APPENDIX Surface Water Drainage Strategy

Transcription

1 APPENDIX 12.2 Surface Water Drainage Strategy

2 London City Airport Proposed Surface Water Drainage Strategy London City Airport Development Programme 12 July 2013

3 London City Airport Surface Water Drainage Strategy Notice This document and its contents have been prepared and are intended solely for London City Airport s information and use in relation to City Airport Development Programme. Atkins/TPS assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents. This document has 62 pages including the cover. Document history Job number: Document ref: Revision Purpose description Originated Checked Reviewed Authorised Date Rev 1.0 Draft Issue for EA comment Atkins KB TPS WH Atkins DH TPS MW Atkins DH TPS AF Atkins MdS TPS AF 14/03/13 Rev 2.0 Rev 3.0 Rev 4.0 Final Planning Submission Final Planning Submission Final Planning Submission Atkins KB TPS WH Atkins KB TPS WH Atkins KB TPS WH Atkins DH TPS MW Atkins DH TPS MW Atkins DH TPS MW Atkins DH TPS AF Atkins DH TPS AF Atkins DH TPS AF Atkins MdS TPS AF Atkins MdS TPS AF Atkins MdS TPS AF 16/05/13 24/05/13 12/07/13 Client signoff Client London City Airport Project Document title London City Airport London City Airport - Surface Water Drainage Strategy Job no. ATKINS TPS Copy no. 1 Document reference LCY-CADP-ATK-TPS-RPT-0001 LCY SWD Strategy, Version 4.0, 12 July 2013

4 London City Airport Surface Water Drainage Strategy Chapter Pages Executive Summary 4 1. Introduction 5 2. Proposed Development Overview 6 3. The Consultation Process 7 4. Existing Surface Water Infrastructure 9 5. Proposed Surface Water Drainage Strategy - Overview 12 Design Basis Proposed Surface Water Drainage Strategy Airside Area 17 Site Constraints 17 Proposed Airside Drainage Strategy 18 Airside Drainage System Design Proposed Surface Water Drainage Strategy Landside Area 23 Overview 23 Landside catchments 23 Landside Area Drainage Strategy 24 Western Terminal Extension Catchment Area 26 Forecourt Catchment Area 26 Dockside Catchment Area 27 Extreme surface flooding Conclusion 36 Appendices 39 Appendix A. Existing Catchment Plan 40 A.1. Existing Drainage Catchment Western Terminal Extension and Forecourt 40 A.2. Existing Drainage Catchment Dockside Sub Catchment 1 41 A.3. Existing Drainage Catchment Dockside Sub Catchment 2 42 Appendix B. SUDS Evaluation 43 Appendix C. Surface Water Drainage Designs 44 C.1. Airside Surface Water Drainage Design 44 C.2. Landside Western Terminal Extension Surface Water Drainage Design Drawing 53 C.3. Landside Forecourt Surface Water Drainage Design Drawing 55 Appendix D. Exceedance Event Drawings (1 in 100 Yr + 20% Climate Change) 57 D.1. Airside Exceedance Drawings 57 D.2. Landside Exceedance Drawings 60 LCY SWD Strategy, Version 4.0, 12 July 2013

5 Executive Summary This Surface Water Drainage Strategy has been jointly prepared by Atkins and TPS on behalf of London City Airport (the Airport ). The Strategy accompanies the proposed City Airport Development Programme (CADP) to extend the infrastructure and passenger facilities at the Airport, including modifications and extensions to the existing Terminal building, the creation of 7 new and 4 reconfigured aircraft parking stands, a parallel taxilane and ancillary infrastructure (proposed within Planning Application CADP1), and a Hotel (proposed within Planning Application CADP2). In preparing the Strategy the following drainage options have been investigated in consultation with key stakeholders and the findings are summarised in this report: The viability of using various types of Sustainable Urban Drainage systems (SUDS), including Porous Pavements and Attenuation Tanks; Discharging to the existing Thames Water sewer systems; Discharging to King George V Dock; and Utilising Infiltration Drainage. The proposed Strategy varies across the proposed CADP surface water catchment areas (as defined in Figure 4) and is summarised as follows: For the Airside Area, where there are a number of airfield operational, safety and space constraints, the usage of Sustainable Urban Drainage Systems (SUDS) is limited and infiltration is not a viable option. It is therefore proposed to provide a pipe system with an attenuation tank with a pumped outfall to the existing airside drainage system. In order to minimise the size of the attenuation tank (which will need to be located within the runway strip), it is proposed to provide a maximum discharge without causing additional flooding of the existing drainage system. The increase in runoff airside will be offset through the reduction in runoff from the landside development. Outside of winter months and only during the winter months when de-icing content is acceptable, there is an opportunity to discharge to KGV Dock, which will be investigated further at later design stages or as part of discharging planning conditions/reserved matters; For the Landside Area it is proposed to provide pipework networks with attenuation tanks. The flows will discharge to the existing Thames Water Network with a significant reduction in the pre-development existing flow rate. The usage of SUDS techniques is limited in most of the CADP surface water catchment areas, due to the proximity of the airport (for open water features), space constraints and heavy traffic, however there is an alternative option for the landside areas of utilising permeable paving and discharging to KGV Dock, or discharging to infiltration systems. These options would also significantly reduce the pre-development flows rates. The options will be investigated further during later design stages or as part of discharging planning conditions/reserved matters. This will be in consultation with RoDMA, the Environment Agency and other relevant consultees; and Rainwater from the roofs of the extended Terminal buildings is proposed to discharge directly to KGV Dock. Due to the archaeological significance of the landside site, potential contamination and high risk of encountering existing buried underground structures (existing retained building foundations in the Dockside Development area), there is also an environmental benefit to reducing the size of the tanks, as far as possible. This will limit the need to dispose of contaminated material and disturbance heritage assets. The Strategy follows the drainage hierarchy stated in London Plan Policy 5.13 Sustainable Drainage, and limits flows to the existing sewers as far as possible. The 1 in 100 yr + 20% climate change storm event has also been considered and the Strategy demonstrates that the surface water associated with this will not overflow into any off-site areas and demonstrates how this will be contained within the Airport.

