In July 2020, the Kyushu region experienced record-breaking heavy rains from July 4-7, causing extreme floods in the Kuma and Chikugo Rivers. This was followed by atmospheric instability over a wide area from western Japan to the Tohoku region, resulting in heavy rainfall on July 13-14 in the Chugoku region, and on July 27-28 in the Tohoku region, and flooding of large rivers including the Go River and the Mogami River. In recent years, record-breaking torrential rainfall disasters have been occurring every year, and the heavy precipitation scale as well as the rainfall intensity has been increasing in space and time, resulting in spatio-temporal expansion of the damage. Furthermore, in 2020, the disaster occurred while the social activities had been restricted due to COVID-19 pandemic. The compilation and dissemination of disaster survey data and lessons are essential toward the sustainable development of society. Therefore, JSCE has planned a special issue on the July 2020 torrential rain disaster in order to share and disseminate disaster information and to contribute to the advancement of technology and science related to disaster prevention and mitigation.

 Heavy rain in July 2020 washed away 17 bridges in Kumamoto Prefecture. This paper calculated the hydrodynamic force acting on the Nishize bridge in Hitoyoshi City during the flood. One-dimensional analysis of riverbed variation was conducted based on the water level observed upstream of the bridge. The results showed that the drag exceeded the estimated strength of the bridge bearings based on the design criteria at the time of construction. The maximum drag was smaller than the bearing strength specified in the seismic design standard of the 2017 Specifications for Highway Bridges. If the truss cross-section was completely blocked by driftwood, the maximum drag was about twice that of the case without blockage.

 The heavy rain in July 2020 caused the record-breaking flow in the Kuma River, which is a first-class river in Kyushu region. This flood flow caused 1) human damages at the special nursing home named Senjuen and 2) the dike break at two places in the Kuma River. In this study, we conducted the field survey to measure the detailed topography and flood mark depths. We also conducted the flood analyses for 1) the entire upstream part of the Kuma River where the inundation damage was concentrated, 2) the river confluence near Senjuen, and 3) the region around the broken points of the embankment. Our results indicated that the backwater of the Kuma River caused a significant water level rise near Senjuen. In addition, the strong return flows from the floodplain to the river occurred for a long time at the low land area where the flow tends to gather in the downstream of the Hitoyoshi City. This strong return flow may have caused the dike break at the two places.

 This study summarizes the rainfall and damage in the Chikugo River system, which was severely damaged by the torrential rains in July 2020. One-dimensional flood flow analysis based on water level information was conducted in the Chikugo-gawa River system of the upper basin, where inundation by river water occurred, to understand the behavior of flood flows in the Chikugo, Kusu, Kuma, Shode, and Kagetsu Rivers. As a result, it was found that (1) the amount of the heavy rainfall was the planned magnitude, (2) in the Chikugo River basin, inundation by river water and inside levee occurred and road and railroad bridges were washed away, and (3) based on the flow hydrographs estimated in this study at Senjo, Ogase, Obuchi, Kuma, Kagetsu and Arase stations, the flow discharge in the Chikugo, Shode, and Kusu Rivers where the water level rose near or exceeded the design high water level was near or above the flow discharge which is maintenance target.

 This paper clears quantitative characteristics of hydro-meteorological warnings, flood forecasting and evacuation protocols, and the relationships between them in consideration of the degree of warning, during the heavy-rain event of July, 2020. An interview survey also cleared what situations and problems staff of municipalities confronted. As results, the situation showed that the degree of flood warning was rapidly increased, although warnings for landslide were announced in advance of those for floods. In details, the warning level of flood forecasting of Kuma river was changed from 2 to 4 in about an hour as the shortest period. In addition, evacuation protocols about “Evacuation Preparation/Start of Evacuation of the Elderly, etc.” were insufficiently issued due to a limit of forecasting and a necessity of the issuance during daytime.

 Facility for elderly care welfare facility have been frequently damaged by heavy rain disaster. Facility users save their lives by moving to social welfare institution evacuation shelter or evacuating vertically.

 The relationship between disaster prevention weather information and evacuation information during the heavy rain event of July 2020. And the actual situation of evacuation at facilities for vulnerable people are investigated by questionnaire. From these, the following became clear. Firstly, information of preparation for evacuation / start of evacuation of elderly people and others is issued only by about half of the local governments. Secondly, it was difficult to make an evacuation decision at a facility using the river water level as an evacuation criterion. Thirdly, the formulation of an evacuation security plan has no direct effect on evacuation.

