APPENDIX 12.2
London City Airport
Proposed Surface Water Drainage Strategy
London City Airport Development Programme
London City Airport
Surface Water Drainage Strategy
LCY SWD Strategy, Version 4.0, 12 July 2013
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 Final Planning
Submission Atkins KB TPS WH Atkins DH TPS MW Atkins DH TPS AF Atkins MdS TPS AF 16/05/13 Rev 3.0 Final Planning
Submission Atkins KB TPS WH Atkins DH TPS MW Atkins DH TPS AF Atkins MdS TPS AF 24/05/13 Rev 4.0 Final Planning
Submission Atkins KB TPS WH Atkins DH TPS MW Atkins DH TPS AF Atkins MdS TPS AF 12/07/13
Client signoff
Client London City Airport
Project London City Airport
Document title London City Airport - Surface Water Drainage Strategy
Job no. ATKINS – 5115752
TPS 112406 Copy no. 1 Document reference LCY-CADP-ATK-TPS-RPT-0001
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 12
6. Proposed Surface Water Drainage Strategy – Airside Area 17
Site Constraints 17
Proposed Airside Drainage Strategy 18
Airside Drainage System Design 19
7. 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 34
8. 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
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:
x The viability of using various types of Sustainable Urban Drainage systems (SUDS), including Porous Pavements and Attenuation Tanks;
x Discharging to the existing Thames Water sewer systems; x Discharging to King George V Dock; and
x 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:
x 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;
x 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.
x 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
x 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.
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:
x A description of the proposed CADP;
x An explanation of the pre-application consultation process that informed the Strategy approach;
x A description of the existing surface water infrastructure systems within the CADP proposal areas; and
x 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.
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.
KING GEORGE V DOCK
AIRSIDE LANDSIDE EXISTING BUILDING
LCY SWD Strategy, Version 4.0, 12 July 2013 6
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.”
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:
x The Environment Agency (EA)
o Pollution Prevention Strategy
o Flood Risk Assessment
o Environmental Impact Assessment
x Thames Water (TW)
o Discharge Agreements
x Royal Docks Management Authority (RoDMA)
o Discharge Agreements
x English Heritage
o Protection of ‘Asset of Heritage Interest’
x London Borough of Newham
o Local Development Framework
x Greater London Authority
o The London Plan
3.2 Preliminary consultation was undertaken with the EA on 8th April 2013.The key points discussed and agreed in the meeting are noted below:
x 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 5.13. (see paragraph 5.15 below);
x 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 pre-development discharges to the existing sewer systems would be considered acceptable;
x 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;
x 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 8th 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 7th February and 12th 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 2013 8 3.5 RoDMA was consulted on 10th December 2013 and 1st 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 6th 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.
4.
Existing Surface Water Infrastructure
4.1 To inform the preparation of this Strategy the following information has been reviewed:x The “Airport” drainage existing record drawings;
x LCY Building Services Report (by Atkins ref 5063669-200-07.01-001 dated Dec 2009);
x LCY Western Terminal Extension area drainage CCTV survey (by Drain Aid Ltd dated 19th Nov 2012) ;
x Preliminary LCY Site Investigation (by RPS ref: HLEI 24974 March 2013 and HLEI 19695/001R [Phase 1 Report]);
x LCY Surface Water Drainage Survey (by MSA Project Number 3676 dated 4th Mar 2013);
x Docklands Light Railway (DLR) ‘as built’ drawings (series 17207-PWD-DUX-00029); and
x 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 7th February and 12th April 2013.
4.4 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_1574785. 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 2013 10
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 C1.
4.6 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 4th 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.
4.9 This results in an estimated pre-development existing flow rate of 1008.4 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 504.15 l/s being discharged to the existing system, as indicated in Table 1 below.
4.10 It should be noted that the existing flow rate assessment is based on the drainage survey undertaken by MSA on 4th March 2013. 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. Details of the Existing Surface Water Drainage Systems and Catchments.
Catchment [2]
Existing Catchment Area (Ha)
Existing Flow Rate (l/s) [1]
50% Reduction in Flow Rate (l/s)
Airside None 0 0
Western Terminal Extension 0.33 46.1 23.05
Forecourt 2.23 309.7 154.8 Dockside Sub-Catchment 1 1.59 220.9 110.5 Dockside Sub-Catchment 2 3.11 431.7 215.8 TOTAL 7.26 1008.4 504.15 Notes:
[1] Existing flow is based on a 50mm/hr storm over the entire existing catchment area. [2] Catchments are detailed on Figure 4.
