First Edition Surface Water Management Plans Thatcham

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First Edition Surface Water Management Plans Thatcham Brian Cafferkey Technical Director WSP

Transcript of First Edition Surface Water Management Plans Thatcham

Page 1: First Edition Surface Water Management Plans Thatcham

First Edition Surface Water Management Plans

Thatcham

Brian CafferkeyTechnical DirectorWSP

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FWR Forum16 March 2010

Thatcham

Located on southern edge of West Berkshire

Significant flooding in 2007 – initial estimates indicated 1100 properties affected

Unique as purely surface water flooding

Prior to 2007 extensive history of surface water flooding, dating back to 1947

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Notes

Slide 2

• Thatcham located on southern edge of West Berkshire, 15 miles west of Reading• Population 23,000 approx and covers an area of approx 23km2• Thatcham badly affected in 2007 floods with initial records of 1100 houses flooded but this was not the

first incidents of flooding – records of flooding date back to early 1900s• Group of interested parties came together to come up with a solution – beginnings of a partnership for the

SWMPSlide 4

• This simulation gives you an idea of the flood risk problems facing Thatcham – based on modelling undertaken as part of the SWMP

Slide 5

• This slide sets out our project plan – we followed the wheel from the guidance.• Steering group engaged throughout the process• Do not underestimate the time taken for data collection and review – a central GIS databse makes life

easierSlide 6

• Project team made up of partners and stakeholders but all attended meetings• Initially tried to set up a memorandum of agreement for each party to sign up to but faced problems on

getting an agreement to this – in the end had to rely on trust• Whilst it was good to have a large number of parties it did present some problems due to size of group

and staff changes etc – will discuss problems later in the presentation• Consultation was made with other parties outside of the project team – this was for both information

gathering and disseminating information• In-house staff used to keep costs down

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Surface Water Flooding

Simulation of the July 2007 event

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Project Plan

Partner/Stakeholder involvement

Data collection and review

Hydraulic Modelling

Identify & Assess mitigation measures

Cost Benefit Analysis

Public consultation throughout project

Reporting

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Consultation also undertaken with the following:

Planning Local Resident Met Office Network Rail Loss Adjusters

Organisations involvedSteering group set up

Project team also included elected members from the councilsAll parties worked together to provide integrated solution

Partners: - West Berkshire Council Stakeholders: - Thatcham Flood

- Thatcham Town Council Forum

- Thames Water - Cold Ash Community

- Environment Agency Partnership

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Data Collection

Collection and screening of data received from the steering group and third parties was uploaded onto a GIS platform:

West Berkshire CouncilEnvironment AgencyThames WaterMet OfficeThatcham Weather StationIn-house staff utilised to undertake surveysInsurance claims from Loss Adjusters

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Notes

Slide 7

Data collection included:• LiDAR data (EA)• Asset data – sewers, pumping stations, syphons (TW)• Highway Drainage (WBC)• s105 Tuflow and ISIS model of River Kennet (EA)• Focued site surveys (In house staff)• Future development areas, core strategy data (WBC)• Thames Catchment Flood Management Plan (EA)• SFRA for West Berkshire (WBC)• Thames River Basin Management Plan (EA)• Rainfall data (Met Office and Thatcham Weather Station)The data were uploaded onto a central GIS platform Gaps in the data meant that in-house staff had to undertaken additional site surveysSlide 10

We were able to clarify the results by a comparison against actual data of the 2007 floods1. Properties identified as having flooded by WBC2. 2007 flood extent3. Insurance claims4. Modelled results

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Modelling – FloodFlow (Micro Drainage)

Thatcham catchment modelled - 15.32km2 - 46% urban, 54% rural

Initial ‘Direct Rainfall’ model built for whole catchment

A ‘Fully Integrated’ modeldeveloped

Catchment split into 3 due tosoftware and hardware limitations

Ran model for return periods5, 10, 25, 30, 50, 100, 100+climate change and 237 return period

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Thatcham model results verses recorded flood events

House to house surveys (WBC)

Estimated Flood Extent (WBC)

Flood Extent Based on Insurance Claims

Modelled Flood Extent

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Surface Water Flooding

Example of depth and hazard for July

2007 event

Legend

Greater than 1.0 m0.75 to 1.000.50 to 0.750.25 to 0.500.05 to 0.25

Danger for some

Danger for most

Danger for all

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Identified Mitigation OptionsIdentified options which included :

Technical 4 No;Maintenance;Development, building control and policy;Awareness;Resilience/resistance; Agricultural land use;Other.

Preferred option a combination of engineering measures and individual property protection.

