開水路において砂堆を形成している砂粒子の流下をParticle Image Velocimetry法(PIV)により計測した．まず，数時間通水して安定的に砂堆が形成・流下する状況を形成した．つぎに，約2分間 2台の小型防水カメラ（アクションカメラ）を水面に固定し，側方から水路床を照明し，個別粒子を確認できる時空間的解像度で，砂堆上を流下する砂粒子を撮影した．PIVに先立つ前処理として，撮影した画像を加工して，砂堆上を流下する砂粒子のみを画像的に抽出し，2台のカメラの撮影範囲を，幾何補正を行ないながら合成して約15×15cm²の範囲を移動する砂粒子の画像を作成した．この画像をPIVで分析して，粒子群の移動速度の時空間変化を計測した．流量を2通りに変えて，2次元的砂堆および3次元的砂堆を形成し，それぞれの状況での砂粒子に移動パターンについて，時間平均および時間変動と砂堆の形状の関連について検討した．分析結果より，2次元的砂堆では，複数のクレスト・トラフでの土砂移動が同時に起きる傾向があるが，3次元的砂堆ではクレストごとに土砂移動の時間変化が異なることがわかった．このような土砂移動パターンの違いが，2次元的砂堆および3次元的砂堆の形状維持・変形やマクロ的な土砂移動量の相違に関連することが示唆された．

中山間地域を対象としてソフト対策・減災などの災害対策の立案への利用を念頭に，計画降雨を大きく超える降雨災害における洪水氾濫被害を，既往の氾濫災害時に生じた被害実態と洪水氾濫流況との関連を元に推定した．本手法を用い，近年大きな洪水被害を受けた瀬戸内地域の中山間地域の二カ所を対象として事例分析を行った．二カ所での分析結果を踏まえ，可能最大降雨レベルの洪水氾濫を対象とした場合，浸水域の大部分で浸水深が2mを超え，流速も大きいため，冠水域での避難や救助活動は不可能となることが想定された．従って，冠水の前後の対応が重要であることを確認した．また，冠水域内の建物や交通ネットワークに壊滅的な被害が生じることを指摘し，そのことを踏まえた，災害後の対応の事前検討等の現実的な減災の方策について考察・提案した．

This paper describes a method of measuring flow rate and size distribution of sediment transported in gravel-bed river during small flood. The device developed is inexpensive, compact and can be installed in the river bed by hand. A steel shell covers the device and impact sound when sediment hits the steel shell is recorded by a digital sound recorder stored in the steel shell. The relationship between sound wave profile and a diameter of the impinged sediment particle was validated based on the result of flume experiment. The proposed method was used to measure processes of sediment transport during a flushing flow released from a dam, then the size distribution and amount of bed-load were successfully analyzed.

近年，気候変動が問題とされており，それに伴う集中豪雨も多発傾向にある．集中豪雨のように，限られた地域において，短時間かつ多量の降雨が発生する場合，内水・外水氾濫，の発生が懸念される．そのため，XRAINのような豪雨を観測する手法とともに，豪雨を予測する手法はその重要性を増している． また，早期に集中豪雨および豪雨災害を予測するシステムを構築するには，数値モデルのような予報時間が長い手法が有効であると考えられる^1）．本研究ではWRF（Weather Research Forecasting model）を用いて，豪雨を事例とした数値実験を実施することで，そのモデルの精度の検証することを目的とする．また，数値モデルの課題である初期値鋭敏性を改善する手法として，局所アンサンブル変換カルマンフィルタ（Local Ensemble Transform Kalman Filter, LETKF）を適用し，その有効性についても検証する．
本研究では，2013年8月23日から25日にかけて，島根県西部を中心に発生した島根県西部豪雨を対象とした．解析領域は西日本全体と韓国を四角形に囲む領域に設定した．LETKFを実行する際に，初期値は適切なバラつきを有している必要性がある．そのため，本研究では初期値摂動の作成法として，異なる日付の同じ時刻を初期値とし，共通の境界条件を用いた2日間のスピンアップ計算を行った．積分時間は，2013年8月21日0時から8月25日9時とした．
本研究で得られた知見を以下の通りである． 1) 島根県浜田市における強降雨が発生することを定性的に再現したが，総降雨量を過小評価した． 2) データ同化を実施することで，同化しない場合よりも，雨量分布が改善され，同化手法の有効性を確認した． 3) 下層の水蒸気輸送に加え，湿舌による上層の水蒸気輸送が豪雨の発生の要因となっていることを示唆した．

