Origins of mud clasts and suspensions on the seabed in Hong Kong

Photographs taken using a seabed profiling camera indicate that the seabed sediments in water depths of 25–30 m in the eastern waters of Hong Kong comprise layers of mud clasts often associated with mud suspensions. The mud clasts and suspensions occur at both a mud disposal site and on natural seabed. At the mud disposal site, mud clasts are often large (commonly >0.05 m across), overlie unbioturbated placed dredged sediment and are commonly associated with extensive mud suspensions. By contrast, mud clasts on the natural seabed are typically less than 0.02 m across, are occasionally associated with suspensions and overlie eroded, intensely bioturbated relict deposits. The generation of the mud clasts and the mud suspensions is attributed to the disposal and reworking of dredged sediment by mass flows and density currents at the disposal site, and to the erosion and redistribution of in-situ muds by trawling equipment on the natural seabed. Hydraulic modelling suggests that waves and currents periodically rework the allochthonous mud clasts at the disposal site and the autochthonous clasts on the open seabed, together with the mud suspensions in both areas, although erosion of in-situ seabed only occurs during severe storms. A seabed profiling camera, in combination with high-resolution side-scan sonar, has been used to estimate the extent and type of seabed impacts arising from mud disposal and trawling.