River Granta, Babraham, Cambs. Channel Narrowing
Client: Babraham Institute and Greenwillows Associates Ltd.
Project Brief: Salix were asked to narrow a 60m section of the river to create greater habitat diversity. The river was silting up and by speeding up the flow the value to fish would be restored.
The Solution: The channel was narrowed from 6 metres wide to 3m wide by two techniques. The upstream section was carried out by installing alternating reed margins to the banks creating a sinuous low flow channel. Pre-Established Reeds in Coir Rolls with Pre-Established Coir Pallets were installed at mean summer water level on a brushwood mattress bed of Faggots. Faggots were also used for the down stream groins which were 400mm diameter, fixed in position with chestnut posts and arranged as shown in the plan view.
The Result: The upstream new reed margins provide an opportunity for water voles and the downstream groins will provide a different habitat as they will accrete sediment on the upside and could show a bed lowering on the downside. The central channel will now have a clean faster flowing water and can generate bed with gravel zones in the future.
Materials used: Pre-Established Coir Rolls, Pre-Established Coir Pallets, Dead Faggots. Chestnut fixing stakes.
River Granta, plan of type and extent of works
River Granta,
typical section channel narrowing
River Granta,
Cambs. Original silt laden channel
River Granta.
New reed margin and narrowed channel
Cannel narrowing with
Faggot groins, River Granta,
photo taken at installation