2013年8月の島根県西部豪雨を対象として，気象モデルWRF（Weather Research Forecasting）とデータ同化手法の一つであるLETKF（Local Ensemble Transform Kalman Filter）を組み合わせた数値解析を実施し，WRF-LETKFの有効性を検討した．また，数値解析結果を用いて豪雨の発生要因を考察した．その結果として，本数値解析が，降水量や豪雨の発生位置を正確に捉えることはできないが，降雨期間の後半における強雨域の移動をある程度再現することを示した．また，地点観測と比較した結果，降雨期間の後半において，アンサンブル平均雨量の方が決定論的計算雨量よりも観測結果に近い結果を算出することを確認した．今回の集中豪雨は，東シナ海から供給された水蒸気フラックスが前線付近で収束したことに起因するが，中国山地の地形の影響も受けたことが推測された．

In this paper, an inundation .ow in the Matsugase district, Sanyo-Onoda City, during the 2010 July torrential rainfall event was analyzed. Topographical data of this area were measured by aerial LiDAR survey. The DEM obtained by the LiDAR survey was veri.ed by .eld survey data measured by RTK-GPS. A shock-capturing shallow water .ow model with a .ne calculation grid (two to 5 meters in resolution) was utilized to represent inundation .ow in the inundated area. The calculated water depth was compared with the water records. The characteristics of inundation .ow, especially the e.ect of the embankments in the inundated area, and those relation to local damage were discussed. E.ects of embankment rise and reduction of energy loss due to a bridge were evaluated.

To evaluate the detachment process of attached algae in rivers, a series of flume experiments were carried out. Time-series analysis of NDVI (Normalized Difference Vegetation Index) image of cobbles covered by attached algae was conducted to estimate the change in Chl-a or the decrease in attached algae density. A new method to convert NDVI to Chl-a was developed to measure the attached algae density on cobbles in a nondestructive and continuous way. Quantitative analysis of the detached rate of attached algae has shown that the detachment ratio can be closely correlated with the work done by sediment impinging on the cobbles.

Abnormal growth of submerged plants in rivers not only interferes with fishery and navigation but also blocks sunlight from arriving at river bed materials, leading to the growth failure of attached algae that living creatures there are fed on. It is important for river engineers to clarify how submerged plans can grow and be washed out because they can propagate in such a way that their lobed leaves survive in downstream reaches. This study aims at making clear the conditions how some submerge plant is washed away by the floods. The field surveys were conducted before and after an artificial flood from reservoir to measure the distribution of Egeria densa and to quantify its wash-out rate through cross-sections. These results have revealed the wash-out condition of the submerged plant.

Growth and detachment of the attached-algae in river bed is one of the key factors in ecological system in the river. The detachment process of the algae can be categorized into three types, namely 1 Natural detachment due to aging, 2 Detachment due to flow and bed material load and 3 Detachment caused by rolling of the bed material. To evaluate the detachment processes during flood, we conducted a series of field survey in a site in the Johge River. In the period this survey had been conducted, a flushing discharge and a flood due to typhoon were observed. The chlorophyll-a densities on the bed material before and after floods were measured. Based on the field survey results, the detachment processes of algae are evaluated, and a new model to estimate an amount of detachment during the flood is proposed.

An estimation method of flood discharge hydrograph is developed that couples a 2-D unsteady flow model considering the effect of vegetation toppling and a 2-D model for bed elevation change due to bed load transport. The method is applied to a flush flood from the Haizuka dam in the Jouge River where vegetation became submerged with deformation in large areas. Measurements of water level hydrographs at several points, the discharge with ADCP at a weir and bed elevation change at a point were carried out during the flood. It is shown that the estimated discharge hydrograph gives reasonable agreement with the measured result and that the reliable data of bed elevation is crucial to accurate estimation of discharge hydrograph in a steep slope river.

