In July 2020, the Kyushu region experienced record-breaking heavy rains from July 4 to7, 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. There was also 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. The heavy precipitation scale has been increasing in space and time as well as in rainfall intensity, resulting in spatio-temporal expansion of the damage. Furthermore, in 2020, the disaster occurred amidst the restriction of social activities due to the COVID-19 pandemic. The compilation and dissemination of disaster survey data and lessons are essential for the sustainable development of society. Therefore, JSCE has planned a special issue on the July 2020 torrential rain disaster 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, Japan. This paper calculated the hydrodynamic force acting on one of the bridges washed away 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 less than the bearing strength specified in the current design standard of the 2017 specifications for highway bridges. If the truss was completely blocked by debris, the maximum drag was about twice that of the case without blockage.

 The heavy rainfall in July 2020 caused extensive damage to the Chikugo River system. In this study, we summarized the rainfall and damage in the system. We also conducted a one-dimensional flood flow analysis based on observed water levels. The analysis covered the Chikugo, Kusu, Kuma, Shode, and Kagetsu Rivers in the upper Chikugo River basin, which were inundated by river water. The results showed the following: (1) The amount of heavy rainfall was almost the same as the design magnitude. (2) In the Chikugo River basin, inundation by river water and inundation inside a levee occurred, and road and railroad bridges were washed away. (3) The flow rate and water level at Senjo, Ogase, Kobuchi, Kuma, Kagetsu, and Arase stations were estimated. The water levels of the Chikugo, Shode, and Kusu Rivers rose to or exceeded the design high-water level. The flow rate in the sections where the water level rose to the design high-water level was at or above the flow rate specified in the river improvement plan.

 It is important to quickly identify the actual conditions of flood damage that frequently occur over a wide area and simultaneously provide appropriate rescue, disaster support, and early emergency restoration. This report demonstrates the results during torrential rains that occurred in Kyushu, Japan in July 2020, using satellite observation data, which is one of the methods to assess the damage and is expected to be used to simultaneously identify disasters over a wide area. This report describes the current situation and issues of assessing the actual damage using satellites by performing a reproduction analysis of the past flooding events. In addition, this paper also discusses the results of verifying the use of the wide-area imaging mode, which is considered to be a promising solution to the current problems identified, and the results of the verification and confirmation of the effectiveness of using different bands and different orbits to identify the flooded areas.

 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.

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

 Flooded areas along the Akiyama River were investigated after Typhoon Hagibis in 2019. The areas that experienced severe flood damage and the two levee failures that occurred were all areas where the river became narrower, and both levee failures occurred upstream of the foot of a bridge. The particularly severe damage to the levee on the right bank of the Kairiku Bridge in Akasaka-cho might have been the result of multiple compounding factors, including its location on the outer bank of a curve and risks, such as the angle at which the girders were built relative to the downstream current. The times when the levee failures occurred were determined through interviews with residents of the damaged areas, and a numerical analysis that took each of the relevant conditions into account indicated that the severe damage to downtown Sano City might have been caused not only by the flooding that resulted from the overflow from the Akiyama River and failures of the levees, but also by multiple compounding factors including the effects of the flooding from nearby rivers, such as the Kikusawa River.

 During the flood event caused by Typhoon Hagibis, a large number of trees on the forested gravel bars were destructed in Kumagaya, on the alluvial fan in the Arakawa River. Parts of the trees were trapped in the downstream bars and formed debris dams. The change in the resistance by a debris dam could affect not only the formation of repelling current, but also the flood propagation downstream. A two-dimensional flood simulation model of the destruction and trapped driftwoods was applied to the site, to clarify the changes in flow pattern due to the destructive flood. The maximum velocity was decreased on the gravel bar where debris dams were formed although the velocity was increased by the destruction in the low channel. The tree destruction situation itself was mitigated in the sheltered area of the forest by the debris dam formation. From the point of the maximum water depth at the embankment, the trapping effect was low in the case of Arakawa River, which has a wide cross-section, but it also reduced the peak discharge by around 10m³/s.

