OUTLINE DRAINAGE DESIGN

Transcription

1 SOUTHSIDE & CITY DEVELOPMENTS LTD OUTLINE DRAINAGE DESIGN Kimpton Road Site, Luton Bedfordshire Document Number: WBC (DR) 001 A01 Revision: A01 ABC Pickfords Wharf Clink Street London SE1 9DG TEL NO: FAX NO: July 2005

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3 CONTENTS EXECUTIVE SUMMARY INTRODUCTION AND POLICY CONTEXT... 2 INTRODUCTION HYDROGEOLOGY... 4 SOIL CONDITIONS... 4 GROUNDWATER EXISTING DRAINAGE SYSTEMS... 6 ON-SITE DRAINAGE SYSTEMS DRAINAGE STATEGY... 7 SURFACE WATER DRAINAGE STRATEGY... 7 Greenfield run-off... 7 Site constraints... 9 Proposed Surface Water System... 9 FOUL WATER DRAINAGE STRATEGY CONCLUSIONS REFERENCES... 14

4 FIGURES AND APPENDICES FIGURE 1: FIGURE 2: FIGURE 3: SITE LOCATION PLAN EA SOURCE PROTECTION ZONE DRAINAGE SCHEMATIC APPENDIX A: APPENDIX B APPENDIX C APPENDIX D APPENDIX E APPENDIX F: APPENDIX G: OS PLAN TOPOGRAPHICAL SITE SURVEY PROPOSED CONTOURS PLAN DEVELOPMENT FRAMEWORK PLANS (OPTIONS 1&2) MASTERPLAN AND AXONOMETRIC VIEW EXISTING DRAINAGE NETWORK CORRESPONDENCE WITH WTU AND THE EA

5 abc EXECUTIVE SUMMARY Waterman Burrow Crocker was commissioned by Waterman Partnership on behalf of Southside & City Developments Ltd in March 2005 to undertake an Outline Drainage Design for the proposed development at the Kimpton Road site. The site is located on the north-eastern outskirts of Luton, northeast of the Luton Airport. The site is located on a former motor vehicles factory. The main conclusion of this study can be summarised as: All new road surface and hard-standing areas are to be drained by a positive drainage system., which will convey water from all impermeable areas at the site to tank sewers beneath main roads and underneath the main boulevard highway. There attenuation will be split in a number of separate severs, with hydrobrakes at the end of each tank sewer to overcome the slope of the road (1:10). From there, surface water will be discharged at a restricted rate to the existing public sewers in Kimpton Road. An off-line attenuation system could be provided in a form of geocellular structure for the 1 in 100 year event within the open space OS4 in front of the hotel. Additional storage could be provided at this location at ground level to for an event in excess of 1 in 100 year return period. Permeable paving could be used for the residential parking areas on north-eastern part of the site, with surface water attenuated in a shallow bed of granular material overflowing into the new highway drainage system via a filter drain. To protect the groundwater below from any specific pollution risk, an impermeable membrane placed within sub-base. The drainage system should be sealed and capable of preventing accidental, unauthorised discharge of contaminated liquid to groundwater. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-1 -

6 abc 1.0 INTRODUCTION AND POLICY CONTEXT Introduction 1.1 Waterman Burrow Crocker was commissioned by Waterman Partnership on behalf of Southside & City in March 2005 to undertake an Outline Drainage Design for the proposed development at Former General Motor site in Luton. The National Grid Reference for the site is East, North and a location plan can be found in Figure The land at Kimpton Road site will provide a mixed use development as shown on the Development Framework Plan, see Appendix D, to include:- up to 1,000 mixed tenure residential units (C3); up to 45,000m 2 office accommodation (B1); up to 12,500m 2 general industrial accommodation (B1/B2); 200 bed hotel and fitness centre; multi-deck car park to provide 5,000 off-airport related parking spaces; the provision of associated infrastructure and services including car parking, roads and drainage; landscaping; 4,000m 2 of local services and neighbourhood retail (A1/A2/A3/A4/A5/D1); formation of two new vehicular access to Kimpton Road; enabling works to include ground remodelling; Together with: Either up to 80,000 m 2 of bulky goods retail floor space (A1) ; up to 10,500 m 2 of casino; and, up to 4,000 m 2 conferencing facility Or up to 1000,000 m 2 bulky goods retail floor space (A1) Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-2 -