Railway and road embankments on a flood plain affect the propagation of inundation flow. In inundation simulation, the embankment in the flooded area is treated as the solid wall that inundation flow is able to overflow. However, the embankment involves the potential to be damaged during a severe flood event, and the breakage of the embankments causes not only physical damage but also a reduction of transportation function. In this study, the damage to the embankment of the Kishin line during the flood event in August 2009 is analyzed. The topographic and hydraulic conditions of the embankment failure are discussed.

Field measurement and numerical simulation are carried out for an artificial flush flood in 2010 in the downstream reach of Haizuka dam located at the Jouge River to quantify the effect of flush flood on the exfoliation of attached algae. Measured items are water level, chlorophyll a (Chl-a), SS and attached algae at several locations. The relation between Chl-a attached to riverbed stones and velocity at the same location before the flood and the relation between detached rate of the algae and accumulated bed shear stress during the flood are obtained based on the field observation and 2-D numerical simulation. Based on these relations the total amount of supplied algae to water over the measurement reach during the flood is estimated to around 15.0kg in which the total amount of exfoliated algae is estimated to around 9.4kg.

Floods accompanied by bed elevation change were observed in a steep reach of the Minochi River in 2009 and 2010. The flood discharge hydrograph in 2009 is estimated by three methods, namely the conventional float measurement, Large-scale PIV method and 2-D unsteady flow simulation. The 2-D flow simulation considers vegetation toppling during flood and bed elevation change due to bed load transport. In the numerical simulation, water level hydrographs observed at various points are used for boundary conditions at upstream and downstream ends and the evaluation of the vegetation drag parameter. The flood discharges obtained by three methods are compared to find that the 2-D unsteady flow model yields good agreement with the discharge by Large-scale PIV method. The selection method of the location of suitable upstream boundary in a step reach is also discussed

Using a surface video record of a large-scale flood in Yoshino River, Shikoku, Japan, some problems related to application method of Large Scale Particle Image Velocimetry (LSPIV) are examined in detail. First, from a hydraulic point of view, a new method is proposed for exclusion of incorrect velocity data and calculation of time-averaged velocity. Second, decision standards are examined with respect to the primary LSPIV model parameters. As a result, it was confirmed that the appropriate time-interval, template size and detection area size for PIV analysis procedure range between 1/3 to 1/2 seconds, between 1.5 to 2 times the maximum size of brightness pattern and about 2 times the estimated displacement distance of templates, respectively.

Open-channel flows with a relatively large-scale side cavity zone exhibits water surface oscillations in spanwise or streamwise directions. Although the side cavity zone is installed for improving the water-friendly environment, such oscillations, once occurred in the actual river in the urban areas, may cause flooding, resulting in a large damage in the residential area. In order to investigate the flow features associated with such surface oscillation, the authors have developed a 3D LES model that utilizes a density function for detecting air-water interface. With the developed model, it becomes possible to simulate water surface oscillation observed in the experiments. In this paper, the flow features related to the water surface oscillation as well as the local flow features along the interface between the main channel and the side cavity zone are investigated for the subcritical flows with relatively large Froude number.

The procedure of extraction of three-dimensional distribution of velocity field from ADCP measurement data is presented in this study. Cross-sectional velocity distributions have been generally used to indicate three-dimensional feature of flow. However complex flow fields, generated by combination of a complex bathymetry, curve of the channel and river structure, are difficult to investigate only by using the cross sectional velocity distribution. In this study, anisotropic averaging and interpolation method is introduced to remove random component caused by turbulence of flow and random error of measurement. The continuity correction using pressure equation is also used to reduce both random and system errors. The validity of our procedure is confirmed by using three-dimensional numerical simulation result. Finally, by using our procedure, the complex flow around spur-dikes is explored.

In Recent years, flood disasters are frequently taking place where relatively steep slope rivers meet in the mountainous area. The cause of the disasters is mainly due to the sudden rise of water level at a confluence, which sometimes accompanies a hydraulic jump in the upstream region. In this research, a numerical simulation based on the shallow water equations and the unstructured grid system is performed to investigate the effect of the discharge ratio to the flow patterns and the riverbed change at a confluence. In order to simulate a river confluence in the mountains, the main channel slope is set at 1/80 with a tributary channel slope of 1/40. Laboratory experiments are also conducted to provide data with respect to water surface profiles and bed variation. Water surface velocity distributions are obtained by an imaging technique, LSPIV, using a high vision video camera. It is made clear that general flow features calculated by the simulation model agree fairly well with the experiments including the locations of hydraulic jumps and local scour.

