Bank Erosion at Brookfields, Ongar, Essex
Client: Epping Forest District Council and Halcrow consultants
Project brief: A report by Halcrow Civil Engineering concluded that the bank between the rear gardens of the adjacent houses and Cripsey Brook was failing due to water emerging high up in the bank profile causing a slump and erosion by the brook removing the bank toe.
The solution: Salix proposed the installation of a “sheet drain” to intercept the wetness escaping up in the bank profile so as to drain the water down the slope without affecting any surface reinstatement soils. The sheet drain releases the water directly into the stream via the Rock Rolls that were installed to secure the toe - see typical section in "Image 1". A line of Pre-Established reeds in a Coir Roll were installed on top of the Rock Roll toe. Behind this a granular wedge was then placed with a soil over layer to create a new stable profile. The soil was seeded and covered with NAG C350 turf reinforcement mat.
The Result: The destabilising groundwater is now controlled by the sheet drain and the new bank profile is safe. As these soils are frequently saturated by the flashy brook the plants are now able to resist pulses of high rates of water flow by locking their roots into the rocks of the Rock Rolls and the permanent fine meshes of the C350. Plants such as reeds have coarse enough stems and leaves to deflect flowing water, protecting the soils from erosion. The Rock Rolls will also buttress the soil behind thereby retaining the intended bank line.
Materials used: Sheet drain geotextile, Pre-Established Coir Rolls, Rock Rolls, Grass seeds
Typical section
Work in progress showing the sheet drain geotextile
in place with Coir Rolls and Rock Roll toe in position
The Rock Roll toe
June 08, The result after 3 months
