In 2019, the Kanto-Koshin area and the Tohoku area suffered devastating damages from floods and sediment disasters caused by the record-breaking heavy rainfall with Typhoon No. 19 (Hagibis). In the same year, the heavy rainfall caused severe damage in northern part of Kyushu-Island such as Saga Prefecture, as well as the wind and rain disaster caused by Typhoon No. 15 (Faxai) in Chiba Prefecture. These disasters also highlighted the danger of a series of disaster factors that could cause extensive damage in large areas with the combinations. It is one of the missions of JSCE to compile and disseminate lessons from disasters in Japan for solving future challenging issues in managing infrastructures and mitigating disaster damages. This special issue on the storm and flood damage 2019 in Japan includes valuable information of the disasters in 2019 from different disciplinary points of view, aiming to contribute the growth and development of interdisciplinary academic fields of civil and environmental engineering.

 Typhoon Hagibis caused serious damage to large area of Japan. This study estimated the future changes in frequency of flood peak discharge and multi-basin flooding probability by the typhoon using bias-corrected d4PDF and a rainfall-runoff model constructed for all the class-A rivers in northeastern Japan (Tohoku, Kanto, and Koshinetsu regions). As the result, the return period of the peak discharges for the two rivers was estimated at 535 and 123 years in the past experiment, and at 32 to 100 years and 16 to 58 years in the 4-degree rise experiment, respectively. Frequencies of multi-basin flooding, defined as annual maximum flood discharge exceeding the river channel’s capacity in the same year, was estimated over all the rivers targeted. Return period of 6 basins (same as Typhoon Hagibis) was 400 years in the past experiment, whereas 20 to 30 years in the 4-degree rise experiment. A typhoon causing flooding in the largest number (13) of rivers in the 4-degree rise experiment showed a similar path with Typhoon Hagibis but more heavy rainfall on shore.

 The 2019 Typhoon Faxai landed on September 9, 2019, in Chiba City of Chiba Prefecture, with a very strong power. It damaged more than 70 thousand houses in Chiba Prefecture. In this study, we used the post-event aerial photographs taken on September 19-20 and September 27-28 to extract bluesheets covering on the damaged roofs in Kyonan Town, Minamiboso City and Tateyama City. First, the visual interpretation was conducted in three small areas. Then four indices were proposed to extract bluesheets. By comparing with the results of the visual interpretation, the best index and the associated threshold value were defined. The ratio of the bluesheets on a roof was calculated by the area ratio of the extracted bluesheet and the building footprint. As a result, the ratios of 46,145 buildings in the study area were estimated. Finally, the functions to estimate the ratio of the bluesheets on a roof were constructed based on the maximum instantaneous wind speed.

 Typhoon Hagibis that occurred in the beginning of October 2019 was one of a largest and powerful tropical cyclone and was considered to be the most devastating typhoon to hit Japan in recorded history. Extreme heavy rainfall caused massive impacts to Japan in general and to Marumori Town, Miyagi Prefecture in particular. In this study, a detailed field observation data obtained immediately after the flood in Marumori has clearly shown that many broken levee locations were caused by water overflow the river embankment at the constriction areas such as the tributaries junction and the intersection of the river embankment. A numerical simulation was performed to investigate the mechanism of embankment breaching in Marumori Town. According to the simulation results, the flooding water from the breached location in the upstream of Gofukuya River caused the broken levees in the downstream of Shinkawa River and Uchikawa River. In addition, the complete or partial damages of the levee are dependent on the duration of overflow and the maximum overflow water depth. The model results have also shown that the initial overflow locations in the downstream areas were occurred at the corner of a constricted areas and agreed well with the observed breaching locations of the levee in Marumori Town.

 Typhoon Hagibis induced massive amounts of rains in eastern Japan. In the down stream of Arakawa basin, the water level reach nearly the inundation bankfull stage. In the future, the inundation risk may increase if a more extreme event than Hagibis strikes the basin. Thus, the increased rainfall data will be necessary as information of evacuation activity. We focused on manipulation of SST (Sea Surface Temperature) because SST takes a role as resource of water vapor which is one of factors for nimbo-developing. This study aims at increasing the rainfall induced by Hagibis and analyzing the impact against change of SST with WRF-ARW model (Weather Research and Forecasting model). Moreover, we validate the reproduction performance for this event to adequately discuss the impact of SST.