 The 2019 Typhoon Hagibis brought heavy rainfall to eastern Japan. In the downstream basin of the Arakawa River, which is located in the Metropolitan area, the water level rose close to the inundation risk level. As flooding is likely to occur in the downstream basin in the event of still heavier rainfall, it is necessary to obtain data on increased rainfall as basic information for the planning of evacuation activities. This paper reports the results of a reproducibility study together with an analysis of changes in rainfall due to manipulation of sea water temperature using a Weather and Research Forecasting (WRF) model and of their impact. The overall shape of hyetograph was reproduced in the study, albeit with differences in peak time and amount of rainfall. In addition, it was verified that the amount of rainfall fluctuates in correspondence with fluctuations in sea surface temperature. The study revealed that this is caused by the effect of the topography of the Arakawa upstream basin, in addition to changes in the course of the typhoon resulting from changes in the atmospheric pressure field and fluctuation of the water vapor flux.

 Land use regulation is a fundamental measure of flood mitigation and climate change adaptation for reducing the amount of exposure in floodplains. In Japan, land use regulation for flood mitigation has been considered in the classification area under the City Planning Act and the Location Normalization Plan under the Act on Special Measures Concerning Urban Reconstruction. However, mitigation effects and operational challenges have not been fully elucidated. In this study, we analyzed the characteristics of flood damage in the urbanization control area in Naganuma District of Nagano City, which was severely damaged by the Chikuma River levee break triggered by Typhoon Hagibis in 2019. The analysis, which adopted building point data, determined that land use regulation by area classification played a role in reducing damage in the area. However, at the community level, it was confirmed that more buildings were located in areas with high flood hazards owing to the development of new houses and businesses associated with the development of infrastructure.

 This study examined the evacuation behavior of two communities—Naganuma and Toyono—in Nagano City, Japan, during typhoon Hagibis. Only two casualties were reported, despite the large size of the inundated area and significant building damage. We analyzed the relationship between (1) the numerical statement of aggregated evacuation behavior, (2) social and hazard circumstances, and (3) the social background, including disaster preparedness plans formulated by the community. The study focused on “the details” of evacuation promotion factors revealed by previous research, rather than “the existence or absence” of such factors. Data were collected from newspaper articles, a post-interview survey conducted by Kansai Television Co. Ltd., minutes from the flood control meetings in the flooded area, and district disaster management plans. The results revealed that the main factors promoting evacuation action differed for the two communities due to their disaster preparedness styles. While Naganuma established a specific (a particular measurement of the water level of the Chikuma River) and socially agreed-upon criterion for evacuation, Toyono had experienced several inland flooding events. It was found that both communities participated in the flood control meetings, formed organizations for voluntary disaster mitigation action, and cultivated common awareness regarding “the sign of a dangerous situation” and “what information is important for evacuation.”

 Typhoon Hagibis caused the serious flood damage on the Naka River, which is one of the major class A 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 of the bank breakage and overflow. In addition, we conducted the runoff analysis for the Naka River Basin 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 Fujii and Tano rivers that are tributaries of the Naka River and 2) the inundation continued for a while (four days) because the drainage capacity was not sufficient. In addition, based on the water level changes of the Naka and Fujii rivers and the traces of the flood flow, it was inferred that the Fujii River was affected by the backwater. Furthermore, the reproduction simulation, which combined the runoff analysis and the flood analysis, reproduced 1) the inundation depth obtained by the field survey and 2) the backwater in the Fuji River.

 In October 2019, a typhoon caused major damage along the Chikuma River, including the structural failure of major roads along the river and of bridge piers. To mitigate flood damage, it is useful to develop a flow pattern evaluation method for detecting high-risk river reaches against bank erosion during a flood. This study focuses on the turbulent energy distribution along the riverbank, which was increased by several flow patterns, such as secondary flows in curved channels, high velocity downstream from a weir, three-dimensional flow around a pier, and water surface fluctuations along the bank — all of which induced bank erosion. The three-dimensional flow structures were investigated using the bottom velocity computation (BVC) method for several instances of bank erosion in three rivers: the Chikuma River, which suffered riverbank erosion damage due to the heavy rainfall brought by Typhoon 19 in October 2019, and the Misasa and Seno Rivers, where massive riverbank erosion occurred due to heavy rain in July 2018. All bank erosions in the reaches in this study are found where the turbulent energy takes a local maximum.

