Closure of estuarine channels in tidal regions

Abstract

Considerations on fluid motion in and around closure gaps, two-dimensional local scour in loose sediments, local scour caused by vortex streets, behaviour of dumping material when exposed to currents, results of model tests applied to an actual project *** The closure of the final gaps is the most difficult part of the construction of the dams in the 'Delta plan' of the Netherlands. In this contribution various factors concerning the water motion in closure gaps are discussed in general terms. These factors are: the tidal motion in the region, the changes of velocities in and near the closure gap during the closure period, and the way in which they can be computed. Remarks are made about the interaction of water motion and soil mechanics. ** The conformity and time scale of local scour is studied from model experiments. A description of the flow pattern in the scouring hole and the development of the scouring process with time is given. A time-scale relationship is derived from experiments with a wide range of scales. The int1uence of the velocity profile and turbulence intensity on the scouring process is demonstrated with some examples. ** In this article considerations are given about three-dimensional local scour caused by vortices, in -~10n-cohesive bed-material. After a general characterization of the scour-patterns for different closing-methods and a description of the flow-pattern, the process of local scour is described more in detail. As the development of the scouring process with time is very important for practical purposes, estimates of the time-scale are given based on reproduction in models and on a systematic research program as well. ** After an introduction, the results are discussed of a series of laboratory model tests, performed in view of establishing a criterion of stability for the various dumping materials exposed to current-action. In working out the test results it has appeared that the critical flow velocities for a certain material could be expressed as a linear function of log h, h being the difference between the downstream water-level and the dam's crest-level. ** The paper deals with the way in which the Hydraulics Division of the Delta Directorate uses the results obtained from model tests in the Hydraulics Laboratory when designing the dams with which the various tidal channels are to be closed. The hydraulic problems that have to be overcome are described in general. as well as the manner in which they are tackled. The final plans for an actual project as treated in this paper are a compromise, since a great number of factors, particularly technical planning and economic considerations are involved.