 As a result, WRF model reasonably reproduced the rainfall compared with radar observation although the calculated hourly rainfall shows some difference in the peak time and amount of rainfall. With respect to impact of SST, we confirmed that change of SST strongly affects the rainfall amount and the peak time. The reasons were as follow: 1) The typhoon tracks for each calculation case show the difference due to change of pressure field. 2) The vapor flux distribution shows that high/low vapor flow into Kanto region. In addition, the mountain terrain located in upstream of Arakawa basin uplift the air mass including moisture.

 Land use regulation is positioned in flood mitigation and climate change adaptation measures as a fundamental measure to reduce the amount of exposure in floodplain. In Japan, land use regulation for flood mitigation has been considered in the area classification under the City Planning Act and the Location Normalization Plan under the Act on Special Measures concerning Urban Reconstruction. However, its actual mitigation effects and operational challenges have not been fully clarified. In this study, we analyzed the characteristics of flood damages in the urbanization control area in Naganuma District of Nagano City, where was severely damaged by the levee breaking of the Chikuma River caused by the Typhoon Hagibis in 2019. The analysis using the building point data revealed that land use regulation by the area classification played a role to reduce the damages in the area. However, in the community level, it was confirmed that more buildings have been located in the high flood hazards area due to the development of new houses and businesses associated with the development of infrastructures.

 Recordable heavy rainfall hit and caused severe floods in a wide area in the eastern Japan due to Typhoon Hagibis in October, 2019. This paper aims at clarifying an effective way to manage large-scale floods by reservoirs by analyzing reservoir flood control practices during this flood event. The paper firstly reviews flood control of reservoirs in this event to discuss effectiveness and issues of current reservoir operation practices. An detailed analysis is then carried out to estimate effects of flood control by two reservoirs in the Sagami River basin, where one of reservoirs conducted the emergency spillway gate operation due to extremely large-scale rainfall. The paper also investigates applicability of a medium-term operational ensemble rainfall forecast to prior release of reservoirs for enhanced management of large-scale floods.

 Investigations of flood disaster on the Akiyama river caused by Reiwa first year east Japan typhoon was performed. The river levee failed at two sites of the Akiyama river. Both levee crevasse was occurred at just the upstreamside of the bridge. Additionally, the both levee crevasse was occurred where the river width decreases steeply. Numerical analysis based on the results of the trace investigation and the hearing survey on disaster sites adjacent to the Akiyama river. Results of the numerical analysis showed that the flood disaster on the Akiyama river was caused by the flooding within the confines of the levee in addition to the flood from the levee crevasse.

 This paper investigated the flood control effect of dams in the upper basin of the Tone River at the Yattajima site during Typhoon Hagibis. In order to investigate the flood control effect, we compared the peak water levels with and without the dams. The presented results showed that the flood control by the dam group reduced the water level by 0.9 m in maximum at the target site. Moreover, the most effective dam among the all dams was Yanba dam, which accounted for approximate 60% of the flood control effect, with a maximum reduction in water level of approximate 0.6m. When dam started test flooding on October 1, 2019, the reservoir was not filled up. However, approximate 100% of the flood was stored in the dam reservoir. We found that the flood control effect was almost the same when the flood control was carried out based on operation manual of Yanba dam.

 At the flood event due to Typhoon Hagibis, large amounts of trees on the forested gravel bars were destructed in Kumagaya, on the alluvial fan in the Arakawa River. Part of the trees were trapped in the downstream bars and formed debris dams. The resistance change by debris dam is supposed to affect not only the formation of repelling current but flood propagation to downstream. A two-dimensional flood simulation model with the destruction and trapping driftwoods was applied to the site, to clarify what kind of differences were caused due to the destructive flood. The maximum velocity was decreased on the gravel bar where debris dams was formed although the velocity increased by the destruction in the low channel. The tree destruction situation itself improved in the sheltered area of the forest by the debris dam. From the point of the maximum water depth at the embankment, the trapping effects is law in case of the Arakawa River that has a wide cross section, but it also reduced the peek discharge around 10m³/s.

 In this paper, we attempted to assess courses of huge slope failures and driftwoods generation in Marumori area in Miyagi prefecture, Japan due to the heavy rain event in the East Japan Typhoon 2019. Firstly, field survey on the damage by the disaster was conducted in Marumori area after the typhoon. Very huge damages due to effluence of a large amount of sediments and driftwoods were observed in the overall area. Next, logistic regression model optimized for slope failures due to the Northern Kyushu Heavy Rain 2017 considering geological, geographical, vegetation, and precipitation conditions was applied for the disaster in Marumori area, and also the model was improved to optimize for it. As a result, it is clarified that geological condition is the most significant factor, and the scale of the present sediments and driftwoods disaster can be considered as same as the heavy rain in 2017 which is the largest driftwoods disaster of recent years.