6 1. Introduction 1.1 This document describes the proposed Surface Water Drainage Strategy for the proposed City Airport Development Programme (CADP). It accompanies Planning Application CADP1 for the extended airfield infrastructure and enhanced associated passenger facilities, and Planning Application CADP2 for a Hotel. Together these applications form the proposed CADP. 1.2 The Strategy is divided into the following sections: A description of the proposed CADP; An explanation of the pre-application consultation process that informed the Strategy approach; A description of the existing surface water infrastructure systems within the CADP proposal areas; and The final sections that describe in detail the Airside Area and Landside Area surface water drainage proposals. 1.3 Atkins has been commissioned by London City Airport ( the Airport ) to design the surface water drainage strategy for the Landside Area. The Landside Area is defined on Figure 1 below. 1.4 TPS has been commissioned to deliver the surface water drainage design for the Airside Area. The Airside Area is also defined on Figure 1 below. KING GEORGE V DOCK LEGEND: AIRSIDE LANDSIDE EXISTING BUILDING Figure 1. Catchment Plan: Airside and Landside 1.5 In undertaking this work, Atkins and TPS have liaised closely throughout to ensure that a holistic Surface Water Drainage Strategy is prepared that covers the whole of the proposed CADP area. LCY SWD Strategy, Version 4.0, 12 July

7 2. Proposed Development Overview 2.1 Two planning applications are proposed: CADP1 seeks detailed planning permission for new passenger facilities and infrastructure; CADP2 seeks outline planning permission for a hotel. 2.2 The proposed description of development is as follows: Planning Application 1 (CADP1) Planning Application CADP1: Works to demolish existing buildings and structures and provide additional infrastructure and passenger facilities at London City Airport without changes to the number of permitted flights or opening hours previously permitted pursuant to planning permission 07/01510/VAR. Detailed planning permission is being sought for: a) Demolition of existing buildings and structures; b) Works to provide 4 no. upgraded aircraft stands and 7 new aircraft parking stands; c) The extension and modification of the existing airfield to include the creation of a taxilane running parallel to the eastern part of the runway and connecting with the existing holding point; d) The creation of a vehicle access point over King George V dock for emergency vehicle access; e) Laying out of replacement landside Forecourt area to include vehicle circulation, pick up and drop off areas and hard and soft landscaping; f) The Eastern Extension to the existing Terminal building (including alteration works to the existing Terminal) to provide reconfigured and additional passenger facilities and circulation areas, landside and airside offices, immigration areas, security areas, landside and airside retail and catering areas, baggage handling facilities, storage and ancillary accommodation; g) The construction of a 3 storey passenger pier to the east of the existing Terminal to serve the proposed passenger parking stands; h) Erection of a Noise Barrier at the eastern end of the proposed Pier; i) Erection of a temporary Noise Barrier along part the southern boundary of the Application Site to the north of Woodman Street; j) Western Extension and alterations to the existing Terminal to provide reconfigured additional passenger facilities and circulation areas, security areas, landside and airside offices, landside retail and catering areas and ancillary storage and accommodation; k) Western Energy Centre, storage, ancillary accommodation and landscaping to the west of the existing Terminal; l) Temporary Facilitation Works including the erection of a Noise Barrier to the south of 3 aircraft stands, a Coaching Facility and the extension to the outbound baggage area; m) Works to upgrade Hartmann Road; n) Landside passenger and staff parking, car hire parking and associated facilities, taxi feeder park and ancillary and related work; o) Eastern Energy Centre; p) Dock Source Heat Exchange System and Fish Refugia within King George V Dock; and q) Ancillary and related work Planning Application 2 (CADP2) Planning Application CADP2: Erection of a Hotel with up to 260 bedrooms, ancillary flexible A1- A4 floorspace at ground floor, meeting/conference facilities together with associated amenity space, landscaping, plant and ancillary works. LCY SWD Strategy, Version 4.0, 12 July

