Flood Alleviation Scheme, BanburyNCE Flood Management Conference 4 Dec 2013

Richard Harding CEng MICE Banbury FAS Project ExecutiveSteven Lee CEng MICE C.WEM MCIWEM MAPM Banbury FAS Project Manager (CPC Project Services LLP) 1

Banbury FAS: BackgroundBanbury has a long history of flooding. In April 1998 125 residential & 35 commercial properties were flooded causing over £12.5m of damage –the flood was estimated as a 1:100 year event.The newly constructed scheme protects Banbury against a 1:200 year fluvial flood reducing the risk of flooding to 441 residential and 73 commercial properties bringing benefits of c.£100m. Total scheme cost c.£18.6m. The scheme has been funded by the Thames Regional Flood & Coastal Committee and 3rd party contributions of over £3m from Cherwell DC, Thames Water, Network Rail and Prodrive.

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Banbury FAS: OptionsHistoric flood review

Spiceball gauging station records

Historic record review

Rainfall hydrographs

Options included

do nothing/maintain

improve conveyance

local defences

diversion channel/tunnel

upstream flood storage

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Banbury FAS: Key facts

The main upstream embankment is 2.85km long with a maximum height of 4.5m. The flood storage area created behind the embankment is capable of holding over 3million m3 of flood water in a 1:200yr event equivalent to 1200 Olympic size swimming pools.Over 200,000m3 of material was dug from the field on the opposite side of the canal to construct the embankment.The 2 flow control structures are identical, contain no moving parts and incorporate the main spillways. We also raised a 860m long section of the A361 to ensure it floods no more frequently than at present.At Wildmere we have constructed a 400m long earth embankment: at Tramways we have built a combination of earth embankments and low level sheet pile walls whilst at Moorfield Brook we had previously constructed a pumping station.

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Banbury FAS: Layout plan

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Banbury FAS: Design challengesClient brief – simple, passive system.

Legal advice was to negotiate alongside a CPO. First CPO was issued in 2004 using ISIS model. Objections received from both landowners and statutory consultees – took several years to resolve statutory objections.

Changes in government priority scoring system in 2006 meant scheme wasn’t funded. Public Inquiry was cancelled at our request as certainty of funding is a prerequisite.

Funding constraints - alternate funding sources/partnership funding confirmed.

New CPO issued in 2007 using new modelling (Tuflow). Objections again received plus 3 subsequent CPO amendments needed.

Public Inquiry in May 2010. Decision on 31 Oct 2010. Last possible day for 2011 start on site. Impact on clearance and mobilisation.

Climate change.

Managing public expectations/interest throughout.

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Banbury FAS: Material win areasReviewed 8 potential local win areas – generally ‘raised areas’ upstream of Banbury

We clearly weren’t the first to recognise their potential as extensive archaeology remains and now many vital as cattle refuges in flood events

Archaeology included Bronze Age settlement patterns, British Romano farmstead settlements with Saxon farmsteads overlaying these.

Medieval ridge and furrow agricultural field patterns throughout the area

Within the Cropredy Bridge Battlefield site from the English Civil War.

But key issue was 2nd World War anti-aircraft battery!

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Banbury FAS: Ideal design of control structure• Flow control structure should throttle river flows to 38m3/s• Control of discharge should be automatic (no attendance)• No reliance on power supplies or remote operation• No moving parts• Minimised impact on upstream landowners

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Banbury FAS: Design innovation: double-baffle orifice

• Uniquely adapted from irrigation schemes • Described in textbooks• Never before used in UK

Battery of double-baffle orifice devices (Neyrpic, 1971)

Double-baffle orifice layout and performance (after Bos, 1989)

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Banbury FAS: Double-baffle orifice design – how it works

Low head, with crump weir controlling the flow

Medium head, with flow controlled by the upstream baffle

High head, with flow controlled by the downstream baffle

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Banbury FAS: Model testing

Hea

d

Flow

Design developed by physical model testing at HR Wallingford

The graph shows the resulting rating curve for each flow control structure

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Banbury FAS: Benefits of innovative design

• Simple design• Passes constant flow• Increased reliability (no moving parts)• Increased life of asset• Reduced operation & maintenance costs• Requires no operation during flood• Increased sustainability (no energy requirements)• Reduced capital costs • Reduced carbon footprint over the life of the asset

All benefits of passive design

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Banbury FAS: Construction

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TRAMWAYS

WILDMEREHUSCOTE FCS

HARDWICK FCSBORROW PIT

MOORFIELD BROOK PS

BANBURY FSR EMBANKMENT

A361 RAISING

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Banbury FAS: Challenges

Availability of materialArchaeologyWorking in four locationsLandowner requirementsMinimising winter workingMinimising waste – 97% recyclingA361 amendments to road width Cropredy FestivalManaging local expectations

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Banbury FAS: Flood storage reservoir borrow pit

Main Borrow Pit located West side of Oxford CanalClay from Borrow Pit suitable for EmbankmentPre-Construction anticipated using Weathered ‘Brown’ ClaySelection Found to be difficult + rock lensesDuring Construction Unweathered ‘Blue Clay’ found to be Suitable

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Banbury FAS: Archaeology

Pre-historic flint tool finds in the RA6Bronze Age settlement patterns British Romano farmstead settlementsSaxon farmsteads overlying the British Romano settlementsMedieval ridge and furrow agricultural field patternsWithin the Cropredy Bridge Battlefield site from the English Civil War. Dummy 2nd World War anti-aircraft battery

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Banbury FAS: Reservoir Embankment -construction photographs

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Banbury FAS: Flow control structures -construction photographs

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Banbury FAS: River realignments -construction photographs

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Banbury FAS: A361 Road Raising

860m of existing road raised to ensure flooding and road closure no more frequent than existingCulverts installed to ensure balanced water levels either side of road during flood eventsTwo local authority boundaries

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Banbury FAS: A361 Road Raising - Construction

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Banbury FAS: Tramways

Protects Industrial Estate & Football Ground from Flooding210m Long Sheet Pile Walls with Concrete Capping Beam100m Long Earth Embankment 2m High

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Banbury FAS: Tramways - Construction

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Banbury FAS: Wildmere

Protects Industrial Estate from Flooding400m of Earth Embankment up to 2m high2 No. Culverts

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Banbury FAS: Wildmere - Construction

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Banbury FAS: Wider scheme benefits

Proposed Country Park in win area, working with Cherwell DC12 hectares of new BAP habitat including wet and dry woodland/scrub planting3 river realignments incorporating new pondsNew hedgerows and trees (planting 3 times more than were cut down)New & reinstated meadowsImproved footpath access

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Banbury FAS: Scheme in operation –November 2012

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Banbury FAS: Aerial view of the completed embankment

29Richard.Harding@environment-agency.gov.uk

Steven.Lee@cpcprojectservices.com