DECEMBER 2008 [ISSUE 2]

Transcription

1 PROPOSED DEVELOPMENT WEST SUSSEX FLOOD RISK ASSESSMENT CLIENT: ARORA MANAGEMENT SERVICES LIMITED [ISSUE 2] Warrington Office Cinnamon House Cinnamon Park Crab Lane Fearnhead Warrington Cheshire WA2 0XP Swindon Office Unit 2 York House Edison Park Dorcan Way Swindon Wiltshire SN3 3RB Tel Fax Tel Fax cec@coleeasdon.com Civil Engineering ~ Highways ~ Transportation ~ Flood Risk Assessment Incorporated in UK as Cole Easdon BMP Ltd No

2 C OLE EASDON CONSULTANTS (CEC) DOCUMENT ISSUE RECORD Client: Project: Arora Management Services Limited Proposed Development at the Sofitel Hotel North Terminal Gatwick Airport West Sussex Job Number: 2671 Document Title: Flood Risk Assessment Issue / Revision: Issue 1 Issue 2 Description / Status: Draft for Comment Planning application submission Date: December 2008 December 2008 Prepared: S Starr BSc (Hons) S Starr BSc (Hons) Signature: Document Check: K Legg K Legg Signature: Technical Check: D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE Signature: Authorised: D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE Signature: File Reference: 2611 FRA (Dec 08) 2611 FRA (Dec 08) Cole Easdon Consultants (CEC)

3 CONTENTS SECTION HEADING PAGE NO. 1.0 Introduction The Existing Site Flood Risk Issues Discussions and Conclusions 18 Appendices Appendix 1 Figures Figure 1 Location Plan Figure 2 EA Flood Zone Map Figure 3 EA Flood Zone Map 1:10,000 Figure 4 Existing Surface Water Drainage Catchments Appendix 2 Surface Water Drainage Calculations Appendix 3 Correspondence Appendix 4 British Geological Survey Borehole Logs Appendix 5 Drawings: Drawing No. 2671/500 - Existing Site Layout (Sheets 1 & 2) Drawing No. 2671/501 - Proposed Development Layout Drawing No. 2671/502 - Existing Site Drainage Drawing No. 2671/503 Compensatory Storage Proposal Cole Easdon Consultants (CEC)

4 1.0 INTRODUCTION 1.1 This Flood Risk Assessment has been prepared by Cole Easdon Consultants on behalf of Arora Management Services Limited in respect of a planning application for commercial development at the Sofitel Hotel Site, North Terminal, Gatwick Airport, West Sussex. National Grid Reference TQ Refer to Figure 1 - Site Location Plan [Appendix 1]. 1.2 This Study considers the issues relating to flood risk associated with the proposals. It has been prepared following discussions with officers from the Environment Agency (the EA) and British Airport Authority (BAA). Development Proposals 1.3 The proposed development is to comprise a new 630 bed, multi-storey Premier Inn hotel with associated access, parking areas and landscaping. The proposal includes for: 1. Ground (50 spaces) and first floor (65 spaces) car parking areas below the hotel; 2. Hotel entry level above the car parking areas; 3. 8 No. floors of accommodation; 4. 1 No. office floor 1.4 The new hotel will be located in the existing Sofitel Hotel car park. Additional car parking areas will be provided around the existing Sofitel Hotel building at ground level (188 spaces) and within a proposed two storey car park (184 spaces). Cole Easdon Consultants (CEC) - 1 -

5 1.5 This study is based on the proposed development layout shown on Drawing No SK031 prepared by Dexter Moren Associates in December 2008, and included in Cole Easdon Drawing No. 2671/501 - Proposed Development Layout [Appendix 5]. Need for Study 1.6 The purpose of this assessment is to demonstrate to the EA that the proposals outlined above can be satisfactorily accommodated without worsening flood risk for the area, as per guidance provided within Planning Guidance document PPS Accordingly, this study has been prepared to: i) assess flood risk to the development from fluvial sources; ii) assess flood risk to the development from other potential sources, including streams, ditches, sewers, groundwater and overland surface water; iii) ensure that the development proposed will fully comply with the requirements of the EA's policy on the safeguarding of floodplains, as detailed within the said Planning Policy Statement 25 (PPS25). iv) prepare a SUDS strategy for management of post development surface water runoff. 1.8 A site topographical survey (referenced to an Ordnance Datum) has been provided to Cole Easdon Consultants and has been reviewed as part of this study. Scope of Study 1.9 In Section 2.0, we describe the characteristics of the development site and surrounding area. In Section 3.0, we assess flood risk issues. Finally, conclusions are presented in Section 4.0. Cole Easdon Consultants (CEC) - 2 -