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Notes

Slide 12

The mitigation works for Option 2 are as follows:7 new detention basins with associated bunding and controls of which four address the main overland flow

routes from the rural areas while the remaining three address urban flooding. The total area of detention basins equate to 15.46ha

One major road crossing along Cold Ash Hill;Proposed new ditch and swale; andProvision of new surface water sewers to be maintained by West Berkshire Council and Thames Water.

Option 3 comprises the following proposed works:11 new detention basins with associated bunding and controls of which 8 address the main rural overland flow

routes while the remaining 3 deal with urban flooding. The total area of detention basins equate to 19.79ha;

Two major road crossing, Cold Ash Hill and Turnpike Road;Proposed new cut off ditch to the north of Thatcham and swale; andProvision of new surface water sewers and up sizing of existing sewers to be maintained by West Berkshire

Council and Thames Water.The proposals in Option 4 replicate the options put forward from Option 2 together with upsizing a number of

critical sewers within Thatcham.Modelling indicated majority of flooding occured at levels below 150mm, therefore in addition to the Cost

Benefit Analysis of the technical options, an analysis was also undertaken for the combination of Option 2 and retro-fitting resistance measures to residential properties (Option 5).

These measures would include flood barriers, air brick caps and sealing external walls etc and would require residents to sign up to an early flood warning system.

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Cost Benefit Analysis

Cost Benefit was undertaken over a 100 year design horizon based on the PAG Guidance, Multi-Coloured Manual and The Green Book

Cost based on damages associated with return period were calculated using the depth damage curves from the Multi-Coloured Manual

Benefit Cost Ratio used to compare options

Chosen option had a Benefit/Cost Ratio of 6.38

Costs and benefits £k

Option 1 Option 2 Option 3 Option 4 Option 5

PV Costs - 12,464 18,246 15,314 28,092

PV Damage 181,179 132,638 134,327 139,331 1,846

PV Damage avoided

(Benefit) 48,541 46,852 41,848 179,333

Net Present Value NPV 36,076 28,606 26,534 151,240

Benefit/cost ratio 3.89 2.57 2.73 6.38

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Notes

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• Cost Benefit Analysis is a key part of the project as it is the point at which true comparisons can be made to determine the preferred option

• Many funding streams look for a Benefit Cost Ratio of 5 or above to be consideredSlide 16

• Due to the sensitive nature of residential property flooding and the extensive flooding experienced in Thatcham, particularly during the July 2007 floods, the SWMP project steering group meetings included representation from the public at all stages of the project. The general public were represented through Thatcham Flood Forum and Cold Ash Community Partnership, both of whom have had a representative present at the meetings.

• The public have been informed of progress throughout the SWMP process through the presentation at key milestones and results of the modelling through graphical interfaces and video simulations at project meetings. In addition ongoing email communication between Thatcham Flood Forum and it’s members, progress of the project reported in the newsletter for Thatcham Town Council, presentations by various members of the steering group at council meetings, a roadshow held by West Berkshire Council and inclusion of the project on various websites.

• It was decided that once all partners and stakeholders had agreed to the flood extent and options to take forward, a more focused public consultation could take place.

Slide 17

• The Thatcham SWMP has been issued as a Work-in-progress. This is due to restrictions on the timing and funding agreements in place for the project.

• SWMPs need to be treated as a living document, with the focus on regular reviews• The Thatcham SWMP concludes with an action plan of how to go forward following the issue of the

SWMP. The main focus is on continuing the project partnership, however is must be recognised that there are resourcing implications on continuing this partnership that not all stakeholders may not be able to sign up to

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Public Consultation

Undertaken throughout process

Thatcham Flood Forum & Cold Ash Community Partnership representedlocal community at steering group meetings

Public informed on progress through graphical interfaces, emails,newsletters, presentations and a roadshow

More detailed consultation will takeplace once all partners agree tooptions to take forward

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Way Forward – Action Plan

Continue existing partnership to ensure success of SWMP

Secure funding to implement options

Set up asset register identifying ownership &maintenance of critical drainage infrastructure

Continue to apply existing planning policy and look to influence national policy

The action plan should be reviewed regularly (6-12 months)

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Summary

Need to ensure partners and stakeholders are engaged and clear roles and responsibilities identified

Detailed information required to assess options

Cost Benefit Analysis undertaken to determine preferred option

Outcome of the SWMP is an Action Plan which is reviewed every 6 to 12 months

Continue to review SWMP every 3-5 years

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Any Questions?

Brian Cafferkey

Technical DirectorWSP Development and

TransportationMountbatten House

Basing ViewBasingstokeRG21 4HJ

Tel: 01256 [email protected]