Difference between revisions of "Case study:River Cleddau"

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|Bank and bed modifications measure=Bank re-grading, New bioengineering elements in order to stabilize slope banks
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|Bank and bed modifications measure=New bioengineering elements in order to stabilize slope banks, Bank reprofiling,
 
|Floodplain / River corridor=Fencing
 
|Floodplain / River corridor=Fencing
 
|Management interventions=Fencing,
 
|Management interventions=Fencing,

Latest revision as of 16:02, 2 January 2019

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Location: 51° 49' 20", -4° 58' 30"
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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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Lessons learnt

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Image gallery


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)
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Catchment and 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) 175
175 m
0.175 km
17,500 cm
Dominant geology Siliceous
Ecoregion Great Britain
Dominant land cover Improved grassland
Waterbody ID GB110061031340



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

Reach length directly affected (m) 500
500 m
0.5 km
50,000 cm
Project started
Works started 2004/07/01
Works completed
Project completed 2004/07/31
Total cost category 10 - 50 k€
Total cost (k€) 29
29 k€
29,000 €
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

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

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

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://www.therrc.co.uk/rrc case studies1.php?csid=22 River Cleddau case study

Supplementary Information

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