Case study:River Cleddau

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Location: 51° 49' 20.49" N, 4° 58' 30.48" W

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Project overview

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Status	Complete
Project web site
Themes	Fisheries, Habitat and biodiversity
Country	Wales
Main contact forename	Richard
Main contact surname	Edwards
Main contact user ID
Contact organisation	Salix River & Wetland Services Limited
Contact organisation web site
Partner organisations	Natural Resources Wales
Parent multi-site project
This is a parent project encompassing the following projects	No

Bank section after works 2005 (courtesy of Salix Water & Wetland Services Ltd)

Project summary

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The River Cleddau in South West Wales is a mobile gravel bed river of moderate to high energy and an important spawning tributary for migratory fish. The project undertaken in 2004 aimed to demonstrate the effectiveness of soft engineering techniques in managing extensive erosion problems, whilst still encouraging natural river processes to continue. The demonstration reach in particular had a history of instability and planform adjustment.

An initial geomorphological survey and detailed design plan were carried out by Salix for the Environment Agency Wales and Pembrokeshire Rivers Trust. The initial survey concluded that overgrazing and poaching were accelerating erosion in the demonstration reach.

Hard engineering techniques, such as blockstone revetments, have been traditionally used in higher energy river systems to control bank erosion. However, the use of such techniques removes any scope for marginal vegetation to establish, thus lowering the ecological value of the river. It can also deflect energy to other areas of the bed or bank, maintaining the overall erosion problem.

As part of a three-year project, entitled “Fishing Wales” the Environment Agency Wales were keen to demonstrate that softer bioengineering methods could be used to control the extensive erosion, whilst maintaining geomorphological processes, and enhancing the physical habitat of the river. The aim of the demonstration was to show that bioengineering techniques can be used as an alternative to blockstone in mobile gravel bed rivers to control erosion, whilst restoring physical habitat to degraded channels, thus helping to ensure the sustainability of fish stocks in Wales.

A range of bioengineering techniques were used based on the local reach scale erosion process, water depth and stream power. In areas with low stream power these included bank grading and toe protection using live willow bundles (faggots/fascines), whilst in areas with greater erosive energy whole upturned tree roots with 2 metres of trunk still attached were installed to act as physical revetments and also deflect flow away from eroding banks (40 tree roots used in total). All the trees used were grey willow (Salix cinerea) or crack willow (Salix fragilis) and have since grown to provide excellent habitat and cover. In addition the entire reach was fenced in order to keep livestock off the banks. Living plant material was obtained from the Cleddau catchments.

The project has been successful in demonstrating soft bio-engineering methods. The Pembrokeshire Rivers Trust and Salix report that 80-90% of planted material has survived, with a mix of other vegetation also colonising the site. Salix have since identified some issues with Himalayan balsam competing with native planting, but this has been acknowledged as a catchment wide problem (see evidence in photos).

The River Restoration Centre would like to thank Salix River & Wetland Services Ltd for providing the content and photographs for this case study.

Monitoring surveys and results

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Image gallery

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Bank section before works 2004 (courtesy of Salix Water & Wetland Services Ltd)

Bank section after works 2005 (courtesy of Salix Water & Wetland Services Ltd)

Bank re-profiling (courtesy of Salix Water & Wetland Services Ltd)

Marginal habitat creation (courtesy of Salix Water & Wetland Services Ltd)

Catchment and subcatchment Select a catchment/subcatchment Edit the catchment and subcatchment details (affects all case studies in this subcatchment) Catchment River basin district Western Wales River basin South West Wales Subcatchment River name W. Cleddau - Anghof conf to Cartlett Brook conf Area category 100 - 1000 km² Area (km2) Maximum altitude category 100 - 200 m Maximum altitude (m) 175175 m <br />0.175 km <br />17,500 cm <br /> Dominant geology Siliceous Ecoregion Great Britain Dominant land cover Improved grassland Waterbody ID GB110061031340 Site Edit site Name River Cleddau, Haverfordwest WFD water body codes GB110061031340 WFD (national) typology WFD water body name W. Cleddau - Anghof conf to Cartlett Brook conf Pre-project morphology Actively meandering, extensive bank erosion Reference morphology Reduced bank erosion, passively meandering 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 Gravel, Sand River corridor land use Improved/semi-improved grassland/pasture, Intensive agriculture (arable), Woodland 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) 500500 m <br />0.5 km <br />50,000 cm <br /> Project started Works started 2004/07/01 Works completed Project completed 2004/07/31 Total cost category 10 - 50 k€ Total cost (k€) 2929 k€ <br />29,000 € <br /> Benefit to cost ratio Funding sources Environment Agency Wales 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 Riparian development Hydromorphology Biology Fish Physico-chemical Other reasons for the project dual purpose of marginal habitat enhancement and reduction in extensive erosion Measures Edit Measures Structural measures Bank/bed modifications New bioengineering elements in order to stabilize slope banks, Bank reprofiling Floodplain / River corridor Fencing Planform / Channel pattern Other Non-structural measures Management interventions Fencing 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 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://www.therrc.co.uk/rrc case studies1.php?csid=22 River Cleddau case study Supplementary Information Edit Supplementary Information