 The aim of this study is to clarify mechanism of rainfall inundation caused by the 2019 Typhoon Hagibis in Iwate prefecture coastal zone, Japan, using probable 1-hour rainfalls estimated for the 2 periods of 1976-2005 and 1989-2018.

 The differences between rainfalls by the 2019 Typhoon Hagibis and 10 year return level of 1-hour rainfall (P₁₀) of 1976-2005, which is generally assumed as discharge standard of drainage system in Japan, at Fudai village, Iwaizumi town, Miyako city, and Yamada town were ranged 40.7-106.6 mm for maximum 3-hours rainfall during the 2019 Typhoon Hagibis, and the estimating gigantic rainfall inundations.

 On the other hands, gigantic rainfall inundations were observed at Kamaishi city, where the amount of overflow from the existing drainage system was only 3.5 mm for maximum 3-hours rainfall during the 2019 Typhoon Hagibis, by rainfall inundations obstructed the huge sediment and woody debris produced landslides at narrow sections in the streams and drainage systems, and by coastal embankments for Tsunami.

 In addition, P₁₀ at all study areas were increased in 8-24 % from 1976-2005 to 1989-2018, which were corresponded with increasing frequency of P₁₀ of 1976-2005 in 1.4-2.1 times. Namely, increasing frequency of rainfall inundations in Iwate prefecture coastal zone was expected in the future.

 Typhoon Faxai passed through the Miura peninsula along with enormous wind and wave in Tokyo bay, destroyed a wide range of seawalls and caused inundation by wave overtopping in Honmoku and Fukuura in Yokohama. Parapets of the seawalls in Fukuura has setback from the caisson front, and it is still difficult to estimate wave pressures and overtopping rate in such complicated cross-sectional shape. In this study, we performed laboratory experiments and numerical simulations to investigate the mechanism of the destruction of seawalls, and estimated wave overtopping discharges. As a result, it was found that large impulsive wave pressures acted on parapets by waves jumping out of the front of the caisson and colliding with the parapets. Also, we found that the wave overtopping rate was about 4.5 times larger when the parapet height after the destruction was 1 m lower than the original parapet height.

 Typhoon Hagibis 2019 recorded the highest precipitation in history, at a lot of area in east Japan. Levee breaching occurred at 7 places in the Arakawa River system. Risk migration to downstream should be discussed when river channel management is conducted. For this purpose, it is necessary to understand the inundation between the river and the flood plain. As a first step, the effects of the levee breaching of the Tokigawa River and the timing on flood volume were considered.

 It was suggested that the levee at junction section of the Tokigawa River and Oppegawa River gradually scoured, considering from the change of water level hydrograph by changing levee breaching timing. In addition, it can be considered that left levee breaching at the Tokigawa River at 6.5 KP occurred after 22:00 on October 12 considering from the change of flood volume by changing levee breaching timing. Since the inundation on the right side of the Tokigawa River in the management section of Saitama Prefecture was caused by the inundation from the Kasumi-tei (open levee), therefore, levee breaching at the 1.4 KP right levee had little effect on the inundation level.

 In recent years, to mitigate the flood disaster caused by the increasing intensity and frequency of torrential rain events, some places have adjusted the retention of the paddy field by installing the runoff control device called paddy field dam. In comparison with other flood control facilities, the installation cost is much lower mainly for drainage control devices at the drain outlet of the paddy field, while the expected effect is significant since paddy fields are widely distributed in Japan. Therefore, this research aims to evaluate the potential flood mitigation effect of the paddy field dam for typhoon 19^th in October 2019 in the Naruse River basin using the Rainfall-Runoff-Inundation model. To consider the potential effect of all paddy fields inside the basin, a simple one-dimensional paddy field dam model was developed, which can be applied for the whole basin. Moreover, the scenario analysis was conducted for different heights of free-drain and proportion of application. In conclusion, the paddy field could store about 90 million m³ water in whole paddy fields in the basin using 15 cm height of free-drain, and it significantly mitigated flood in the downstream of the Naruse River basin.