令和2年7月，九州地方では4日から7日にかけて記録的な大雨となり，球磨川や筑後川では記録的な洪水が発生した．その後も西日本から東北地方の広い範囲で大気が不安定となり，江の川，最上川など大河川においても氾濫が相次いだ．近年では，毎年のように記録的な豪雨災害が発生しており，降雨強度だけでなく降雨のスケールが時空間的に大きくなっており，被害が時空間的に拡大している．さらに，令和2年度は新型コロナウィルスによる感染症対策のため，人々の活動が制限される中での災害となった．災害調査データを取り纏め，情報発信することは今後の持続可能な社会を検討する上で不可欠である．土木学会論文集では，災害情報を共有・発信し，防災に関する技術および学術分野の進展に資するために令和2年7月豪雨災害に関する特集を企画した．

The float method is a standard method for flood discharge measurement but safeness and efficiency are limited. LSPIV (Large-scale Particle Image Velocimetry), as a flow visualization method, can avoid these problems. Johnson and Cowen (2016) suggested a method to calculate the local flow depth based on LSPIV results and proofed the method through laboratory experiments. This study utilizes the method to the Uji river for predicting the local flow depth. Large-scale velocity fluctuation modes in the river disturb the calculation of the integral-length scale. Proper Orthogonal Decomposition has applied to remove the large-scale velocity fluctuation.

The growth of aquatic mosses attached to cobbles composes bed material in riffles was confirmed in a river in Japan. Covering bed material with aquatic mosses reduces the biomass of attached algae that is a source of energy for commercial fish. This study focuses on how much and how the aquatic mosses keep their coverage on the river bed in riffles after suffering flood disturbances. Resistance to disturbances due to high-speed water flow and sediment impingement on mosses and the rolling of the cobble attached with mosses was investigated by a laboratory experiment. We observed that intermediate disturbance seems to remove silt in the biofilm but the biomass of mosses had tend to be maintained. Based on visual inspection of moss cover following the experiment, we found that the remaining moss consists of short fresh stems. The relationship between the rate of moss detachment and work due to sediment impingement has investigated and the existence of the optimal work per sediment impingement has been clarified.

Application of image-based method for flood flow measurements has become popular in recent years. There are several issues reducing the accuracy and applicability of the image-based flood measurements, and some of them may have been overlooked. Based on our experience, this paper discusses the issues related to video format and orthorectification, and measures to manage the issues are also presented.

洪水時に河床波が形成されると，土砂運動や通水抵抗が変化することは広く認識されている．しかし，河床波上を移動する個々の砂粒子の運動と河床波の発達や変遷過程との関連性を捉えた研究例は限られる．本研究では実河川において頻繁に見られる河床の3次元性に着目し，一定流量の条件下において，平坦河床から2次元砂堆を経て，3次元砂堆が形成される変遷過程を対象として，河床近傍流速と河床波上を移動する個々の砂粒子の運動を直接計測した．その結果，平坦河床から2次元砂堆が形成される期間においては，粒子の移動速度と流速成分の変動が2次元砂堆の形成に伴って抑制されていき，2次元砂堆から3次元砂堆に切り替わる時刻では特異的に流速成分の変動がほぼなくなることが確認された．その後，発達した3次元砂堆の形成においては，横断方向の土砂移動ではなく，流下方向の土砂移動の空間的な不均一性が重要であることが示唆された．

河道での植生を伴う河岸侵食は，河川動態の主要機構でありながらそのメカニズムについては未だ研究途上であり知見の収集が望まれている．本研究では，植生化した高水敷を，模擬植生を用いた複断面流路として単純化し，植生の有無，密生度，水深などを系統的に変化させた計6ケースの水理実験を行い，植生流出を伴う河岸侵食のメカニズムを検討した．特に，植生基盤土砂の流送とともに流出する植生モデルとレーザー変位計を用いた流水中の地形計測に工夫を凝らしたことが特徴である．その結果，植生は侵食による低水路の拡幅を一旦は抑制し側岸下部の洗掘を促進させるが，流出後には側岸の侵食が再開するというプロセスを経て，間欠的に侵食が進行することが確認された．

Embankments on a floodplain, usually treated as solid walls in inundation simulations can cause considerable damage during severe flood events. The damage to road or railway embankment system not only changes the propagation of inundation flow but also destroys the transportation infrastructure. In this study, the failure of the embankment system of the Kishin Railway Line during the flood event in August, 2009 is analyzed. The factors in causing embankment failure are analyzed from the geographic and hydraulic view points. A framework to estimate the risk of the embankment failure, which is based on the analysis of the structures of the geography and inundation flow, is proposed in this work.