8 3. The Consultation Process 3.1 The preparation of the Surface Water Drainage Strategy has been directly informed by relevant policies from the Development Plan and through dialogue with the following key stakeholders and their associated remits: The Environment Agency (EA) o Pollution Prevention Strategy o Flood Risk Assessment o Environmental Impact Assessment Thames Water (TW) o Discharge Agreements Royal Docks Management Authority (RoDMA) o Discharge Agreements English Heritage o Protection of Asset of Heritage Interest London Borough of Newham o Local Development Framework Greater London Authority o The London Plan 3.2 Preliminary consultation was undertaken with the EA on 8 th April 2013.The key points discussed and agreed in the meeting are noted below: The CADP proposals should have a holistic approach to reducing surface water discharge, with an ideal aim of achieving Greenfield runoff rates in compliance with London Plan Policy (see paragraph 5.15 below); The EA accepted if this could not practicably be achieved, for instance due to space limitations, aircraft operations or ground conditions, then a 50% reduction in the predevelopment discharges to the existing sewer systems would be considered acceptable; The EA acknowledged that part of the development is built over the existing KGV Dock and therefore the strategy to reduce flow rates would be different from a typical land based development; The surface drainage hierarchy stipulated in London Plan Policy 5.13 should be followed, with the use of Sustainable Drainage Systems (SUDS) systems and a preference to infiltrate and connect to the existing sewer only if unavoidable. 3.3 The EA was sent a draft Surface Water Drainage Strategy Report and it provided comments on 8 th April 2013 (EA reference: NE/2013/117338/01-L01). The EA agreed in principle with the proposed Strategy and requested that it be updated to address how the surface water is managed in landside areas during a 1 in 100 year plus climate change storm event. It also requested clarification on the necessity of providing an airside surface water pump station. These clarifications have been provided in this final version of the Strategy. 3.4 Thames Water advised at meetings on 7 th February and 12 th April 2013, that it had no objections to the proposed drainage strategy, and to discharge to the existing surface and combined water connections, provided that the flow is equal or less than existing. TW advised that the existing system connections would require tracing to prove that they currently serve the site, when the formal application to connect is submitted. TW also advised that if a new connection is required, this would also be acceptable, if the flows do not exceed 10% of the existing pipe capacity. If this flow is exceeded than a capacity check may be required. All connections (existing or new) will be subject to formal agreement with Thames Water at later design stages or when discharging conditions/reserved matters. LCY SWD Strategy, Version 4.0, 12 July

9 3.5 RoDMA was consulted on 10 th December 2013 and 1 st May 2013, and it advised that there were no objections in principle to discharging to King George V Dock (KGV Dock), provided that the surface water discharges are clean and do not have an unacceptable adverse environmental impact on the Dock waters. RoDMA also stated that KGV Dock is an impounded body of water and water is pumped into the Dock on a regular basis from the River Thames to maintain the Dock water level. Any surface water discharge to the Dock would therefore help reduce the amount of pumped water required. It also advised that in an extreme condition of a coincidental highest astrological tide and storm surge conditions, their lock gates could be pushed open, and an inflow from water from the River Thames could make the Dock level rise higher. 3.6 A meeting was held with English Heritage on 6 th March 2013, who advised that it had no objections to pipework penetrating through the existing KGV Dock wall, provided that a record is provided of the wall prior to any construction works. The modifications to the Dock wall to accommodate the airside deck drainage infrastructure were also explained. English Heritage advised that it would advise the London Borough of Newham (LBN) (as the Local Planning Authority) to secure it s requirements, in relation to the recording of the Dock wall, through the placing of planning condition(s) on any planning permission. LCY SWD Strategy, Version 4.0, 12 July

10 4. Existing Surface Water Infrastructure 4.1 To inform the preparation of this Strategy the following information has been reviewed: The Airport drainage existing record drawings; LCY Building Services Report (by Atkins ref dated Dec 2009); LCY Western Terminal Extension area drainage CCTV survey (by Drain Aid Ltd dated 19th Nov 2012) ; Preliminary LCY Site Investigation (by RPS ref: HLEI March 2013 and HLEI 19695/001R [Phase 1 Report]); LCY Surface Water Drainage Survey (by MSA Project Number 3676 dated 4 th Mar 2013); Docklands Light Railway (DLR) as built drawings (series PWD-DUX-00029); and Thames Water Sewer Records (dated 2009). 4.2 From the above information, it has been determined that the existing CADP areas are drained via separate foul and surface water gravity collection systems. 4.3 Based on Thames Water asset plans, the existing drainage networks within the CADP area are private. This was also confirmed by Thames Water at the meetings held on 7 th February and 12 th April The Airside Area is drained by a number of existing surface water drainage sewers, which are shown on Figure 2 and described below: a) The airfield is typically drained from the east to the west due to the constraint of the surrounding Docks present to the north, south and east. There are four main runs that travel east to west as follows: o North of the runway (300 to 525 diameter); o South of the runway (300 to 525 diameter). This also picks up the runway hold drainage; o Eastern Stand Drainage (stands 21 to 24) and runway link Delta (225 to 750 diameter); and o Original stands Drainage (Stands 1 to 10) (600 diameter). b) These four runs connect into the same drainage system at manholes SWS44 and 45 to the north of the Airport fire station. An easterly drain run from the western part of the airfield also connects to the system around SWS44. c) From SWS44 the drain increases to 900 diameter and drains south east towards aircraft stand 14 where the drainage from stands 12 to 14 connect to the system. This 900 diameter drain connects to the Thames Water Network at manhole 8304 shown on Thames Water plan titled ALS/ALS Standard/2009_ This 950 diameter sewer soon increases in size to 1800 diameter and is named the Royal Docks Surface Water Sewer (Connaught Branch) d) The Airport is also implementing a small development at the western end of the airport to reinstate aircraft parking Stand 11 and to widen Taxilane Yankee. This provides new pavement to the airfield. The drainage for this pavement will have attenuation on the outfall and the outfall is expected to have an additional discharge in the order of 10.5 l/s. An additional connection to the airfield drainage system of this magnitude will not affect the proposed Strategy. 4.5 The Airside Area and proposed Eastern Terminal Extension is substantially built over KGV Dock, and therefore has no existing catchment or flow rate as rain falls directly into the Dock. In the Airside Area there are small areas of new pavement which were previously grass although due to their minor nature the existing flow characteristics has not been included in the assessment. LCY SWD Strategy, Version 4.0, 12 July

