|Hydrography and cohesive sediment modelling: application to the Rømø Dyb tidal area|In: Journal of Marine Systems. Elsevier: Tokyo; Oxford; New York; Amsterdam. ISSN 0924-7963, more
Bathymetry; Cohesive sediments; Estuarine sedimentation; Hydrodynamics; Modelling; Sediment transport; Settling rate; Shear stress; ANE, Denmark, Lister Dyb [Marine Regions]; Marine
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Estuaries act as sinks for fine-grained sediments and because of the cohesive properties of these sediments, heavy metals and nutrients tend to accumulate in estuaries. In order to quantify the erosion, transport and deposition of these pollutants, modelling of cohesive sediment dynamics is a very useful tool. However, the description of cohesive sediment dynamics through numerical analysis is a difficult task since the physical properties of cohesive sediments are of complex nature. In this paper, setup and calibration of a cohesive sediment transport model covering the period from October 20, 1999 to December 13, 1999 are described and an interpretation of the results is carried out. Further, a comparison with measured suspended sediment concentrations and bed level measurements from the modelling period is presented. A detailed bathymetry is used. This is necessary in order to describe the water movements in a realistic way. A bottom description is created in a way so that differences in erodibility of the sediment can be described. Further, a spatial differentiated description of critical shear stress, both for erosion and deposition of the cohesive sediment bottom, is made in order to describe the processes of settling and scour lag. The hydrodynamic simulation is shown to be very reliable, and therefore, it has been possible to extract key values for the area. Thus, the maximum current velocities for the Lister Dyb area are modelled to 1.2 and 0.93 m s−1 for the flood and ebb period, respectively. The tidal prism has likewise been computed to 620×106 m³. The cohesive sediment transport modelling has shown that the highest sediment concentrations at a given site appear when onshore winds are prevailing. Further, it can be recognized in the results that an inward sediment transport direction is prevailing, especially after a windy period with waves has mobilized considerable amounts of sediment. A detailed investigation of the cohesive sediment's settling velocities collected in the area is used to give a site-specific description. Since the description of the settling velocity changes with temperature, several simulations using different descriptions have been carried out. These simulations have shown that this parameter is very influential for the net deposition result. Thus, an increase of the settling velocity will increase the deposition rates considerably. The modelling results and the comparisons to measured data presented in this paper show that it has been possible to calibrate the hydrodynamic model in accordance with observed values. In continuation of this, the deformation of the tidal wave has also been modelled satisfactorily. Modelling of the cohesive sediment dynamics has been more complicated and the modelling results show divergence from the measured results. However, the levels of the sediment concentrations and the overall net sedimentation pattern show accordance with observed values.