Description • Monitoring • Effectiveness of the scheme • Further information

Lymington River

Description

In conjunction with the Lymington Harbour Authority, in 1994 New Forest District Council (NFDC) decided to implement a small-scale trial to defend the leading edge of an eroding length of saltmarsh from wave attack. Instead of using traditional rock revetments to protect the leading edge of the marsh, a new approach using rolls of coconut fibre (coirs) was employed. If it proved to be successful, the main justification for using this technique would be the significant cost savings made compared to the cost of installing rock revetments.

The trial scheme was carried out on a 160m eroding length of saltmarsh and consisted of a double row of wooden stakes pushed vertically into the mud. Within these stakes, rolls of coir were placed and secured into position with twine. To investigate how distance from the cliff edge affected the performance of the coir fence, the fence was installed at three different distances: tight against the cliff edge, a few metres from the cliff and further away to form an embayment.

Monitoring

Subsequent monitoring of the trial site demonstrated that the scheme was ineffective, as the saltmarsh edge behind the constructed fence line continued to retreat at 1m/year.

Effectiveness of the scheme

According the NFDC, the ineffectiveness of the scheme according to NFDC related to the following problems:

• Wave action worked apart the coir rolls allowing water to flow through the gaps, causing scour.
• During periods of submergence, wave reflection between the cliff edge and the rolls exacerbated erosion.
• Scour and subsidence caused the coir rolls to sink.

Following the project, NFDC suggested possible improvements (see figure below) to the scheme which included:

• Tying the ends of the rolls together to form one long unbroken roll, (see figure below). This would prevent gaps from forming and may help to reduce scour.
• Scour around the base of the fence could be reduced by changing the orientation of the fence from the vertical to an angled face.
• Wave reflection between the cliff edge and the coirs could be reduced by securing the rolls tightly against the eroding cliff edge.

Possible improvements to the use of coir rolls (from Carpenter and Brampton, 1996)

In recent years a significant amount of research has been undertaken by NFDC to determine the rate and extent of saltmarsh loss over the last 60 years in their district, and how this may impact on the shoreline, existing defences, the economy of the towns and region, and nature conservation. The culmination of this research will result in a coastal defence strategy for the Western Solent which will explore the various options for coastal management between Hurst Spit and Calshot Spit. The strategy is due for completion towards the end of 2005.

Further information

Andrew Colenutt
Coastal Protection
New Forest District Council
TownHall
Avenue Road
Lymington
Hampshire
SO41 9ZG

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