The impact of dredge spoil dumping on coastal morphodynamics monitored by high-resolution acoustic measuring instruments (outer Weser Estuary, German Bight)

Abstract

The continuous dredging of shipping channels is a commom problem in most estuaries. Shoals constitute navigational hazards, which have to be removed to guarantee a safe access to the adjacent harbors. Dredging is always associated with the dumping of dredge spoil at declared sites, which are normally located close to the dredged channel. In this context, a main problem is the uncertainty, whether the dumped material remains at the site or is further transported by currents and redistributed in the surrounding estuarine area. In the worst case, the previously dredged channel might be re-infilled. However, knowledge about the fate of dumped sediments and their interaction with the existing morphological and sedimentological conditions is still fragmentary, especially in a dynamic tidally influenced environment. The goal of this work is to contribute to a better understanding of the natural dynamic processes and the consequences of dredge spoil dumping in a coastal area. A dumping site situated in the seaward exposed outer Weser Estuary (German Bight) was repeatedly surveyed with a shallow-water multibeam echosounder and an acoustic seabed classification system, which enable to investigate large seabed areas in a cost- and time-efficient mode, and thus the monitoring and investigation of morpho- and sedimentdynamical processes in a high spatial and temporal resolution. The aim of the present thesis was to answer following questions concerning the naturally and artificially changed state of the dumping area: How does the sediment budget change due to the supply of sediment by dredge spoil dumping? What are the predominant sediment transport pathways? How strong is the impact of dumped material on morphological seabed features? How variable is the spatial distribution of sediments in the dumping area?