7 abc 1.3 The site is steeply terraced down from the northeast to the south with a number of steep, densely vegetated slopes present on the northern part of the site. The difference in levels on the entire site is between 156m on site north-east AOD and 116m AOD on the south-west corner,, which gives an overall difference of 40m. 1.4 The largest cliff at the site, located at the northeastern perimeter is elevated 17-20m above the road running at the base of the slope. The slope falls southwards at approximate gradient of 1:60 and westwards at approximate gradient of 1:40. The slopes are covered by dense bushes and woods. 1.5 In the northwest corner of the site, there are three subsequent slopes. The first one, located at the western end of Zone 1, falls from 140m AOD to 137m AOD at an approximate gradient of 1:30. The next range of slopes falls inwards from the west, north and east creating a low lying area. A road was built on an approximately 10m high embankment surrounding the cut from north and west. It falls from 135m to 120m AOD at an approximate gradient of 1: The site is currently covered entirely by hard standing except the vegetated slopes, and comprises of large production buildings and office building and number of large surface car parking. 1.7 The proposed development will be split into plateaus to overcome this steep gradient with the main highway layout designed to take the difference in level into account. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-3 -

8 abc 2.0 HYDROGEOLOGY Soil Conditions 2.1 A ground Investigation has been carried out and report has been prepared by Wardell Armstong in 2005 for the former IBC Vehicle site, Kimpton Road. This paragraph provides a summary of geology and hydrology relevant for the purpose of Drainage Strategy (Ref 1) 2.2 Made Ground consisting mainly of clays, sand, silt and gravels of various proportions with occasional fragments of brick, clinker, chalk and charcoal was encountered within the site ranging from 0.25 to 5.55m bgl. 2.3 The strata is underlain by solid strata associated with the Upper Chalk and Middle Chalk Formations consisting generally of very weak to moderately weak, white chalk bearing flints (recorded as silty to sandy gravels). 2.4 There are several areas within the site of known or suspected contamination. The overwhelming majority of this contamination was found to be of hydrocarbon nature, which, depending upon further analysis could be amendable to on site bio-remediation. These concentrations are not mobile or hazardous to health. There are however, some other areas of oil/kerosene contamination. Furthermore, there are appreciable concentrations of petroleum contamination in the groundwater on site indicating the possibility of yet undiscovered petroleum soil contamination. This organic contamination may be harmful to human health and some form of remediation is essential and will be undertaken. 2.5 Remediation treatment of the contaminated upper layer of soils will be undertaken on impervious pavements within the site. Contaminated infiltration water (leachate) should be collected and appropriately treated/discharged to prevent cross contamination of the underlying soil strata. The leachate should be treated by an oil/petrol interceptor, prior to appropriate discharge which is considered to be a sewer (Ref 2). Groundwater 2.6 The groundwater was observed in eight monitoring locations in gas/water standpipes in November December 2004 and January March 2005) by Wardell Armstong. The observed groundwater levels within these boreholes show groundwater flow primary to the south. The ground water was sampled at 102.0m AODN in December to 100.5m AODN. The hydraulic gradient underneath the site has been calculated as range from 2.66 x10-4 to 1.15x10-3. (Ref 1). This supersedes previous report by Golden Associates (U.K) Ltd. 2.7 Four in-situ soakage tests and two falling head tests undertaken on site indicate a range of infiltration rates of 1.2x10-4 ms -1 to 9.4x10-5 ms -1 and probabilities of 4.3x10-7 and 4.4x10-7. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-4 -

9 abc 2.8 According to Environment Agency s policy and Practise for the Protection of Groundwater Vulnerability Map Sheet 31 Bedfordshire, the middle chalk formation is classified as Minor Aquifer (highly permeable strata). The Chalk Formation underneath the site is highly productive and supports a number of abstractions for commercial and public supply. The major aquifer is shown to be vulnerable as the site is located in an urban setting. 2.9 The Environment Agency website indicates south-eastern corner of the site lies within the within the groundwater source protection zone I (SPZI) and the eastern part of the site within the SPZ II (Ref 3). (See Figure 2) Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-5 -

