Dredging, navigation and revetment construction • Sediment extraction •Turf cutting • Hay making • Reed cutting • Samphire gathering • Spartina

Dredging, navigation and revetment construction

Both dredging and navigation have been suggested as factors contributing to saltmarsh erosion in the UK. Dredging can have the effect of increasing flow velocities and the proportion of total tidal discharge in the main channel, thereby, reducing the velocities and average shear stresses over the adjoining tidal flats. Under these circumstances, net stability or accretion may be expected in the intertidal area. Dredging can also cause the progressive net movement of sediment from the intertidal flats into the dredged area, due to slumping, rill incision or increased wave erosion over the steepened edges of the channel. The dredged section may also act as a sediment sink, reducing the sediment volume available for deposition over adjacent intertidal areas.

The permanent loss of sediment from the system may then occur as a result of the disposal of sediment at sea during the maintenance dredging of navigation channels in estuaries. Vessel movement along navigation channels can also result in erosion of fringing saltmarsh by ship/boat wash, while settling of mud on intertidal flats may be impeded by the increased turbulence generated by ship propellers (Pye, 2000). Solutions to these problems include restricting speeds and limiting the depth and frequency of dredging in estuaries, as well as returning sediment from the navigation channels to the intertidals (sediment recharge).

Dredging and the construction of revetments or training walls can also affect the position and migration of deep water channels in estuaries, thereby creating centres of relatively high or low energy where bank sediments are likely to erode or accumulate. Training walls and revetments act to artificially constrain the natural tendency for tidal channels to migrate. While this may act locally to prevent erosion, the non-equilibrium situation often created is likely to transfer the erosion problem to some other point downstream.