 Typhoon Hagibis caused the most flood damage on the Nakagawa River, which is one of the major first-class rivers in the Kanto region since the flood damage in 1986. In this study, we recorded the disaster situation and conducted field survey in the damage concentrated segment from the bank breakage and overflow. In addition, we conducted the runoff analysis for the Nakagawa River System and the flood analysis for some regions near the Nakagawa River. As a result, it is clarified that 1) the large-scale inundation (average inundation depth: 1.9m) occurred near the tributaries Fujii, Tano rivers and 2) the inundation continued for four days because the drainage capacity was not sufficient. In addition, based on the water level changes of the Nakagawa Fujii rivers and traces of the flood flow, it was inferred that the Fujii River was affected by backwater. Furthermore, the reproduction simulation, which combined the runoff analysis and the flood analysis, reproduced the inundation depth obtained by the field survey and confirmed the backwater in the Fuji River.

 In October 2019, a typhoon caused major damage in the Chikuma River, including structure failures of major roads along the river and bridge piers. In order to mitigate the flood damage, it is useful to develop a flow pattern evaluation method for detecting the high-risk river-reaches against bank erosion during a flood. This study focuses on the turbulent energy distribution along riverbank, because it was increased by several flow patterns inducing bank erosion, including secondary flows in curved channel, high velocity downstream from a weir, three dimensional flow around a pier and water surface fluctuations along the bank. The three dimensional flow structures are investigated with bottom velocity computation (BVC) method for several bank erosions cased in the three rivers: Chikuma River, which has been suffered riverbank erosion damage due to the heavy rain fall brought by typhoon 19 in October 2019, Misasa River and Seno River, where massive riverbank erosions have occurred due to heavy rain in July 2018. All the bank erosions in the reaches in this study are found where the turbulent energy takes a local maximum.

 This research assessed the heavy rainfall risk of Typhoon Hagibis (2019) from the view point of a typhoon track. A typhoon track data, an ensemble forecast experiment and an large ensemble climate dataset were used in order to grasp statistical relationships between typhoon track and rainfall volume, potential rainfall volume of Typhoon Hagibis and influence of climate change to rainfall, respectively. These three data enabled to estimate regions where the influence of typhoon track shifting is larger and influence of climate change. Heavier rainfall tends to occur in some areas in case of typhoons passed through further east of Typhoon Hagibis. Typhoon induced rainfall volumes increase statistically under warmer climate condition regardless of typhoon track. The rainfall volumes due to typhoons which are similar tracks of Typhoon Hagibis and that of due to typhoons which 2 degree shifted in the longitudinal direction under warmer climate are statistically almost same.

 This study examined the evacuation behavior of two communities in Nagano City; Naganuma and Toyono, Japan, during the typhoon Hagibis. Despite the width of the inundated area and significant building damage, there were only two victims in the area. We analyzed the relationship between (1) the numerical statement of the aggregated evacuation behavior, (2) social and hazard circumstances, and (3) social background, including the disaster preparedness formulated by the community. The research focused on “the minutia” of factors which have been revealed by previous researches, rather than “the existence or absence” of such factors. The analysis was based on newspaper articles, a post-interview survey conducted by Kansai Television Co. Ltd., the minutes of the flooded area’s past flood control meetings, and community disaster management plans. The results showed that the main factor promoting evacuation action differed in the two communities. This difference lied in each community’s disaster preparedness. Indeed, Naganuma had set a specific and socially agreed upon criterion for evacuation: a specific water level of the Chikuma River. Meanwhile, Toyono had experienced several inland flooding. On the other side, both communities had participated in the flood control meetings and formed organizations for voluntary disaster mitigation action and a common awareness of “what constitutes a dangerous situation” and “which information is important for the evacuation”.

 On 12-13 October 2019, Typhoon Hagibis (called as Reiwa First East Japan Typhoon or Typhoon No. 19 in Japan) attacked and caused heavy rain in the wide areas of Japan. In the upper Abukuma River basin, the flood with record breaking water-levels, rising very rapidly and lasting long-time, caused numerous damages including 31 breaches of national and prefectural levees. This paper provides a study on proper flood information service, including irregular and untypical service like a direct hot-line to the mayors, as well as regular service as the legislative flood forecast. It is based on an overview of the disaster and damages with the response of Ministry of Land, Infrastructure, Transport and Tourism and an analysis of actual records of flood forecasts and other flood information services to the municipalities, comparing with actual evacuation support activities by the municipalities.

 Prompt and safe evacuation is important to prevent casualties from heavy rain disasters. When heavy rain is predicted, the government is required to encourage residents to take appropriate evacuation actions by transmitting accurate information.