The confluence region downstream of the Katsura River, the Kizu River and the Uji River in Kyoto displays complicated flow patterns depending on which watershed undergoes larger precipitations. The flow characteristics at this location have not been clarified so far due to the complexity of the combining streams. In the present study, in order to obtain the flow features in normal flow conditions, surface flow measurement by using a helicopter is performed. The surface flow is visualized by biodegradable tracers and they are videotaped from a helicopter hovering at about 300 meter height following the cloud of tracers. A high-vision video camera is used for recording the images and the velocity of the tracers are analyzed by applying PIV with subtracting background movement. At the same time, distributions of temperature, pH, and other parameters related to water quality are measured by multi-purpose instrument installed on a boat. It was found that water quality of the Ktsura River is quite different from the other rivers and the longitudinal dispersion of tracers is evident at the location.

In Recent years, flood disasters are frequently taking place where relatively steep slope rivers meet in the mountainous area. The cause of the disasters is mainly due to the sudden rise of water level at a confluence, which sometimes accompanies a hydraulic jump in the upstream region. In this research, a numerical simulation based on the shallow water equations and the unstructured grid system is performed to investigate the effect of the discharge ratio to the flow patterns at a confluence. In order to simulate a river confluence in the mountains, the main channel slope is set at 1/80 with a tributary channel slope of 1/40. Laboratory experiments are also conducted to provide data with respect to water surface profiles. Water surface velocity distributions are obtained by an imaging technique, LSPIV, using a high vision video camera. It is made clear that general flow features calculated by the simulation model agree fairly well with the experiments including the location of hydraulic jumps.

It has been investigated that an open-channel flow with a relatively large-scale side cavity exhibits water surface oscillations in spanwise or streamwise directions. Such oscillations, once occurred in actual river installing a side cavity zone as a water-friendly facilities in the urban areas, may cause flooding, resulting in a large damage in the residential area. In order to investigate the flow features associated with such surface oscillation, the authors have developed a 3D LES model that utilizes a density function for detecting air-water interface. With the developed model, it becomes possible to simulate time-dependent water surface variation with a reasonable accuracy as compared with the experiments. In this paper, local flow features along the interface between main channel and side cavity zone are investigated by distributing virtual tracers following the 3D flow pattern.

When a portion of a straight open-channel is replaced by a channel with a side cavity zone, water surface in the main channel in front of the cavity as well as that within the side cavity zone begins to show significant surface oscillation for specific experimental conditions. The mode of the oscillation changes with hydraulic condition and aspect ratio of the side cavity shape. When the aspect ratio is small, only a seiche-like oscillation within the cavity zone can be observed. However, as the streamwise length of the cavity increases, i.e. for a large aspect ratio, the oscillation in the cavity zone propagates into the main channel and a longitudinal oscillation becomes evident. In order to understand the mechanism of such mode-change, we performed a stereoscopic measurement of the water surface flow oscillation by gradually changing the aspect ratio of the side cavity.

There occurred a levee break of the Izushi River near the district of Torii in Izushi-chou, Hyogo Pref. on Oct. 20, 2004, which is caused by the heavy rainfall induced by Typhoon No. 23 (TOKAGE). In order to perform a high-resolution inundation simulation of this area, detailed topographical data is measured by using a kinematic GPS system. Two dimensional shallow water equation based on the unstructured grid system is used for the simulation. Since the grid size is adjusted so that respective houses can be resolved in the simulation, it became possible to estimate hydrodynamic forces exerted on the houses, which is found to have a close relation to the actual house damages.

