Acoustic seabed classification in a coastal environment (outer Weser Estuary, German Bight)-a new approach to monitor dredging and dredge spoil disposal

Acoustic seabed classification provides a sophisticated technique to discriminate seabed characteristics and to map their distribution at high spatial and temporal resolution. In the present study, the seabed classification system QTC View/Impact was applied to investigate a 9-km2 area at water depths between 6 and 20 m in the outer Weser Estuary (German Bight, southeastern North Sea). The survey area comprises parts of a routinely dredged shipping channel as well as of a dredge spoil disposal site. The acoustic data, collected by means of a singlebeam 200-kHz echosounder combined with a QTC system, were classified into three acoustic classes. These classes are identified with (1) fine to medium sand, with a low content of shell fragments; (2) medium sand, with a moderate content of shell fragments; and (3) medium to coarse sand, with a high content of shell fragments. By comparing this point-related acoustic classification with sidescan sonographs, it can be convincingly demonstrated that the acoustic data obtained by the QTC system reflect more than simply sediment type distribution, but that rather the occurrence of bedforms also appears to be important. Furthermore, in a time series of three QTC surveys in a period of 14 months, it is apparent that dredging as well as the disposal of dredge spoil constitute a strong impact on the sediment distribution and seabed morphology of the study site.