensity (i) for 360min storm duration was utilised for the relevant return period based on FEH rainfall data. An additional 20% betterment over the additional rate has been included The results of Infiltration testing to BRE365 were not available at the time of writing this report, should the ground not be suitable for the use of infiltration SUDS methods and discharge to the public sewer network is required the storage requirement for the lowest discharge rate allowable for a self-cleansing system (5 l/s) has been calculated as a worst case scenario A summary of the results is provided in the table below, with full Micro-Drainage WinDes calculations provided in Appendix F. RCEF R Mill Lane, Welshpool 20

21 Event Discharge rate (l/s) Storage required (m 3 ) 30 year year + 20% climate change year + 20% climate change (Worst Case) Table 2: Summary of Attenuation Requirements Consideration of Sustainable Drainage Systems 10.8 The potential for the use of Sustainable Drainage Systems (SuDS) has been considered at this stage. Swales, detention basins and ponds 10.9 The proposed development buildings, car parking areas, peripheral landscaped areas and access roads take up the entire site footprint. As such it is considered that it is not possible within the scope of the proposed development to incorporate SUDS techniques such as swales, detention basins and ponds on the site. Soakaways Reference to BGS mapping indicates that the site is underlain by the Chalk Clay-with-flint Formation, which are likely to be of a moderate permeability. The soils are described as slightly acid loamy and clayey soils with impeded drainage by the National Soil Resources Institute. Given the reported geological conditions beneath the site, it is considered that soakaways could provide may provide some means of soakaway a feasible method for the disposal of surface water runoff from the site. Soakaway testing will be required at the detailed design stage. 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. Porous / Permeable Paving The car parking areas may be suitable for the use of permeable paving. Surface water storage would be provided within the sub-grade material prior to controlled discharge to the receiving storm water sewer; however the internal access road to the northern and eastern end of the site leading through the car park will be trafficked with frequent HGV and heavy vehicle movements. RCEF R Mill Lane, Welshpool 21

22 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 footpaths, car parking spaces within the car park and the internal access road not utilised for HGV traffic provides a potential area of 2,035 m 2, based on a depth of 0.3 m with a voids ratio of 30%, the car parking area could provide up to 183 m 3 of attenuation prior to infiltration to the ground or discharge to the sewer. Storage would be provided within the sub-grade material prior to controlled discharge to the receiving storm water sewer. Further investigation into the use of porous paving will be undertaken at the detailed design stage. Modular Underground Attenuation Tanks Storage could be provided within underground attenuation tanks or oversized pipes within the access road and car parking area towards the northern end of the site. Conceptual Surface Water Attenuation Scheme On the basis of the calculations above, a total attenuation volume of 92.6 m 3 will be required to achieve the proposed 92.6 l/s surface water runoff rate which includes 20% betterment from the existing rate or a total attenuation volume of m 3 will be required to achieve the worst case 5 l/s surface water runoff rate The entire balance of the required storage limiting the rate to between 5 l/s and 92.6 l/s could be provided utilising porous paving and/or source control techniques such as oversized pipes or a small underground attenuation tank within the car parking area towards the southern end of the site In either case the proposed surface water drainage scheme represents betterment over the existing drainage through the reduction of overall hardstanding area with soft landscaping towards the peripheral areas of the site. Currently, during events of a greater magnitude than 1 in 5 year event, it is likely that runoff will flow over the site following the topographic gradient to the neighbouring Marston s public house. The proposed drainage network will ensure that runoff will be retained on site, thereby potentially reducing flood risk to off-site locations. Event Exceedence The proposed indicative surface water drainage concept provides underground storage up to the 1 in 100 year plus climate change event. In an event exceeding this magnitude, detailed drainage design will identify mitigation measures to ensure that the resulting above-ground flooding will be confined to temporary shallow flooding of the on-site road network and will not affect the buildings on site or significantly increase flood risk to off-site locations. RCEF R Mill Lane, Welshpool 22

23 11 JUSTIFICATION TEST 11.1 TAN 15 states that new development should be directed away from Zone C and towards suitable land in Zone A, otherwise to Zone B where river or coastal flooding will be less of an issue. In Zone C2, highly vulnerable development and emergency services should not be permitted. All other new development should only be permitted within Zone C1 and C2 if determined by the local planning authority to be justified in that location The majority of the application site is currently located within Zone C2 with a small area of Zone A identified to the south-western boundary of the site. Following the implementation of the flood alleviation scheme the sites current flood classification will be revised. The flood alleviation scheme will provide at least a 1 in 100 plus climate change standard of protection therefore removing the site from NRW s Flood Zone 3, and as the site is not situated within the natural floodplain of the Lledlan Brook the site will no longer be classified as being within the TAN15 Development Advice Map Zone C Appendix A.14 of TAN 15 states that retail development should remain flood free in up to the 1 in 100 year fluvial event. Beyond this threshold frequency, development would be expected to flood under extreme conditions. As discussed in Section 6 Hydrology of this FCA, following the implementation of the flood alleviation scheme the site will be located wholly outside of Flood Zone 3 and would remain flood free in the 1 in 100 year fluvial event therefore satisfying the justification test. RCEF R Mill Lane, Welshpool 23

24 12 SUMMARY AND CONCLUSIONS 12.1 The aim of the Flood Consequences Assessment is to outline the potential for the site to be impacted by flooding, the potential impacts of the development on flooding both onsite and in the vicinity, and the proposed measures which can be incorporated into the development to mitigate the identified risks. The report has been produced in accordance with the guidance detailed in the NPPF. Reference has also been made to the CIRIA SUDS manual (C697), the Strategic Flood Consequences Assessment and the Surface Water Management Plan and following consultation with Natural Resources Wales and Powys County Council The potential flood risks to the site, and the measures proposed to mitigate the identified risks, are summarised in the table below: Source of flooding Identified Risk Mitigation proposed Residual risk L M H L M H Following the implementation of the Welshpool flood alleviation scheme the site will be removed from the 1 in 100 fluvial floodplain. Fluvial Raising the building slab level 300 mm above surrounding ground levels will place the building above the existing 1 in 1000 year flood level. Tidal No mitigation necessary. Sewers No mitigation necessary. Slightly raising slab levels or sloping yard areas away from buildings. Surface Water SuDS techniques; porous paving, soakaways and underground attenuation tanks may be utilised. Groundwater No mitigation necessary. Other Sources (e.g. reservoirs, water mains) No mitigation necessary. RCEF R Mill Lane, Welshpool 24

25 12.3 The site is set to benefit from a series of flood alleviation measures to be developed by PCC, with works programmed to commence in September of The plans include raising a section of the towpath adjacent to the north-western end of the site to a height of m AOD, bringing the section to consistent height with the rest of the canal towpath as well as constructing new trash screens to prevent blockages of the Montgomery canal aqueduct. Following the implementation of the flood alleviation scheme the sites current flood classification will be revised. The flood alleviation scheme will provide at least a 1 in 100 plus climate change standard of protection therefore removing the site from NRW s Flood Zone 3, and as the site is not situated within the natural floodplain of the Lledlan Brook the site will no longer be classified as being within the TAN15 Development Advice Map Zone C The proposed development would result in a reduction of the impermeable area on the site from 100% to 86% providing betterment over the existing situation It has been demonstrated that the development meets the Justification Test under TAN Overall, the report demonstrates that the development will be safe without resulting in an increase in flood risk to the site or the surrounding area. It is therefore considered that the development meets the requirements of TAN 15. RCEF R Mill Lane, Welshpool 25