Description • Monitoring • Effectiveness of the scheme • Further information

Wellhouse, Strood Channel, West Mersea, Essex

Description

Lateral erosion of the saltmarsh edge at Wellhouse of up to 0.6m a year was putting at risk the integrity of a concrete block armoured earth bank. The width of the remaining saltmarsh was between 0-20m. In places the toe of the bank showed signs of failure and some of the concrete blocks had been ripped out, allowing waves to gouge hollows in the earth bank. As the sea defence protected 37ha of farmland it was determined that maintenance works needed to be carried out. However, to reduce the medium term maintenance costs, it was decided to combat further saltmarsh erosion and attempt to reverse saltmarsh loss by encouraging sedimentation using sedimentation fences.

Rows of double wooden stakes filled with brushwood were constructed running perpendicular to the shore. The fences were not of uniform length, varying between 20-80 metres. Some of the perpendicular fences were connected with fences running parallel to the shore to enclose areas of mudflat, forming fields or ‘polders’.

Detail of the sedimentation fence (from Carpenter and Brampton , 1996)

Monitoring

Although the sedimentation fences were installed in 1986, post-project monitoring did not begin until six years later. Subsequent monitoring, between 1992 and 1995, involved twice yearly surveys carried out in April and October and involved measuring the following:

• Vertical accretion/erosion using a grid of bamboo canes.
• Sediment density using a graduated brass bar.
• Position of seaward extent of saltmarsh (gives an indication to the degree of success in controlling the lateral erosion of the saltmarsh).

Effectiveness of the scheme

Unfortunately as monitoring did not begin until six years after the fences were installed it is difficult to assess what the initial rate of accretion was following the inception of the scheme. Subsequent monitoring has shown that there is a significant difference in levels between the polders and the groynes, with the former showing higher accretion rates. However, the regression of the saltmarsh has not been prevented and the scheme is regarded to have been unsuccessful. Furthermore, even If the scheme had been successful in promoting the stability of the saltmarsh cliff, the economic justification for it would be questionable, as the cost and maintenance of installing the fences was similar to the value of the land they were trying to protect.

To adequately gauge the success/failure of the scheme, a baseline survey should have been carried out to establish existing conditions with monitoring following at regular intervals directly after installation.

Further information

Carpenter, K.E. and Brampton , A.H. (1996). Maintenance and Enhancement of Saltmarshes. Environment Agency R&D Note 473.