Tomonari Okada

Horizontal and residual circulations driven by wind stress curl in Tokyo Bay

This study investigates the horizontal and residual circulations in Tokyo Bay using field observations, numerical simulations, and theoretical analysis. Numerical simulations show that the observed deepening of isopycnals and associated anticyclonic horizontal circulation in the bay head are mainly driven by negative wind stress curl. The effects of river discharge, surface heat fluxes, and tides are found to be small. Under strong wind events, the wind stress curl over the bay head can be large enough to make the surface Ekman layer strongly nonlinear. Theoretical and numerical analyses show that, under large negative wind stress curl, the nonlinearity tends to induce positive pumping velocity (at the base of the surface mixed layer) that counteracts the Ekman pumping; however, the typical duration of wind events in the bay head is not long enough to induce positive pumping under negative wind stress curl. These results and historical wind data suggest that the average horizontal circulation and residual circulation immediately below the surface mixed layer in Tokyo Bay are, respectively, cyclonic and convergent in summer but anticyclonic and divergent in winter.

Current profiles around the mound under different stratification conditions

Generally, currents vary with density conditions. This paper describes mechanisms of currents around the mound at Ohfunato Bay in accordance with density difference between inside and outside the bay. It is revealed that mechanisms of currents are greatly related with the rainfall depend on the results of the field measurement in summer. Immediately after rainfall, low density water due to rainfall remains at the surface layer outside the bay. This water body mixes with the high-density water body at bottom layer outside the bay, and so the density of water body outside the bay becomes small. Under this condition, the inflow of low-density water at middle layer and the outflow of the bottom water inside the bay are stronger than usual. Thereafter, the density above middle layer inside the bay becomes small uniformly due to the inflow of low-density water. When low-density water body disappears outside the bay, the density of water body going into the bay becomes big.Therefore this water body comes to go into the bottom layer inside the bay. This current supplies oxygen to the bottom layer, thereby the depleted water body inside the bay recovers rapidly.

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?

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