Laboratory observations and calculations of the depth averaged flow patterns in a square harbor on a tidal river

Abstract

Many harbors in the world suffer from siltation in their basins and in many cases removal of the deposited sediment leads to high costs. This siltation results from a net transport of sediment into the harbor caused by the water motion in the harbor entrance. The water motion is very complex and of a three-dimensional nature. Three main mechanisms can be distinguished: (1) exchange in consequence of water flowing along the mouth of the harbor and the resulting eddies in the harbor entrance, (2) exchange in consequence of variations in water level of the adjacent waterbody (e.g. sea, estuary or river) , and (3) exchange in consequence of a density difference between harbor and adjacent waterbody. For a more extensive discussion of the problem see Langendoen (1988). The first two mechanisms are being examined in a physical model in the Laboratory of Fluid Mechanics of the Delft University. Measurements were made to generate a dataset by which numerical models of the flow in harbor entrances can be tested. A more distant goal of this project is to obtain insight in the influence of the geometry of the entrance on siltation of the harbor. The first part of the research is discussed in this report. It concerns the water motion in a square harbor due to an oscillatory flow in the adjacent waterbody (here a tidal river). The time-varying depth averaged flow patterns in the harbor have been measured. These flow patterns are then compared to the results of preliminary calculations with a numerical model that solves the shallow water equations. In this report a simple geometry of the tidal river and the harbor was considered as a first step. Depth averaged flow patterns were measured and calculated. The phenomena which occurred in the harbor entrance around and after slack water may be important for the exchange of matter between harbor and river. Large parcels of water from the river are exchanged with water from the harbor. This convective exchange is much larger than the exchange around maximum current in the river, when there is only a turbulent transport through the shear layer at the transition between harbor and river. The mathematical model ESTRA, which has been used to study the exchange of a well-mixed solute, also predicted this behavior.