 Against frequent flooding occurring simultaneously in a wide area, the early detection of flooding damage leads to the appropriate rescue, disaster relief, and early recovery. This paper reports on the results of the demonstration of the satellite use during the heavy rain of July 2020 in Kyushu and the current issues of satellite-based damage assessment resulted from the reproduced analysis of past floods. To solve the current issues of satellite application for damage assessment, we verified two possible solutions; the use of wide-area imaging mode and the use of images taken with different bands and orbits. Based on the results of the verifications, this paper also reports the effectiveness of those satellite-based damage assessment methods.

 Early July in 2020 corresponding to the end of rainy season, lots of warm air containing massive water vapor flowed into the seasonal rain front being stationary over Kyushu Island, located at western part of Japan through the so-called “atmospheric river” from Indian Ocean and East China Sea. Therefore, large scale “Senjo-Kousuitai” (quasi-stationary band-shaped precipitation system) generating over basins of Kuma River and Chikugo River which are class A rivers in Kyushu Region caused serious flood and sediment disasters. Global warming has enlarged Senjo-Kousuitai, and has become Typhoon gigantic and slow-moving. Geographical features in Kyushu Island increases large scale flood disaster risks in each river basin. As the countermeasure for future large scale flood disaster in Kuma River basin with a main branch, Kawabe River, a hybrid type large-sized dry dam is recommended in order to achieve both natural environment preservation and flood disaster prevention.

 In recent years, unprecedented torrential rains have caused flood damage in many parts of Japan. Even under such severe conditions, expressways are required to work as "roads of life". The torrential rain that occurred in July 2020 caused severe damage mainly in Kumamoto Prefecture, Japan. Due to this rain, though the expressway was not closed for a long period of time, some damages were caused, such as the lifting of concrete pavement of a tunnel and collapse of the slope. West Nippon Expressway Company Limited (NEXCO-West Company), which manages this section of the expressway, worked to restore the road as quickly as possible to minimize the impact on customers. In addition, in order to support the stricken areas, emergency gates for the residents and emergency vehicles has been opened and free tolls between the areas has been provided. This report summarizes these measures implemented by NEXCO-West Company.

 In this paper, we attempted to conduct field survey of driftwoods generation in the Kuma River basin and the Chikugo River basin, where were damaged by the floods due to the heavy rain event in July, 2020. We focused on the upper area of Ushifusa Dam and several tributaries in the Kuma River and the upper area of Shimouke and Matsubara Dams in the Chikugo River. We carried out a photogrammetric survey using UAV (drone). The driftwoods generation was estimated from the measurement data and the tree species dataset in each slope failure site. Total amount of driftwoods from our survey was much smaller than the estimation by the public administration. In addition, the logistic regression model optimized the heavy rain in 2018 was applied to the heavy rain in the Kuma River basin in 2020, to express the driftwoods generation. As a result, it was shown that there was small possibility to generate a lot of driftwoods in the upper area of Kawabe-gawa Dam which was planned to construct after the floods in 2020.

 Recordable heavy rainfall hit and caused severe floods from the Kuma River in Hitoyoshi and Kuma regions in July, 2020 due to an active frontal rain system. This paper aims to clarify challenges and issues in measures taken by core medical institutions against large-scale floods, which have been identified in the flood event in Kuma River in July 2020, based on the interview with the core medical institution in those regions. The paper then discusses directions for effective flood countermeasures of core medical institutions in order to have more robust Business Continuity Plan (BCP) for large-scale flood disasters.

 The July 2020 flood caused damage such as inundation of river water, inundation of landside water, scouring of river terrace, and damage to revetments in the section managed by the Ministry of Land, Infrastructure, Transport and Tourism of Gounokawa river. On the other hand, in the lower reaches of the Gounokawa river on the Shimane Prefecture side, the Ago district, where the residential land raising work has been completed, and the Yakami district (upstream block), where the levee is being developed, were protected from flood damage. This paper reports on the flood damage that occurred in Gounokawa river and the effects of hydraulic control work. In addition, we will introduce river basin management that is being promoted together with the Ministry of Land, Infrastructure, Transport and Tourism, prefectures, cities and towns.

 Comparing the 2020 flood in the Kuma River to the 1965 flood, the inundation of the upstream floodplain has decreased, while the downstream urban areas have suffered from more significant flood damage. In this study, therefore, the decline in the flood retention function of the upstream basin was investigated using the geographical data and the aerial photographs. The result suggested that the intensive farmland improvement on the floodplain and the terrace behind as well as the levee construction along the river possibly caused the decline in the flood retention effects. Considering the change of land condition, numerical simulation of the flooding was carried out on the simplified topography of the basin, and in addition, the effect of an “open levee system” for runoff control was examined using the same topographic model. The calculation results showed that the flood peak discharge was increased with 800 m³/s by the land condition change, but it would be recovered by the open levee system.

