Description • Monitoring • Effectiveness of the scheme • Further information

South Ferriby, Goxhill and Barrow Haven, Humber Estuary

Description

Rock sills were placed in front of eroding saltmarshes at three locations along the Humber estuary. At all three sites, the top of the mounds were level with the saltmarsh surface. However, the Barrow Haven scheme (1991) differed to those at South Ferriby (1986) and Goxhill (1988) (see picture below), as the mounds in the latter two cases were positioned approximately 0-15m from the eroding saltmarsh cliff edge, whereas the rock barrier at Barrow Haven (following recommendations from English Nature) was constructed so that it abutted the leading edge.

Construction of the rock beach sill, Goxhill on the Humber (from Toft et al., 1995)

Monitoring

At each of the three sites, the rate of accretion behind the rock mounds was monitored. This was achieved by pushing 2m lengths of metal tube about 1m into the sediment. The change in level was recorded every 2 weeks for over two years by measuring the distance between the top of the tube and the surface of the mud.

Over the two year monitoring period all three sites showed a net increase in sediment accretion. At South Ferriby, where the rock sill was placed at a distance of 0-15m from the saltmarsh cliff edge, an increase of 30cm on the leeward side of the barrier was measured. However, levels did not increase further as the system reached equilibrium in relation to the height of the rock mound. Three years after the first placement of rocks, a second scheme was initiated where additional rocks were placed on the rock mound, resulting in an additional increase in level of 25cm. Subsequent colonisation by saltmarsh plants between the rock mound and cliff edge has been successful in widening the saltmarsh and, hence, increasing the degree of protection to the earth embankment.

Accretion at Goxhill was greater with over 80cm being deposited in the two year period. This resulted in reducing the height of the saltmarsh cliff from 1.5m to 0.8m, which would have a significant effect on supporting the toe of the seawall. Saltmarsh plants also colonised the accreted mud between the mound and the saltmarsh cliff. Monitoring carried out at Barrow Haven also revealed an increase in sediment levels.

Effectiveness of the scheme

The schemes do appear to have been successful in that sediment has accreted in between the mound and the saltmarsh cliff, and this has been colonised by various species of saltmarsh plants. However, as the monitoring program did not include an adjacent control site, it is not possible to conclude what proportion of this accretion was due to the presence of the rock mound (although a significant proportion is likely to have been). The schemes would have benefited from a more extensive post-operation monitoring program based on the following:

• Burial/exposure stakes arranged in a transect to include sites on the saltmarsh surface, the accreting mud between the saltmarsh cliff and rock mound, and the foreshore on the estuary side of the rock mound.
• A similar transect on a control site
• Measurements of the change in the position of the saltmarsh cliff behind the rock mound and at the control site.

When the schemes were originally constructed concerns was expressed by English Nature that the rock mounds could decrease the potential feeding area available for foraging wading birds. Although the schemes were successful in meeting their objectives (i.e. halting saltmarsh erosion/promoting accretion), English Nature were of the opinion that mudflat habitat was more desirable than the saltmarsh which replaced it and that if saltmarsh loss was a ‘natural’ process they did not favour intervention. Subsequently, there have been no further attempts on the site to halt saltmarsh erosion using hard engineering techniques.

Further information

Chris Noble
Guy Gibson Hall
Manby Park
Manby
Louth
Lincolnshire
LN11 8UR

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