Case study:River Valency at Boscastle

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Location: 50° 41' 24", -4° 41' 32"
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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
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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


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
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Catchment and 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) 304
304 m
0.304 km
30,400 cm
Dominant geology Siliceous
Ecoregion Great Britain
Dominant land cover Improved grassland
Waterbody ID GB108049007170



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

Reach length directly affected (m) 300
300 m
0.3 km
30,000 cm
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

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


Measures

Structural measures
Bank/bed modifications Excavation of bedrock
Floodplain / River corridor Construction of flood wall
Planform / Channel pattern Change in bed profile, Creation of pools, embayments and cascades created
Other
Non-structural measures
Management interventions
Social measures (incl. engagement)
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
Fish No Yes No Yes No Improvement

Physico-chemical quality elements

Element When monitored Type of monitoring Control site used Result
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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



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Supplementary Information

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