 This study aims to clarify the mechanism of rainfall inundation caused by the 2019 Typhoon Hagibis in Iwate prefecture coastal zone, Japan, using probable one-hour rainfalls estimated for the 2 periods of 1976-2005 and 1989-2018. The differences between rainfalls by the 2019 Typhoon Hagibis and the ten-year return level of one-hour rainfall (P₁₀) of 1976-2005, which is generally assumed as the discharge standard of drainage system in Japan, at Fudai village, Iwaizumi town, Miyako city, and Yamada town were in the ranged of 40.7 mm–106.6 mm for maximum three-hour rainfall during the 2019 Typhoon Hagibis, and the estimated massive rainfall inundations. On the other hand, massive rainfall inundations were also observed at Kamaishi city, although the amount of water overflow from the existing drainage system was only 3.5 mm for a maximum of three-hour rainfall during the 2019 Typhoon Hagibis. These were caused by water overflow at narrow sections in the streams and drainage systems by huge sediment and large wood and debris from landslides dammed up, and by damming up rainfall inundation at coastal levees for tsunami. In addition, P₁₀ at all study areas were increased by 8–24% from 1976-2005 to 1989-2018, which corresponded with increasing frequency of P₁₀ of 1976-2005 by 1.4-2.1 times. Thus, increasing frequency of rainfall inundations in the Iwate prefecture coastal zone is expected in the future.

 Typhoon Hagibis 2019 recorded the highest precipitation in history, in several areas of East Japan. Levee breaching occurred at 7 places in the Arakawa River system. The risk of downstream migration should be discussed when conducting river channel management. For this purpose, it is necessary to understand the inundation between the river and the flood plain. Therefore, the effects of the levee breaching of the Tokigawa River and the timing on flood volume were considered as a first step.

 It was suggested that the levee at the junction section of the Tokigawa River and the Oppegawa River gradually scoured, considering the change in water-level hydrograph by changing the levee breaching timing. In addition, it can be considered that the left levee breaching at the Tokigawa River at 6.5 KP occurred after 10:00 PM on October 12th, noting the change in flood volume by changing the levee breaching timing. The inundation on the right side of the Tokigawa River in the section managed by Saitama Prefecture was caused by the inundation from the Kasumi-tei (open levee); therefore, the levee breaching at the 1.4 KP right levee had little effect on the inundation level.

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

 This study investigated the rainfall characteristics of Typhoon Hagibis (2019) by focusing on the relationship between the typhoon track and the amount of rainfall. Analysis was performed based on the data of past typhoons, Typhoon Hagibis ensemble forecast experiments, and massive ensemble climate data. Each analysis revealed the relationship between typhoon tracks and rainfall, and the potential rainfall amount caused by Typhoon Hagibis based on short-term weather forecasts (several days). The effect of the progression of global warming on rainfall was also evaluated. Analysis based on past typhoon cases suggests that rainfall may have increased at several locations when Typhoon Hagibis shifted to the east. Furthermore, the amount of rainfall has tended to increase regardless of the typhoon track since the progression of global warming. Moreover, at the Chichibu weather station, the amount of rainfall was similar to that in conventional weather, even with a 2° shift toward the east or west direction from the track, similar to Typhoon Hagibis.

 The 2019 typhoon No. 19 hit Japan on October 12. It caused severe damage to Kanto-koshin and Tohoku areas upon landing. The authors conducted damage investigations on all the breached sections of river dikes in Ibaraki prefecture in total five times after the disaster occurred. The survey results showed that: 1) the breach of river dikes in Ibaraki prefecture might be attributed to the erosion of levee slopes caused by the overflow; 2) no signs of soil ejection and piping were 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 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%.

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

 This report investigates the characteristics of heavy rainfall and flood inundation area due to heavy rain caused by Typhoon No. 19 in 2019. We obtained data on rainfall, inundated areas, and topographic characteristics of the affected area. We also examined the relationship among them. This study confirmed the following: (1) In prefecture-managed rivers, inundation occurred due to insufficient flow capacity within areas of heavy rainfall. Furthermore, reduced flow capacity was caused by rising water levels in the main river at the confluence. (2) In rivers under the jurisdiction of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), rainwater due to heavy rain upstream flowed into the river channel and downstream. It caused the water level in the river channel to rise in the downstream area where rainfall was relatively low. The rise of the water level caused inundation with levee failure. (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 was used as a field and where the inundation flows were stored, may have served as a flood-retarding basin.