6 1.10 The following abbreviations are used in this report: EA Environment Agency SUDS Sustainable Urban Drainage Systems FZM Flood Zone Map prepared by the EA PPS25 Planning Policy Statement 25: Development and Flood Risk AOD Above Ordnance Datum BGS British Geological Survey SFRA Strategic Flood Risk Assessment LPA Local Planning Authority BAA British Airport Authority Cole Easdon Consultants (CEC) - 3 -

7 2.0 THE EXISTING SITE Refer to Drawing No. 2671/500 - Existing Site Layout [Appendix 5]. 2.1 The development site is brownfield land currently occupied by the Sofitel Hotel and associated car park. The site is located 150m east of the North Terminal, Gatwick Airport, West Sussex. It covers some 2 hectares and comprises the hotel building, associated car parking, access roads and landscaping. An elevated railway linking the North & South Terminals passes over the site. The site is accessed from the highway known as Northway. 2.2 The site is bounded on all sides by highways. Northway to the north and west and Crossway and North Terminal Approach to the south and east. Land use in the vicinity of the site is related to the operation of Gatwick Airport. 2.3 The existing topography of the land within the airport complex is flat. Development site ground levels vary from 56.00mAOD to 57.00mAOD. Existing Surface Water Drainage 2.4 Surface water runoff from the existing hard areas and buildings at the site is piped to a private surface water drainage system comprising of a 2900mm x 1500mm culvert at a depth of some 4 metres below the site. 2.5 The culvert outfalls to an attenuation pond (known as Pond D) at the northern extent of the airport, some 600 metres north of the development site. The total contributing area draining to Pond D comprises of the North Terminal area and is some 327 ha, of which 244 ha is impermeable surfacing. BAA relate that the pond has been designed to accommodate runoff from the 1:5 year event. The pond outfalls to the River Mole, situated 30m north, via Hydrobrake flow controls. Cole Easdon Consultants (CEC) - 4 -

8 2.6 The drainage network within the airport is privately owned and maintained by BAA. All drainage infrastructure located within the development site is under the joint ownership of BAA and Sofitel. Nearby Watercourses/Drainage Features 2.7 The River Mole, a tributary of the River Thames, drains the local catchment. All surface water runoff from the airport is directed to the River Mole. Attenuated flows from various storage ponds are discharged directly to the watercourse, or to Gatwick Stream or Crawter's Brook, both tributaries of the River Mole. Surface Water drainage catchments within the airport are shown on Figure 4 (Appendix 1). 2.8 The River Mole is located approximately 0.5km north of the site, at the northern boundary of the airport. The watercourse skirts around the western edge of Crawley, then flows north across some 1.5km of farmland before entering a culvert beneath Gatwick airport runway. It emerges as an open channel to the north of the runway and proceeds to flow in a north easterly direction, through the northern extent of the airport. The watercourse flows beneath the A23 highway in the north eastern corner of the site, then follows a northerly course through Surrey. Its confluence with the River Thames is near Hampton Court Palace, London, some 80km north of the site. 2.9 The Gatwick Stream flows north along the airport's eastern boundary. It flows under the A23 highway adjacent to Gatwick South Terminal before heading in a north easterly direction, following the route of the A23. It joins the River Mole downstream of the culvert, which conveys the Mole under the A23, some 600 metres north east of the site. Cole Easdon Consultants (CEC) - 5 -