 Under the global warming condition, it is a great concern that a large scale linear heavy rainfall system causes severe flood disaster in a wide area. In such a case, it would be important to consider simultaneous floodings from multiple rivers. In this study, numerical inundation simulations were implemented under the consideration of simultaneous levee breaches in Kakehashi River and Tedori River in Ishikawa Prefecture, Japan. Based on the simulation results, water-related disaster risk was evaluated. At the same time, based on simulated inundation depth and flood water velocity at each simulation time, potential window time for evacuation was estimated. In the case of simultaneous levee breaches at the two rivers, in the right bank of the Kakehashi River, the maximum inundation depth became larger than in the case of levee breach at a single river. The window time for evacuation became shorter in the same area. In an area affected by the two rivers, arrival times of inundation flow from the two rivers are usually different. Depending on time lags between floods in two rivers, evacuation window time has to be estimated based on the time when the later flood level reaches the evacuation warning water level.

 In recent years, there has been a tendency toward increased rainfall intensity, and sediment inflow disasters caused by mountain streams are increasing in number. As there is a significantly large number of mountain streams along railway lines, time and effort is spent on local surveys for the purpose of extracting dangerous streams. Additionally, as there is a mixture of diverse factors related to mountain streams, such as terrain, geological features, risk level assessments are differ among technicians. Above all, the risk level assessment of upstream areas located in positions vastly separated from the railway tracks and dispersed over a wide area require a huge amount of time and effort, and there may be a large disparity among engineers. In this paper, therefore, we describe a simple method of evaluating the risk level of mountain stream upstream areas using uniform criteria, without requiring local surveys. In concrete terms, our activity was the development of a scoring sheet that can assess the risk level mainly using the quantitative data of digital evaluation model. It was shown that the evaluation by the scoring table was almost the same as the evaluation by the local surveys.

 The confluence improvement project has progressed, but the management accuracy of the quantitative control facility that is the base in the sewer management is low, and there are still issues in environmental load reduction and heavy rain measure effective in the pipe line system through utilization of stock and will be become problems in terms of maintenance management and project operation in the future. In this study, based on the hydraulic engineering theory and hydraulic experiment verification enabling diversion control with high accuracy, and the previous studies aimed to put them into practical use on extension of the knowledge of sewer standard books and practical cases, there is presented a method for planning a rational sewer pipe line system relating to the environment and safety in an urban area, such as a pollution load in a public water area and a heavy rain measure in a residential area still having issues even after the confluence improvement project. Promoting the efficiency of the sewer project requires reliable sewage flow rate management and the pipe line system through effective utilization of the stock, and the pipe line system utilizing the sewage flow rate control technique of this study contributes to the effective achievement of the project purpose with quantitative evaluation and cost reduction.

 In this paper, an attempt is made to formulate the upstream water level of the flow over weirs with rectangular longitudinal cross-section in order to give basic information for flood manage planning of the river with weir(s). Experimental data by Govinda Rao and Muranlidhar, which gave the discharge coefficient relating the flow pattern over the crest, were selected as base data. Equations for the upstream water level as the functions of the critical depth, hc, were obtained with application limits almost correspondent to those of the formula by Govinda Rao and Muranlidhar. The obtained formulas gave proper evaluation of the upstream water level in spite of the need for a little correction.

 In a two-layer fluid, when pycnocline thickness is negligible, an internal solitary wave (ISW) breaking over a slope can be categorized into four breaker types: surging, collapsing, plunging, and fission breakers. The latest classification into four breaker types is based on wave slope, bottom slope gradient and an internal Reynolds number. However, it remains unsolved if this classification can categorize the breaking of an ISW under thick pycnocline conditions. This study uses numerical simulations to investigate the applicability of the classification under changing pycnocline thickness. We found that the classification can categorize all breaker types even when pycnocline thickness varies. Additionally, thicker pycnoclines result in more significant energy loss and onshore mass transport energy loss due to ISW breaking.

 Long-term records of flood damages are important for studying flood countermeasures and predicting future flood damages. Although the best record of flood damages in Japan is the Flood Statistics Survey started in 1961, there are no long-term statistics which record the magnitude, scale, area, and factors of flood before the survey. In this study, we developed a database on flood damages from 1900 to 1965 with using the flood records prepared by the local meteorological stations and others. This database is the most detailed and long-term records related flood damages in Japan. The results of the database analysis show that major flood events occurred between 1941 and 1945, which have not been recorded in the previous records, and the frequency and intensity of flood events have clearly increased since 1950s.

 The 1-D flood flow calculation method coupled with existing runoff models has a limitation in the accuracy of water level prediction, because of disability to take the effect of the river vegetation resistance into account. This study proposed a method to improve the accuracy of water level calculation during floods by the 1-D flood flow calculation method in the RRI (Rainfall-Runoff-Inundation) model. We derived the surface flux vector using the 2-D diffusion wave approximation from the shallow water flow equation. From 2-D analysis results, it is clarified that the roughness coefficient considering vegetation can be expressed as a function of water depth. The modified RRI model is confirmed to reproduce well the water level hydrograph during a big flood event in the Nuta River basin.

