Wednesday, February 09, 2011

BW use both old and new technology to battle bank erosion on the K&A Canal

At Hungerford on the Kennet & Avon Canal BW have gone back to the original John Rennie technique of protecting the bank using tufted sedge which will also provide a home for water voles without the risk of damage to the bank.  Whereas a few miles to the east at Newbury they are using the latest high tech fabric in the battle against the bank erosion caused by the non-native American Signal Crayfish.
Hungerford — water vole homes
Water vole gets new home: Picture by BW
At Hungerford Marsh the challenge was to create natural-looking stable canal banks that are also water vole friendly as part of the recent £600,000 project there. As well as the extensive dredging, one and a half miles of bank needed to be rebuilt.

Tussock sedge planted at Hungerford Marsh
to protect the bank and provide a home 
for the water vole: Picture by Derrick Hunt©
Antonia Zotali, BW's senior project engineer, said, “Over the years the banks have been eroded by boat wash, cattle grazing and even burrowing crayfish. It would be very easy to just say we are going to rebuild the banks using concrete and steel, but this wouldn’t reflect the special canal heritage and environment so we are using more natural methods to do the job.”

BW contractors have rebuilt the bank by recycling the sediment dredged from the bottom of the canal. The dredged material is held by a shelf below the water level which is supplemented by a reed planting scheme that includes a line of tussock sedges,  as used by John Rennie more than 200 years ago.

BW ecologist, Oda Dijksterhuis said: “Tussock sedges are brilliant plants to use as part of this scheme to rebuild the waterway banks. By planting the sedge we will create a fringe of greenery along the water’s edge. John Rennie originally planted tussock sedges when the canal was first built because as the plant grows, and its roots form, they help to knit the soil and sediment together, stopping erosion and defending the bank. The tussock sedge provides a great habitat for water voles. As well as providing a good food source the voles can also make their homes in the beds of sedge. 

Newbury — crayfish problem
The contractors have now moved on to Newbury where the bank is being damaged by American Signal Crayfish.

American Signal Crayfish:  Picture byTrevor Renals©

Between Bull's Lock and Hambridge Road Bridge the American Signal Crayfish are being blamed for causing extensive damage by burrowing into the canal banks using their tails which, says BW Engineer David Berezynskyj, can affect the stability of the bank. He said, “Some of these banks are like Swiss cheese and we need to act to avoid any serious damage and reduce the risk of the embankment breaching.” 

Posts with geotextile fabric stretched between them ready to be back-filled: Picture by Bob Naylor©

Pre-planted coir rolls ready for use: Picture by Bob Naylor©
The work in Newbury, which will take about 10 weeks to complete at a cost of £250,000, involves creating a barrier using a geotextile fabric that is resistant to all naturally occurring soil acids and alkalis, is unaffected by bacteria and fungi and which the crayfish cannot penetrate — but it will allow water to drain through it. This will be back filled with material dredged from the canal and planted with native water plants to create a stable canal bank.

During this work a much needed landing stage has been built for boaters operating Bull's swing bridge.

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