9 2.10 Crawter's Brook flows north through Crawley, then flows west along the southern boundary of the airport. It meets the River Mole upstream of the culvert beneath the runway. Existing Ground Conditions Refer to Borehole Logs [Appendix 4] 2.11 An intrusive site investigation has yet to be undertaken. However, borehole data has been obtained from the British Geological Society. The data relate to three boreholes established along the M23 highway, adjacent to the airport. The boreholes were sunk to depth of 6m, 9m and 14.6m The boreholes indicate that clay strata are present to approximately 4m below ground level at the site. The clay strata are underlain by a mixture of clay, siltstone and shale material, encountered to depth in each borehole Groundwater was struck in two of the boreholes at levels of 4.4m and 5.6m below ground level. Cole Easdon Consultants (CEC) - 6 -

10 3.0 FLOOD RISK ISSUES 3.1 This section presents an assessment of flood risk to the development from: a) external sources; b) surface water discharge from the proposed development. 3.2 Recommended flood risk mitigation measures appropriate to the level of perceived risk are included in the assessment. The mitigation measures are summarised in Table 3.1 on page 14. The Sequential Test 3.3 PPS25 requires that LPAs adopt a sequential approach when allocating land for development. The sequential approach should be used in order to direct development to the lowest Flood Risk Zone available before higher risk areas are considered. 3.4 We can confirm that hotel use is classified as more vulnerable development, and is therefore considered to be an appropriate use within Flood Zone 2, as informed by Table D.3 of PPS25. However, this means only that the proposed development will not be required to pass the Exception Test. As footnote 22 (p. 27) of PPS25 explains, even though a development may be considered an appropriate use of a flood zone, it still requires the application of the Sequential Test to steer development first to areas at the lowest probability of flooding. 3.5 In this instance, the application of the Sequential Test has revealed that there are no other appropriate development sites in the locality for the following reasons: A significant proportion of Gatwick Airport lies within Flood Zones 2 and 3, including parts of the area for the proposed second runway. Cole Easdon Consultants (CEC) - 7 -

11 3.5 Contd. There are no feasible alternative sites for hotel development within lower risk Flood Zone 1 and only limited opportunities are available for hotel development within Flood Zone 2; The development site is located at the edge of Flood Zone 2. As such, the development site location conforms with the requirements of the Sequential Approach, which requires that development is directed to areas of lowest flood risk; This is the only available hotel site with pedestrian access to the North Terminal at Gatwick Airport; The development site makes efficient use of brownfield land at a very accessible public transport hub A) Assessment of Flood Risk to the Development Site from External Sources Ai) Assessment of Flood Risk from Fluvial Sources 3.6 The Flood Zone Map (FZM) for the locality as produced by the EA is shown on Figures 2 & 3 within Appendix 1. The FZM shows that the site encroaches upon Flood Zone 2 (medium risk). 3.7 The EA relate that although flood level data is available for node points along the River Mole and Gatwick Stream, these flood levels are not relevant to the development site due to their distant location from the site. Cole Easdon Consultants (CEC) - 8 -

12 3.8 Consequently, the EA has advised that in the absence of site specific flood level data related to the 1:100 year + 20% flood event, the Flood Zone 2 floodplain should be overlaid the development site. Refer to Drawing No. 2671/503 (Appendix 5). The 1:100 year + 20% floodplain extent is approximated by the 56.75mOD contour. At this level, some 50% of the proposed car parking areas would be sited within floodplain. The Crawley Borough Council Strategic Flood Risk Assessment 3.9 Crawley Borough Council Strategic Flood Risk Assessment states that much of the Gatwick Airport precinct (including the proposed development site) is within Flood Zone 2 (Medium Risk). The source of flood risk is identified as fluvial flooding from the River Mole. According to the assessment, the nature of this flooding is anticipated to be limited to shallow sheet flow. Fluvial Sources Flood Risk Mitigation Measures Finished Floor levels 3.10 The hotel entry level will be sited above the two levels of decked car parking at a finished floor level of some 5.7 metres above finished ground floor level at approximately 61.95mOD. Proposed Car parking in Floodplain 3.11 The following measures are proposed so as to mitigate flood risk posed by the car park development in floodplain: 1. Existing ground levels will be maintained (except in car park areas where ground level will be lowered so as to provide compensatory storage for floodplain lost as a result of the proposed hotel footprint); Cole Easdon Consultants (CEC) - 9 -