11 Figure 2. Existing Airside Surface Water Catchment Plan and Existing Sewers. A more detailed drawing (SK023) showing the existing airfield drainage system is included in appendix C The Landside Area is drained by a number of existing surface water drainage sewers, which are described below and identified in Figure 3: a) An existing surface water sewer crosses the Landside Area, flowing from east to west, and ultimately discharging to Ø1800 Royal Docks Surface Water Sewer (Connaught Branch) further west, which then runs north and east and discharges to the River Thames. The pipe sizes vary, starting near the existing Airport KGV building (opposite Fernhill Street) at Ø300mm and leaving the Landside area through the Western Terminal Extension site at Ø600mm. b) Further east from Fernhill Street (to the south of the DLR), there are a number of combined sewer connections which run north/south perpendicular to Newland Street, Brixham Street and Woodman Street, connecting to an Ø1200 Main Line combined sewer to the south in Factory Road. The survey undertaken by MSA indicated 13 surface and foul drainage routes from the proposed development area (1 to the west and 12 to the south, crossing the DLR line), however the exact outfall location points and connections to the Thames Water sewer will be agreed with Thames Water at later design stages or as part of discharging conditions/reserved matters. The connections are believed to be private extensions/stubs to the boundary of the Dockside Development ( Dockside ) area (see Figure 8 for catchment breakdown), via the adjacent DLR drainage system, which pick up runoff collectors, such as gullies and combined kerb drainage units. 4.7 In the Landside Area, a large proportion of the Dockside area does not have any positive drainage system. However, after a review of the existing topography and MSA drainage survey (dated 4 th March 2013), it is evident that a proportion will drain to the sewers in Hartmann Road, by overland flow. These areas contribute to the existing flow rate from the CADP area and have been included in the existing drainage catchment. 4.8 A number of the existing outfalls to the combined system south of the DLR, appear to have flow control units, and therefore in some instances flows have already been attenuated. This report makes no allowance for these devices, which are proposed to be modified/reduced (if required) to allow for the flow rates stated in this report. Further investigations will be undertaken to access these chambers in later design stages or as part of discharging conditions/reserved matters. LCY SWD Strategy, Version 4.0, 12 July

12 4.9 This results in an estimated pre-development existing flow rate of l/s draining to the existing sewer networks surrounding the CADP area, at a 50mm/hour rainfall event. If this flow rate was reduced by 50% upon redevelopment, this would result in l/s being discharged to the existing system, as indicated in Table 1 below It should be noted that the existing flow rate assessment is based on the drainage survey undertaken by MSA on 4 th March This survey determined the existing catchment as far as is reasonably practical for the Dockside area, any assumed piped connections will need to be verified at later design stages or as part of discharging conditions/reserved matters. Table 1. Catchment [2] Details of the Existing Surface Water Drainage Systems and Catchments. Existing Catchment Area (Ha) Existing Flow Rate (l/s) [1] Airside None 0 0 Western Terminal Extension Forecourt Dockside Sub-Catchment 1 Dockside Sub-Catchment TOTAL Notes: [1] Existing flow is based on a 50mm/hr storm over the entire existing catchment area. [2] Catchments are detailed on Figure 4. 50% Reduction in Flow Rate (l/s) 4.11 The extent of the existing CADP Landside catchment is shown in outline on Figure 3 and in detail in Appendix A. Figure 3. Existing Landside Surface Water Catchment Plan and Existing Sewers LCY SWD Strategy, Version 4.0, 12 July

13 5. Proposed Surface Water Drainage Strategy - Overview Design Basis 5.1 The surface water drainage network proposals included within the proposed CADP are based on the following: City Airport Development Programme Design and Access Statement London Plan (Major of London - Spatial Development Strategy for London July 2011); and LBN s Local Development Framework. Design Standards 5.2 The design standards employed for the networks and collection systems will meet the following design criteria: Sewers for Adoption 7 th Edition, August 2012 published by WRc plc; No surface flooding in 1 in 30 year rainfall return period plus 20% allowance for Climate Change (Climate Change not included in airside design to minimise tank size see Section 6); Consideration of a 1 in 100 year rainfall event plus 20% allowance for Climate Change, to ensure that extreme flood flows are directed away from critical Airport infrastructure and offsite areas; Maximise the employment of SUDS techniques for new installations within site constraints; Minimise flows to existing sewers to Greenfield runoff rates, as far as practicable [1]; Consideration of long-term maintenance and operation; Consideration of future system ownership; Consideration of minimising the abandonment and removal of existing assets; Incorporation of Petrol Interceptors to control hydrocarbon discharges in accordance with Environment Agency Pollution Prevention Guidance PPG5; and Infiltration Drainage (if adopted) to CIRIA SUDS Design Manual. Notes: [1] For the purposes of this Strategy, Greenfield runoff is assumed to be a conservative value of 5 litres/second per hectare up to a 1 in 30 yr storm event. Catchments 5.3 The proposed CADP has been divided into two main catchment areas consisting of Airside and Landside Areas. These areas have been split further into smaller catchments to suit the proposed future development and existing drainage regime as illustrated on Figure 4. Proposed Surface Water Drainage Strategy 5.4 The Surface Water Drainage Strategy base intent is to reduce the existing flow rate from the proposed CADP to Greenfield runoff rates using SUDS techniques, wherever practical. However due to space constraints, Airport operations, possible heritage assets and potential land contamination, the options to reduce the existing flow rate to Greenfield runoff rates are constrained. This is further described in Section 7 of the Strategy, which includes a detailed strategy for each individual catchment and how the overall reduction of the existing flow rate is achieved. 5.5 The potential use of infiltration drainage (to Landside Areas only) is discussed in Paragraphs 5.6 to 5.13 below, which will be investigated further at later design stages or as part of discharging conditions/reserved matters. LCY SWD Strategy, Version 4.0, 12 July

