In large-scale tanks used for shrimp farming, establishment of a method for efficiently accumulating and removing waste deposits such as shells of shrimp is an important requirement for successful farming. In this study, we investigated the structure of the forced circulation flow generated by a pump and the advection characteristics of the sediment associated with the circulation flow in a flat square water tank simulating an aquaculture farm, using a hydraulic model experiment and numerical analysis. As a result, the secondary current of the first kind due to the circulation flow accumulates the sediment near the center of the tank, and this effect is enhanced by increasing the pump flow rate. Since the initial vortex immediately after starting the pump and the backlash of the water surface immediately after stopping pump contribute to the accumulation of sediment, repeated starting and stopping of the pump makes the sediment accumulation more efficient.

 In recent years, Japan has experienced a series of river floods caused by levee failures. For disaster mitigation, it is necessary to precisely reproduce the progressive behavior of levees, such as seepage, overflow, and erosion by numerical analysis. In this study, the 3D flow model that can simultaneously simulate overtopping flow and seepage flow was coupled with a soil deformation model discretized by the GIMP method to establish an analytical method that can reproduce the erosion failure of the back slope due to overflow water flowing on the embankment surface. An algorithm was developed to pass the pressure in the overtopping and seepage flows and the shear force in the overtopping flow to the soil deformation model. It is shown that the algorithm can qualitatively reproduce the progression of erosion and embankment failure from the back side.

 In recent years, due to climate change caused by global warming, record-breaking torrential rains have been occurring frequently. In particular, river levee breaches cause serious damage, so investigation of the mechanism of the levee breach is an important issue. In Typhoon No. 19 in 2019, the levee failures caused sediment deposits on the protected lowland, causing extensive damage. Although many studies have focused on sediment transport, the process of topographic changes during a levee breach, including clay and gravel, has not yet been revealed. In this study, we conduct levee breach experiments with simulated levee using colored gravel, and observed in detail the topographic changes during the overflow failure. As a result, it was revealed that the differences of the overflow failure process have a great effect on the shape of levee and surrounding topography.

 This study presents a method for analyzing channel fluctuations, including bank erosion, in a river dominated by suspended sediment using 2-D flow model and applies it to the lower reaches of the Sangu River in Bangladesh to understand the charteristics of river channel change. The analysis is conducted for an area of the lower Sangu River basin, using the 2018 riverbed as the initial condition, and the computation results are compared with the actual channel change data. The results of the computation generally evaluate the trends of actual riverbed fluctuations and riverbank erosion. At the meandering area where the flow concentrates, suspended sediment is deposited on the inner bank due to the separation eddy from the main channel, while this such eddy is less formed on the outer bank, causing riverbank erosion when the flood flow returns to the main river channel. Thus, we clarified the process of river channel change caused by the deposition of suspended sediment and riverbank erosion.

 In phases of selective sediment transport such that sand was supplied gravel bed, characteristic sediment transport and riverbed fluctuation conditions occur, such as sheltering by exposed gravels, deposition of fine sediments in voids, and easy movement of fine sediments on the surface. Conventional exchange layer models cannot adequately represent such phenomena, and a model that can represent these phenomena is needed from the viewpoint of sediment management and river environment management. In this study, the modeling of the area rate of mobile sediments in the surface layer and the sediment exchange between modeling layers in the exchange layer were updated and applied to a sediment supply experiment in order to confirm the effectiveness of the model. As a result, the reproducibility of riverbed fluctuation and longitudinal profile of water surface was improved, and the model was confirmed to be useful.

 The local scour at bridge pier has known to be caused by the formation of three-dimensional separated vortices, i.e., the horseshoe vortex at the movable sand bed and the impinging down-flow of water at the upstream of the bridge pier. This horseshoe vortex removes sand from the bed, particularly at the region where the piers are in contact with the bed material. Countermeasures against local scouring of bridge piers include bed-armoring for protection against scouring of the river bed and control of horseshoe vortices with strong downflow by means of flow-conducting structures. In this study, focusing on the latter, which has less environmental impact, three types of submerged group piles as flow-conducting structures were installed directly upstream of the piers to reduce scour of the piers by controlling the horseshoe vortex around the pier and the downflow in front of the piers. The experimental results showed that the maximum scour depth around the piers was reduced by about 40% and the volume of scour holes was reduced by about 6% when group piles were installed, compared to the local scouring of the piers without reinforcement, and that the scour holes around the piers and at the top of group piles showed independent forms if their length in the downstream direction was more than four times the pier diameter.

