River Cleddau, Haverfordwest
Client: Pembrokeshire Rivers Trust
Background: The River Cleddau is a mobile gravel bed river, one mile north of Haverfordwest, South West Wales, and is an important spawning tributary for migratory fish. The Cleddau is a mobile gravel bed river of moderate to high energy and in the demonstration area has a history of instability and planform adjustment.
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 the 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 could work in applications where blockstone would usually be used.
Salix undertook a detailed geomorphological survey to ascertain the erosion processes at work, rates of planform adjustment, estimated discharge at bank-full and an estimation of the erosive energy potential of the river by calculating stream power and shear stress applied to the bed and banks of the river during bank-full 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 Cleddau catchments. The desired vegetation types were deemed to be a mix of marginal grasses (Phalaris arundinacea, Juncus effusus, Iris pseudacorus, Carex riperia and Sparganium erectum) and tree cover (Salix cinerea and Salix fragilis).
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 a physical revetment 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 crack willow (Salix fragilis) and as such have started to grow providing excellent habitat and cover.
Results:
The Pembrokeshire Rivers Trust 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.
Materials used: Willow Faggots(Fascines)
River Cleddau eroded banks before
works
River Cleddau, Haverfordwest, before works
River Cleddau, Haverfordwest, before
works
River Cleddau after works 26-08-05
Afon Cleddau before works (left) and one year after (right)