4.11 The extent of the existing CADP Landside catchment is shown in outline on Figure 3 and in detail in Appendix A.
LCY SWD Strategy, Version 4.0, 12 July 2013 12
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:
x City Airport Development Programme Design and Access Statement
x London Plan (Major of London - Spatial Development Strategy for London – July 2011); and
x 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:
x Sewers for Adoption 7th Edition, August 2012 published by WRc plc;
x 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);
x 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;
x Maximise the employment of SUDS techniques for new installations within site constraints;
x Minimise flows to existing sewers to Greenfield runoff rates, as far as practicable [1];
x Consideration of long-term maintenance and operation;
x Consideration of future system ownership;
x Consideration of minimising the abandonment and removal of existing assets;
x Incorporation of Petrol Interceptors to control hydrocarbon discharges in accordance with Environment Agency Pollution Prevention Guidance PPG5; and
x 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.
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.
5.10 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 2013 14 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.
5.11 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.
5.12 Infiltration is not deemed appropriate for the Airside Area due to the following considerations:
x Increased hydrostatic water pressures behind the Dock wall structure;
x Potential for de-icants within the surface water runoff concentrating within the groundwater; and
x The preferred option for discharge into the Dock during permitted conditions.
5.13 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.
5.15 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.
5.16 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. 5.17 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.
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 2013 16 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.
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.
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
Royal Albert Dock
LCY SWD Strategy, Version 4.0, 12 July 2013 18 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:
x Ducting for airfield electrical systems;
x Historic concrete pavement with railway lines; and
x 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:
x 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;
x Aircraft refuelling is performed on the aircraft stands and therefore the runoff should pass through oil separators which are not practical on the deck;
x 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
x This area of land to the north is constrained due to its location within the runway strip. 6.11 Given the requirement to connect the drainage to the north, the following key constraints were
identified:
x The new development should be designed to prevent any increase in flooding of the existing airfield drainage system;
x The drainage systems in the runway strip are constrained due to airfield safeguarding. This rules out the majority of SUDS techniques;
x The attenuation tanks installed within the runway strip should be minimised where practical to limit the airfield safety and operational implications; and
x Any installations within the runway strip would need to be checked for their possible impact on the Airport’s navigational aids.
6.12 Given these constraints the following Airside Area surface water strategy has been chosen:
x Minimise the size of any attenuation tanks and offset the increase in runoff airside through the reduction in runoff from the landside development;
x Attenuation tanks to be sized in accordance with sewers for adoption 7th edition (1:30 year storm);
x Assess surface flooding for 1:100 years +20% for climate change; and
x 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.
6.14 This is due to the site being extremely flat with very little fall being achievable throughout the system.
6.15 Based on a potential upstream invert level (eastern end) of 4.400 AOD, and the existing outfall manhole at 3.200 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.
6.16 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 2008. 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.
6.18 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.
6.19 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 2013 20
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.
6.21 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.
6.23 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.
6.24 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 m3 1000 m3 640 m3(Therefore
approx 360 m3overflow)
North of runway 492 m3 830 m3 2900 m3
South of runway 1330 m3 1865 m3(+ approx 360
m3overflow from stands)
2900 m3
6.25 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.
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:
x 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;
x 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;
x Aircraft washing is prohibited at the Airport during the winter season and is controlled at other times having regard to operational constraints; and
x 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 2013 22 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.
6.30 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.
6.31 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.
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.
7.10 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 2013 24 7.12 The proposed sewers within the Landside Area will not be adopted by Thames Water, and will
remain private.
7.13 The proposed Landside catchment areas are indicated in Figure 8 below.
Figure 8. Proposed Landside Catchment Areas
Landside Area Drainage Strategy
7.14 The proposed Landside Area drainage strategy of reducing the existing flow rates is indicated in Figure 9.
7.15 Discharge options are summarised below and described in detail for each catchment in Paragraphs 7.18 to 7.50.
7.16 The surface water from the Landside Area is proposed to discharge to either:
x The existing Airport private sewer which ultimately discharges into the Ø1800 Royal Docks Surface Water Sewer (Connaught Branch) – the new surface water network will discharge via an attenuation tank with flow control units to the existing Ø300mm to Ø600mm private sewers;
x The existing combined Thames Water sewers in Newland Street, Brixham Street, Woodman Street (that pass under the DLR), and Albert Road. This will be via existing connections and all flows will discharge via attenuation tanks with flow control units; or
x The existing combined Thames Water sewers in Albert Road via new connections. All flows will discharge via attenuation tanks with flow control units.