 In Typhoon No. 19 of 2019, many houses were flooded in Sano City, Tochigi Prefecture, as a result of the breakage of two banks of the Akiyama River in the Tone River system. In this report, we conducted an interview survey of Sano City staff to verify the occurrence of damage, the transmission and communication of evacuation information, and the evacuation of residents.

 Since a strong typhoon was expected to approach, evacuation centers were opened and evacuation information was disseminated early so that residents could evacuate before it became dark. However, many residents evacuated after heavy rains or after the inundation began. In order to promote quick and safe evacuation behavior of residents, it is important to improve disaster prevention literacy of residents.

 THE 19^th Typhoon (2019) caused a heavy rainfall in the northeastern part of the Japan, and caused a great damage to each place. Especially in Fukushima Prefecture, the largest level of damage with a occupancy ratio of about 24 to 49% of the whole country was concentrated. Among the damages, 32 dead and damages at 50 river bank breakages indicate issues to be corresponded with in basin management future disaster prevention against heavy rainfall.

 In this paper, we reported disasters characteristics in Fukushima prefecture, which suffered enormous damage based on the results of field surveys, and summarized the consideration of disaster damages. For the use of future disaster prevention in planning, we prepared archive information of the enormous damage situation by the description in this paper.

 The heavy rainfall from 10th to 13th October 2019 in various parts of eastern Japan occurred due to Typhoon Hagibis and caused numerous disasters. In Tochigi Prefecture, the maximum 1-hour rainfall was 58mm and 52.5mm, and total rainfall for 3 days was 423mm and 512.5mm at the Shioya and Okunikkou weather station respectively. 27 levee breach in 13 rivers occurred due to this heavy rainfall. The field survey and numerical simulation were performed to clarify behavior of the inundation in Arakawa river in Nakagawa river system. The levee breach occurred due to the overflow of inundation flow from landside to riverside. Moreover, this flow influenced the opposite-side levee breach.

 This report investigates the characteristics of heavy rainfall and flood inundation in the area by Typhoon No. 19 in 2019 by examining the heavy rainfall, flood inundation, and breached rivers in the affected areas. From this study, it is clear that (1) In prefecture-managed rivers, flooding occurred due to insufficient flow capacity in heavy rainfall areas and a decrease in flow capacity with rising water levels in the main river, (2) In nationally managed rivers, the rainwater in the upstream basin flowed downstream, and the water level in the river channel increased in areas with relatively little rainfall with causing levee flooding, (3) Depending on the topographical characteristics, there were two types of floodplains, one that attempted to divert floodwater back into the river channel and the other that acted as a retarding basin, (4) The area where the levee breach occurred in the Kanto region, where most of the land use was a field and where the inundation flows was stored, may have roled as a flood retarding basin.

 The Typhoon Hagibis (the East Japan Typhoon of 2019) caused flooding in many areas, which also disrupted lifeline services such as electricity, gas and water supply. In this paper, I focus on the electric and gas utilities in the area along to the Chikuma River in Nagano Prefecture where the typhoon damages are particularly severe. And their damage and recovery processes are described with their impact-response relationships by reviewing the information from the central / local governments and these service providers. The maximum number of houses that were outages of electricity and gas were respectively about 63,500 (electricity) and about 900 (gas) in and around the Nagano City, in the early morning of October 13 2019, immediately after the Chikuma River breaching. According to my findings, most of these damages are caused by the failures of supply facilities such as electrical substations and gas pressure governors due to being submerged in flooded water. Furthermore, in order to mitigate these outages of electricity and gas supplies, I could suggest that the preparedness measures preventing from flooded water are important not only for each of the aboveground facilities, but also for their connecting apparatuses via their supplying networks.

 In this study, the specifications and rainfall conditions of the reservoirs in Miyagi, Fukushima, and Tochigi prefectures, which were collapsed by Typhoon No. 19 in 2019 and later heavy rains, were arranged. After that, the damage situation of the embankment of reservoir and the characteristics of the collapsed part were investigated by the field survey. In addition, the physical properties of the embankment materials were investigated by laboratory soil tests. As a result, it was clarified that the collapse of the reservoir occurred mostly around the structure of the embankment, and that many of the collapsed embankments of reservoir were constructed of sandy soil containing much sand. Furthermore, the factor of the collapse of the embankment of reservoir was estimated based on the field survey results. As a result, it was presumed that the shortage of the length of the impermeable sheet was the cause of the collapse in the surface impermeable type embankment. In addition, in the homogeneous type of embankment, it was presumed that the collapse history, the reduction of the cross section due to slip failure on the downstream surface, and the settlement of the embankment surface due to piping were the causes of the collapse.

