DRAINAGE MANAGEMENT PLAN. HERTFORDSHIRE RESIDUAL WASTE TREATMENT PLANT Hertfordshire County Council

Transcription

1 DRAINAGE MANAGEMENT PLAN HERTFORDSHIRE RESIDUAL WASTE TREATMENT PLANT Hertfordshire County Council EMPLOYER: Veolia Environmental Services (UK) Plc Veolia House 154A Pentonville Road London N1 9PE Copy No Rev. A03 Date: 14th January 2011 ISFT submission

2 Herts EfW Drainage Management Plan Revised December 2010 Client Name: Reference: Issue: Vinci Construction C11758 (DR) Herts EfW 0001 A02 3 rd issue, revision by VINCI for ISFT Quality Assurance Approval Status This document has been Prepared and checked in accordance with Waterman TDL IMS (BS EN ISO 9001: 2000 and BS EN ISO 14001: 2004) Prepared by: Harry Chetty Position: Associate Director Checked by: Harry Chetty Position: Associate Director Approved by: Richard Simmonds Position: Divisional Director Pickfords Wharf, Clink Street, London, SE1 9DG t f

3 A VINCI revised for ISFT layout change VINCI A Greywater Recycling added HC HC A Preliminary Issue HC HC Rev. Date Description Prepared Checked Approved Waterman Transport & Development Limited have prepared this report in accordance with the instructions of the Client, Vinci Construction. It is prepared solely for their specific use. It is nonassignable and Waterman Transport & Development Limited accept no liability for its use by a third party.

4 CONTENTS EXECUTIVE SUMMARY INTRODUCTION DEVELOPMENT PROPOSALS THAMES WATER REQUIREMENTS ENVIRONMENT AGENCY REQUIREMENTS SURFACE WATER DISPOSAL METHODOLOGY FOUL WATER DISPOSAL METHODOLOGY RECOMMENDATIONS REFERENCES...9 Tables Table 1 Table 2 Thames Water Sewers around the vicinity of the Site Take-off of Proposed Areas Appendices Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Site Location Plan Development Proposals Topographical Survey Thames Water Sewer Plans Correspondence with the Environment Agency/Thames Water Proposed Drainage Layout Drawings

5 EXECUTIVE SUMMARY This Drainage Management Plan has been prepared by Waterman Transport and Development Limited, and revised by VINCI for the ISFT layout, for the proposed development at New Barnfield Site. This report presents the drainage strategy for the site, identifies any areas that are susceptible to flood risks and indicate the locations of surface water attenuation and connections to the public sewer. Thames Water need to be consulted with regard to discharging both foul water and surface water to the adjacent sewers. The total surface water attenuation within the site based on discharge to the surface water sewer is an average of 1500 m 3 for the 1 in 100 storm events plus climate change. The number of deep borehole soakaways required will be confirmed once the permeability tests in the chalk layer are carried out. Based on permeability tests for a different site in Hertfordshire, a deep borehole soakaway solution has been provided. However the effect of dissolution in the chalk will need to be considered by a Geotechnical Engineer. Based on the flow rate from an industrial plant, the foul water discharge rate has been estimated to be a peak flow rate of 5 l/s. A 24 hour storage requirement of 110 cubic metres may be required if at the detailed design stage it is demonstrated that the discharge needs to be pumped to the outfall.

6 1. INTRODUCTION 1.1 Waterman Transport and Development Ltd (WTDL) have been commissioned to carry out a Drainage Management Plan for the Stage D submission for proposed drainage works at the Hertfordshire EfW development at the New Barnfield Site. 1.2 This report examines the strategy in conveying both surface water and foul water from the site to the surface water and foul water networks adjacent to the site and soakaways for the surface water only. 1.3 A Flood Risk Assessment (FRA) carried out by White Young and Green has already been submitted to the Local Planning Authority as part of the outline planning application. The Site 1.4 The Herts EfW development in Hatfield (thereafter referred to as the Site ) refers to a plot of land, approximately hectares (ha) in area, located between Hatfield and Welham Green. The Site is centred on National Grid Reference , and is shown in Appendix A. 1.5 The site is located to the south west of Travellers Lane and bounded to the north by a school and to the east by a distribution centre. The western and southern site boundary is formed of greenfield areas. The site boundary is presented in Appendix A. 1.6 A topographical site survey, shown in Appendix C, which forms part of the FRA, indicates a west to east fall with ground levels ranging from approximately 108.0m Above Ordnance Datum (AOD) towards the west of the site to 96.5m AOD to the east of the site. The existing building footprint appears to be carved into the hillside to a varying floor level of to m. 1.7 The site is located is located in an area designated by the EA as a major aquifer and is within the Total Catchment of the Source Protection Zone. 1.8 The EA flood maps indicate that the site is located within a Flood Zone I, which is considered to be an area with low probability from flooding. 1.9 From the intrusive site investigation, the geology of the site consists of a made ground over Boulder Clay ranging between 6.1 and 8.6mm depth over Reading Beds ranging between 5.1m and 12.3m depth over white chalk, comprising of sandy, silty chalk gravel. 2. DEVELOPMENT PROPOSALS 2.1 The proposals involve the total demolition of the existing site and redevelopment into an Energy for Waste plant. An architect s proposal has been attached in Appendix B. 2.2 The proposed building will encompass from administrative offices, to the waste bunker, turbine and steam generation halls and all other processes that is required to form an EfW plant.

