Sewer Flooding Alleviation in the Counters Creek Catchment

Transcription

1 Sewer Flooding Alleviation in the Counters Creek Catchment London Borough of Hammersmith and Fulham Monday, 14 November 2011 at 7.00pm The Small Hall, Hammersmith Town Hall, King Street, W6 9JU

2 Agenda for this meeting Drainage overview Thames Water engagement Summary of progress since the meeting in June 2011 Independent Advisory Group Short term mitigation measures (the FLIPs programme) Basement surveys and data collection Modelling progress and solution development Sustainable urban drainage (SUDS) pilot studies Programme Questions? 2

3 TW engagement in Counters Creek 3

4 Drainage overview: flooding in London Active participant of GLA Drain London Forum looking at surface water management across all 33 boroughs. Potential contributors to Drain London pilot projects. Identified pilot sustainable drainage projects in conjunction with local authorities, local stakeholders and customers. Contributing to Surface Water Management Plans and Preliminary i Flood Risk Assessments across our region. Regular weekly dialogue and site meetings with the planning, environment and highways departments at RBKC, LBHF and City of Westminster. Joint project on planning database for SUDS on small developments with RBKC.

5 Progress since last meeting To date we have installed and commissioned 263 FLIPs units. We have evidence of 2017 flooded properties and believe a further 5000 may be at risk. If anyone is aware of other property flooding, we still need to know. The hydraulic model has been independently audited and is fit for purpose. We are continuing to enhance the detail and accuracy as the project progresses. We have had two further workshops with our Independent Advisory Group, with Ofwat representatives attending, held on 17 June and 11 October to review and challenge our progress. The outcome of these workshops is extremely positive. We have identified pilots to install Sustainable Drainage Systems (SuDS) at a number of sites identified within RBKC and LBHF. 5

6 Independent Advisory Group: Who are we? Professor Bob Andoh Director of Innovation, Hydro International Visiting Professor, Liverpool John Moores University IWEX 2003 Innovation Award Winner Professor David Balmforth Executive Technical Director, MWH Visiting Professor Imperial College Vice President, Institution of Civil Engineers Professor Adrian Saul Professor, University of Sheffield Leading academic in Flood Risk Management Research Consortium 6

7 Independent Advisory Group: What we are doing? Answering the key question: Is Thames Water s proposal to construct a series of new storm relief tunnels in the Counters Creek area the most cost effective and sustainable long-term solution, or are there other more sustainable alternatives that should be developed and appraised? 7

8 Independent Advisory Group: How we are doing it? We are determining if - All historic options considered to date to alleviate the risk of flooding in the area have been explored Whether Thames Water s forward programme includes all of the activities that are required to develop and appraise sustainable solutions Whether the assumptions made within Thames Water s hydraulic model to identify properties at risk of flooding are reasonable Whether there are sustainable alternatives to a programme of new storm relief tunnels that should be considered and developed Whether there are sustainable elements that could be incorporated in a storm relief tunnel design that should be considered Whether Thames Water could do more to either alleviate or mitigate customers at the highest risk of sewer flooding in the interim period 8

9 Short term mitigation measures (the FLIPs programme) FLIP stands for flooding local improvement projects. Essentially a mini pumping station concealed within a manhole chamber, protecting one or more properties from flooding. Waste water from the property is pumped to levels above the maximum water level in the main trunk sewer at times of heavy storm rainfall. A FLIP device prevents raw sewage from backing up from the main sewer and flooding basements. We are aiming to protect 634 properties which are most at risk of flooding using FLIP devices. The contract to install the FLIPs devices will take about two years to complete and is currently scheduled to finish by the end of spring

10 FLIPs legal agreements and installation numbers 10

11 FLIPs locations - LBHF 11

12 Basement surveys and data collection We have completed phase 1 of the basement surveys. Phase 2 order has been placed and we are targeting ti a further 1350 properties. We currently have 2017 properties with reported flooding but we believe there are more that we haven t been told about. We have installed a number of short and long term flow monitors to further enhance our model accuracy. We have completed a number of asset surveys (manholes) at key hydraulic locations in the catchment and are about to collect more. All data collection will be complete by January

