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 and Harbertonford Parish Council.
Parent multi-site project
This is a parent project encompassing the following projects	No

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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.

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Catchment and subcatchment

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Site

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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	No
Invasive species present	No
Species of interest
Dominant hydrology
Dominant substrate	Slate/shale, overlain by alluvium.
River corridor land use	Agriculture.
Average bankfull channel width category
Average bankfull channel width (m)
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Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)
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Average unit stream power (W/m2)

Project background

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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

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Mitigation of a pressure	Flood risk management
Hydromorphology
Biology
Physico-chemical
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Measures

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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

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quality elements

Element	When monitored		Type of monitoring		Control site used	Result
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Biological quality elements

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quality elements

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Physico-chemical quality elements

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Any other monitoring, e.g. social, economic

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Monitoring documents

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Image gallery

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Additional documents and videos

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Additional links and references

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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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