Thursday, 22 October 2015

Curing the bank erosion problems at Hilgay

Nick Carter, Wetland Project Officer

Bank erosion
One of the main tasks last winter at Hilgay was to fill the storage lagoon in stages up to the maximum level 1.59m above sea level. During the filling process the banks had to be checked to ensure there were no leaks or subsidence. The filling passed without incident but then the maximum level had to be held for at least a month while the checking continued. It was during this month, with the water at the same level, that wave action resulting from the south westerly winds caused some bank erosion along the northern banks of the two arms of the lagoon. The sandy soils prevalent in the area and the poor grass establishment added to the problem.

The erosion matting

A plan was hatched to solve the problem by using erosion matting over a layer of peat sown with a grass mix. The peat would ensure good growth of the grass which would help strengthen the integrity of the bank. At the bottom of the matting a line of reeds would be planted that would grow to form a natural barrier that would break up the action of the waves to reduce the erosion pressure. Fen Group was employed to deposit peat from the Methwold site, where they were already working, along the two eroded banks, 75-100m in length. The grass mix was sown and the two sets of erosion matting were laid over the top and pinned down in position. Well-developed reeds were sown by Broadwood Conservation, who had done reed planting on the Hilgay and Methwold sites this year, and then protected by chicken wire cages to prevent grazing by the resident wildfowl.

The grass has germinated in the mild conditions we have had so far this autumn and is becoming established through the erosion matting. The cages appear to be working in protecting the young reeds which should establish, which combined with the natural vegetation that has developed along the lowered shoreline should protect the bank. Water levels will be raised gradually during the winter to ensure the grass and reeds have the maximum time to develop while temperatures are still mild. The matting will prevent any erosion this winter and as it rots away it will be replaced by the hopefully well-established grass sward and the fringe of reeds.

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