River Irwell Bank Stabilisation
Feb 11, 2008 - 4:20:17 PM
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| Background: |
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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. |
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River Irwell before stabilisation works.
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| Design: |
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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. |
| Eroding footpath at River Irwell |
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was important to protect the toe of the bank and as such 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 ( 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. |
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| Results: |
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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.
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| Well established
vegetation at River Irwell (above left and right).
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PROJECT BY SALIX
