River Derwen, Preston, Lancashire. Spring 2008
Client: National Grid Plc. and RSK Environmental
Project Brief: National
Grid guidelines require a minimum of 10m from bank top to a distribution gas pipe. Erosion of the river bank had reduced to less than the minimum width and recovery of the soil mass was required. Giant Hogweed and
Himalayan balsam had dominated the bank vegetation, which dies back for the winter leaving the sandy soil exposed to erosion. The Environment Agency would not allow a "hard engineering" solution, ruling out tipped stone.The river floods up to nearly bankfull regularly each year.
The solution: The bank was built out with layered subsoil and Tensar geogrids at 0.5m centres. The founding layer was built up to water level with granular fill behind the Rock Roll pyramid. The pyramid with the filter fabric also acted as a sediment control measure during construction. The new profile was soiled, seeded and finally armoured with the NAG C350 turf reinforcement mat. The base of the slope was secured by reinstating a robust new reed margin using Pre-Established Coir Roll at the water line and set on a pyramid of Rock Rolls under the water. The Rock Rolls were reduced to nil as the river moved out of the bend. As the NAG C350 resists flows of 4m/s to short periods of 6m/s the perimeter of the matting was secured with Rock Rolls up the slope to ensure it was not undermined in flood. 120m of the outside bend was treated.
The result: The required bank width was reinstated and secured against future erosion. The system allowed for plants to contribute to the safety of the system as well as fulfilling the environmental aspect demanded.
Materials used: Rock Rolls, Reed Established Coir Rolls, Separation fabric, Tensar geogrid, seed mix, North American Green C350 TRM..
Typical cross section
Installing the Rock Rolls
Reinstating the bank width
Installation on completion, summer 2007
Close up of the slope late summer 2007