Field measurements using an image analysis technique, the large-scale particle image velocimetry (LSPIV), are conducted for a river reach having a series of groins in the Uji River. The measurements are performed using a digital video camera from a height of 27m of the nearby tower for meteorological survey. The river reach with a streamwise length of about 150m and the width of about 60m is cover by three-angle recordings. The measurement results from each angle are successfully connected with each other, by which the general flow feature of the region can be revealed with a reasonable accuracy. The measurement by an ADCP is also performed to obtain the internal three-dimensional flow structure. In addition, a numerical simulation based on the unstructured grid system of the shallow water equation is conducted to compare with the field measurement results and to evaluate the effect of the groins to the main flow.

Open-channel flow with a side cavity is investigated with the aid of a high-vision video camera having 1440 by 1080 pixel resolution. The experiments are performed by fixing the aspect ratio of the side cavity at one to five and by changing the Froude number of the incoming flow. At the same time, longitudinal side slope with several angles are installed within the side cavity to simulate the actual river situation and its effect on the surface flow is examined in detail. It is made clear that the introduction of a new video camera improves the measurement accuracy of instantaneous velocity fields with a smaller template size in PIV. Installation of a side slope is found to have large effects on the recirculating flow pattern at the surface.

When a relatively large side cavity zone is installed in an open channel, the influence of separated vortices and recirculation flow generated at the zone is sometimes exerted to the main flow as water surface oscillation. The mechanism of the oscillation is still not clearly understood because of the complexity of the phenomena. In this research, by using a stereoscopic surface measurement system developed by the authors, time-dependent water surface configurations are successfully measured at a sampling rate of 12Hz, from which general features of the surface oscillation is made clear for a wide range of the Froude number. We then applied the proper orthogonal decomposition (POD) method to the measured data and found out that the surface oscillation can be clearly decomposed into a Seiche-like major mode and sub-modes with smaller wavelengths.

This paper compares the accuracy of the novel PIV technique based on the optical flow concept and the conventional PIV with the Gaussian sub-pixel fitting. In the new technique, we take into account of image gradient terms with three consecutive frames in the optical flow treatment in order to improve the accuracy for complex flows. This paper includes: i) the fundamental concept and formula of the new method, ii) experiments with standard PIV images, video sequences of the meandering channel flow in a Japanese old garden and flow images of the Uji river, and iii) the discussion on the measurement accuracy. The result shows that the presented method is superior to the conventional PIV especially in the measurement of instantaneous flow field with rotational motion.

A ceremony that a few poets sitting alternately on the both banks of a meandering channel are expected to make poems during the limited time when drink-filled cups flow down and finishing poem making, poets pick up cups to drink is called Kyokusui-no-En. This sort of ceremonious game has its origin in ancient China and was introduced to Japan where it prospered in 8th through 11th centuries.
In this paper, the channel flow of Kyokusui-no-En is observed from the view point of fluid mechanics. The actual flow was studied using PIV measurement and numerical simulation. According to these results, it was made clear that the meandering channel creates the separate flow, reverse flow and vortices to make the wine cups movement elegant.

Field observations on the surface flow were carried out in a straight reach of the Uji River. Velocity distributions on the water surface were obtained based on the captured video images by means of large-scale particle image velocimetry (LSPIV). Flow structures around spur dykes are studied for both non-submerged and submerged cases.
It is clearly seen that, for both cases, the dykes are working as an additional roughness. Their effect on channel conveyance capacity is estimated in terms of the effective channel width calculated from the velocity distributions. It is also of interest that the maximum velocity filament appears at a different position as the water depth varies in case of the channel with single-side dykes such as studied here. In addition, large-scale boils shedding from the submerged dykes are clearly observed and their origin and advection mechanisms are also evaluated.

A new two-dimensional simulation model for urban rivers applicable to complicated sidewall geometry was developed. The proposed model is based on the finite volume method (FVM) with flux differcncing splitting (FDS) under unstructured grid system. Firstly, the performance of the model was evaluated by the comparison with experimental results for a straight open-channel with a long side concavity having an aspect ratio of ten. The calculated results showed fairly good agreements with the experiments, indicating the good performance of the model. Secondly, the model was used to examine the effects of various sidewall geometries on the local water-level increase at the side concavity section. It was shown that water depth variations within the side concavity section can be controlled by a separation wall and the optimum arrangement of the separation wall for the present side concavity shape was proposed.