7.17 An alternative option of draining the high level catchments to the KGV Dock, via permeable paving using gravity pipework and multiple Ø150mm outfalls has also been included. This proposal is anticipated to restrict the velocity to 0.5m/s and have a discharge flow rate not exceeding 5l/s. This will be investigated further in later design stages or as part of discharging conditions/reserved matters, as highlighted in Paragraphs 7.5 to 7.7.
LCY SWD Strategy, Version 4.0, 12 July 2013 26
Western Terminal Extension Catchment Area
7.18 The Western Terminal Extension catchment area is constrained by the existing Terminal and proposed Western Energy Centre in the north, the WTE building and diverted services corridor in the east and DLR station and assets in the south west. The service yard consists predominately of a heavy vehicle loading area with a pedestrian footway to the south.
7.19 Due to the heavy loading of the yard, it is not deemed practicable to provide a porous paving solution and there is insufficient room for any SUDS features, except for providing a cellular attenuation tank outside of the heavily loaded area, in the pedestrian footway. This will provide storage and control the outflow from the site and discharge to the existing Ø600 sewer that drains westwards to the Thames Water System.
7.20 It is proposed that the flow rate to the sewer will be limited to 50% of the existing flow rate from this catchment area as there is insufficient space available to provide attenuation to meet the Greenfield runoff rates.
7.21 The level of the existing Ø600 sewer at the proposed connection point is +1.4m Above Ordnance Datum (AOD) and the catchment area can be drained by gravity.
7.22 Appendix C.2 indicates a drainage solution for the Western Terminal Extension catchment area and Table 2 summarises the existing flow rate, anticipated runoff and required attenuation volume. The viability of using an alternative infiltration drainage solution will also be investigated in later design stages or as part of discharging conditions/reserved matters.
Table 2. Proposed Western Terminal Extension Flow Rates Catchment
Area
Existing Proposed [3] 50% Reduction in Existing Flow Rate Catchment Area (Ha) Flow (l/s) [1] Catchment Area (Ha) Non-attenuated Flow (l/s) Runoff Flow (l/s) Required Attenuation Tank Volume (m3) [2] Total Reduction in Existing Flow Rate (l/s) Western Terminal Extension 0.33 46.1 0.48 66.6 23.05 140 43.6 Notes:
[1] Existing flow is based on a 50mm/hr storm over the entire existing catchment area.
[2] The attenuation tank volume is based on a storm of 1 in 30 year return period with a 20% climate change. [3] Calculations assume a 100% runoff from redeveloped site areas.
Forecourt Catchment Area
7.23 The Forecourt catchment area consists of car, taxi and bus stopping areas and associated pedestrian and landscaped areas.
7.24 Due to heavy loading of the parking/stopping off areas and the number of utilities required under the pedestrian areas, the use of porous paving is not considered a viable option. There is insufficient space to provide any other SUDS features, except for cellular attenuation tanks, which will located under the landscaped areas in front of the proposed extended Terminal. 7.25 An attenuation tank will be provided and will discharge to the existing Ø300 private Airport
sewer. This ultimately discharges to the Ø600 Thames Water Sewer in the west. It is proposed that the flow rate to the sewer will be limited to the Greenfield runoff rates.
7.26 Appendix C.3 indicates a conceptual drainage solution for the Forecourt catchment area and Table 3 summarises the existing flow rate, anticipated runoff and required attenuation volume. 7.27 The viability of using an alternative infiltration drainage solution will also be investigated in later
Table 3. Proposed Forecourt Flow Rates Catchment
Area
Existing Proposed Greenfield Runoff Criteria Catchment Area (Ha) Flow (l/s) [1] Catchment Area (Ha) [4] Non-attenuated Flow (l/s) Runoff Flow (l/s) [2] Required Attenuation Tank Volume (m3) [3] Total Reduction in Existing Flow Rate (l/s) Forecourt 2.23 309.7 2.75 334.7 12.1 1215 297.6 Notes:
[1] Existing flow is based on a 50mm/hr storm over the entire existing catchment area. [2] Greenfield runoff flow is based on an allowable discharge of 5 litres/second/hectare.
[3] The attenuation tank volume is based on a storm of 1 in 30 year return period with a 20% climate change. [4] Calculations assume a 100% runoff from redeveloped site areas.
