Juichiro Akiyama

Prediction of velocity profiles and Reynolds stress distributions in turbulent open-channel flows with adverse pressure gradient

Turbulence measurements in open-channel flows with adverse pressure gradient were conducted with an X-type hot-film anemometer. The velocity profiles in whole depth are described well by the log-wake law which involves a wake strength parameter π. The wake strength parameter π is controlled only by the pressure gradient parameter β in the equilibrium region, i.e., normalized velocity profiles are similar at all streamwise distances. A new relationship formula between the wake strength parameter π and the pressure gradient parameter β is suggested on the basis of experimental data. The equation of motion and continuity equation are transformed to predict the velocity profiles and also the Reynolds stress distributions under the conditions that the flow is in equilibrium and the coefficient of velocity is constant in the streamwise direction. The wake strength parameter π can be predicted with high accuracy from the obtained equations. The predicted Reynolds stress distributions correspond to the experimental data when the pressure gradient parameter β is low. On the other hand, the predicted ones do not correspond to the experimental data when the pressure gradient parameter β is high.

Relationship between Pressure Fluctuations on the Bed Wall and Free Surface Fluctuations in Weak Hydraulic Jump

The relation of time averaged conjugate depth and the characteristic of the length of hydraulic jump have been almost clarified. The time averaged pressure distributions around the end sill are also measured by several researchers. On the other hand, bed materials near the roller of hydraulic jump are sometimes picked up by the pressure fluctuations, so that it is quite important to investigate characteristics of the pressure fluctuations in and around the hydraulic jump. In this study, the fluctuations of bed pressure in and around roller and also instantaneous flow depth are measured with the pressure gauges and supersonic wave gauges, respectively. It was found that oscillations of roller influences on the instantaneous flow depth and bed pressure and also that the instantaneous bed pressure concerns with the instantaneous free surface fluctuations.

EFFECTS OF SPANWISE WIDTH OF EXPANSION AREA IN OPEN-CHANNEL FLOW ON RESTING CHARACTERISTICS OF OPSARIICHTHYS PLATYPUS

中村 は魚が川にすめる条件として,流量の確保,水 質の確保,餌の確保,天敵からの保護,産卵場の確保, 回遊路の確保,避難場所の確保を挙げている.この内, 避難場所の確保は,次の観点から必要とされる.魚の筋 肉には血合筋と普通筋とがあり,平時には疲労の蓄積が ない血合筋を使用し,疲労の蓄積がある普通筋は使いた がらない .遊泳時に血合筋のみを使用する維持速度は, 魚種,遊泳型,成長段階,水温などに影響を受けるが, 体長倍流速で 2~4(1/s)程度といわれている .ここで, 体長倍流速(1/s)は流速(cm/s)を魚の体長(cm)で除したもの である.一方,遊泳時に主として普通筋を使用する突進 速度も魚種などの影響を受けるが,体長倍流速で 10(1/s) 以上といわれている .河川の流速が魚の突進速度を 超えると,魚は突進速度で上流方向に遊泳しても下流に 流される.そのため,低流速な避難場所が必要となる. また,低流速域を遊泳していても,捕食の危険が迫った 場合は突進速度で遊泳するため,疲労の回復が可能な避 難場所が必要となる. 洪水時における魚の避難場所の利用形態について,多 くの研究がなされている.綾らは淀川において水位が 上昇すると,フナなどの18魚種がわんどに避難すること を解明した.佐川らは自然共生研究センターの実験河川 で人工洪水を発生させ,洪水時に仔魚等がわんどに避難 することを観測した.傳田らは信濃川水系千曲川の一 次水域および本川における魚類調査を行い,洪水時にア ユ,ウグイ,オイカワ,ギンブナが一次水域に避難する ことを示した.東らは青森県を貫流する平川において, 出水で流下したウグイがわんどや植生帯に避難すること を,テレメトリー法を用いて確認した.佐々木らは山 口県を貫流する大田川において,洪水中はフナがわんど 的な低流速域に留まることを,テレメトリー法を用いて 確認した.傳田らは過去の地形データに基づいて洪水 時の流況を再現し,魚の避難可能エリアの経年変化を推 定した. 平水時に主眼をおいた魚の避難場所の利用形態につい ても多くの研究がなされている.福留らは高知県を貫 流する安田川に水制を設置した結果,水制設置の半年前 および半年後の魚の生息密度が約2尾/mから約4.5尾/mに 変化したと報告した.Hartman & Titusは米国バージニア 州の河川に連続水制を設置した結果,様々な魚種の生息 空間が確保されるため,魚種数および尾数が増加するこ と,また,稚魚が顕著に水制間を利用していることを示 した.Gaudinnd & Sempeskiはヒメカワマスの仔魚は本流 ではまったく確認されず,側岸凹部のみで確認されたと

ASSESMENT OF FLOOD CONTROL EFFECTS OF DRAINAGE PUMPS AND THE MUTABE RETARDING BASIN IN THE ROKKAKU RIVER BY NUMERICAL ANALYSIS

A retarding basin is one of the most effective countermeasures to mitigate existing downstream flooding problems. Effectiveness of the basin is strongly dependent on design of an overflow levee. Experimental works have been commonly employed for the overflow levee design. The primary objective of this study is to demonstrate that a numerical model, which are comprised of the distributed hydrological model, the 2D unsteady flow model based on flux-difference splitting scheme as well as unstructured finite-volume method, and operation of drainage system, is an effective tool to evaluate the stage-discharge relationship of the Mutabe retarding basin in the Rokkaku River under the given conditions of rain-fall and tidal level of the Ariake Sea.

