Beneficial use of dredged sediment
Crouch Estuary (Essex)
This work was done within the Allfleet's Marsh Managed Realignment site to create saltmarsh habitat on land that would otherwise only produce mudflat. A 4km-long clay bund was formed in sections in front of a new sea wall. This created an impounded area for sediment retention. Maintenance dredged material from the Port of Felixstowe approach channels were then pumped in. A total of around 550,000m3 (700,000 tonnes) of dredged material was placed on site. The clay bunds were required to maintain the dredged sediment in situ until it consolidated. Over the long-term these retaining bunds are expected to be erode slowly as the whole site evolves over time and becomes more natural in its morphological appearance. The bund was made of less suitable engineering clays and as a result did present problems with risks of failure. The first stage where failure was anticipated was during the placement of dredged material and in the final event the volume of material placed in the bund differed according to the estimated strength of the retaining bund. Each bund section was constructed to a height of 130mm above design height to allow for compaction, and included shallow grips (channels approximately 1m wide cut to allow water runoff) at 50m intervals along the lip of the bund to allow run-off of water as the dredged materials consolidated. Subsequent experience showed that there was a need for gripping at 25 m intervals. Remedial action was necessary and it was also necessary to lower the lip of the bund after the material had settled behind it. Land levels were increased by 2m to ensure that the recharge area was then set at the correct height for saltmarsh development. Two years after the sea walls were breached the previously barren sediments had been rapidly colonised by saltmarsh. Within 4 years there was an almost 100% coverage of plants.