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Use dredged material - Harwich Haven
Dredgings from the Harwich Haven for the Port of Felixstowe have been placed
at several sites in the River Orwell (North Shotley Marsh and Trimley Marsh),
Hamford
Water (Horsey Island), the Blackwater Estuary (Cob Marsh Island,
Old Hall Point and Tollesbury Wick) and the River Crouch (Wallasea Ness). At
all these sites the aim was to improve the sea defence capability of the foreshore.
Invertebrate monitoring
At the same time monitoring took place of the extent of invertebrate colonisation
providing an indication of the biological value of the restored mudflats. The
results so far suggest that using dredged material can enhance the rate of restoration
of both tidal flats and saltmarsh. The effects are different for individual
sites. Based on the results of the invertebrate monitoring the following conclusions
have been reached at those sites where high
density slurry was placed on the existing mudflats:
| Site |
Conclusion |
Reference |
| North
Shotley, R. Orwell |
Initial reduction invertebrates due to 'smothering'. Enhanced
development thereafter, possibly due to use of consolidated mud. |
Posford Duvivier 2000 |
| Trimley
Marsh, R. Orwell |
Deposition site greater diversity and abundance of
invertebrates than adjacent control site. |
Environment Agency 1999 |
| Horsey
Island, Blackwater |
At this deposition site the diversity and abundance of invertebrates
was much lower than at the control site. |
At the other sites 'normal' slurry i.e. diluted with sea water was used. Here
the results were intermediate between the extremes noted above.
| Guidance: It is possible to enhance the development of tidal
flats by the deposition of dredged material and it is a method used extensively
in the USA.
It is a useful, but by no means certain, way of enhancing the biodiversity
and abundance of invertebrates or for the development of saltmarsh. Using
high density slurry
may be a useful improvement to the technique. See also English Nature
Reports on Capital
and Maintenance dredging for useful reviews of the general
approach. |
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