14 Figure 4. Proposed Catchment Areas Infiltration Potential 5.6 The proposed CADP is located outside of the Environment Agency Source Protection Zone for Potable Water. 5.7 The preliminary intrusive site investigation carried out by RPS in the Landside Area comprised 22 boreholes drilled to depths of up to 5m below ground level. The purpose of that survey was to investigate the potential for infiltration drainage. 5.8 Made Ground was encountered in all of the 22 boreholes to depths of between 4.7 and >5.0m below ground level. The Made Ground typically consisted of reworked natural materials (Alluvium and River Terrace Deposits) with localised fragments of anthropogenic material including brick, concrete, ash and clinker. The Made Ground was predominantly granular in nature, although localised pockets of sandy clay (reworked Alluvium) were encountered. Preliminary falling head tests were undertaken in monitoring wells installed into eight boreholes. In three of the holes water drained too quickly to perform the tests. In the other holes K values varied from 3.87e -04 to 2.05e -06. This is indicative of relatively rapid drainage rates that would be appropriate for infiltration drainage. This is however subject to further site investigation at later design stages or as part of discharging conditions/reserved matters. 5.9 A slight hydrocarbon odour was detected in shallow Made Ground sampled from one borehole (WS4) to the west of the existing Terminal. TPH compounds, predominantly in the range C12- C35 were detected in a sample of Made Ground collected from this borehole. Trace concentrations of hydrocarbons were also detected in a number of other samples of Made Ground. However, these were typically the longer chain, less mobile TPH compounds and at the measured concentrations are unlikely to pose a significant risk as a result of leaching if infiltration were to be increased. Concentrations of PAH compounds, metals and other inorganic determinants were typically low within the samples analysed, consistent with the fill comprising predominantly reworked natural material Depths to water during monitoring in the boreholes ranged from 1.96m below ground level at the eastern extent of the site to up to 4.22m below ground level in the vicinity of the existing LCY SWD Strategy, Version 4.0, 12 July

15 Terminal. Overall, due to the predominantly granular nature of the Made Ground and the absence of any significant contamination, infiltration may be viable at shallow depth Further investigation will be undertaken of the potential usage of infiltration in the Landside Area in later design stages or as part of discharging conditions/reserved matters, where additional soakage and contamination tests will be undertaken at any proposed infiltration drainage locations. The potential to use infiltration will also be constrained by heavily trafficked areas and the lack of space to accommodate such features, especially in the proposed Western Terminal Extension Service Yard and Forecourt Infiltration is not deemed appropriate for the Airside Area due to the following considerations: Increased hydrostatic water pressures behind the Dock wall structure; Potential for de-icants within the surface water runoff concentrating within the groundwater; and The preferred option for discharge into the Dock during permitted conditions This report demonstrates that even if widespread infiltration is not implemented, the aspirations of the EA, London Plan and LBN Local Development Framework can be achieved using a range of other SUDS techniques to reduce the existing flow rate. Sustainable Urban Drainage Systems 5.14 The objectives of SUDS are to minimise the impacts of development on the quantity and quality of site runoff and maximise amenity and biodiversity opportunities The London Plan Policy 5.13 Sustainable Urban Drainage Systems states the following: POLICY 5.13 SUSTAINABLE DRAINAGE Planning decisions A Development should utilise sustainable urban drainage systems (SUDS) unless there are practical reasons for not doing so, and should aim to achieve Greenfield runoff rates and ensure that surface water runoff is managed as close to its source as possible in line with the following drainage hierarchy: 1 store rainwater for later use 2 use infiltration techniques, such as porous surfaces in non-clay areas 3 attenuate rainwater in ponds or open water features for gradual release 4 attenuate rainwater by storing in tanks or sealed water features for gradual release 5 discharge rainwater direct to a watercourse 6 discharge rainwater to a surface water sewer/drain 7 discharge rainwater to the combined sewer. Drainage should be designed and implemented in ways that deliver other policy objectives of this Plan, including water use efficiency and quality, biodiversity, amenity and recreation This Strategy has been informed by a full evaluation exercise of all available SUDS features and their suitability to be employed in the proposed CADP and is included in Appendix B The proposed CADP is an operational airport, which is constrained by a number of features, including KGV Dock and the Docklands Light Railway (DLR), and therefore there is limited space for redevelopment. As a result there are a number of SUDS techniques that are not deemed practical. LCY SWD Strategy, Version 4.0, 12 July