 Efficient sand removal from settling basins or tanks is increasingly being studied using 3D sediment transport simulations. However, the applicability of the pick-up rate equation is still unclear under the high bed shear stress conditions. In this study, we first reviewed previous experimental data on pick-up rates and examined the suitability of the pick-up rate equation. Next, the applicability of the pick-up rate equation was verified through 3D simulations of sediment release tests at the actual settling basin. The literature review and simulation result showed that the pick-up rate equation proposed by van Rijn simulated has good accuracy for sand under the high bed shear stress conditions. In addition, the result implied that the grain size dependence has non-negligible effect on the non-dimensional pick-up rate.

 In this study, an experimental investigation was conducted to examine the response of a river channel with a risk for failure. We used the condition of a stable channel occurrence as a criterion. We conducted experiments where the river width decreased toward the stable channel. We examined the differences in channel response within and outside the stable channel condition. We also conducted experiments when the flow rate varied from the channel-forming flow. We examined the response characteristics of the river channel. The results were compared with stable channel conditions. The results showed that the risk potential for damage to riverbanks and revetments increased in the following cases. (1) When the K value exceeds the stable channel condition due to increased flow rate, the riverbank or revetment restrains the channel's response to widening. (2) When the K value is near the boundary of the stable channel area with two rows of sandbars, the river channel transitions to the single-row sandbar area and the riverbed is degraded.

 In the reclaimed land in Wakasu district, Hikawa town, Kumamoto prefecture, some groundwater is used for irrigation, but poor growth of crops has been reported when groundwater is used for crop cultivation. Thus, we evaluated the characteristics of groundwater chemistry and its suitability for irrigation. In this study, shallow and deep wells were investigated, and the dissolved ion concentrations were higher in shallow wells. The Na-Cl type water in shallow wells and the (Na+Ca)-Cl type water in deep wells were mainly showed, indicating different water chemistry characteristics depending on the depth of the wells. The USSL and Wilcox diagrams were used to evaluate the suitability of groundwater for irrigation. Most of the collected groundwater could be used for irrigation, but it was considered unusable depending on the salt tolerance of the crops and the drainage environment of the soil.

 We classified dams in the Chubu region into three patterns with similar characteristics in the upstream areas, based on the influence of upstream dam discharge operations. By constructing deep learning models for representative dam in each pattern and evaluating their prediction accuracy, we assessed the validity of the model construction method based on the input conditions and its applicability to predicting dam inflow volume. To address the challenge of limited data for low-frequency and unprecedented floods, we supplemented the training data with large-scale flood events during periods with-out radar rainfall data, utilizing ground-based rainfall data. Additionally, we analyzed the impact of using different types of rainfall data in training, verifying the effectiveness of our approach.

 The purpose of this study is to clarify the differences in flood inundation analysis results when different elevation data are used in flood inundation analysis in low-lying areas. The elevation data used in this study showed an average difference of 2 m to 4 m in low-lying coastal areas compared to the elevation data obtained by airborne laser scanning. The results of the flood inundation analysis at a spatial resolution of about 250 m using these data indicate that the maximum inundation depth is 0.86 m in the case of the smallest average difference between the calculated and measured values, and 0.66 m using the airborne laser scanning elevation data. And we conclude that this difference must be tolerated in the present situation, and that the accuracy of the elevation data is not necessarily proportional to the accuracy of the analysis.

 In urban and urbanized areas, it is important to understand the actual status of runoff. In this study, water level-flow curves were developed for regulation ponds in Nagoya City, and field observations were conducted at flood discharges of regulation ponds to determine the runoff volume from the reservoir water level data. The runoff and inflow rates of the reservoirs were calculated using the water level data of the reservoirs in actual rainfall events to evaluate rainwater runoff in the regulation ponds and urbanized watersheds, and to understand the regulation pond storage effects and runoff characteristics in the regulation pond watersheds. As a result, we confirmed the difference in the storage effect of the two regulation ponds and the runoff characteristics of the reservoir watersheds, as well as the quick rainfall-runoff response due to the biased spatio-temporal distribution of rainfall even in small watersheds.