Dockside Catchment Area
7.28 As previously noted, the Dockside catchment area has been split into 2 logical sub-catchments (areas 1 and 2), depending on the site characteristics and road access points. Both of the sub-catchments have a high and low elevation area.
Dockside Sub - catchment 1
Higher Level Catchment
7.29 It is proposed that the high level catchment will discharge to the existing Ø300mm private Airport sewer located in Hartmann Road via an attenuation tank. The discharge will be limited to Greenfield run off rates using flow control unit. The attenuation tank will be located at the higher level and the design proposals are indicated on Figure 10.
7.30 A possible alternative solution would be to drain directly to the KGV Dock. This would be via a gravity system, using a permeable paving solution within the car parking areas (refer to Figure 11). The permeable paving would provide attenuation and would also reduce the outfall sizes, improve the quality of the water (by filtering out any pollutants) and would allow for a gravity discharge with a small discharge flow to the Dock.
7.31 The raised parking deck could possibly drain to the sub-base of the underlying porous paving via rainwater down pipes.
7.32 The water in the Dock is impounded and levels normally vary between 3.44m AOD and 4.24m AOD. It is likely, if this alternative option was adopted, that the permeable paving would need to discharge to the Dock via multiple Ø150mm outfalls. The proposal would restrict the velocity to 0.5m/s and flow rate to not exceed 5l/s. The outfalls invert level would need to be set at 4.3m AOD, just above the highest normal water level in the Dock.
7.33 RoDMA have advised during preliminary meetings, that in a Highest Astronomical Tide, the Dock level could rise to approximately 4.45m AOD, and that tidal surges can occur due to storms in the North Sea. If the right conditions coincided, the lock gates might be forced open and the level within the Docks could rise accordingly. This would need to be investigated further in later design stages or as part of discharging conditions/reserved matters, and if implemented, suitable mitigation measures (such as flap valves) would likely be required.
7.34 Other uncertainties associated with this alternative solution are the penetration of the Dock wall detail and water proofing issues. These would need to be investigated and agreed with RoDMA during the later design stages or as part of discharging conditions/reserved matters, and confirmed by a structural survey of the existing Dock wall. The anticipated traffic loads may also potentially affect the selection of porous pavements and underground obstructions may limit gravity pipework routes.
7.35 The high level catchment consists of the car parking areas only; the proposed Hotel, car parking entrance ramp and existing KGV house will form part of the lower level catchment.
LCY SWD Strategy, Version 4.0, 12 July 2013 28 Lower Level Catchment
7.36 The lower level catchment including Hartmann Road, proposed Hotel, car parking entrance ramp and KGV house, will be discharged into the existing Ø300mm private Airport sewer located in Hartmann Road via an attenuation tank with a flow control unit to limit the flow rates to Greenfield runoff rates. This sewer drains westwards through the forecourt area and ultimately discharges to the Thames Water sewer system.
7.37 However, there is another existing Ø225mm private sewer, which is believed to serve Hartmann Road and drain southwards under the DLR. This network could be utilised as an alternative outflow and flows would be limited to Greenfield runoff rate flows. This will be investigated further in later design stages or as part of discharging conditions/reserved matters.
Figure 10. Proposed Dockside Drainage -S ub-Catchment 1
Figure 11. Proposed Dockside Drainage -Sub-Catchment 1 -Alternative Solution
Dockside Sub – catchment 2
Higher Level Catchment
7.39 It is proposed that High Level Catchment will discharge to the existing sewer connections that currently serve Hartmann Road and pass under the DLR, prior to connecting to the combined Thames Water sewer systems in Brixham Street and Woodman Street. The existing connections will be traced and verified prior to obtaining a formal indirect connection approval from Thames Water. This will be undertaken at later design stages or as part of discharging conditions/reserved matters. All flows will pass through attenuation tanks with flow control units, which will limit the flows to the Greenfield run off rates. The design philosophy is illustrated in Figure 12.
7.40 The far eastern portion of the catchment will connect via new connections to the existing Thames Water sewer in Albert Road. All flows will pass through attenuation tanks with flow control units, which will limit the flows to the Greenfield run off rates. The new connections will be subject to formal approval with Thames Water in later design stages or as part of discharging conditions/reserved matters, and are not anticipated to exceed 10% of the capacity of the existing Thames Water system. The design philosophy is illustrated in Figure 12.