16 5.18 The proposed Strategy follows the above drainage hierarchy (London Plan Policy 5.13) and the detailed approach for each CADP catchment area is described in sections 6 and 7. Notes relating to each drainage hierarchical solution (as set out in Policy 5.13) are described below: Hierarchy #1 store rainwater for later use Rainwater harvesting consists of a system to collect rainwater runoff, which can be stored within an underground storage tank for reuse. Such systems are not normally recommended to collect rainwater from potentially contaminated catchment areas (such as from car parks) and are normally limited to the collection of runoff from new building roofs. In terms of attenuation, such systems do not reduce the amount of storage required, as any design accounts for the tanks being full, however they contribute to significant reduction in potable water usage. A rainwater harvesting system is proposed on the Landside Area, which stores rainwater collected from the extended Terminal roof and provides water to irrigate the landscaping in the proposed Forecourt. It may be also possible to drain the Dockside car parking areas to the impounded KGV Dock. This provides a benefit of reducing the amount of pumped top-up water required by RoDMA, which is sourced from the River Thames. This will be investigated further at later design stages or as part of discharging conditions/reserved matters. A discharge from the Airside Area to KGV Dock is being considered if suitable permitted conditions can be agreed. Further agreement on permits with the EA and RoDMA would be needed to see if suitable conditions and associated monitoring regime can be agreed. This is discussed further in Section 6. Hierarchy #2 use infiltration techniques, such as porous surfaces in non-clay areas As stated in Paragraphs 5.6 to 5.13, there is a possible opportunity to utilise infiltration drainage on the Landside Area that will be investigated further in later design stages or as part of discharging conditions/reserved matters. Hierarchy #3 attenuate rainwater in ponds or open water features for gradual release It is considered that the usage of bioswales or open water attenuation ponds would conflict with Civil Aviation Authority safeguarding principles and the Airport s Safety Management Plan with regard to bird habitat management, by attracting wildlife and increasing the risk of bird strikes. There is also limited space to incorporate such features. Therefore the use of open water features has been discounted. Hierarchy #4 attenuate rainwater by storing in tanks or sealed water features for gradual release Cellular storage is proposed to be provided to create attenuation tanks, which receive surface water from the Landside Area. The outfall from the tanks to the downstream sewers will be controlled by a flow control device such as a hydro brake. The tank will be designed to suit the vehicular loading and will consider the ground water levels in the vicinity. A more robust storage solution such as culverts will be used on the Airside Area to provide storage where the tanks need to resist loading from fire appliances. A possible alternative would be that the attenuation tanks could possibly also be used as infiltration drainage devices, (if ground conditions are found to be suitable). Porous pavements are a possible alternative to conventional paving and can be made from materials such as gravel, porous (no fines) concrete blocks or porous asphalt. Pollutant removal rates are high, as the majority of the removal occurs as a result of the infiltration of the water through the aggregate sub-base. Porous paving also reduces the requirements for drainage channels, slot drains and gullies. Porous pavements are normally utilised in pedestrian areas or lightly trafficked areas, as they are not normally suitable for heavy vehicular loading. Porous pavements are however relatively high maintenance surfaces requiring regular jet washing to prevent silt and moss from sealing the surface. LCY SWD Strategy, Version 4.0, 12 July

17 Porous pavements are a viable option for the lightly trafficked Landside Areas and could possibly also be used as infiltration drainage devices (where ground conditions are favourable). Hierarchy #5 discharge rainwater direct to a watercourse There are no open watercourses in the vicinity of the site. KGV Dock is impounded. Hierarchy # 6 discharge rainwater to a surface water sewer/drain It is proposed to attenuate flows prior to discharging to the existing surface water sewers, as indicated in Hierarchy #4 above. Hierarchy # 7 discharge rainwater to a combined sewer It is proposed to attenuate flows prior to discharging to the existing combined sewers, as indicated in Hierarchy #4 above. LCY SWD Strategy, Version 4.0, 12 July

18 6. Proposed Surface Water Drainage Strategy Airside Area Site Constraints 6.1 The Airport has a unique environment due to its location surrounded by the Royal Docks. This location is close to the centre of London and Canary Wharf. This resulted in the original Airport being developed within a constrained site with minimal existing land to expand and infrastructure that contained certain restrictions. Recent developments at the Airport have been over the waters of KGV Dock. This construction technique is important to the development of the Airport although the technique does pose constraints on the services infrastructure. The existing airfield layout is shown in Figure 5 below. 6.2 The runway is located on the strip of land between the Royal Albert and the King George V Docks. This area of land was naturally suited to its current use as a runway as the width of the strip of land is very similar to the width of the runway strip. The runway strip is an area enclosing a runway intended to reduce the risk of damage to an aircraft running off the runway and to protect aircraft flying over it when taking-off or landing. This and other safeguarded areas are regulated by local and international aviation standards. Royal Albert Dock King George V Dock Figure 5. Existing Airfield Layout 6.3 The proposed extended airfield layout for the 4 reconfigured and 7 new aircraft parking stands and taxilane over KGV Dock is shown in Figure 6. The proposed deck construction comes up to the edge of the existing runway strip, which is generally along the line of the existing northern Dock wall on KGV Dock. 6.4 Outside of the runway strip there is a protected surface within the airspace, angling upwards, called the transitional surface which must be free from obstructions. This surface starts at the level of the runway and extends up at a slope of 1:6 aligned perpendicularly to the runway centreline. 6.5 The construction methodology for the proposed stands and taxilane over KGV Dock is to provide a platform over the water which is supported on piles which are founded into the ground beneath the Dock bed. The construction methodology is tailored to suit the environment close to the runway. This utilises as many precast elements as practical to create the deck before placing an in-situ topping to provide the finished surface. These large precast elements typically have to be installed at night when the runway is not operational due to the restrictions LCY SWD Strategy, Version 4.0, 12 July

