* Wave Laboratory, Port and Harbour Research Institute, Ministry of Transport 1-1, 3- chome, Nagase, Yokosuka, Kanagawa 239, JAPAN
** Yokohama Port Office, Yokohama Branch, Toyo Construction Co., Ltd. 6, 1- chome, Kominato; Naka, Yokohama, Kanagawa 231, JAPAN
Design Department, Wakachiku Construction Co., Ltd. 23- 18, 2- chome, Shimo- Meguro, Meguro, Tokyo 153, JAPAN
ABSTRACT
Characteristics of waves, currents, and sand movement around a submerged breakwater is studied in a large scale three-dimensional experiment.
The wave height decreases on the submerged break water by plunging breaker. The waves on the submerged breakwater consists of the overtopped incident waves and waves diffracted at both heads of the breakwater. The experimental formula to estimate the wave height on and at the backward of the submerged breakwater is proposed in this paper. The wave height calculated in the formula is compared with measured in the experiment. The applicability of the Boussinesq model to estimate wave profiles on the submerged breakwater is also dicussed in this paper.
The velocity of current can be divided into steady and un-steady components. The un-steady component follows small amplitude wave theory at the forward of and on the submerged breakwater. The velocity of the steady current at the backward of the submerged breakwater is much larger than that at the forward.
The movement of tracer put on the submerged break water crown is observed in photograph analysis. The movement depends on the current velocity which has the relation with the wave height and water depth above the submerged breakwater. The direction of tracer movement can be estimated with the average of the significant current directions.
1. INTRODUCTION
It is reported that serious erosion has occurred behind a submerged breakwater. The sand behind the submerged breakwater is transported by strong currents. To prevent the erosion, it is necessary to study the characteristic of the wave heights, current velocities, and sand movements around the submerged breakwater.
Large scale three-dimensional laboratory simulations must be carried out to reproduce wave and current conditions similar to the sea state. The wave height, period and current velocity measured in the laboratory simulation can be compared with those estimated in the numerical model. The Boussinesq equation is widely employed to estimate wave profiles in shallow water. The applicability of the computation model is discussed in this paper. The tracer movement on the submerged breakwater crown is also observed to investigate the relation with the wave and current condition.
2. EXPERIMENTAL FACILITY AND MODEL
The scale ratio of this experimental model is 1/100. The model submerged breakwater has a length of 11.0m, width of 2.0m, height of 0.3m above the bottom of the wave hasin, and slope of which gradient is 1:3. The model is made from sand mat and covered with mortor. The water depth above the submerged breakwater crown varies from 2 to 4cm.
The regular waves, uni-directional irregular waves, and multi-directional irregular waves are reproduced with a multi-directional random wave generator. The Mitsuyasu-type directional spectrum is employed for the multi- directional irregular waves. The angular distribution of wave energy depends on the angular spreading parameter Smax in the Mitsuyasu model. In this study, Smax is 25. The incident wave height varies from 2 to 6cm. Thediscrepancy of propagating wave direction from the axis of