 The 2019 typhoon No. 19 hit Japan on October 12. Severe damage to Kanto-koshin and Tohoku areas was brought due to its landing. The authors conducted damage investigations for all the breached sections of river dikes in Ibaraki prefecture in total five times after the disaster has occurred. The survey results showed that: 1) the breach of river dikes in Ibaraki prefecture may have been attributed to erosion of dike slope caused by the overflow; 2) the signs of soil ejection and piping were not found around the breached sections of river dikes; 3) some river dikes with the crest pavements and revetment blocks for both side surfaces demonstrated that they could occasionally be collapsed by the overflow.

 At the time of the above-mentioned survey, geomaterials were retrieved from both slope surfaces of each undamaged dike section, which was adjacent to the damaged area, for the laboratory tests. The test results revealed that most of the retrieved materials were non-/low-plastic sandy or gravelly soils with fines content less than 20%.

 Typhoon No. 19, which landed in Japan on October 12, 2019, caused record heavy rainfall in the Kanto, Koshin and Tohoku regions. Flood damage such as swept away of abutment and inundation due to levee breach occurred in the Chikuma River of the Shinano River system. The authors conducted a field survey on the 23rd and 24th October 2019 at the damaged area of the river structure in the Chikuma River. The survey methods are visual observation by field reconnaissance and aerial photography by UAV (Unmanned Aerial Vehicle). This paper reports the damage situation of river structures obtained from the field survey and the lessons learned.

 A one-dimensional model was developed in order to clarify hydraulic characteristics of rubble mound weir. Theoretical solutions of water surface profile and discharge rating curve were obtained for every categories of flow regime and they were verified by comparing with a laboratory experiment. The flow structure was classified into three categories of flow regime depending on water level and weir height. They are “Regime-S” in which the weir is submerged and the stream is running completely over the weir, “Regime-P” in which the weir is partially submerged and the stream is buried to the weir body in a middle reach of weir and “Regime-E” in which the weir is emerged and the stream is buried along the whole reach. Each flow regime was divided further into two categories depending on the influence of backwater, which are the flow controlled by backwater and the flow governed by critical depth, respectively. The six categories of flow regime were systematically displayed in a diagram of discharge contour map in which discharge is plotted as a function of the water depths at the cross section of upstream and downstream boundaries.

 This paper presents the inverse analysis of boundary conditions and parameters in river flow analysis, using the adjoint sensitivity method adopted as data assimilation method for weather forecasting and contributing to the improvement of forecasting accuracy. The adjoint equation and sensitivity coefficient were derived for the cross-section-averaged one-dimensional unsteady flow. The numerical simulation method was shown, and the applicability for the actual river was verified. The data assimilation using multi-point water gauges successfully estimated discharge at any point, and the accuracy was changed with the numbers of water gauges. In addition, in the data assimilation using water gauges at 8 points for the river channel network, the data assimilation results also showed high accuracy, reproducing observed discharge and water level. Furthermore, the forecasting simulation using the assimilation results as initial values showed highly accurate predicted water level up to 2 hours ahead. Next, its method of data assimilation was applied to the channel optimization. In the optimization of the channel shape considering two cases of river-bed excavation and channel widening, where the river water level is below the levee height, the inverse analysis was successfully applied to determine the optimized channel shape by one time simulation.

 This study investigated sediment release from bottom outlet at dam by 3D numerical simulation of turbidity current and bed deformation. The numerical model was developed by customizing the open-source CFD library OpenFOAM. The erosion of cohesive sediment was modeled using the past experimental result. The numerical model was validated in the field measurement of sediment release. The field data taken after sediment release operation showed the existence of scour hole expanding laterally in front of the inlet of the sediment release conduit. The numerical simulation reproduced the scour hole with sufficient accuracy. The concentration of turbidity (i.e., SS) was also well simulated quantitatively. Contribution of the scour process to SS was evaluated using the result of the simulation. The numerical simulation revealed the phenomenon that, in some time interval, sediment is temporarily efficiently eroded during sediment release. The numerical model and findings of this study will be useful to optimize operation of sediment release gate to mitigate reservoir sedimentation of cohesive sediment.