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

 Typhoon 19, which landed on the Japanese Islands on October 12, 2019, brought record-breaking heavy rain to the Kanto-Koshin and Tohoku regions. In the Chikuma River basin of the Shinano River system, severe damage to bridges and large-scale inundation occured due to a levee breach occurred. On October 23 and 24, 2019, about 10 days after the disaster, we carried out a field survey at the damaged sites of river structures in the Chikuma River. The investigation techniques used field exploration and aerial photography by Unmanned Aerial Vehicle (UAV). This paper reports the damage situations obtained from the field survey at the sites where the abutment backfills were washed away and the levee breach occurred. It also cites issues for future consideration.

 Semi-underground structures are often constructed for storing water. Most parts of their bodies are placed underground and the roofs are covered with thin or no soil layers. The current earthquake design standard for semi-underground structures mainly considers the earth pressure, which is proportional to ground acceleration and displacement. Detailed mechanisms of active and passive earth pressures are not considered in the current design method. In this paper, we study the dynamic interaction mechanism between ground and side wall of semi-underground structures during earthquakes based on FEM analysis. Earth pressures during and after the earthquake, which result from shear-failures in the surrounding soil, are especially examined. The results show that effects of soil failure enlarge the earth pressure. Passive earth pressure only acts on the shallow part of the wall, whereas active earth pressure acts on the deeper part as well, during earthquake. The residual earth pressure acts after earthquake, which is intermediate between the maximum and minimum values during earthquake.

 In this paper, a new type of foundation composed of micropiles and soilbags is proposed. The foundation is characterized by laying soilbags between the pile head and footing on which a superstructure stands. The expected effects of the foundation are: (1) eliminating the fixed junction between the piles and the footing, (2) reducing the diameter of piles, and (3) reducing the response acceleration of structures. In this study, a seismic response evaluation method for the proposed structure in real-scale is also proposed, and the seismic responses of the proposed structure and pile foundation are compared with different conditions of input acceleration and height of pier. From the calculation results, except for the case where the pier height was low, it was found that, compared to a pile foundation, the prolonged period due to the uplifting of footing during rotation may cut off peak response acceleration of the pier and suppress the bending moment at the base of the pier. It was also found that the response displacement of the proposed structure was almost the same as that of the pile foundation, and that the residual displacement of the proposed structure was smaller than that of the pile foundation. Moreover, it became clear that laying the soilbags could suppresses the bending moment of piles.

 In this paper, we present an alternative formulation of the plastic flow rule using a yield function, to facilitate the understanding of the rule and to possibly provide a clue to improve the stability of the numerical analysis of elasto-plastic problems. Without making any physical assumptions, such as the decomposition of strain rate or the form of plastic strain rate, the alternative formulation produces an elasto-plastic tensor consistent with the conventional formulation and governing equations for displacement rather than displacement rate. In addition, the treatment of hardening is studied, and its potential application to numerical analysis for the improvement of stability is discussed.

 In this paper, the axial-force sharing of cross-sectional loss in steel members repaired by patch plates with high-strength bolts under axial loading is estimated by applying the displacement compatibility conditions between cross-sectional loss steel members and patch plates between the distance of high-strength bolts near the defects. The estimated axial-forces sharing of cross-sectional loss in steel members with patch repair are in good agreement with that calculated by the finite element result, as well as that given by the tensile test results. Additionally, when the cross-sectional loss was deep and its length short, it fractured at the defective area even after the repair by the patch plates.

 In recent years, there have been many reports of bridge fire incidents caused by either accidental or deliberate actions, vehicle accidents, etc., in Japan and overseas. When bridges are exposed to fire, it is necessary to promptly and accurately judge the state of damage and serviceability, and to aim to open bridges as soon as possible. However, a diagnostic method for fire-exposed bridges have not been established yet, and reports and research data on steel bridge fires are inadequate. In this study, a parametric analysis was carried out assuming that fire occurs in a riverbed below the steel girder through numerical simulation by CFD and heat transfer analysis by FEM. Based on the maximum temperature characteristics in the steel bridge subjected to a fire, which are obtained from the analysis, a simple estimation model of the maximum temperature of the steel bridge is proposed.

