Reducing Harbor Siltation. II: Case Study of Parkhafen in Hamburg

Sedimentation in harbor basins necessitates continuous maintenance dredging to keep the bed level of the harbor at the desired depth. Costs involved in the dredging and disposal of the material are often significant, and Port Authorities therefore search for adequate methods to mitigate the influx of sediment into harbors. In the 1990s, a current deflecting wall (CDW) was constructed in the entrance of the Kohlfleet harbor in Hamburg. Field observations on dredging volumes showed that siltation in the Kohlfleet could be reduced by about 40%. To investigate the feasibility of a CDW for other harbor basins in Hamburg, experiments for the Parkhafen were carried out in a nondistorted physical scale model. Water exchange between the Parkhafen and the river Elbe is governed by tidal effects and horizontal entrainment and possibly sediment-induced density currents. The first two effects were addressed in the hydrodynamic scale model. Various design parameters, such as curvature, length, and location of the CDW, were investigated under steady state as well as tidal conditions by means of visual observations of float tracks and dye injection to arrive at an optimum configuration. The spreading of dye was assessed with digitized video images, which were analyzed in terms of dye concentrations. This technique appeared to be very adequate, as high-resolution spatial information was obtained. From the dye experiments under steady state conditions, it was concluded that with the selected CDW design it is possible to reduce the gross inflow of river water into the harbor by 25-45%. The application of a sill, connected to the upstream side of the CDW, may contribute another 10% as near-bed water with high suspended sediment concentration is deflected into the river. The tidal filling of the harbor is then accomplished with water carrying a reduced sediment load.