 In this study, we firstly developed an "agricultural-pond sub-model" based on an inundation analysis model to evaluate the flood mitigation effects of agricultural pond complex on a watershed scale, and this was applied to the Manganji River basin of the Kako River system. The results presented that the peak reduction rate of river discharge was determined by the sum of the catchment area of ponds with free capacity at the peak of rainfall and the effective rainfall at the peak of rainfall. From the relationship between these determinants and the peak reduction rate, an equation that is independent of the rainfall magnitude and rainfall pattern was derived. This equation can easily evaluate the peak reduction rate of a river from the characteristics of reservoirs in a watershed without the need to construct a numerical simulation model.

 This study evaluated the impact of population decline on stream-flow using the distributed runoff model in the three small- and medium-sized river basins in the Tohoku region, assuming changes in land use and river channel characteristics in response to the progressive stages of population decline. This study applied the Monte-Carlo simulations to create various land cover change scenarios so that the streamflow changes can be evaluated with its uncertainty. As a result, the maximum annual flow decreased by up to 1.0% due to the increase of forests and the decrease of artificial land use, induced by the population decline in the target watershed. Further, streamflow in the dry period was found to increase by up to 6.6% due to the land cover and river characteristics changes. The findings of this study will contribute to future river management in watersheds where population decline is significant.

 One of the important factors for highly accurate inundation analysis is the elevation resolution and the grid resolution. In this study, we conducted inundation analysis using Hyogo 1m resolution elevation data compared with 5-meter resolution elevation data by the Geospatial Information Authority of Japan (GSI), and examined the differences by the resolution of inundation analysis. It was found that using 1m elevation resolution data resulted more accuracy to the roads than 5m. In addition, we indicated that inundation analysis by using the accurate building shapes and 5-meter resolution elevation becomes similar to 1-meter. Afterwards, it was found that the importance of taking into account the characteristics of individual buildings in the collapse evaluation.

 This research focused on the rainfall runoff characteristics of Typhoon Hagibis (2019) and Typhoon Ida (Kanogawa Typhoon)(1958), and evaluated flood control effectiveness of kanogawa river diversion channel. As a result of a numerical experiment, it was showed that even if the total rainfall in the period is the same, the amount of runoff to the river channel is larger in a rainfall event in which about 50 mm/h rain continues for more than half a day in the Kanogawa River basin. In addition, It is estimated that the flood control function of the overflow embankment of the Kano River drainage channel is almost nonexistent, and that the function of the gate is significant. Moreover, When flood occurs in the area upstream from the branch point of the Discharge Channel, such as in the case of a natural dam during Typhoon Ida, the flood control function of the discharge channel cannot function sufficiently because the flooding spreads to the middle part of the river. Therefore, it is shown that the importance of improving the flood control function in the upstream area and the main river maintenance.

 Data assimilation of overhead wind speeds is necessary to improve the accuracy of forecasting weather phenomena that have been aggravated by global warming in recent years. We proposed a new method of measuring the overcast wind field by continuously capturing cloud images with multiple inexpensive timelapse cameras and identifying the advection velocity of clouds. However, the time synchronization and angle of view of the cameras used affected the accuracy of the wind speed calculation. Therefore, we constructed a GPS-equipped imaging system with an extended angle of view and a high dynamic range image sensor. Simultaneous observations were made with a previous camera system and the constructed imaging system. With the new camera system, more wind vectors were identified than the previous one due to wider angle lens and image sensor.

 We investigated whether the rainfall intensity could be estimated with rainfall sound hitting on an umbrella. The rainfall intensity measured by Laser Precipitation Monitor (LPM) was regressed with sound pressures of various wave lengths.The rainfall intensity was evaluated with the accuracy of 0.28 mm/h in RMSE basis for rainfall events of 14mm/h maximum rainfall intensity irrerespective of umbrella materials and position of sound recorders as long as the regression equation were made to that umbrella-and-recorder system specifically.