10 abc 3.0 EXISTING DRAINAGE SYSTEMS 3.1 Thames Water Utilities (TWU) was contacted in order to ascertain the location of the foul and surface water drainage in relation to the development site. The plans received from TWU revealed that: There are surface water sewers to the west of the site, within the Kimpton Road serving the existing GM Site. A 450mm pipe conveys surface water from the land southeast of the Kimpton Road to an outfall (River Lea), which is situated to the west of Kimpton Road. The nearest foul water sewer can be found in Kimpton Road, serving properties adjacent to this street. A single 750mm diameter pipe runs from northwards and then turns west. The existing on site Trade Effluent Treatment Plant will be retained and serve the remained Van Production Plant. On-site Drainage Systems 3.2 Surface water from the development site currently discharges to the public surface water sewer in Kimpton Road at five locations. All the outfalls taking surface water from car parking areas pass through petrol interceptors prior to discharge. Those outfalls which carry surface water from roofs and / or road - do not. 3.3 Foul water discharges to the public foul sewer also in Kimpton Road at three locations and to a Thames Water adopted foul sewer through the Vauxhall site into an existing foul water sewer within Kimpton Road. 3.4 A trade effluent drainage system exists on the site but this is redundant and will remain so following completion of the development. 3.5 The area to the southeast of the site under consideration is to remain part of the Vauxhall plant. At this time it is understood the sewers along the south-eastern boundary between the retained land and the land sold for development are to be modified or cut off as part of the development. There are no details of which specific services are to be severed. 3.6 There existing drainage network on site will be removed and replaced with new drainage network on site designed to Sewers for Adoption 5 th edition and BS 752: See Drainage Strategy below. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-6 -

11 abc 4.0 DRAINAGE STATEGY Surface Water Drainage Strategy 4.1 PPG25 identifies how built development can cause flooding by increasing run-off. Development usually increases the area of impermeable surfaces, promoting rapid run-off to surface water sewers/watercourses rather than percolation into the ground. The effect can be to increase both total and peak water flows, contributing to flooding. 4.2 The majority of the site is currently impermeable. Surface water run-off from the site currently is collected by the surface water drainage on site which drains to Kimpton Road. 4.3 Based on the currently available drawings (see Appendix D & E), the new development will result in an overall increase in soft landscaping by approximately 6,000m 2 from 23116m 2 existing soft landscaping to 29150m 2 : Majority of the new soft landscape areas will be located on the newly created slopes with the exception of an area in front of the hotel, identified on the attached Development Framework as OS4, see Appendix D. Greenfield run-off 4.4 Although it a brownfield site and all existing surface water goes to public sewers, the Environment Agency has stated that it expects surface water discharges from the site to be limited to the greenfield runoff rate and requested that there should be no increase in surface water run-off above that of a 1 in 1 year greenfield site. We have investigated this to ascertain if a Greenfield runoff will be feasible in this instance. 4.5 Consideration has been given as to whether a Greenfield run-off approach to the assessment of source control was feasible. 4.6 The Greenfield runoff rate has been calculated using the methodology given by the ADAS Reference Book 345 (Ref 2). 4.7 Q o = S T x F x A Where Qo: Peak Flood Flow S T : Soil Type Factor F includes: Catchment characteristic Average annual rainfall Crop type (grass) A is Catchment Area (ha) For the 22.7 ha catchment area, Greenfield Runoff Rate Q o = 200 l/s. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-7 -

