Kazuo Fujita

FIELD OBSERVATION ON TIDAL CURRENT STRUCTURE ALONG THE NORTHERN COASTAL AREA OFF SHIMABARA PENINSULA

Recently, reduction of fish catch and poor harvest of cultured laver in the Ariake Sea have been a serious social problem. It is suspected that construction of the Isahaya Sea-Dyke causes deterioration of aquatic environment in the Ariake Sea. Although several studies on the aquatic environment have ever been carried out, the causes of the deterioration have not been understood clearly. In this study, in order to understand the role of tidal current in the tidal exchange phenomena in northern coastal area off Shimabara Peninsula, in situ measurements on tidal current by ADCP (Acoustic Doppler Current Profiler) and sea level by tide gauges were carried out.As a result of this study, it was observed that the phase of the tidal current near the coast proceeds. It is considered that it is brought about by oscillatory boundary layer. Findings of the present study imply that it is very important to carry out high resolution field observation and numerical simulation in order to understand tidal transport well.

STUDY ON EFFECTS OF THE SPECIFIC PIPE WITH DIRECTIONAL RESISTANCE PROPEREIES ON SEAWATER EXCHANGE

The water in the pipe installed in a breakwater flows back and forth because of waves and a tidal current, so that if wall roughness with directional resistance properties is established in the pipe, the roughness behaves differently in response to the direction of flow. As a result, an one-way residual flow is produced and contamination in the port is swept away to the outer sea, and the water quality in the port is expected to be improved rapidly. In this paper, it was tried to make an one-way residual flow across the breakwater in the port by using a pipe with particular devised shape of wall roughness. At first, a square pipe with the Wall Roughness which has directional resistance properties was examined experimentally. Next, 2-dimensional numerical simulations were carried out to estimate the validity of this method in a model port. From these results, it becomes clear that this method can make an one-way residual flow and is quite efficient to improve the water quality in a semi-closed port

AN APPROACH TO THE APPLICATION OF THE METHOD TO IMPROVE WAFER QUALITY BY USING BOTTOM ROUGHNESS

Nowadays water pollution caused by eutrophication in a semi-enclosed bay has been a serious problem. In order to solve this problem, a method has been proposed to activate the water exchange between an inner bay and its outer sea area by using Bottom Roughness with directional resistance properties. In this paper, a possibility for practical use of this method was examined from many angles. First, the basic effect of Bottom Roughness on substance transport was examined by a laboratory experiment Next, 2-dimensional numerical simulations were carried out to estimate the validity of this method in the Nagoya Port that has a complicated topography. Lastly, it was it-ported that four kinds of Bottom Roughness were sunk to investigate their influence on the scour of seabed and the water environment. From these results, it becomes clear that this method is efficient to improve the water quality in real sea areas.

A FIELD OBSERVATION ON WATER PURIFICATION SYSTEM IN MAN-MADE LAKES AND RESERVOIRS USING WAVE ENERGY

We proposed a method to make an unidirectional flow by making use of the natural wave energy. The unidirectional flow in a dam lake or a reservoir becomes possible to send water of a surface layer including a large amount of dissolved oxygen (DO) into a bottom layer in which DO is not enough. The following important items for applying the method to practical use have been examined.(1) how large are the scale and the occurring frequency of wind waves in man-made lakes or reservoirs?(2) how much is the volume of wave overtopping gained on a floating structure?(3) how much is the concentration of DO at the bottom layer changed when water of the surface layer is sent into the bottom layer?

DEVELOPMENT OF A CONCRETE BLOCK FOR CREATING ECOSYSTEMS ALONG RIVERBANKS

Very recently, attempts have been made to restore damaged ecosystems in rivers and to improve watershed management. We have proposed the Mothers Rock which maintains or creates ecosystems along riverbanks. The block consists of two spaces, both are directly connected to the land. The upper space is filled with stones to drain well, so that banks should be protected from failure. The lower space is planned to be filled with soil suitable for stabilizing plant vegetations. The blocks can be useful in stabilizing the bank, creating a wildlife habitat and in providing quasi-natural landscapes.

AN EXPERIMENTAL STUDY ON THE EFFECTIVE TRANSFORMATION OF WAVE ENERGY INTO POTENTIAL ENERGY

波浪エネルギーの有効利用の一つとして, 3次元構造物を用いて波を効率的に越波させることにより獲得された水位差を利用して, 一方向流れを発生させる技術が提案されている. 3次元構造物は一様法面勾配をもつ越波堤部とV字状集波堤部から構成されている. 波浪エネルギーから効率良く位置エネルギーを獲得するには, 入射波に対して構造物の最適寸法を決定することが重要となる. 入射波が規則波の場合と不規則波の場合に対して構造物の形状と寸法を広範囲に変える実験を行い, 越波量と獲得される位置エネルギーを定量的に評価した. さらに設計波に対し所要越波量を得るための構造物の設計指針を提案した.

Volume of Wave Overtopping Gained Effectively by a 3-D Structure

For the utilization of ocean energy, we are demonstrating how to gain the difference of water head by wave overtopping and how to discharge the sea water by using the head difference. The devised artificial structure is composed of two parts. One is the dam to activate the wave overtopping and to gain effectively the head difference. The other is the walls to converge the widely distributed wave-energy. The evaluation has been given for the volume of wave overtopping gained effectively by using the structure

A Study on Amplification Rate of Two-dimensional Regular Waves by V-shaped Con verging Walls

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.