ABSTRACT:

A rectangular container is used in studying vortical flow and material transport in a harbor with a breakwater induced by a tidal motion. A careful analysis for the similarity laws suggests that we can choose the water depth and the period of the tidal motion without severe restrictions. Flow visualizations are made" on the water surface and on the container bottom: A light particle is also used to study the material transport of the harbor flow. A peculiar result contrary to the common sense is found in the effect of the material transport.

INTRODUCTION

We present the results of an experimental study on the dynamics and the structure of vortical flow and the characterstics of the material transport in a harbor with a breakwater. Consider a harbor with a straight breakwater, whose primary role is "breaking" the water wave. Needlessly to say, the breakwater-gap space should be as small as possible to satisfy its primary object. There exist however other factors which must be considered in determining the relative size of the gap., Apparently, the current inside the basin has a fundamental effect on the transport of materials, e.g. the migration of sediments and the movement and deformation of pollutants. Among several types of currents in harbors, the residual flow induced by the tidal motion is the" most persistently effective because of the, relatively "narrow" gap; that is, the periodic rise and fall of the water level outside the harbor causes a relatively strong current through the gap, and in general the current becomes stronger as the gap size is decreased; on the other hand, the other type such as the wind-driven current may also be effective but not always. Yanagi (1976) conducted an experiment using a plunger type generator at one end of a shallow rectangular container.

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