Case study:Harbertonford Flood Alleviation Scheme

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Contents

Location: 50° 23' 27" N, 3° 43' 20" 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.

Monitoring surveys and results

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Lessons learnt

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Image gallery


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.
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Catchment and 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
0.375 km
37,500 cm
Dominant geology Siliceous
Ecoregion Great Britain
Dominant land cover Arable and Horticulture
Waterbody ID GB108046005170



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

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€
3,500,000,000 €
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

Mitigation of a pressure Flood risk management
Hydromorphology
Biology
Physico-chemical
Other reasons for the project


Measures

Structural measures
Bank/bed modifications 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).
Floodplain / River corridor 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.
Planform / Channel pattern
Other Flood wall installed along a portion of the river, through Harbertonford. Replaced earth bank. Surface drainage systems improved.
Non-structural measures
Management interventions
Social measures (incl. engagement) Local primary school visited site during construction of dam and have continued to visit 'reservoir' site, to study ecological development.
Other


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

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

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative


Monitoring documents



Additional documents and videos


Additional 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

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