Description • Monitoring • Effectiveness of the scheme • Further information

North Shotley, Orwell Estuary

Description

In December 1997 a trial recharge scheme on the Orwell Estuary at Shotley was undertaken by the Harwich Haven Authority and the Environment Agency. Here, a 2km earth wall, protecting low lying grazing land, had undergone severe erosion following the near complete loss of fronting saltmarsh. Several unsuccessful attempts were made to halt the erosion using hard engineering techniques, such as gabions and concrete facing.

Consequently, approximately 22000m³ of maintenance dredgings, mostly silt (20µm in size), was pumped behind a retaining bund of coarse, poorly sorted gravel. The placement extended over 450m of the foreshore with a maximum width of 70m. Mud was pumped at a density of approximately 1.3gcm^-3, leading to an initial elevation of between 1.0 and 1.3m above Ordnance Datum (OD). For comparison, the level of local Mean High Water Neap (MHWN) and Mean High Water Spring (MHWS) was 1.4m and 2.0m respectively, with the remnants of the old marsh system lying between 2.0 and 2.2m OD.

Monitoring

Monitoring was undertaken at the site from January 1998 until the scheme was subsumed into the larger Shotley Foreshore Habitat Enhancement Scheme in 2003. Monitoring was undertaken on a quarterly basis to determine the in situ stability and morphodynamics of the deposited mud and enclosing gravel, and the extent of invertebrate and halophyte colonisation. Results have shown that due to the low elevation of the raised surface relative to MHWN and MHWS limited colonisation of saltmarsh plants occurred. Although very sparse Salicornia and isolated Spartina plants have established, the recharge is clearly not direct replacement for the saltmarsh fringe, whose remnants lie above MHWS.

Recharge scheme at North Shotley (Environment Agency)

Prior to recharge, the sediment infauna of the site was impoverished relative to other foreshores elsewhere in the estuary. Post-recharge monitoring, however, showed that the site was rapidly colonised by benthic invertebrates, such as polychaete and oligochaete worms, gastropods such as Hydrobia and a few bivalves. As expected, substantial short-term and seasonal variability was evident on the site. However, it seems clear that the pumped recharge mud quickly acquired sufficient structure to support the recruitment and development of a varied infauna.

Effectiveness of the scheme

The establishment of a diverse sediment infauna at substantially higher elevations compared to the pre-existing tidal flat led to an increase in its use by wading birds. Although no quantitative bird monitoring was undertaken, it is clear from qualitative observations that birds are able to feed on the recharge mud until much later into the tidal cycle than was previously the case. For example, Brent Geese were observed at the Shotley site in 1999 along with small numbers of shorebirds, including Ringed Plovers, Oystercatchers, Grey Plovers, Dunlin, Redshanks and Turnstone; whereas, prior to the scheme, only two shorebirds (Grey Plover and Oystercatcher) and no geese were recorded on the site.

The recharge scheme was unsuccessful in raising the height of the mudflat surface sufficiently enough to allow saltmarsh vegetation to colonise. In addition, the gravel bund that protected the lower margin of the site subsequently rolled landward under wave action (raising its height) and provided suitable conditions for a second phase of recharge at a higher elevation over the inner region of the site.

Further information

French, J.R., Watson, C.J., Moller, I. , Spencer, T., Dixon , M. and Allen, R. (2001). Beneficial use of cohesive dredgings for foreshore recharge. Proceedings 35 th MAFF Conference of River and Coastal Engineers. 5 th-7 th July 2000 , Keele.

Posford Duvivier Environment (2000). North Shotley mud placement: analysis of effects on benthic invertebrate community. For the Harwich Haven Authority.