Dredging and disposal: Changes to hydrodynamic regime and geomorphology

General statements about the impact of maintenance dredging on the hydrodynamics and geomorphology of a site cannot be made as the effects are site specific, very difficult to isolate from other 'forcing effects', such as sea level rise or reclamation, and are often little understood. Although all dredging activities can cause some change to the hydrodynamic flow, the magnitude and type of effect will be related to the overall size of the excavation compared to the overall size of the system. Most reported adverse effects of dredging on hydrodynamics and geomorphology of coastal and estuarine areas are associated with capital dredging operations. Examples of knowledge of possible effects of dredging on the hydrodynamics and geomorphology on selected marine SACs and other estuarine sites are summarised below.

Reported possible effects of dredging on the geomorphology in selected marine SACs

The current pattern of dredging in the Solent is reported to have altered the sediment regime and environment, however there is no evidence of long-term damage (Solent Forum 1997).

The Falmouth Bay and Estuaries Initiative (Cornwall County Council 1995) states that the impact of dredging on the sediment budget of the area is unknown and there is a need for more information.

Major channel deepening works in the approach to Harwich Harbour has altered the sediment transport regime (HR Wallingford & Posford Duviver Environment 1998). The capital dredge increased siltation in the harbour, which subsequently reduced the amounts of sediment input into the Stour/Orwell Estuaries and increased the requirement for maintenance dredging. The net effect is to increase mudflat and saltmarsh erosion in the estuaries, with adverse effects on intertidal morphology. In this case the capital dredge has created the conditions for increased erosion, which is sustained by the regular removal of sediment from the harbour for disposal at sea. A mitigation package has now been devised to offset this effect.

Capital dredging operations in an estuary may permit a saltwedge intrusion to travel further upstream than previously, increase shoreline wave action, change tidal range, tidal currents, suspended sediment load and suspended sedimentation in areas away from the deepened part of the river (PIANC in preparation). The hydrodynamic changes and their effect on sediment erosion, deposition and transport may cause secondary geomorphological changes away from the dredge location, including the potential erosion of intertidal areas. These processes are affected by the sea bed sediment characteristics, underlying geology and, particularly on mudflats, the flora and fauna.

Since in many cases maintenance dredging is routine small changes in depth (relative to capital schemes) which for the majority of ports has taken place over a long period of time, the operation itself will have become part of the 'equilibrium' of the system. In such cases, a cessation of maintenance dredging could cause greater environmental change than continuing to dredge. However, this can not be used as justification for the continuation of dredging activities that are damaging designated features.

The overall effect of maintenance dredging on the hydrodynamics and geomorphology of a site has all the complexity of a capital scheme but the impacts are much smaller. In many cases the magnitude of dredging related alterations may fall well within the range of naturally occurring phenomena and probably impose little or no additional stress to marine features (IADC/CEDA 1998).

The siting of the disposal site could, however, cause a regular removal of sediment from the transport system which could affect the erosion and sedimentation processes and ultimately the form of the estuary, possibly depriving downstream coastal areas of sediment required to maintain coastal stability (Bray, Bates & Land 1997). Equally if the sediment is placed back within the same system, although the net change is insignificant the locations of maximum sediment concentration may change promoting additional siltation in specific areas. Increased erosion of mud and sand flats may have numerous implications on the ecology of marine habitats and species. For example a reduction in the lower intertidal area may lead to reduced intertidal communities and a subsequent loss of bird feeding grounds, to the possible benefit, however, of a better fish breeding grounds (Nedwell & Elliott 1998). By contrast, careful design of disposal can result in intertidal areas being increased.

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