Client : Environment Agency
Date : May 2004
Scope of Work : 500 metres of river enhancement using innovative bioengineering techniques
Value : £30,000
Background:
The River Dulais is a tributary of the River Towy, one mile east of Llandeilo, South West Wales, and is an important spawning tributary for migratory fish. The Dulais is a mobile gravel bed river of moderate to high energy and in the demonstration area has a history of instability and planform adjustment.
River Dulais before works
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River Dulais bank erosion
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Traditionally, blockstone revetments would be used in higher energy rivers to control bank erosion. However, blockstone removes any scope for marginal grasses and trees to establish, thus lowering the ecological value of the river and also reflects the flow of water, which causes additional instability downstream. Environment Agency Wales were keen to demonstrate that softer bioengineering methods could be used that would control the extensive erosion, whilst maintaining geomorphological processes, and enhancing the physical habitat of the river. The ultimate aim of the demonstration was to show that bioengineering techniques can be used as an alternative the blockstone in mobile gravel bed rivers.
River Dulais eroded banks
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Salix undertook a detailed geomorphological survey to ascertain the erosion processes at work, rates of planform adjustment, estimated discharge at bankfull and an estimation of the erosive energy potential of the river by calculating stream power and shear stresses applied to the bed and banks of the river during bankfull events. The survey also concluded that overgrazing and poaching were greatly accelerating the erosion in the entire reach.
Design:
Salix also provided a detailed design plan for restoring the river using bioengineering techniques. In order to maintain and enhance local biodiversity all living plant material was harvested from the Dulais and adjacent catchments. The desired vegetation types were deemed to be a mix of marginal grasses (Phalaris arundinacea, Juncus effusus) and tree cover (Salix cineria & Salix fragilis).
River Dulais works in progress
- placing tree root wads in river bank
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River Dulais - one year on
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Several bioengineering techniques were employed based on the local reach scale erosion process, water depth and stream power. For example, in reaches with slack bed gradients and therefore reduced stream power simple low cost techniques were used, such as bank grading and toe protection using live willow bundles (faggots/fascines). The entire reach was fenced in order to keep livestock off the banks.
In deeper areas, outer meander bends and in reaches with steeper bed gradients and therefore more erosive energy more robust techniques were used. 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. In total some 40 large tree roots were installed in the deepest and most unstable reaches. All the trees used were grey willow (Salix cinerea) and as such have started to grow providing excellent habitat and cover.
Results:
The Environment Agency are delighted with the results and are using the project as a test site to demonstrate the use of sustainable river management techniques. Approximately 90% of all living material planted has survived and a diverse mix of other vegetation species has colonised.
Afon Dulais Restoration works before and after (8/7/05)
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