Characteristics of waves and currents induced when a strong wind blows shoreward in a nearshore zone have been investigated experimentally. The drag coefficient of wavy surface has been related to the ratio u*a/Cp, where u*a is the air friction velocity on the water surface and Cp the phase velocity of the predominant wind waves. Though the relation between the frequencies of the predominant waves and fetch is very similar to that for deep water, the fetch-relation of the wave energy is a little complicated because of the wave shoaling and the wave breaking. The dependence of the energy spectra on the frequency f changes from f-5 to f-3 in the high frequency region with increase of the wind velocity. A strong onshore drift current forms along a thin layer near the water surface and the compensating offshore current is induced under this layer. As the wind velocity increases, the offshore current velocity increases and becomes much larger than the wave-induced mass transport velocity which is calculated from Longuet-Higgins' theoretical solution.
For water purification of man-made lakes and reservoirs, we propose a method to make an one directional flow by making use of natural wave energy. A devised artificial floating structure has a dam with a slope to activate the wave overtopping and to gain effectively the head difference. How much the volume of overtopping is gained has been investigated. The volume obtained and sent to a bottom layer is estimated briefly for the water purification. It is shown that the wave energy in man-made lakes seems to be large enough to be used for the water purification.
The sea-sand mining has been carried out in the coast of western part of Japan since the early 1970's. Mined sea-sand is mainly used as fine aggregate of concrete structures. Mining a large amount of sea-sand near the coast induces the possibility of beach erosion and devastation of fishing grounds. In order to evaluate its influence on coastal environment, the present situation of sea-sand mining has been investigated in seven prefectures of Kyushu island in Japan. The sea-sand mining in Kyushu Island is classified into two types. One is the mining in the sea areas where sea-sand reserves are limited. In these areas, the compatibility between the sea-sand mining and the protection of coastal environment will become very important in future. The other is the type that sea-sand has been supplied constantly from rivers or other sea areas. In this case, it will be the most important to keep the balance between the amount of mined sea-sand and that of supplied one.
A project has been pursued to generate uni-directional flow in the sea by utilizing the natural enegy. This flow is applicable to the development of artificial fishing grounds or to the purification of stagnant polluted sea-water. The utilization of water-head difference has been devised as a method to generate the flow, in which the head difference is effectively gained by overtopping amplified waves through converging walls. In this study, it has been investigated how two-dimensional surface waves are amplified by propagating the converging walls. The amplification rate H/Ho is strongly dependent on l /L0, where is the length of the converging walls and Lo the wavelength in deep water. The amp l ified waves are progressive at l/L0 ‹ 0.4. Partial standing waves are formed at l/L0 › 0.6. The maximum values of H/Ho take about 2.4 for the progressive waves, and they are seen at 0.7 l from the entrance of the converging walls.
The quantity of spray transported by strong wind has been measured experimentally when the wind blows over waves propagating on a sloping bed. The leeward variation of spray concentration is much smaller than that in the vertical direction because of the quasi uniform supply of spray from the water surface. In the theoretical development, therefore, the balance has been analyzed between the upward flux of the concentration by turbulent diffusion and the downward flux due to the spray precipitation. Three characteristic quantities introduced in the analysis have been obtained by superposing the experimental data on the theoretical solutions. They are related to the wave and wind parameters to predict quantitatively the spray concentration supplied from sea to shore.
A landward transport process of spray generated from a wave absorbing sea wall under a strong wind has been investigated experimentally. Near the wall, the concentration of spray decays exponentially both in the leeward direction and in the upward direction. The profiles are determined by the equilibrium between the advection of spray and the sedimentation. The characteristic quantities on the profiles have been related to the experimental parameters.
