The effect of dock length on harbour siltation

摘要

Density-driven exchange flows between estuaries and harbour docks are influenced by the length of the dock. As a result, increasing dock size through its lengthening, not necessarily results in an increase in sedimentation rates. The propagation of a low-salinity surface patch into the dock is blocked at the head of a relatively short dock, resulting in a reversal of density-driven flows, and a reduction of the hydrostatic pressure gradients in the entrance of the dock. A reduced hydrostatic pressure in the dock, in turn, promotes near-bed inflow. When this increased near-bed inflow coincides with a high sediment supply on the adjacent river, the sediment transport into the dock increases. This has been tested with an extensively validated high-resolution numerical model developed for the Deurganckdok in the Port of Antwerp. In the Deurganckdok, siltation rates are expected to decrease when the dock is fully excavated compared to the present half-opened dock. Whether exchange flows between estuaries and harbour docks are influenced by the length of the dock, depends on the tidal variation in salinity. For small tidal density variations (around 0.5 kg/m(3)), the dock length is expected to influence exchange flows in a short dock (approximately 1 km), whereas the dock should be much longer (4 km) when the tidal density variation is higher (around 5 kg/m(3)). Whether these changing exchange flow result in a lowering or increase of sediment import, depends on the phase difference between sediment concentration peaks on the adjacent river/estuary and the salinity variation, and on the vertical distribution of sediment.