19 of the transitional surface. This construction over water is not suited to the installation of attenuation tanks or oil separators within the deck structure. 6.6 Within the runway strip there is a cleared and graded area which is defined in aviation standards. CAP168 Licensing of Aerodromes by the UK Civil Aviation Authority defines this area as part of the Runway Strip cleared of all obstacles except for minor specified items and graded, intended to reduce the risk of damage to an aircraft running off the runway. This cleared and graded area extends 45m from the runway centreline. For safety and regulatory reasons it is not practical to install significant drainage systems such as attenuation tanks and oil separators within the cleared and graded area of the runway strip. 6.7 Works within the runway strip, but outside of the cleared and graded area, are typically kept to a minimum but there is a possibility of installing larger services during out of hours working as long as the strip is returned to a safe condition before operations re-commence. This zone is between 45m to 75m from the runway centreline. At the Airport there are already constraints within this area which include the following: Ducting for airfield electrical systems; Historic concrete pavement with railway lines; and The Dock wall structure. 6.8 With these constraints there is only a thin strip of land (see drawing CAOL-430 in Appendix C) remaining for the installation of drainage systems. 6.9 It is also considered that the usage of open water features (such as bioswales or ponds) would conflict with Civil Aviation Authority and airport safeguarding principles, and represent a bird strike risk. As discussed in Paragraphs 5.6 to 5.13, infiltration drainage is not suitable for the Airside runoff. Figure 6. Proposed Airfield Layout Proposed Airside Drainage Strategy 6.10 The proposed Airside Area drainage system needs to consider the constraints of the environment in which it is being developed. These include the following: The levels on the proposed deck need to reflect the existing infrastructure at their tie in on their western edge. This requires a slot drain positioned at a low point between the taxilane and the stands; LCY SWD Strategy, Version 4.0, 12 July

20 Aircraft refuelling is performed on the aircraft stands and therefore the runoff should pass through oil separators which are not practical on the deck; The runoff collected in the slot drains needs to be piped off the deck in carrier drains. The only interface with the airfield deck and the ground is to the north; and This area of land to the north is constrained due to its location within the runway strip Given the requirement to connect the drainage to the north, the following key constraints were identified: The new development should be designed to prevent any increase in flooding of the existing airfield drainage system; The drainage systems in the runway strip are constrained due to airfield safeguarding. This rules out the majority of SUDS techniques; The attenuation tanks installed within the runway strip should be minimised where practical to limit the airfield safety and operational implications; and Any installations within the runway strip would need to be checked for their possible impact on the Airport s navigational aids Given these constraints the following Airside Area surface water strategy has been chosen: Minimise the size of any attenuation tanks and offset the increase in runoff airside through the reduction in runoff from the landside development; Attenuation tanks to be sized in accordance with sewers for adoption 7 th edition (1:30 year storm); Assess surface flooding for 1:100 years +20% for climate change; and Provide a system with the maximum outflow without causing additional flooding on the existing drainage system. Airside Drainage System Design Connection to the existing system 6.13 A gravity solution for the drainage outfall connection into the existing airfield system would not be a practical solution This is due to the site being extremely flat with very little fall being achievable throughout the system Based on a potential upstream invert level (eastern end) of AOD, and the existing outfall manhole at AOD the resulting fall is only 1.200m. With a distance of 930m through the system this gives a potential gradient of only 1:775. However, due to the requirement to provide attenuation and light liquid separators, the additional drop through these items will exceed the fall within the system. See drawing SK 029 in Appendix C As such the options have been based on a pumped outfall to the existing drainage system. Assessment of the existing system capacity 6.17 The Airside Area drainage network has been modelled in WinDes Micro Drainage utilising the as built network produced on completion of the Eastern Stands Development project in The Eastern Stands Development Project constructed the existing deck over the KGV Dock for stands 21 to 24 and the associated taxilane and pier. The proposed 7ha site has been input into this model as a constant flow rate to simulate the maximum pumped outflow The system was initially assessed to ascertain the flood volumes for a number of scenarios. Flow rates of 50, 300, 400, 500 and 600 litres/sec with return periods of 2 & 30 years. The network has also been modelled to check the extent of surface flooding for a 1:100 year event + 20% for climate change The above assessments indicate that for the proposed 30 year design scenario there is limited impact on the network for flows up to and including 400 l/sec. LCY SWD Strategy, Version 4.0, 12 July

21 Attenuation tank size 6.20 The attenuation volumes required for 300 and 400 l/sec are 1155 and 985 cubic metres respectively. As these volumes are not vastly different, it is proposed to utilise the 300 l/sec rate on the basis that smaller pumps will be more economical to install and run It is proposed that this additional flow to the Thames Water network will be offset by modifications to the landside drainage which connects to the same section of the system. Refer to the Landside Area drainage proposals in Section 7. Extreme surface flooding 6.22 The 1:30 year and 1:100 year +20% storm return periods have been checked in WinDes to assess the extent of surface flooding. This indicates surface flooding of the existing and proposed stands and also an increase to the existing flooding to the north and south sides of the runway The proposed stands have potential flooding of approximately 1000 cubic metres, of which around 640 could be contained before overflowing would occur at the western end. Surface fall would direct this across the taxilane and into the grassed area south of the runway The additional flooding adjacent to the runways and taxilane is not excessive in terms of the potential containment within the adjacent low areas. The volumes are summarised as follows:- Area 1:30 year (as existing) 1:100 year +20% Potential Volume Proposed Stands 0 m m m 3 (Therefore approx 360 m 3 overflow) North of runway 492 m m m 3 South of runway 1330 m m 3 (+ approx 360 m 3 overflow from stands) 2900 m Refer to SK025 and SK026 for details of areas of surface flooding (Appendix D). An extract of SK025 is included in Figure 7 below for context. LCY SWD Strategy, Version 4.0, 12 July