7 3. THAMES WATER REQUIREMENTS 3.1 The sewer records (See Appendix D) shows that Thames Water sewers are present in the roads adjacent to the site as indicated below: Locations South Way Sewers 225mm Surface Water Sewer 225mm Foul Water Sewer Table 1: Thames Water Sewers around the vicinity of the Site 3.2 The Thames Water sewer plans do not appear to indicate any foul or surface water sewers in Travellers Lane, and therefore it likely that the sewers indicated by the topographical survey in Travellers Lane are private. 3.3 There will be a requirement to confirm the outfall location of the foul and surface water sewers from the proposed site into the private sewers in Travellers Lane. 4. ENVIRONMENT AGENCY REQUIREMENTS 4.1 There is a requirement to seek the Environment Agency approval to use soakaways at the above site. It is noted that the site is within a Source Protection Zone and that the use of deep borehole soakaways may need detailed negotiation with the Environment Agency. 4.2 Permeability tests using deep borehole soakaways will need to be carried out to confirm that this type of soakaway is feasible on this site and the depth of the borehole will need to be confirmed to the Environment Agency. 4.3 The site investigation carried out by White Young Green indicated that there was a possibility of chalk dissolution taking place which can lead to features being formed within the chalk. This needs to be fully investigated by a geotechnical engineer with regard to discharging water into the chalk layer which is located 15 to 18m below ground level. 4.4 The Environment Agency consent will be required to discharge rainwater run-off into deep borehole soakaways as the site is wholly located with a Source Protection Zone III.

8 5. SURFACE WATER DISPOSAL METHODOLOGY 5.1 The surface water sewer system shall be designed to convey effluent separately. Sewers will be designed to comply with BS EN 752 and Building Regulations Part H. 5.2 As the proposed development site is above surrounding ground level of adjacent properties, the surface water system has been designed not to flood for a 1 in 100 year storm event plus climate change as any surface water flooding is likely to flood adjacent properties. 5.3 Local Planning Authorities have recommended in the outline planning permission that all Sustainable Urban Drainage Systems (SUDS) should be considered and where appropriate, should be implemented to reduce the risk of flooding. 5.4 The implementation of sustainable water management through sustainable drainage systems (SUDS) and rainwater harvesting is becoming more common in an effort to use and manage water sustainably. The philosophy of SUDS is to mimic the natural drainage patterns of the land prior to development as closely as possible and treat runoff to remove pollutants. The use of vegetative features to treat pollution and reduce flow rates provides the opportunity to enhance the landscape and provide wildlife habitat. 5.5 SUDS provide a flexible approach to drainage with a wide range of options available. Individual techniques should be used in a management train that reinforces and, where possible, follows the natural drainage patterns. The management train incorporates a hierarchy of techniques, as detailed below:- 5.6 The following SUDS techniques that can be implemented on this site as shown in the in the Surface Water Drainage Layout are as follows: Rainwater Harvesting Tanks This SUDS method involves collecting rainwater from the roofs of buildings and storing this run-off in underground storage tanks. This rainwater shall undergo a filtration process before being recycled back into the building, to be used to flush toilets for example. To keep within the budgetary constraints, the rainwater harvesting tanks can be used for irrigation purposes only, which means that the capital cost of providing UV filtration and separate pipework within the building does not have to be considered. An overflow from the rainwater harvesting tank has been provided flowing directly into the pond for extreme rainfall events, where they may not be enough storage within the rainwater harvesting tank Greywater Recycling This SUDS method involves collecting rainwater from the showers and wash basins and storing this discharge in underground storage tanks. This water shall undergo a filtration, disinfection process before being recycled back into the building to be used to flush toilets. The most advantageous of this system is that it reduces the usage of potable water in the long term Swales System Swales are primarily used as conveyance systems, so they can be laid at very shallow longitudinal gradients. This is extremely important so that erosion cannot take place, so velocities are restricted to 1.5m/s. The swales will be dry during dry weather but during a rainfall event, flows will enter the swale and move slowly through the grass channel. The flow of surface water is retarded and filtered by the grass. Swales can be increased in depth to provide additional attenuation for the extreme storm events. Within this site, a swale can be provided between the existing pond and the visitors centre, with a discharge to the existing pond on site, and from there into the existing surface water drainage system at the site boundary.

