The River Derwent is an important habitat for various fish species including Atlantic Salmon, Lamprey, Sea Trout and Brown Trout as well as an important drinking water supply.
Recent flood events accelerated bank erosion on a reach of the River Derwent, east of Workington, Cumbria compromising an access road to the Yearl pumping station and threatening the structural stability of the Yearl weir.
Salix, the appointed contactor, were enlisted the appointed contractor to reinstate the 160m length of bank in this ecologically sensitive site identified that strong environmental protection would be required throughout the duration of the project including silt control.
The risk of silt pollution associated with the disturbance of the fine, unconsolidated mud depositions on the river bed and weak alluvium soils on the bank was high.
frog environmental worked alongside the Salix team to advise on the method of work to minimise silt and find a low-cost silt control solution that did not use chemical treatments.
The decision was to surround the work areas with Silt Wattles, untreated Floc Mat and Silt Mats which, in combination, created a pool where flow could be limited to enhance the settlement of silts and provide a natural fibre filter, restricting the suspended solid load entering the main river flow.
In addition to this a further silt barrier was installed in the form of a bubble curtain, this is made up of two lines of ¾” diameter Bubble Tubing®. An air compressor located on the bank pushes air through tiny holes in the Bubble Tubing® to create a wall of bubbles. The bubbles create a flow pattern that hold the sediment back, whilst the centre of the bubble curtain baffles and augments the water to prevent the transition of silts downstream.
Silt control in river restoration schemes is a critical part of any project’s success. It relies on assessing and addressing environmental risk in the planning stages. Salix exemplify good environmental practice during their river works, mitigating the potential impact of silt on the river and maintain high dissolved oxygen levels that benefit the salmonid populations.
frog environmental can provide silt control guidance and materials. Contact us to discuss your project.