Description • Monitoring • Effectiveness of the scheme • Further information

Horsey Island, Hamford Water (southeast corner)

Description

This scheme involved the direct application of dredged material on top of an eroding saltmarsh in the southeast corner of the island. The objectives of the experiment were to assess the impacts of dredged material on saltmarsh vegetation and to see whether a high marsh community would develop in place of a low marsh community. In 1992, 1000m³ of dredged silt from Harwich Harbour was deposited on a 0.5ha plot of heavily grazed saltmarsh dominated by Spartina and Puccinellia. The work was carried out during the late summer of 1997 so that the sediment had time to consolidate before the main release period for the seeds of saltmarsh plant species. Material was deposited on the marsh via rainbow discharge using a fixed distributing nozzle.

Monitoring

Unfortunately no pre- or post-monitoring data is available for this scheme. However, it has been confirmed by Ian Black (English Nature Site Manager) that most of the applied silt was washed off the sites during the first set of spring tides following the placement. Due to the lack of monitoring data, however, it is not known what quantity silt was washed away or remained.

Effectiveness of the scheme

As most of the sediment was washed off on the following spring tides the scheme was generally regarded to have been unsuccessful. Recommendations which followed the initiative included the potential use of retaining bunds to trap the fine silt material. A possible suitable material for this would be rolls of coconut matting, similar to those used in Lymington River Case Study, secured onto the saltmarsh surface to form a continuous low wall around the receiving area. The bund may also serve to enhance accretion.

The Environment Agency has scheduled further saltmarsh recharges at Horsey Island for October 2005. Following on from the original trial scheme, the use of a retaining bund will be used in an attempt to retain the sediment (Gemma Costin, pers. comm.).

Further Information

Karen Thomas
Coastal Processes Engineer
Environment Agency
Iceni House
Cobham Road
Ipswich
IP3 9JE

Gemma Costin
Coastal Processes Coordinator
Environment Agency
Iceni House
Cobham Road
Ipswich
IP3 9JE

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