9 5.6.4 Permeable Paving Permeable paving is a load bearing construction surfaced with materials that allow surface water to enter the underlying construction. It consists of block paviours that have slots cut into their sides or porous asphalt, allowing surface water to drain through the surfacing and into a granular sub base, from where infiltration into the underlying subsoil can occur. Permeable paving can successfully remove oil and other contaminants in the water via a membrane which breaks down hydrocarbons as it flows through the granular sub base layers, improving the quality of water discharged into the surface water sewer and from there into the river. In areas where permeable paving was installed, there would be no need for traditional drainage gullies network ironwork or for oil interceptors. Confirmation from the Environment Agency will be required to ensure that this surface water is clean enough to discharge to the ground via infiltration methods. Due to the underlying chalk, direct infiltration to this material through deep borehole soakaways may be accepted by the Environment Agency. Further consultation will be required to be sought. Permeable paving can only be used in the footway and car parking areas as they have been found to be unsuitable for HGVs manoeuvring areas. Exact details of the permeable paving will be provided based on the architectural layouts Green roofs Green roofs can intercept water and attenuate it within the soil or through an irrigation system, reducing the volume of runoff from buildings. Green roofs contribute to attenuation storage as they can prevent run-off from the roof for small rainfall events. For larger rainfall events the storage element is minimal however the time it takes for water to enter the surface water network is increased Cellular Attenuation or Infiltration Units Cellular attenuation units are plastic tanks that are installed below ground and can be used to provide storage before discharge or infiltration. Cellular units can be placed in all external areas where a minimum cover of 600mm can be accommodated but will need to be located close to the outfall. The soft landscaping will retain the predevelopment or greenfield surface water run-off profile. These areas will not require any attenuation as these areas will not positively drain into the surface water network. 5.7 A take-off impermeable areas has been carried out and the table below indicates these areas: Surfacing Area (m 2 ) Green Roof (beneath 260 main roof) Roof 20, 100 External Hard Paving 8, 000 Permeable Parking 700 Soft Landscaping 23, 900 Total 52, 700 Table 2: Take-off of Proposed Areas 5.8 Surface Water run-off will be restricted to the private sewer in Travellers Lane based on the full bore capacity of its last connection, which is a discharge rate of 23 l/s. 5.9 Permeability rates in the chalk have not been carried out as part of the site investigation and therefore the deep bore hole soakaways that can be used have been based on the permeability rates obtained at other sites. It is noted that the permeability rate used is 1 x 10-5 m/s with a safety

10 factor of 5, which a requirement under Clause of CIRIA 156 Infiltration Drainage-Manual for Good Practice The proposed drainage network option has been indicated on the drawing given in Appendix E Attenuation will be provided in a new pond, to be located close to the existing outfall For external areas which will be trafficked by HGV vehicles, Class 1 petrol interceptors have been provided to reduce the risk of contamination from petrol and oil. Surface Water Drainage Layout Option Gravity discharge to the outfall, via attenuation in ponds and / tanks Surface water attenuation provided based on discharge of 23 l/s Surface water attenuation of approximately 1500 cubic metres is required based on storm duration of 1 in 100 years with 20% climate change. Surface Water Drainage Option Gravity discharge to the soakaway field with pumping station provided at the manhole prior to the soakaways Surface water attenuation provided based on discharge into the chalk using deep borehole soakaways Based on the permeability rates, it is calculated that there is a requirement of 184 number of 1m diameter deep boreholes at a depth of 20m into the chalk layer, therefore overall borehole length of 40m This option is deemed to be not economically feasible and a drawing has not been provided.

11 6. FOUL WATER DISPOSAL METHODOLOGY 6.1 The foul water system shall be designed to convey effluent separately. Sewers will be designed to comply with BS EN 752 and Building Regulations Part H. 6.2 Sustainable forms of foul water drainage such as reed beds have not been considered due to the size and area required and the presence of the Thames Water sewer in the adjacent road to the site. 6.3 The total peak foul water flow from the site is 5.0/s. Foul Water Layout 6.4 Gravity discharge to the outfall. 6.5 Based on Building Regulation requirements, 24 hour storage is required where foul water is being pumped. Based on an average industrial flow of 550 cubic metres per 100 square metres, the foul water storage required for a day is 110,580 litres or cubic metres.

12 7. RECOMMENDATIONS 7.1 Direct connection into the surface water and foul water sewers in Travellers Lane are required based on the full bore capacity of the existing connections. 7.2 The site investigations carried out to date have not provided any permeability tests in the chalk layer. There is a requirement for the permeability test to be carried out in accordance to BRE 365 and to confirm that deep borehole soakaways can be used. 7.3 The proposed external levels throughout the site have been assumed without any groundworks modelling being carried out and therefore it is noted that the drainage strategy as indicated may be revised. 7.4 We have assumed at this stage that some green roof will be used but have not considered any possibility of storage within the green roof. 7.5 We will need confirmation from Thames Water that the peak foul water discharge of 5.0 l/s is acceptable from the proposed development. 8. REFERENCES 1. Communities and Local Government (December 2006), Planning Policy Statement 25: Development and Flood Risk. The Stationary Office, Norwich. 2. Communities and Local Government (June 2008), Planning Policy Statement 25: Development and Flood Risk. The Stationary Office, Norwich. 3. SUDS Manual (2007), CIRIA 697. Classic House, Old Street, London.