12 abc 4.8 A preliminary storage estimation undertaken on WinDes Software indicated that restricting the discharge to a greenfield runoff rate of 9.2 l/s/ha would require approximately 6600m 3 of storage for a 1 in 30 year return period and 9100m 3 for a 1 in 100 year return period. It would not be possible to provide this volume of storage on site. Therefore the proposed surface water strategy is to provide as significant reduction in surface water runoff, although not to greenfield runoff rate. 4.9 The Environment Agency, in accordance with PPG25 (Ref 4), seeks to reduce flood risk, not keep the status quo. The EA is looking for a significant reduction in the runoff from the site and an increase in attenuation To assess the feasible volume of attenuation we have first assessed the capacity of the existing drainage outfalls from the site. The existing entire GM site has three surface water outfalls. The proposed development covers a proportion of this site but due to the location at these existing outfalls can only use one of these three The existing drainage network was assessed using standard drainage design method and undertaken using the Micro Drainage software. The rainfall was based on the Wallingford Procedure, using The Modified Rational Method The principle of the strategy is to use the available capacity in the existing outfall and attenuate the remainder on site. This will reduce the peak discharge from the site to less than currently experienced by using a feasible volume of storage on site. However it will not reduce runoff to as low a level as Greenfield runoff Based on the Modified Rational Method, we calculated the current run-off from the site which is 2.78 CIA. = 2.78 X X 50 X 0.85 = 2306 l/s. The entire impermeable area of site currently drains to public water sewers via three existing outfalls The existing outfall that we propose to use as the single outfall from the new development passes beneath Kimpton Road via manhole numbers (as per Thames Water Records) will form the basis of the proposed drainage discharge. Based on the Hydraulic Research - Tables for Hydraulic Design of Pipes and Sewers, for the existing pipe diameter, 600mm, hydraulic gradient 1/27 and assumed pipe friction k s = 0.6, the capacity of the sever has been calculated as 0.96m 3 /s (960 l/s); with the velocity 5.167m/s. The drainage strategy proposes to use one existing outfall beneath Kimpton Road at manholes Flows in excess of 960 l/s will be attenuated on site. This is a significant improvement on existing peak discharge at 2306 l/s as outlined in 4.7 above. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-8 -

13 Site constraints abc 4.16 The proposed development is partly situated within a Groundwater Source Protection Zone (SPZ), The Environment Agency's Policy and Practice for the Protection of Groundwater states that no soakaway will be allowed in SPZ The site lies on a major aquifer (The Chalk) with a southwestern groundwater flow direction. Additionally there is a public water abstraction point about 700m down gradient. The EA does not therefore recommend the use of soakaway for surface drainage on this site. Only roof drainage should be discharged directly to soakaways without bypassing the petrol interceptor Additionally the Guidance states that no soakaways shall be constructed in contaminated lands. The attached soil and groundwater chemical analysis show elevated levels of Contaminants of Concern (CoC) Violating the above recommendations could constitute a direct discharge into controlled waters which contravenes the Water Resources Act 1991 and the Groundwater Regulations The existing topography is steeply terraced down to the south. Although the proposed development will be split into plateaus to overcome this steep gradient on the north-western part of the site, the steepness of the site limit the use of underground attenuation tanks. Proposed Surface Water System 4.21 Under the Sewers for Adoption standards, the surface water system should be designed under pipe full bore conditions to accept the 1 in 2 year design storm without surcharging above the pipe soffit (BS EN 752: 1996). For a 1 in 30 year return period, the system can be designed to surcharge but not to flood under these conditions on any part of the site Storage should be provided to protect from a 1 in 100 year critical duration storm event including increased run-off due to the future effects of climate change. The 1 in 100 year flows would need to be attenuated within the site without by-passing the flow control structure and without causing flooding to buildings The main characteristics of the drainage system are as follows (See Figure 3, Drainage Schematic): The site naturally falls east to west and a future positive drainage system for the site would need to drain by gravity to the existing connection on Kimpton Road, manhole number Assuming 1 in 30 year return period and based on 0.96m 3 /s restricted discharge the volume of attenuation required to attenuate surface water runoff from the impermeable areas in whole site will be 2970m 3. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A03-9 -

