River Irwell Bank Stabilisation
Client: Groundwork Trust
Background: Approximately 50 metres of riverbank had collapsed during flood events and a public footpath (right and below) had started to slip into the river. The soil consisted of a highly erosive sandy soil which is non-cohesive and easily scoured once exposed. The steep banks had been subject to both surface scour erosion and deeper rotational failures in the steeper sections.
Design: Salix tendered for the contract which was based upon a “riprap” revetment design with willow stakes and cuttings. Salix offered a conforming bid and also an alternative design which did not require the use of riprap. The bank was to be re-graded to an inherently stable angle, so the footpath was moved 10 metres back from its existing position. The bank was then re-graded to a 1:25 slope and protected using bioengineering techniques.
It was important to protect the toe of the bank and therefore Rock Rolls and Coir Rolls were installed. These were staked into place to prevent any toe scour which would then undermine the rest of the bank. The Coir Rolls contained native marginal plants that are known to tolerate regular inundation whilst providing an erosion control function (Phalaris arundinacea, Carex riperia, Iris pseudacorus and Juncus effusus). The rest of the bank was tilthed, seeded and covered with an erosion control matting (North American Green C125 (http://www.nagreen.com/)). Over 200 number long willow stakes were installed at 1m centres (Salix caprea and Salix fragilis). These would help to stabilise the bank longer term by reinforcing the soil with their deep fibrous root systems. The willows will also help to remove moisture from the soil, reducing the bulk weight of the soil and increasing the shear strength of the soil.
Results: The willow establishment rates have been very good with an estimated “strike rate” of over 80%. Two large flood events occurred during the first winter, with no damage to the vegetation or without noticeable erosion. Some maintenance will be necessary in order to prevent the willow out-shading the ground cover grass species which will provide the surface erosion control. However, the maintenance will be minimised with 1-2 days every other year.
Materials used: Willow Faggots(Fascines), North American Green C125
River Irwell before stabilisation works
Eroding footpath at River Irwell
Well established vegetation at River Irwell (above left and right)
