Case study:Harbertonford Flood Alleviation Scheme

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Location: 50° 23' 27.36" N, 3° 43' 19.76" W

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Project overview

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Status	Complete
Project web site
Themes	Flood risk management, Social benefits
Country	England
Main contact forename	Nick
Main contact surname	Elbourne
Main contact user ID	User:NickRRC
Contact organisation	River Restoration Centre
Contact organisation web site
Partner organisations	Environment Agency, Defra, South Hams District Council & Harbertonford Parish Council
Parent multi-site project
This is a parent project encompassing the following projects	No

Illustration of proposed works. Courtesy of Bradley et al.(Available at http://www.britishdams.org/2004conf/papers/bradley.PDF).

Project summary

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The River Harbourne flows through the village of Harbertonford, which has been flooded on 21 separate occasions over the past 60 years - of which 6 flood events occurred between 1998 and 2000. The catchment is highly 'flashy', with little warning for residents prior to a flood.

The Harbertonford Flood Alleviation Scheme was a joint project funded by DEFRA, the Environment Agency, South Hams District Council and Harbertonford Parish Council, costing £2.6 million. Its aim was to reduce the risk of flooding through the implementation of engineered flood alleviation solutions. A zoned clay-core embankment dam ('Palmer's Dam') with a 4.1 ha (150,000 cu m) reservoir storage area was constructed 2km upstream of Harbertonford, in conjunction with 600mm of bed level lowering and channel widening through Harbertonford to increase channel capacity (to accommodate flows up to 28 cumecs - 10 year flood. A flow control system was installed at the reservoir outlet to permit normal river flow, with automated flood control gates to throttle flows during a flood event. In addition, a flood defence wall was installed and surface drainage systems improved. Together, these measures were built to prevent flooding in Harbertonford up to a 1 in 40 year flood event.

On 7th July 2012, a 1 in 40 year flood event occurred and Palmer's Dam was overtopped for the first time since its construction. This resulted in the flooding of some properties in Harbertonford, however flooding was limited to a few lowlying homes and the flood peak was significantly delayed, giving increased warning time.

As part of the bed-lowering technique, a series of pool-riffle sequences were constructed within the channel through Harbertonford - a best practice approach for habitat enhancement, proposed by the River Restoration Centre (www.therrc.co.uk). These would provide both improved spawning habitat and habitat for adult fish.

Works included:

• Channel lowered by 600mm through Harbertonford using pool-riffle sequences, to increase channel capacity, to withstand 28 cumec flow (1:10 year event). Also widened in areas to further increase capacity. Two weirs lowered to minimise backwater effect (Crowdy Mill and Mill Leat).
• Zoned clay-core embankment dam constructed 2km US of Harbertonford, with 4.1 ha flood storage reservoir (150, 000 cu. metre capacity), capable of withstanding up to 1:40 year event.
• Flood wall installed along a portion of the river, through Harbertonford. Replaced earth bank. Surface drainage systems improved.

Community Involvement - Local primary school visited site during construction of dam and have continued to visit 'reservoir' site, to study ecological development.

Monitoring surveys and results

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Lessons learnt

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Image gallery

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View look upstream towards outlet of flood storage reservoir, with flow control structure. Courtesy of the River Restoration Centre.

View looking downstream towards dam and outlet. Courtesy of the River Restoration Centre.

One of a number of pool-riffle sequences installed on the advice of the River Restoration Centre. Courtesy of the River Restoration Centre.

View from newly constructed flood wall in Harbertonford - looking downstream. Courtesy of the River Restoration Centre.

Catchment and subcatchment Select a catchment/subcatchment Edit the catchment and subcatchment details (affects all case studies in this subcatchment) Catchment River basin district South West River basin South Devon Subcatchment River name HARBOURNE RIVER Area category 10 - 100 km² Area (km2) Maximum altitude category 200 - 500 m Maximum altitude (m) 375375 m <br />0.375 km <br />37,500 cm <br /> Dominant geology Siliceous Ecoregion Great Britain Dominant land cover Arable and Horticulture Waterbody ID GB108046005170 Site Edit site Name Harbourne at Harbertonford WFD water body codes GB108046005170 WFD (national) typology Low, Small, Siliceous WFD water body name Harbourne River Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body No National/international site designation Local/regional site designations Protected species present Yes Invasive species present No Species of interest Otter and common dormouse (protected); Atlantic salmon, bullhead, sea trout, brown trout. Dominant hydrology Dominant substrate Slate/shale, overlain by alluvium. River corridor land use Agriculture. Average bankfull channel width category Average bankfull channel width (m) Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) Project started 1999 Works started Works completed 2002/09/01 Project completed Total cost category more than 10000 k€ Total cost (k€) 35000003,500,000 k€ <br />3,500,000,000 € <br /> Benefit to cost ratio Funding sources Defra, South Hams District Council, Harbertonford Parish Council Cost for project phases Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname Investigation and design Stakeholder engagement and communication Works and works supervision Post-project management and maintenance Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Flood risk management Hydromorphology Biology Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Bed lowering, Creation of pools and riffles, Riverbed widening, Weir modification Floodplain / River corridor Containment bund to prevent flooding, Construction of flood wall Planform / Channel pattern Other Surface drainage systems improved, SUDS Non-structural measures Management interventions Social measures (incl. engagement) Engagement with schools, Community Education Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Physico-chemical quality elements Edit Physico-chemical quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Any other monitoring, e.g. social, economic Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description http://ec.europa.eu/ourcoast/download.cfm?fileID=950 Environment Agency - 'Harnessing the Harbourne' http://harbertonford.org/index.php/Local/HowItWorks Harbertonford News http://www.britishdams.org/2004conf/papers/bradley.PDF Integrating design with the environment to maximise benefits from a flood storage dam: successful implementation at Harbertonford. Supplementary Information Edit Supplementary Information