Beneficial use of dredged sediment
Orwell Estuary (Essex)
Suffolk Yacht Harbour (SHY) is situated at Levington on the east bank of the Orwell (6km upriver of Felixstowe). At this site, there is a long history of beneficial recharge work being undertaken over the last 20 years or so (Jonathan Dyke, SYH 29 July 2016). The harbour has generally disposed of its maintenance dredge material near to the marina but prior to the late 1990s it was placed on the landward side. The SYH has, however, always been keen to place the regularly available maintenance material on the adjacent foreshore to help address losses of nearby intertidal habitat. The first licence for such marine deposition was granted by MAFF/MFA in the 1997/98. Since then maintenance dredge sediments have regularly been excavated (at around 10,000m3/year) using a suction dredger and pumped to the top of the shoreline at 3 or 4 different nearby intertidal sites. Until recently the foreshore recharge sites were located immediately adjacent to the marina (either upstream or downstream). In 2014/15, however, the marina owners Suffolk Wildlife Trust, the Environment Agency and the Suffolk Coast and Heaths AONB Unit worked together to place the material at a more distant and more sheltered degraded marsh (North Marsh) which lies some 500-600m away. At this site the soft fluid sediment was retained in the creek system of a using coir logs held in place by wooden posts. At the deposit sites, pumped sediment is generally retained, as much as possible, using wattle hurdles or faggots (fencing with bundles of twigs) or coir logs on the foreshore. The dredged material is very fluid though (perhaps 10% solids) and there is inherent sediment loss into the estuary system. However, that exported material will still stay within the estuary and within the local sedimentary system. There are signs that sufficient material has remained to gently raise the tidal height on the foreshore and in some locations allow some saltmarsh plants to colonise. During a ABPmer site visit in February 2016 for instance it was clear that fluid mud was still being retained in the creeks behind the coir logs at the North Marsh site. For the SHY site, beneficially using maintenance dredge sediments on the adjacent marshes in this manner is manifestly cheaper than exporting the materials to a more distant location (even within the Orwell).