Factors influencing boat and propeller induced increases in suspended sediments

• The shallower the water, the greater the suction created by the vessel moving through the water at the bed resulting in increased re-suspension of sediment. The depth of influence will vary on a site by site basis.

• Where a vessel passes through a narrow channel, occupying a large proportion of its cross sectional area, the wave generated and the ‘under-keel clearance’, can result in a greater mobilisation of sediment from the bottom and margins of navigation channels.

• The extent and duration of vessel activity is related to the resuspension of sediment within an area. Areas that are heavily used offer little time for the sediment to resettle and as a result receive more disturbance than an area that is not used frequently. It has been observed that one hour after vessel activity has stopped the sediment will begin to resettle, although this will depend upon the size distribution of the material stirred up and the hydrodynamic regime.

• Sediment characteristics have an important influence on turbidity. Finer materials such as silt will be more easily re-suspended than larger particles.

• The hydrodynamic conditions at the site determine where suspended sediments will be re-distributed.

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