Author(s): Rolf Riethmuller; Andreas Wurpts; Peter Mewis
Linked Author(s): Peter Mewis
Keywords: Weser estuary; German Bight; Dumping plumes; Hydro-acoustical measurements; Optical measurements; Suspended sediment concentrations; Fine-grained content; Morphodynamical model
Abstract: The propagation of dumped material was studied in the partially mixed outer Weser estuary, Germany. The material was dredged in a deposition area close to the center of the Weser turbidity maximum, shipped downstream to an area at the outer edge of the turbidity maximum and dumped there. It consisted mainly of mud and contained differing amounts of sand. The dispersing material was followed by a moving boat equipped with a 600 kH z broad band ADCP and a CTD profiler which also carried both optical attenuation (660 nm) and backscatter (880 nm) sensors. Concentrations of the suspended material were derived from the optical signals and the ADCP backscatter intensities in combination with repeated sampling. In the case of the ADCP an inverse modeling technique was applied. The ratio of optical attenuation to optical backscatter intensity was used as a proxy to characterize the nature of the suspended material with respect to particle size distributions and to discriminate dumped material from the natural background. Several phases for the entrainment and propagation of the dumped material can be documented. The dumping-process induces a massive local entry of momentum which leads to strong turbulent mixing in the near-field of the dumpsite. This causes strong local fluctuations both in concentrations and particle sizes. At the interface between lower suspended matter concentrations in the upper water column and high concentrations in the lower water column these fluctuations were observed to persist to more than half an hour. In parallel, a significant particle-size segregation of the material was deduced from the optical proxy. The present paper shows measurements for one specific survey close to ebb slack water. The observations were compared to the output of a numerical morphodynamic model. The model reproduces the observed plume dispersal qualitatively quite well and provides additional spatial and temporal information. Model results indicate that both vertical settling and density induced lateral transport govern the fading of the observed plume.
Year: 2004