13 3.11 Contd. 2. Ground floor car parking below the hotel will be subject to flooding to depths of up to 500mm. However, vehicles will be prevented from being washed offsite by the walled confines of the car park itself. 3. Other car parking areas will flood to depths not exceeding 300mm and no mitigation is proposed. Emergency Planning 3.12 The following measures are proposed: 1. The developer will need to formulate and implement an Emergency Plan that details emergency procedures in the event of an extreme flood event; 2. As part of the emergency procedures, the proposed hotel will be linked to the Environment Agency's Flood Warning Directory service, ensuring that customers and staff can remove their vehicles from the site well in advance of any flood at the site; 3. adequate passive flood warning and clearly signed evacuation routes will be provided, with signs highlighting the susceptibility to flooding. All signage will be maintained by the landowner. The landowner will also be required to keep evacuation routes clear. Flood Resilient Construction 3.13 The developer will give consideration to the use of flood resilient construction practices and materials in the design and build phase of the proposed hotel and decked car parking. The following measures will be considered for implementation. The measures will protect the proposed development where floodwaters exceed the design floor level up to a depth of 1 metre. Water exclusion measures are described below. For flood depths exceeding 1 metre, water exclusion is not recommended. Cole Easdon Consultants (CEC)

14 3.13 Contd. In general, building materials with good resistance to water penetration, good drying ability and the ability to retain pre-flood characteristics should be selected up to floor slab level. Examples include engineering bricks and dense stone; Where ground conditions are favourable, foundations should be ground supported not suspended types. Floodwater and waterborne contaminants can become trapped beneath suspended floors. If ground supported flooring is unsuitable, a suspended concrete slab floor is recommended; Concrete blockwork should not be located below floor slabs. Blockwork provides a potential path for water ingress into wall cavities; Where possible, masonry walls should be constructed using engineering bricks up to floor slab level. Blocks and dense facing bricks have much improved resistance to water penetration if rendered; Insulation in the form of external insulated render or internal boards is preferable to cavity insulation; Door thresholds should be raised, whilst complying with level access requirements. Sealed PVC external framed doors should be used where possible; Periscopic air vents should be used; Fittings that will not be significantly affected by water should be installed. Fittings should be easily cleaned. Fittings such as electrical appliances should be placed on plinths; All service entries including electricity, communications, gas and water should be sealed with expanding foam or similar closed cell material; Water, electricity and gas meters should be located above the predicted flood level; Electrical sockets should be installed above flood level. Electric ring mains should be installed at a higher level with drops to sockets and switches; Heating systems such as boilers should be located above the predicted flood level; Cole Easdon Consultants (CEC)

15 3.13 Contd. Electrical switchgear and plant should be placed on raised plinths above the predicted flood level; Non-return valves are recommended in the drainage system to prevent back flow of diluted sewage resulting from potential foul sewer surcharging. Maintenance of these valves is required to ensure their effectiveness; Where possible, stored supplies should be placed on raised plinths/racking; Any floor level glazing should be strengthened so as to resist floodwater pressure resulting from water depths of up to 0.5m, if possible. This will also provide resistance to damage from floating debris; Compensatory Storage Refer to Drawing No. 2671/503 Compensatory Storage Proposal [Appendix 5] 3.14 Compensatory storage will be provided for the loss of floodplain resulting from development within the 1:100 year + 20% floodplain. Floodplain loss (170m 3 ) is due to ground floor development comprising of plant rooms and linen store within the hotel footprint area Lost floodplain volume will be compensated for on a 'level for level' basis by the removal of an existing landscaped bund located in the proposed car park area at the southern end of the development site, adjacent to the existing Sofitel Hotel. Some 450m 3 floodplain volume can be made available by the removal of the bund, thereby providing a 280m 3 net increase in floodplain storage. Cole Easdon Consultants (CEC)