7.41 Similarly to Sub-Catchment 1 it may be possible to drain the higher level car parking areas directly to KGV Dock via a gravity system, using a permeable paving solution. This would discharge to the Dock as Sub-catchment 1 at Greenfield runoff rates and provide the same benefits. This option will be investigated further in later design stages or as part of discharging conditions/reserved matters, and is illustrated in Figure 13.
7.42 The proposed entrance ramps to the high level area will drain to the lower level catchment. Lower Level Catchment
7.43 The runoff from the low level catchment will be attenuated and discharge will be controlled by a flow control unit. It is proposed to provide a single tank located to the east of the site and reduce the existing flow rates to Greenfield runoff rates, prior to discharging to the Thames Water sewer in Woodman Street This is based on an existing connection point level being at maximum height of -0.24m AOD (interpolated from Thames Water sewer records), which will be verified in later design stages or as part of discharging conditions/reserved matters.
7.44 The low level catchment could also mimic the existing regime and discharge by gravity to the existing outfalls (that are utilised for the high level catchment). These pass under the DLR and connect to the combined Thames Water sewer in Brixham Street/Woodman Street. Multiple attenuation storage areas/cellular tanks or oversized pipework with flow control units would be required to limit the flows to Greenfield runoff rates. This is subject to further investigation of the existing connections, at later design stages or as part of discharging conditions/reserved matters.
Figure 1 . Proposed Dockside Drainage -Sub-Catchment
Figure 1 . Proposed Dockside Drainage -Sub-Catchment -Alternative Solution
LCY SWD Strategy, Version 4.0, 12 July 2013 34
Dockside area - Summary
7.46 The proposed design solution for the Dockside area is to reduce the existing flow rate from the redeveloped site, by utilising attenuation systems which will discharge via flow control units at Greenfield run-off rates. The attenuation systems have been minimised as far as possible to limit the disposal of contaminated material, disturbance of heritage assets and to avoid underground existing building foundations.
7.47 Further investigations will be undertaken at the later design stages or as part of discharging conditions/reserved matters, to determine the viability of the alternative option of possibly discharging the high level catchment to the KGV Dock.
7.48 The existing connections that pass under the DLR and connect to the existing combined Thames Water sewers in Newland Street, Brixham Street and Woodman Street will be traced and verified prior to obtaining a formal indirect connection approval from Thames Water. These could possibly also be used as an alternative discharge point for the low level catchments. 7.49 Table 4 summarises the required attenuation volumes and anticipated reduction in existing flow
rates. It is considered that the design solutions and alternative options follow the drainage hierarchy within the London Plan Policy 5.13 and provide similar reductions in the existing flow rates discharging to the existing sewer systems. These flows have been limited as far as possible, within the constraints of the site.
7.50 The viability of using infiltration drainage solution will also be investigated in later design stages or as part of discharging conditions/reserved matters.
Table 4. Proposed Dockside Flow rates
Notes:
[1] Existing flow is based on a 50mm/hr storm over the entire existing catchment area. [2] Calculations assume a 100% runoff from redeveloped site areas.
[3] Greenfield runoff flow is based on an allowable discharge of 5 litres/second/hectare.
[4] The attenuation tank volume is based on a storm of 1 in 30 year return period with a 20% climate change.
Extreme surface flooding
7.51 The 1:100 year +20% storm return periods have been checked in WinDes to assess the extent of surface flooding.
Catchment Area
Existing Proposed Greenfield Runoff Criteria
Existing Catc hm en t (m 2) T o ta l Existi ng F lo w Ra te (l/ s ) [ 1 ] Catchment Breakdown Non-attenuated Flow Rate (l/s) [2] A tten u ati on (m 3) [ 4 ] Gr e enfi e ld F low R a te (l/ s ) [3] T o ta l Re du ctio n in Existing F low Ra te (l/s) [4 ] Poro us Pa ving (disc har gin g t o King G e o rg e V Dock) (m 2) Har d Sur fa c e A rea (m 2) Har tm a nn R o a d (m 2) TO TA L Catch me nt A re a disch ar ging to sewe r ( m 2) Design Solution Sub-Catchment 1 15912 220.9 - 18850 2300 21150 293.7 1000 10.6 210.3 Sub-Catchment 2 31081 431.7 - 49100 6300 55400 769.4 2500 27.7 404.0
Alternative Solution (‘High Level’ catchments discharging to KGV Dock) Sub-Catchment 1 15912 220.9 8400 10450 2300 12750 - 600 6.4 214.5 Sub-Catchment 2 31081 431.7 36600 12500 6300 18800 - 820 9.4 422.3