 Bangladesh is one of the most disaster-prone countries in the world, with great negative consequences being associated with various natural and human-induced hazards. Therefore, the evaluation of long-term variation of recorded meteorological and hydrological time-series data is necessary for infrastructure development in the future. In this study, we collected the 32 years monthly precipitation data at 36 locations in Bangladesh, the 116 years monthly precipitation data at 7 areas in India using and 30 years data of the daily water level at 4 locations in the Ganges Brahmaputra Meghna and Haor in Bangladesh. We have conducted the trend analysis by using these data and also conducted the correlation and causality analysis in order to find the relation to global scale phenomena such as SOI and DMI. We employed Mann-Kendall test and Seasonal Mann-Kendall test for long-term trend test of precipitation and river water level, the wavelet and cross-wavelet coherence analysis for correlation and VAR-LiNGAM for causality between the precipitation and SOI, the precipitation and DMI.

 From the Mann-Kendall test and Seasonal Mann-Kendall test a significant decrease trend in the central part of Bangladesh is shown. Meanwhile the decrease trend with no significant is shown in northern part and no trend is shown in the other part. It is found that the slope of trend in 32 years is bigger than that in 116 years from the data of India. The daily water level of three major rivers have significant decrease trend in recent 30 years. Especially, the slope of the trend in Brahmaputra is bigger than the others. For the relation to global scale phenomena, the cross-wavelet coherence analysis shows the correlation of DMI to monthly precipitation is stronger than that of SOI. The causality from DMI to precipitation in Bengal area with 5 to 6 months lag is found by VAR-LiNGAM analysis.

 This paper proposes fragility curves for asset damage based on questionnaire replies from companies which were affected by floods after the heavy rain event of July 2018. Fragility curves are a probabilistic method for analyzing structural vulnerability and production facility of companies. Although a fragility curve is a basic model of disaster damage assumption, there has been no case of estimating the asset damage rate at the time of flood damage. In this study, the expected values of the damage rate and the damage cost obtained by the probability distribution of the asset damage rate are smaller than the estimation results by Manual for Economic Evaluation of Flood Control Investments. These results may reflect the progress of the flood control and the BCP of companies, and it is important to verify the validity of the evaluation model while continuously updating it.

 In recent years, a riverbed excavation project is planned and implemented for safety against frequent flood in many rivers of Japan. It is necessary to predict the impacts on the growth and habitat of aquatic organisms because riverbed would be largely altered by the excavation. In this study, we developed an ecosystem model, which can do the effects assessment of a project. Then, we examined the effects of the riverbed excavation project on the riverine ecosystem in the Takatsu River with the model. As a result, the ecosystem model has sufficient reproducibility for physical environmental variables and the biomass of aquatic organisms. Moreover, the ecosystem model estimated that the distribution of water depth and velocity changed due to the riverbed excavation and the longitudinal distribution of water temperature also changed. In addition, the biomass distribution of aquatic organisms changed in response to the differences in the physical environment. When total biomass in the calculated interval was examined, the attached algae and benthic invertebrates were maximally up 12.7% and 26.3%, respectively. In contrast, the biomass of fishes did not significantly change (~2% increase/decrease) in the total biomass.

 Agriculture-based revitalization of rural socioeconomy in Japan has been tried throughout the country by branding agricultural products for high added value and promoting farmers’ diversification into processing and distribution, in other words, “sixth sector industrialization”. The existing method for economic evaluation of flood-control projects uses the country-wide average of their prices, regardless of the efforts for branding in each area, to estimate the damage to agricultural products due to inundation of farms. Moreover the inundation of farms can cause not only the damage to agricultural products but also indirect impacts via the sixth sector industry, threatening sustainable development and stability of the rural socioeconomy. To estimate flood-caused damages in agriculture more properly, this study proposed to modify the existing economic evaluation method of flood-control projects by considering the product branding and the sixth sector industrialization as factors characterizing current circumstances in agriculture. In its application to a flood event in Yamagata prefecture, Japan, the modified method demonstrated 15% increase of the damage to paddy fields compared to the conventional method, implying that the above factors are not negligible for economic evaluation of flood-control projects.

 Future changes in annual maximum river discharge in the Southeast Asia region were estimated using “database for Policy Decision making for Future climate change (d4PDF)” taking the Red River basin, Vietnam as a case study. A distributed Hydrologic Model 1K-DHM, based on a kinematic wave approximation, was used for rainfall-runoff simulation. In addition, future changes of the atmospheric environments resulting largest-class flood peak discharge were evaluated. The results show that 1) the frequency distributions of annual maximum 15-days rainfall for d4PDF of the historical experiment match with those for APHRO_MA. 2) In the return period of 10 years or more, a significant difference was detected in the probability distributions of the annual maximum discharge between the past and 4-degree rise experiments. 3) In the 4-degree rise experiment, the instability of a moist atmosphere increases, which may enhance the convergence of vertically integrated water vapor flux and thus, result in stronger precipitation.