14 abc For 1 in 100 year return period and based on the 0.96m 3 /s restricted discharge, the required volume of attenuation will be 4500m 3. When taking into account climate change and adding 20% of rainfall intensity, the required attenuation will raise to 5840m 3. A combination of tank sewers and stormcell units will be used to accommodate the required storage. Approximately 6800m 3 storage volume is required to attenuate the storm water runoff from the whole site for up to the 1 in 100 year event including climate change. An off-line attenuation system could be provided in a form of geocellular structure for the 1 in 100 year event within the open space area OS4 (see Appendix D, Development Framework Plans). A 2.5m deep Stormcell structure of dimensions 54m x 45m, could be provided within the area shown on the attached Drainage Schematic to provide around 6000m 3. Additional storage could be provided by oversized pipes and tank sewers up stream within the site. Additional storage could be provided at this location at ground level to for an event in excess of 1 in 100 year return period. Thames Water confirmed that tank sewers and culverts will be the only form of attenuation storage units that they will adopt (Ref 5). Due to soil contamination, there could be no soakways designed on site (Ref 6). All new road surface and hard-standing areas are to be drained by a positive drainage system which will convey water from all impermeable areas at the site to 900mm sewers beneath main roads and underneath the main boulevard highway. There attenuation will be split in a number of separate parts, with hydrobrakes at the end of each tank sewer to overcome the slope of the road (slope 1:10). From there, surface water will be discharged at a restricted rate to the public sewers. The discharge rate will need to be agreed with Thames Water and the EA at the detail design stage. All surface water from roads and hard standings susceptible to petrol contamination shall be passed through an appropriate interceptor tank designed and constructed to have capacity compatible with the site being drained. On-going maintenance of the interceptor shall be provided in accordance with the manufacturers instructions. The drainage system should be sealed and capable of preventing accidental, unauthorised discharge of contaminated liquid to groundwater. It would be advisable to run a cut off ditch along the edge of the northern parameter of the site to prevent overland flooding from areas above the site and on the base of the slopes Permeable paving could be used for the residential parking areas on north-eastern part of the site, with surface water attenuated in a shallow bed of granular material overflowing into the new highway Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A

15 abc drainage system via a filter drain. To protect the groundwater below from any specific pollution risk, an impermeable membrane placed within sub-base There is the opportunity to recycle rainwater from roof run-off within the development but it will be plot specific and depend on building form and user requirements Under present legislation there is no method for adoption of SUDS. The most appropriate method of achieving implementation and long-term maintenance of SUDS is an agreement under Section 106 of the Town and Country Planning Act or a site management company. This could apply to the permeable paving and the off-line cellular tank. However on line pipes up to 1m diameter are acceptable to TWU for adoption. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A

16 abc Foul Water Drainage Strategy 4.27 The site will require a wholly new foul water drainage system. This would discharge to the existing public foul sewers in Kimpton Road at manholes 3801 and Whilst the majority of the site would be a gravity system, the far east end of the site may require a small package pump station depending on the final location of foul connections Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A

17 abc 5.0 CONCLUSIONS 5.1 All new road surface and hard-standing areas are to be drained by a positive drainage system., which will convey water from all impermeable areas at the site to tank sewers beneath main roads and underneath the main boulevard highway. There attenuation will be split in a number of separate severs, with hydrobrakes at the end of each tank sewer to overcome the slope of the road (1:10). From there, surface water will be discharged at a restricted rate to the existing public sewers in Kimpton Road. 5.2 An off-line attenuation system could be provided in a form of geocellular structure for the 1 in 100 year event within the open space OS4 as shown on the Development Framework Plans. Additional storage could be provided at this location at ground level to for an event in excess of 1 in 100 year return period. 5.3 Permeable paving could be used for the residential parking areas on north-eastern part of the site, with surface water attenuated in a shallow bed of granular material overflowing into the new highway drainage system via a filter drain. To protect the groundwater below from any specific pollution risk, an impermeable membrane placed within sub-base. 5.4 The drainage system should be sealed and capable of preventing accidental, unauthorised discharge of contaminated liquid to groundwater. Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A

18 abc 6.0 REFERENCES 1. Wardell Armstong, (April 2005), Former IBC Vehicle site, Kimpton Road, Luton Bedfordshire, Ground Investigation Report; 2. Wardell Armstong, (May 2005) Outline Remediation and Reclamation Strategy; 3. Ministry of Agriculture, Fisheries and Food, ADAS Reference Book: The Design of Field Drainage Pipe Systems; 4. Department of Transport Local Government and the Regions (DTLR) (July 2001) PPG25, Planning Policy Guidance Note 25-Development and Flood Risk. The Stationary Office, Norwich; 5. Correspondence with Thames Water Utilities; 6. Correspondence with Environment Agency; Kimpton Road, Luton July 2005 Outline Drainage Design WBC (DR) 002 A

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