Case study:River Valency at Boscastle

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Location: 50° 41' 23.89" N, 4° 41' 32.37" W

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Project overview

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Status	Complete
Project web site
Themes	Flood risk management
Country	England
Main contact forename	James
Main contact surname	Burke
Main contact user ID
Contact organisation	Environment Agency
Contact organisation web site	http://www.environment-agency.gov.uk/
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

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Project summary

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Boscastle village is located in a steep sided and narrow valley through which the River Valency flows down to the harbour. In 2004 an intense storm centred over the small wooded catchment caused massive erosion of sediment and river-side trees. This,combined with high flows, inundated the village with water and debris causing extensive damage.

Enlargement of the river channel offered the only viable way to reduce flood risk and improve the flood capacity and sediment conveyance of the river. The scheme aimed to avoid producing a deep geometrically uniform’channel. The channel was designed in such a way that it simulated the natural features found higher up in the undisturbed reaches of the river by engineering features into the excavated bedrock.

A detailed topographical survey of the longitudinal profile of the river was an important design tool. This enabled a new, lower bed gradient to be superimposed onto the original one, giving the ‘best fit’ with the levels upstream and downstream of the reach. The natural valley slope at Boscastle is 1 in 45 and the channel bed upstream of the village has a depth of approximately 2.5m. Extending this channel depth down through the village to the harbour, the longitudinal section showed the original bed to be typically 1 metre higher. This helped to explain the loss of capacity through the village and its propensity to flood. Bed regrading to this 2.5m depth profile was therefore considered feasible (Figure 6.4.2). The design of the cross section and longitudinal profile of the lowered bed involved close study of the natural characteristics of the rock visible in the upper river and in the harbour. The rock featured strong bedding planes that typically dip from left to right bank, angled downstream at about 45°. It had vertical fracture lines as well as regular intrusions of much harder quartz. Concept drawings were provided to show how the rock was to be removed. An engineer worked closely with machine operators to obtain the desired result of the left side sloping with the dip and the right side vertical along the fractures. Both sides were zig-zagged to stay within the'character' stone retaining walls.

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Image gallery

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Reference reach upstream of Boscastle

Flat and featureless prior to works - 2006

Breaking out the bedrock

Post works

Excavation of bedrock

Formation of beaches post works - 2008

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 North Cornwall, Seaton, Looe and Fowey Subcatchment River name VALENCY Area category 10 - 100 km² Area (km2) Maximum altitude category 200 - 500 m Maximum altitude (m) 304304 m <br />0.304 km <br />30,400 cm <br /> Dominant geology Siliceous Ecoregion Great Britain Dominant land cover Improved grassland Waterbody ID GB108049007170 Site Edit site Name River Valency at Boscastle WFD water body codes GB108049007170 WFD (national) typology WFD water body name VALENCY Pre-project morphology Bedrock cascade with lower gradient Reference morphology Bedrock cascade with higher gradient 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 Atlantic salmon, European eel (Anguilla anguilla) Dominant hydrology Dominant substrate Gravel River corridor land use Urban (village) 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) 300300 m <br />0.3 km <br />30,000 cm <br /> Project started 2007/03/01 Works started Works completed Project completed 2008/09/01 Total cost category Total cost (k€) Benefit to cost ratio Funding sources 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 Supplementary funding information The exact cost of the river restoration element of this project is unknown. The project was part of a £6.3M (€7.3M) flood risk management scheme. 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 Excavation of bedrock Floodplain / River corridor Construction of flood wall Planform / Channel pattern Change in bed profile, Pools, embayments and cascades created Other Non-structural measures Management interventions Social measures (incl. engagement) 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 Fish No Yes No Yes No Improvement 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 Supplementary Information Edit Supplementary Information