 Future climate change will impact river discharge and its temporal variation as well as biological processes in rivers due to change in temperature. This study incorporated a water temperature model into the Rain-fall-Runoff-Inundation (RRI) model developed by ICHARM, PWRI of Japan and estimated temperature distribution in the Kako River basin, Japan using a model output from a climate model of Japan Meteorological Agency. Additionally, we evaluated the effects of climate change on the habitat and upward migration timing of sweetfish. The incorporated model showed a good performance in the current water temperature simulation by comparing the daily mean and seasonal variations in the river. Water temperature from 2077 to 2095 was projected to increase in annual mean by 2.1°C and by 2.9°C in July compared with those from 1981 to 1999. However, because August precipitation in the future is greater than now, the increase in water temperature by 1.3°C became smaller than July. This increase reduced the habitat area of sweetfish. The stream length where the water temperature more than 28°C lasts 200 hours or longer increased from present 4.3% of the total length to 12.2% in the future. The upstream migration of sweetfish at the river mouth, which peaked in mid-March currently, was estimated to start earlier and peak in late February.

 In this study, the Self-Organizing Map (SOM) was applied to the meteorological data from 1954 to 2020 to classify the patterns of the meteorological field in the South Kyushu region. In addition, the heavy rain indexes of each unit were created by using the analytical precipitation data from 2006 to 2020. As a result of classifying 32,696 meteorological fields into 900 units and 55 groups, heavy rain disasters caused by typhoons were classified into neighboring groups around one group. On the other hand, heavy rain disasters caused by the seasonal rain front were widely classified, but the groups in the heavy rain disasters in the Kuma River basin and the groups in the heavy rain disasters in Kagoshima prefecture were different. By comparing the characteristics of the meteorological fields of the heavy rain of July 2020 and July 1965, which caused flood damages to the Hitoyoshi region of Kumamoto prefecture, the behaviors on the SOM of the meteorological fields corresponding to the time were different. It was observed that both meteoro-logical fields were in the same pattern when the rain was the heaviest.

 We developed a new storage discharge relationship based on water retention curve whose parameters are determined by experiments or observations. The proposed method assumes the vertical equilibrium distribution of pressure head and uses Brooks Corey and Mualem model for estimating water retention curve and unsaturated hydraulic conductivity. The new storage discharge relationship was applied to a distribute runoff model. It was found that the model can reproduce observed discharge with realistic soil parameters. Difference in soil storage effect based on four representative soil types and initial conditions with four durations of no rain appeared clearly. Storage amounts depended on moisture situation in basins and soil porosity.

 One third of Joso City area was under water due to the flood caused by the Kanto-Tohoku torrential rain of September 2015, during which numerous delays and isolation of evacuation were experienced. Considering the above, we started a project of TimeLine by Everybody, aiming at reducing the number of people who fail to evacuate to zero, under which the concept of “My-TimeLine” was developed. To disseminate “My-TimeLine”, we developed the “Nige-Kid” as the teaching tools that help elementary and junior high school students to create their own “My-TimeLine” in less than one hour. In the process of developing the “Nige-Kid”, we conducted disaster resilience education to Joso-City elementary school children of producing their “My-TimeLine” using the prototype “Nige-Kid”. The results of education indicated that the knowledge of flood risk and preparedness were properly enhanced through the consideration process of the “My-TimeLine” using the “Nige-Kid”.

 In 2015, Japan’s Ministry of Land, Infrastructure, Transport and Tourism published the vision for rebuilding a society that is more aware of water-related disaster prevention. It suggests that our entire society should be prepared at all times for water-related disasters, assuming that floods occur. As part of this vision, low-cost and easy-to-install water level measuring devices have been developed for flood monitoring. These water level measuring devices have been installed along the river. And the systems that provide information on river water levels on the website have been launched. Aichi Prefecture has begun installing these water level measuring devices on small and medium-sized rivers managed by Aichi Prefecture. Its purpose is to observe the rivers in flood and to support the evacuation of residents of the flooded area. Aichi Prefecture has been providing information on river water levels on the website since June 2018. Aichi Prefecture and associated cities should use additional new information on river water levels for disaster reduction. In September 2019, Aichi Prefecture and the city of Gamagori began a study on the use of new information on river water levels. Information on water levels of the Ochiai and Nishida rivers in the city of Gamagori has been provided since June 2019. Based on the study, this paper shows the usefulness of this information on water levels to monitor floods and the method to strengthen the system of observing rivers on floods.