ON THE EXAMINATION AND EVALUATION OF BREACH DISCHARGE IN INUNDATION ANALYSIS

The breach discharge by overtopping flows is examined experimentally and numerically. The process of breach erosion is reproduced, employing laboratory-scale non-cohesive homogeneous embankments. The relationship between breach discharge and effective width of breach cross-section is identified by using experimental results. The relationship is compared with the one obtained from the on-site overflow experiments at Chiyoda conducted in 2010~2011. Breach discharge is calculated by the numerical model that simultaneously solves flood flows in a channel and inundation flows in a flood plain with the dynamic inundation model, which comprises the finite volume method on unstructured grid using FDS technique. It shows that breach discharge can be predicted comparatively by the use of the model with the h-q boundary condition and effective width of breach cross-section.

ECONOMIC LOSS ESTIMATION IN IIZUKA CITY USING NUMERICAL SIMULATOR FOR RAINFALL-RUNOFF/FLOOD INUNDATION PROCESS IN URBAN AREA WIT RIVER AND SEWER NETWORK

Economic loss estimations for flooding in the Iizuka-city area in the Onga river basin were performed using numerical simulator for rainfall-runoff in watershed and flood inundation process in urban area with rivers and sewer network. The rain-fall runoff was simulated by a distributed hydrological model. The flood flow in rivers and inundation flows on the ground were simulated simultaneously by the 2D freesurface flow model. The dynamic network model for free-surface-pressurized flow combined with the Preissmann slot was used for the sewer network. The economic loss estimation based on the simulated inundation area and depth was compared with the reported value. The effects of flood disaster-reduction system were also examined based on the numerical results.

Study on Reason Why Ayu Migrates From Sea to River

The ayu, Plecoglossus altivelis altivelis, migrates from the sea to the river in spring. Hotta(1953) pointed out that the reason why the ayu migrates is coincidence of water temperatures in the river and sea. Wada(1990) pointed out that the water level at the sea greatly affects on the migration number. However, a collective view has not been obtained. In this study, the water level, water temperature, pH, DO, COD, electric conductively, turbidity when the ayu migrates and those when the ayu does not migrate were compared. The ayu wants to migrate when the water temperature is high, the sea water level is high. On the other hand, the ayu does not want to migrate when the discharge is high, due to the water temperature decreases. In such a situation, sometimes DO becomes high.

Effects of Transmitted Light and Bubbles in Open-Channel Flows on Fish Behaviors

Fish sometimes enters into an intake gate of dams. Some techniques with electricity, transmitted light and air bubbles have been developed to avoid fish from the intake gate. However, a lot of fisheries co-operative associations pointed out that most of such techniques have no effect. In this study, the color types of transmitted light are changed at both sides of a rectangular water tank and swimming area of Zacco platypus are observed. Further, the air bubbles are injected from the channel bottom and trajectories of fish are observed.

Simultaneously Measurements of Velocity and Pressure on the Bed Wall in a Backward-Facing Step Flow

A revetment block which is located just downstream of a weir is sometimes destroyed during the flood. The one of the reason why the revetment is destroyed is pressure fluctuations near the bed. The drag and lift forces of bed materials on the bed are measured by many researchers. Turbulent structure in open-channel flows behind a backward-facing step is also investigated with a PIV and LDA. In contrast, pressure fluctuations on the bed were not measured, due to its difficulty. In this study, simultaneously measurements of velocity and pressure on the bed wall in a backward-facing step flow were conducted with a PIV and pressure transducer. It was found that the instantaneous pressure increases when the K-H instability vortex which is generated behind the step is falling toward the bed wall. The kolk-boil vortex is generated after the K-H instability vortex hit on the bed wall.

Effects of Notch Ratio on Migration Rate in Pool-And-Weir Fishways

Most of fishways constructed beside of weirs and dams in Japanese rivers belong to pool-and-weir type. Usually, a notch is placed at the weir. Ministry of Land, Infrastructure and Transport recommends that the ratio between the notch width and fishway one set in the range between 0.17 and 0.20 without any verification. In this study, the notch ratio in pool-and-weir fishway was changed in the range between 0.1 and 0.7 and migration rates of the ayu, Plecoglossus altivelis altivelis, were obtained with the aid of two sets of digital video cameras. Further, three components velocities were measured with a 3-D electromagnetic current meter.