Siltation

About: Siltation is a research topic. Over the lifetime, 1420 publications have been published within this topic receiving 20983 citations.

Interacting effects of multiple factors on the morphological evolution of the meandering reaches downstream the Three Gorges Dam

Abstract: Elucidating the influence of dams on fluvial processes can inform river protection and basin management. However, relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches. Using hydrological and topographical data, we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam (TGD). Our conclusions are as follows. (1) The meandering reaches can be classified into two types based on their evolution during the pre-dam period: G1 reaches, characterized by convex point bar erosion and concave channel deposition (CECD), and G2 reaches, characterized by convex point bar deposition and concave channel erosion (CDCE). Both reach types exhibited CECD features during the post-dam period. (2) Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars. However, changes in the river regime, river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches. (3) Flood discharges ranging from 20,000 to 25,000 m3/s result in greater erosion of convex point bars. The point bars become scoured if the durations of these flows, which are close to bankfull discharge, exceed 20 days. In addition, the reduction in bedload causes the decreasing of point bar siltation in the water-falling period. (4) During the post-dam period, flood abatement, the increased duration of discharges ranging from 20,000 to 25,000 m3/s, and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features. Our results are relevant to other meandering reaches, where they can inform estimates of riverbed change, river management strategies and river protection.

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

Abstract: 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.