22 Figure 7. Airside Surface Flooding (1in 100 Year + 20% Climate Change Storm Event) 6.26 This surface flooding will affect airfield operations but will not pose a risk to buildings or flow out of the Airport boundary. Further detailed surface modelling can be performed to quantify the full extent of surface flooding at the later design stages or as part of discharging conditions/reserved matters. Environmental Strategy 6.27 The Airport is in advanced discussions with the Environment Agency and Thames Water regarding acceptance of the environmental strategy for the existing airfield drainage. The strategy for the new aircraft stands will be in line with this strategy which includes the following: Potassium formate based de-icant used for the airfield pavements. This has better environmental characteristics than the de-icants used at most other UK airfields. The Airport source the product from overseas to obtain these environmental improvements; Aircraft De-icants need to meet the requirements of international aviation standards. The Airport use mobile glycol recovery units to capture the product when it is applied to the aircraft to minimise any environmental impact; Aircraft washing is prohibited at the Airport during the winter season and is controlled at other times having regard to operational constraints; and The airfield drainage has been subject to monitoring of de-icant content, enabling it to go to the Thames Water surface water drainage network. Such monitoring could be built into an on-going strategy. Dock Outfall 6.28 A possible opportunity to outfall some of the airfield surface water drainage to the Dock during permitted conditions has been identified, e.g. outside of the winter months and only during the winter months when de-icant content is acceptable. This would need further engagement with the relevant stakeholders to determine the appropriate conditions and permits. LCY SWD Strategy, Version 4.0, 12 July

23 6.29 Currently it is only deemed to be suitable if practical conditions and an associated reliable monitoring system can be agreed. Outfall permit and conditions would need to be agreed with the Environment Agency and RoDMA. These permits would need to align with a monitoring system that could be installed within the runway strip. Outside of the permitted conditions outfall to Thames Water Surface Water drain would continue It is intended that the outfall control to divert to Thames Water would be by a Biological Oxygen Demand (BOD) sensor (and possible other sensors dependent on conditions) and manual override. The manual override will be written into de-icing operational procedures The alternative proposal to discharge to KGV Dock is indicated on the system schematic drawing shown on SK024 (Appendix C). It is still subject to discussion with both RoDMA and the EA to confirm acceptability. Extended Terminal Roof Drainage 6.32 The extended Terminal roof drainage is intended to discharge directly to KGV Dock due to the clean nature of this discharge subject to agreement from RoDMA. LCY SWD Strategy, Version 4.0, 12 July

24 7. Proposed Surface Water Drainage Strategy Landside Area Overview 7.1 As described in Paragraphs 5.6 to 5.13, preliminary ground investigations indicate that infiltration drainage could possibly be implemented, subject to further investigation at later design stages or as part of discharging conditions/reserved matters. Section 7 however demonstrates that the aspirations of the Environment Agency and London Plan can also be achieved (as far as practical) using a range of other SUDS techniques to reduce the existing flow rate. 7.2 The objective of Section 7 is to illustrate how this flow rate reduction could be maximised, to compensate for the increase in the Airside Area runoff rate. This will result in the net overall reduction in flows to the existing sewer systems. 7.3 To maximise the reduction in the existing flow rate it is proposed to provide attenuation tanks with controlled outfalls to limit the flows to the existing surrounding sewers. 7.4 Attenuation tanks will be provided for the Western Terminal Extension, Forecourt and Dockside Areas. 7.5 It may be possible to discharge the higher Dockside parking areas to KGV Dock by utilising a porous/permeable paving solution with a shallow gravity piped outfall. It is not possible to drain the lower Forecourt, Western Terminal Extension or Hartmann Road to KGV Dock by gravity. 7.6 The benefit of draining to KGV Dock is that it will reduce the amount of Dock top-up water required to be pumped from the River Thames to maintain the Dock high water level. This will provide some energy savings and will reduce the quantity of pumped water required. It will also have the benefit of reducing the discharge to the existing sewer to zero for these catchments areas. 7.7 There are methodological questions to be addressed related to drilling the outfalls through the existing Dock wall, which will be assessed fully in later design stages or as part of discharging conditions/reserved matters, if this alternative option is pursued. 7.8 Due to the potential for buried archaeology in the deeper/undisturbed ground, potential contamination and high risk of encountering existing buried underground structures (existing retained building foundations in the Dockside area), there is an environmental benefit to reducing the size of the tanks, as far as possible. This will limit the need to dispose of contaminated material and minimise disturbance to the heritage assets. 7.9 Paragraphs 7.14 to 7.56 describe in detail the proposed solution for each Landside Area catchment It is considered that the usage of open water features (such as bioswales or ponds) would conflict with Civil Aviation Authority and airport safeguarding principles, and represent a bird strike risk. Landside catchments 7.11 The total proposed Landside catchment area consists of 3 main development areas: the Western Terminal Extension, Forecourt, and Dockside area. The Dockside area has been split into 2 logical sub-catchments due to the site characteristics and road access points. The landside catchments are illustrated in Figure 8. LCY SWD Strategy, Version 4.0, 12 July