Channel dredging and coastal alteration

Case studies UK

Case Studies Elsewhere

In order to renew or enlarge navigational channels, extensive dredging may take place. This involves removing the seabed, which results in the suspension of the fine silt and clay fractions of the sediment. This fine sediment may be deposited by the inshore currents either locally or at a considerable distance from the dredging operation. The additional sediment load will increase local turbidity. The addition of breakwaters, promenades and sea defences to EU coasts is becoming commonplace. These constructions inevitably result in changes in the depositional and erosional patterns of the local coastal area. These changes may be gradual and continuous or may be catastrophic (storm-related) but intermittent. Gradual but continuous changes are the norm on mobile depositional shorelines such as much of the east coast of England. Where an area of shore is protected with solid defences, erosional scouring increases adjacent to the ends of the protected area. Where constructions result in the formation of tide driven or wind and wave driven eddies, the scouring may take place at a considerable distance from the structure.

There have also been a number of reports of coastal eutrophication problems arising as a result of hydrographical alterations, construction work, sand movement etc. limiting tidal interchange (Fletcher, 1996).

Case studies UK

None known. However, in the North Sea the construction of sea defences and other man-made structures provide additional substrata which are colonised by kelp species.

Case studies elsewhere

South Africa

Changes within kelp beds as a result of increased silt load have not been studied. Within Saldanha Bay (South Africa) the construction of an ore-loading facility resulted in a major increase in the silt load within the bay which led to the loss of the commercial harvest of Gracilaria sp. Despite the return to normal sediment loads in the water column, this species had not recovered to harvestable populations after 10 years (J. Bolton, pers.comm.).

Italy

The previously dominant large brown algae in the Venice Lagoon were adversely affected when channel excavations limited the water exchange patterns and this led to a build up of nutrient levels and eutrophication within the lagoon. The sewage-related floristic changes included a decline or disappearance of a number of algae belonging to the Cystoseira association, and their replacement by green algae such as Codium fragile and Ulva fasciata, which are more readily adaptable to the high nutrient loadings.

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