Concentration-dependent settling velocities of cohesive sediments from the Port of Hamburg based on ultrasound velocity profiling

The settling behavior of cohesive sediments plays a key role in estuarine sediment dynamics and harbor maintenance. This study aimed to assess the applicability of Ultrasound Velocity Profiling (UVP) to capture the spatial and temporal evolution of the settling process and to derive concentration-dependent settling velocities. Laboratory experiments were conducted in a settling column with initial sediment concentrations (C0[jls-end-space/]) of 0.3-80 g/L, using cohesive sediments from the Port of Hamburg. A 4 MHz transducer was utilized to measure the vertical particle velocities with a resolution of 0.08 mm/s and a sampling volume height of 1 mm. Effective settling velocities were derived from the UVP measurements and compared to the settling velocities obtained from the lutocline evolution. Subsequently, common mathematical models were fitted to the data. The results demonstrate that UVP is a suitable, non-intrusive method for high-resolution imaging of settling processes, revealing concentration-dependent characteristics in particle movement in unprecedented detail. The calculated effective settling velocities increased approximately linearly in the flocculation settling range until a maximum of ws,max = 2 mm/s was reached at C0 = 3.5 g/L, followed by an asymptotic decline in the hindered settling range. The associated deposition flux showed a broad maximum of approximately 0.01 kg/(m2s) for C0 in the range of 5-15 g/L. The results provide previously unavailable, site-specific data on the settling behavior of the studied cohesive sediments. Potential application areas of UVP include the execution of field experiments and the automated high-resolution tracking of the lutocline or bed level.