 It is well known that sediments and seeds are deposited between roughness elements, which induces the development of thick growth of trees. So, it is important to reveal turbulence structure and its effect on capturing process of particles between roughness elements. In the present study, two kinds of laboratory flume experiments were conducted using hemisphere roughness elements. First, we measured the flow velocity within roughness layer using PIV. The roughness elements were made of hydrogel. The technique enabled us to reveal the mean-flow and turbulence within roughness layer. Then, river plant seeds are added to the flow and we tracked the motion of the seeds over the rough bed to investigate the mechanism that seeds are trapped between the roughness elements. The PIV results indicated that the vertical velocity is mostly negative between the roughness elements. We examined the effect of coherent motion on the mechanism that seeds emerge from the rough bed. The results revealed that in rough bed flow, the bursting event responsible for particle entrainment is sweep.

 We evaluated rainfall predictability of a data-driven machine learning model, LSTM (Long short-term memory) model, which is one of the deep learning architecture and excels at learning time-series data. We configured the LSTM model for rainfall prediction and trained the model with AMeDAS ground observed data in the Kyushu region for providing every 1-hour forecast.

 In many observational sites of AMeDAS, the LSTM model predicted accurately more than persistence method (PER) and Mesoscale model (MSM) did, which is one of the physically-based numerical weather models by Japan Meteorological Agency. The three models were compared their accuracy in terms of RMSE, threat score, and prediction accuracy of beginning of rainfall. The features of the LSTM model were as follows.

 1. The prediction accuracy was higher in the eastern side and inland of Kyusyu than in the northern, southern and the western region near seacoast. That is possibly because the LSTM effectively learned the pattern that the precipitation moves from west to east, from north to south and from south to north. As a result, the better prediction was found in cold and warm frontal rain which were patterned by westerly wind.

 2. Difficulty found in outlier events such as unprecedented torrential rain which is lack of learning data.

 3. Among the various meteorological elements, only the rainfall played the important role in prediction accuracy possibly because the AMeDAS observational density was not high enough to capture cold outflow by rainfall, warm-air convergence precedent precipitation as well as surface wind were affected much by topography.

 This study estimated future changes in extreme river discharge in all the class-A rivers in Japan using d4PDF. A distributed rainfall-runoff model was constructed for all the rivers accounting flood control of major dams. A novel bias correction method was developed and applied to the annual maxima of basin rainfall in d4PDF with thousand-year data using the Radar AMeDAS Rainfall. As the results, the simulated river discharges corresponding to the return period of design rainfall are in good agreement with design flood discharges. Extreme flood discharge will increase in all the rivers, especially in the Kanto region and the Pacific side of the Hokkaido, Tohoku and Kyushu regions. Rainfall-runoff simulation with/without dam operation in the Oyodo River basin indicated that its dam control effect for peak discharge remains for around 1000-year flood in the past experiment. Such floods will occur once in 100 years in the 4-degree rise experiment; consequently, the dam control may become less effective for events over a 100-year flood.

 The objective of this study was to clarify characteristics of runoff and stream water chemistry in forest and grassland watersheds in the headstreams of the Okuragawa River, Sado Island, Niigata Prefecture, Japan. The factors controlling the difference in characteristics of runoff and stream water chemistry between two watersheds were also discussed. Continuous monitoring of precipitation and discharge, periodic stream water sampling, and the water quality analyses of major ions and stable isotope (oxygen and hydrogen) composition were carried out in both watersheds. In the forest watershed, the response to precipitation was slower, the peak discharge was clearly lower, and the decrease in discharge after rainfall was more gradual than those in the grassland watershed. For the relationships between the discharge and the stream water chemistry, concentration of Ca²⁺, Mg²⁺, Na⁺, and HCO₃^- and Electric Conductivity (EC) decreased with increasing discharge in both watersheds. The degree of decrease in stream water chemistry of the forest watershed was markedly smaller than that of the grassland watershed. The result was consistent with the difference in runoff characteristics between both watersheds and also suggested that dilution effect on the stream water chemistry during rainfall was not significant in the forest watershed compared to the grassland watershed.