16 Aii) Assessment of Flood Risk from Existing Sewers/Drains 3.16 In the event of existing site drainage flooding, overflow will be routed off-site in a westerly direction, due to topography. Two no. existing potential ponding areas adjacent to the existing Sofitel Hotel will be removed under the development proposal, due to the presence of the decked carparking area. Sewer Flood Risk Mitigation Measures 3.17 No mitigation measures proposed. Aiii) Assessment of Flood Risk from Overland Flow Refer to Drawing No. 2671/500 - Existing Site Layout [Appendix 5] 3.18 The site is bounded by highways, which are served by an appropriate highway drainage system. In any event, the grade of the bounding highways is such that any highway drain overflow will be routed along the highways and away from the site. Accordingly, flood risk to the site from overland flow is considered low. Overland Flow Flood Risk Mitigation Measures 3.19 No mitigation measures are proposed. Aiv) Assessment of Flood Risk from Groundwater 3.20 Clay strata exists to depth in the site vicinity, based on nearby BGS borehole data. As this strata is impermeable, a groundwater table is unlikely to exist. Accordingly, flood risk to the site from this source is considered low. Cole Easdon Consultants (CEC)

17 Groundwater Flood Risk Mitigation Measures 3.21 No mitigation measures proposed. Table Assessment of Flood Risk to the Development Site Arising from External Sources Source of Potential Flooding to the Development Site Fluvial Flood Risk Medium. Flood Zone 2 Mitigation/Comments Above ground decked car parking below hotel entry level Finished floor levels in utility rooms to be set 300mm above 1:100yr+20% flood level Compensatory storage Dry access Flood resilient construction Emergency planning Sewers Low No mitigation measures proposed Groundwater Low No mitigation measures proposed Overland Flow Low No mitigation measures proposed B) Assessment of Flood Risk Arising from Surface Water Discharge from the Proposed Development Refer to Drawing No. 2671/501 Proposed Development Layout [Appendix 5] 3.22 In order to mitigate flood risk posed by post development runoff, adequate control measures will be required within the site. Cole Easdon Consultants (CEC)

18 Bi) Surface Water Runoff Control 3.23 The proposed development will introduce areas of impermeable surfacing, which in turn will generate additional surface water runoff. In order to manage the disposal of surface water in a sustainable manner and in accordance with the requirements of PPS25, the EA advises the developer to accommodate excess surface water on-site and control its release from the site according to the following criteria: i) the rate at which surface water is discharged from the site must not exceed the existing site runoff rate for a given storm event up to and including the 1:100 year storm event; ii) excess surface water from storm events up to and including the 1:100 year storm event must be stored on-site and released to watercourses and drains at existing discharge rates; iii) Surface water flows are estimated to increase by 30% over the lifetime of the development due to climate change. Therefore, the impact of climate change on the surface water drainage system and flooding should be assessed for the 1:100 year storm event plus 30%. Mitigation measures should be implemented as necessary to ensure that property is protected from surface water flooding under extreme storm conditions Notwithstanding the above, BAA has advised that all surface water discharge from the development site shall be restricted to the existing 1:5 year discharge rate for runoff entering the BAA owned drainage system. This, on the basis that the downstream BAA owned balancing pond has storage capacity to attenuate runoff from storms up to the 1:5 year design event only The existing 1:5 year discharge rate has been calculated to be 78 l/s. Refer to Drawing No. 2671/502 - Existing Site Drainage [Appendix 5] and Appendix 2 for surface water drainage calculations. Cole Easdon Consultants (CEC)

