Description • Monitoring • Effectiveness of the scheme • Further information

Tollesbury realignment site

Description

The realignment site is located on the Tollesbury fleet, a tributary of the Blackwater estuary, Essex. The site is owned by English Nature and is part of the Blackwater Estuary Nature reserve (www.english-nature.org.uk). In August 1995, a 60m breach was made in the sea defences and land previously under cultivation was opened to tidal inundation for the first time in over 150 years. Tidal inundation was limited to the 21ha site by the construction of a counter wall built on the 3m contour. The elevation of the site ranged from 0.96m OD to 3.0m OD, although most is less than 2.0m OD. The mean tidal range for Bradwell, the nearest reference point, in the Blackwater Estuary is 4.7m on spring tides and 3.0m on neap tides.

Monitoring

To record the development of saltmarsh vegetation a transect 20m wide was laid out in each of the three fields within the realignment site, starting at the foot of the counter wall on the highest part of the site and extending 125m to lower ground. Each transect was divided into 2500 1m² cells, where plant species presence and percentage cover were recorded. Monitoring started in 1997, two years after the breach, and took place in September each year.

Tollesbury realignment site at High tide (from Living with the Sea, 2003)

Invertebrates were sampled at seven sampling sites within the realignment area annually from 1995 to 1998 and again in 2001. Nine 10cm diameter core samples were taken from each of the seven sites to a maximum depth of 15cm, sieved through a 0.5mm mesh and preserved in 5% formal saline.

Effectiveness of the scheme

By 2001, approximately 6ha of the 21ha site had been colonised by saltmarsh vegetation. The lower limit of the vegetation corresponded with the 1.5m contour. The dominant vegetation community established within the site was annual Salicornia saltmarsh, which composed of large strands of Salicornia eurpaea agg. (common glasswort) at high density, often with no other species. Puccinellia maritima (common saltmarsh-grass) and Atriplex portulacoides (sea purslane) species that dominate the vegetation communities of the adjacent marshes were restricted to the highest land in the area near the foot of the new sea wall (>2.2m OD).

During the monitoring period, accretion rates showed no indication of declining, with an average of 23mm accretion per annum. However, there was considerable within-site variation, ranging from 8mm to 258mm of sediment build up over the original agricultural surface. Most of the variation was accounted for by differences in elevation.

Initial invertebrate colonisation of the site was rapid. Fourteen species of intertidal invertebrate were recorded within the site after two months of tidal inundation, with between 18 and 19 species recorded in each survey thereafter. Colonisation occurred only in the newly accreted sediment and not the original agricultural substrate. Numbers and distribution of most species increased year on year, with the most abundant species, the mud snail Hydrobia ulvae, occurring in every sample in 2001.

Further information

Reading, C. J. (1996). Colonisation of the Tollesbury 'Set Back' site by intertidal animals (Draft Report). Institute of Terrestrial Ecology.

Reading, C. J., O. A. L. Paramor, et al. (1999). Managed realignment at Tollesbury and Saltram. Annual report for 1998, Institute of Terrestrial Ecology.

Reading, C.J. (2002a). Colonisation of the Tollesbury realignment site by intertidal animals. In: Managed realignment at Tollesbury and Saltram. Final Report. Defra/NERC contract. CSA 2313. Defra London.

Garbutt, A., Gray, A., Reading, C. & Brown, S. (2003). Saltmarsh and mudflat development after managed realignment. 38th Defra Flood and Coastal Management Confeence. Keele University. Defra London.