 We investigated the possibility of solar radiation measurement using four kinds of pin photodiodes which have different photosensitive areas and sensitive wavelength bands. As a result of the fixed-point observation with the use of passive-active measurement circuits, it was showed basic measurement circuits are capable of effectively measuring solar radiation with pin photodiodes. Pin photodiodes with a large photosensitive area and sensitivity across the visible to near-infrared range exhibited exceptional accuracy in measuring solar radiation. The mobile observation revealed the accuracy of pin photodiodes in measuring upward solar radiation declined in areas with concrete paved pathways and vegetation. Additionally, measuring reflected solar radiation from concrete structures presented certain challenges.

 In recent years, the harmful impact of the heat island phenomena and torrential rains are becoming apparent in cites, thereby increasing the demand of the dense ground meteorological data. In this research, we developed an IoT device that can measure temperature, humidity, wind direction and speed, and CO concentration, and transmit data in real time. The prototype IoT device was developed and installed in vehicles to compare its measurements with stationary observations. The results confirmed that while there were influences of solar heating and sensor biases during stationary observations, the IoT device consistently provided reliable meteorological measurements (nighttime air temperature RMSE = 0.55 ℃). During mobile observations, the device revealed higher temperatures along major roadways, which were attributed to localized atmospheric heating from vehicular exhaust emissions, as indicated by the newly employed gas sensor, as well as high sky view factor.

 Procrustean bed is an epigram to admonish a hard and fast rule. In the conventional manner to estimate the return levels of the precipitation and the river discharge, we prepare the numerous candidate frequency distributions including the extreme value distribution and the other ones, and we try to fit them to the records and select one of the best ones by means of the goodness-of-fit. It will be a procedure to avoid imposing a certain prejudice. However there's room for improvement in the examination of the distributions. Cut-and-try is an inevitable path in scientifical and engineering judgements but comparing various distributions is difficult like doing apple to orange. The result based on the extreme value theory should be accepted with help of Theseus' wisdom and ingenuity.

 We applied the bivariate extreme value theory to extreme river discharge data calculated from d4PDF historical climate experiment in 5, 886 pairs of class-A rivers in Japan and analyzed their tail dependence and joint flood probability. The results found various pairs having joint floods at return period less than 500 years over Japan, especially in the Chugoku region where more than three rivers could flood as well. Occurrence months of joint floods in d4PDF data depicted their seasonal patterns such as Baiu front and typhoons. Interestingly, distant pairs such as Koyoshi and Shimanto Rivers have certain tail dependence, which is found in typhoon seasons in d4PDF.

 In this paper, we propose a inference model for prediction of water level using the HMLasso (least absolute shrinkage and selection operator with high missing rate) algorithm. The HMLasso algorithm enables the learning of models directly from data sets that contain missing values. In the collection of river data, there are several factors that can induce missing data. These factors encompass the closure of telemeter, their installation, and observation errors. We conducted a comparative analysis between conventional method and the HMLasso model. The analysis was carried out on the Mogami River during a flooding event in August 2022. To facilitate this comparison, we artificially increased the missing data rate up to a maximum of 50% and performed multiple analyses. As a result, when trained using actual values, the Nash-Sutcliffe coefficient was 0.876. However, even with a 50% data missingness rate, the coefficient reduced only marginally to 0.842. This results that the HMlasso model's performance degradation due to missingness rate is relatively minimal.

 Estimating global precipitation fields is important for disaster forecasting and water resource management. This study proposes a new methodology to estimate global precipitation fields from ground rain gauge observations using advanced ensemble data assimilation techniques. Here, we use the algorithm of local ensemble transform Kalman filter (LETKF) in which the first guess and its error covariance are developed using reanalyzed precipitation data from the European Center for Medium-Range Forecasts. Our estimates have better agreements with the independent reference data than an existing product issued by the National Oceanic and Atmospheric Administration, as demonstrated by the verification against an independent rain gauge observation. This improvement would be achieved by improved error variance and covariance owing to the ensemble-based error covariance estimation. In addition, limiting the number of assimilated observations within the LETKF was beneficial to improve the precipitation estimates.