19 3.26 In accordance with requirements of the EA, PPS25 and BAA, surface water runoff from the development will be restricted to the existing 1:5 year brownfield site runoff rate for storms up to the 1:100 year + 30% event by means of attenuation and flow restriction facilities. This will be achieved at the development site through the implementation of oversize pipework to be sited below the ground level car parking adjacent to the existing Sofitel Hotel. Development discharge to the BAA owned culvert (2900mm x 1500mm) will be restricted by the use of a Hydrobrake flow control. Refer to Drawing No. 2671/502 [Appendix 5] It has been calculated that some 140m 3 attenuation storage will be required for the development site. This is based on a proposed 0.69ha impermeable area. The storage can be provided by a 1.8m wide x 1.2 m deep box culvert (50 m long). Table 3.2 summarises existing and proposed discharge rates, together with proposed attenuation volumes for storm up to and including the 100 year storm + 30% climate change event. Refer to Appendix 2 for storm runoff calculations. All computer calculations have been undertaken using MicroDrainage software (W.11.2). Table Existing and Proposed Surface Water Discharge to the 2.9m x 1.8m box culvert (BAA owned) Storm Existing Hard Area Runoff Contributing Area (ha) Runoff (l/s) Permissible Development Discharge into Glastonbury Mill Stream (l/s) Proposed Impermeable Area (ha) Required Volume for On- Site Retention (m 3 ) 1:5 Year :30 Year 0.37 > :100 Year + 30% 0.37 > Cole Easdon Consultants (CEC)

20 Bii) Soakaway Drainage 3.28 Made ground, peat and clay sub-soils exist to depth beneath the site. These strata have poor infiltration potential and will prevent the efficient operation of infiltration devices. Additionally, groundwater has been encountered close to the surface in one part of the site during the intrusive soil investigation. Accordingly, this report does not promote the use of such devices at the site. Refer to BGS borehole logs [Appendix 4]. Biii) Water Quality 3.29 The proposals include 188 No. ground level outside car parking spaces around the existing Sofitel Hotel. Due to the large number of spaces, discharge from external car parking areas will pass through an oil interceptor prior to discharge to off-site. Bv) Adoption 3.30 It is proposed that all on-site drainage facilities will be maintained privately. Bvi) Design Exceedance 3.31 The surface water drainage system will be designed so as not to flood during the 1:100year + 30% storm event. In the event of storms exceeding this magnitude or in the event of blockage, drain overflow will gravitate across the site in a westerly direction without ponding. Cole Easdon Consultants (CEC)

21 4.0 DISCUSSION AND CONCLUSIONS Sequential Test 4.1 The application of the Sequential Test has revealed that there are no other appropriate development sites in the locality Assessment of Flood Risk from External Sources 4.2 Flood risk to the proposed development from various sources, such as fluvial, overland flow, sewers, groundwater and overland flow have been considered in this study. 4.3 Fluvial flow has been identified as a possible flood risk to the site. The site is located within medium risk Flood Zone Accordingly, in order to mitigate fluvial flood risk arising from the 1:100 year + 20% event, the following measures are proposed: 1. Two levels of decked car parking (ground & first floor) will be sited below hotel entry level. This will ensure that floodwaters will not enter the hotel; 2. Compensatory storage will be provided for loss of floodplain storage arising from the hotel footprint; 3. Flood resilient construction measures will be incorporated into the development; 4. Emergency planning measures will be incorporated into the development; 4.5 Flood risk arising from sewer flooding, overland runoff and groundwater is considered to be low. However, the measures proposed in order to mitigate fluvial flood risk will also provide residual risk protection from these sources of flooding. Cole Easdon Consultants (CEC)

22 Assessment of Flood Risk Arising from Surface Water Discharge from the Proposed Development 4.6 The existing strata beneath the site have poor infiltration potential. Infiltration SUDS is not appropriate for the development site. 4.7 In accordance with requirements of the EA, PPS25 and BAA, surface water runoff from the development will be restricted to the existing 1:5 year brownfield site runoff rate for storms up to the 1:100 year + 30% event by means of attenuation and flow restriction facilities. This will be achieved at the development site through the implementation of oversize pipework to be sited below the ground level car parking adjacent to the existing Sofitel Hotel. 4.8 The proposals include 188 No. ground level outside car parking spaces around the existing Sofitel Hotel. Due to the large number of spaces, discharge from external car parking areas will pass through an oil interceptor prior to discharge to off-site. 4.9 It is proposed that all on-site drainage facilities will be maintained privately This study has been undertaken in accordance with the principles set out in PPS25. We can conclude that providing the development adheres to the conditions advised in paragraphs 4.1 to 4.9 above, the said development proposals can be accommodated without increasing flood risk within the locality in accordance with objectives set by Central Government and the EA. Cole Easdon Consultants December 2008 Cole Easdon Consultants (CEC)