www.managingwater.co.uk Managing & Maintaining our water channels Stabilising eroding spoil


Reconstruction of water courses in post-industrial Britain often requires stabilisation of soil in locations where storm events can cause significant damage. For example, in South Wales, old mineral spoil dumped at the bottom of small river valleys has frequently been swept away in spates with a heavy environmental impact down stream.


Calverton post installation permanently reinforced vegetative channel


The site engineer had favoured replacing the original bank with rock filled gabions, an expensive and low sustainability option which did nothing for aesthetics of the area.


Salix proposed a value engineered solution which used a combination of elements to achieve a natural appearance while the fully established vegetated banks which would easily withstand bank high flood erosion. The banks were covered with Tensar Vmax turf reinforcement matting over seeded topsoil. The erosion control matting was anchored in a trench at the top of the bank, while at the toe it was anchored with a pre-established vegetated coir roll and a rock roll staked into the river bed below the average water level.


As well as the rapid installation, there was less disruption of the gardens by this method than using gabions, and the aesthetics and sustainability met local planning requirements. The new vegetation also reduced sediment from surface water running off the gardens behind the bank.


When on the River Ebbw, initial proposals for stabilising 1500 metres of loose colliery shale river bank featured heavy blockstone armouring, engineers sought more cost-effective bio-engineered alternatives. As well as economic advantages offered, two criteria were imposed by the consultants: that any solution had to enhance the location’s landscape aesthetics, and the design had to demonstrate its stability under the expected water velocities of 4.2m/sec for a 1:100 year storm event.


The adopted solution used Tensar Vmax matting over the steep 1 in 1.5 slopes, with a length of up to 13 metres and a water depth of up to 4 metres. The slopes were prepared with preseeded and fertilised topsoil to ensure rapid vegetation establishment; erosion protection from the day of installation was provided by pinning back the matting into the slope at 0.5m intervals.


The key area at the toe of the slope was protected by rock roll trenched into the river bed and staked; next, live willow bundles (fascines) secured the wet margins next to the bank. Rapid growth of the willow ensured mitigation of the scour effects of water at these margins, while grass germination of the slopes occurred within a week and established full cover to the bank before the end of the summer.


“The value of this approach becomes more and more evident with time,” concludes David. “Five years after the project was completed, this is now a valued riverside landscape by the side of a popular sewin (sea trout) fishery, and the banks prevent the colliery residues threatening the river environment down stream.”


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