13 Density of basements LEGEND Basement Density (No./km 2 )

14 Density of basements with flooding history LEGEND Flooding Basement Density (No./km 2 )

15 Explaining the system The system is complicated and the flows loop through it. As the rainfall varies, so does the magnitude and direction of flow in the system. To help us understand this, we have broken the problem down in to small parts for assessment. We are considering each part in turn using a systematic methodology joining up the parts to make the whole. This process has been agreed with the IAG. 15 Part of this process includes understanding the mechanism causing flooding Local capacity, Strategic capacity or a combination of the two

16 Regions and problem simplification 16

17 Flooding mechanism results modelled and reported

18 Flooding mechanism results modelled to be at risk if connected to the sewer

19 Identification of hydraulic constraints Key Hydraulic pinch point Flood cluster Zone of influence 19

20 Options available for consideration Isolation e.g. a FLIP device installed locally at a property Flow transfer moving flow from one location to another System optimisation e.g. Real Time Control systems to allow the maximum use of the existing asset base by active flow management. Storage tanks to provide capacity to attenuate flows in the system which then drain as the system permits. Upsizing increasing the capacity of the existing system along the alignment of the current pipework to a location where capacity exists. Overflow e.g. new CSO or pumping station Source control e.g. SUDs preventing or controlling the rate rainwater enters the system at its source. 20

21 Long term solution: our current view 21

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26 SuDS progress since June 2011 Put a lot of energy into building momentum and buy-in across stakeholder group Establishing a robust approach through building partnerships Formed a technical working group Confirmed that engagement takes a long time, even with the keenest stakeholders Identified and agreed six specific sites in principle, plus back up list Green roof and rain garden monitoring ongoing Allocated capital and Greenstreets project negotiations 26

27 SUDS options for catchment We have identified potential for SUDS in the catchment a key part of the solution for the alleviation of sewer flooding. Pilot trials needed to assess: How much? Reliability of reducing peak flows in sewer. Data for national evidence base and Council projects Where? Suitable locations for installation How acceptable? Costs, opportunities and barriers - how readily will homeowners and businesses install measures Alignment with planning policy? Designs must be realistic, sympathetic and accepted by the community Can they be retrofitted? Installations must work in the existing urban environment, not just for new build applications

28 SUDS measures roof drainage Green roofs (businesses/housing blocks) Water butts (houses) or rainwater tanks (commercial) where space and downpipes permit Downpipe diversion to modular rain garden (or soakaway where practical)

29 SUDS measures driveways, minor roads Reversing the urban creep of impermeable driveways Using permeable paving or grasscrete for minor paved areas Driveway cutoff drains to adjacent permeable areas or soakaway Permeable highways or controlled run-off from highways

30 South Kensington proposed site

31 Customer engagement Over the coming months we need to engage with customers to establish your views. We anticipate this being in March 2012 when we will have a good understanding of the most likely solution and the available alternatives. We anticipate this process involving a number of facilitated focus groups. The timescales are tight and to meet our commitment to you, we need your support to the preferred solution. 31

32 Programme to end of 2012 Today FLIPs Installations Data Collection / SUDS desk studies SUDS Pilot Studies Basement Surveys and Data Collection Hydraulic Modelling Outline Design / Engineering Studies / Cost Analysis Planning g& Environmental Studies Detailed Design Environmental Surveys Review of Suitable Sites Independent Advisory Group Inputs Liaison with Local Authorities / Stakeholders Public Public Public Meetings Meetings Meetings Public Consultation Submission to OFWAT Public Meetings Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 32

33 Long term programme Design development Submission of proposals to Ofwat for funding (July) Agree solution costs with Ofwat Apply for planning approval Start construction Final AMP6 Determination (Nov) Six monthly updates to customers Ongoing communications Planning and development Construction 33

34 Next Public Meetings on the Counters Creek sewer flooding alleviation scheme We are planning to hold the next public meetings on: Monday 28 May 2012 at Hammersmith Town Hall and Thursday 31 May 2012 at Kensington Town Hall 34

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