 Temporal and spatial variations in tsunami coda waves in medium- to long-term decay processes were investigated based on theoretical analysis and observations. In this paper, medium- and long-term were defined as approximately τ ≤ t ≤ 2τ and t ≥ 2τ, respectively, where t is elapsed time from the earthquake origin time, and τ is the coda energy e-folding decay time. In the medium- to long-term decay processes, the energy and amplitude of tsunami coda waves may become small so that the energy dissipation due to turbulent friction can be negligible compared to the directional spectrum of coda waves. It was found that the energy balance equation without dissipation for the directional spectrum, D(f,θ), should be approximately satisfied in a quasi-steady state, and the quantity CC_gD(f,θ) should hold for any two points, where C and C_g are the wave phase speed and group velocity, respectively. The latter relationship indicates the inverse proportion of the tsunami coda energy to the water depth, and the similarity of the frequency spectrum of coda waves between any two points. The water depth dependency of tsunami coda was confirmed over a wide frequency range by using the 2011 Tohoku Earthquake Tsunami data. However, the tsunami coda energy with periods of 30–70 min did not follow the h^-1 law, because continental seiche and edge waves were generated at Kochi, and the amplitude of tsunami coda waves at other stations did not show exponential decay in time. The estimated frequency spectra of coda waves at depths larger than 100 m were found to be inversely proportional to frequency squared, f², for a wide frequency range. The f^-2 law of frequency spectrum was theoretically derived by using linear shallow water equations with linear friction and dispersion terms. The effects of linear friction and dispersion on the frequency spectrum were deduced from the spectral solutions.

 In urban rivers where combined sewer overflows occur, scum appears several days after rainfall, causing bad odor and adverse effects on the landscape and ecosystem. To clarify the generation and floating behavior of scum, we conducted a detailed observations of scum using multiple fixed-point cameras and sediment surveys in the Hirano River, an urban tidal river in Osaka, Japan. Organic mud derived from sewage was deposited in two separate areas, of which the upstream was the main source of scum in the river. The floating behavior of scum was mainly dominated by tide, and scum accumulated at the stagnant point at rising tide. Scum was frequently observed at the bend of the river channel, suggesting that the local flow structure had a considerable influence on the behavior of scum over a wide area of the river.

 This study developed an autoregression-driven deep neural network model using deep learning techniques and nonlinear time-series analysis to estimate water quality variations in coastal areas. This local prediction model analyzes the autoregression characteristics of the nonlinear water quality system combined with an extrinsic deep learning model to express the relationship between water quality items and external factors such as tides and weather.

 By combining both models, two water quality items were estimated: the electrical conductivity in a tidal river and the dissolved oxygen concentration in the bottom layer of an enclosed bay. Both models showed high accuracy in estimating the two water quality items. However, the autoregression-driven deep learning model was superior, particularly for water quality items affected by several internal state variables, such as physical, biological, and chemical processes.

 We improved the fundamental equation and the data assimilation algorithm of the DIEX-Flood, which can estimate the longitudinal profile of the water level in the current and future situation in order to reduce the computational cost and maintain the robustness of the model. As a result of applying the improved DIEX-Flood to some flood events of the Kinu River, we can simulate the longitudinal profile of the water level in the current situation with high accuracy, good stability and low computational cost. Furthermore, we proposed a new flood forecasting method combining the DIEX-Flood and deep learning. This prediction method can calculate the longitudinal profile of the water level in the future, so it makes the evaluation of the spatial and temporal distribution of flood risk possible. As a result of the flood forecast simulation using this system, we can predict water level profiles 6 hours ahead with high accuracy.

 This study carried out a transient wave overtopping and overflow modeling to improve the accuracy of simulated inundation due to extremely large storm surges, like in the worst-case scenario of a tsunami. Preparing an inundation hazard map for such extreme storm surges is essential for risk management and evacuation plan. Since existing studies considering the transient phenomena of wave overtopping and overflow are scarce, the present study proposes such modeling using two kinds of wave overtopping models that integrate wave runup and overtopping. The present model can be applied to seawalls from vertical and gentle slopes installed at deep water to very shallow water, even on land. The combined wave overtopping and overflow model was implemented into a wave-surge coupling simulation model to show the importance of the transient phenomena in inundation for simple cases of typical bathymetry and typhoon conditions.

 A one-dimensional two-layer model was developed in order to examine the hydrodynamics of rubble mound weirs. First, depending on the water depth relative to the weir height, the flow was classified into three regimes: (a) “Regime-S, ” in which the weir was submerged along the whole reach, (b) “Regime-P, ” in which the weir was submerged only in the upstream reach, and (c) “Regime-E, ” where the weir was emerged in the whole reach. Second, each regime was further grouped under two sub-categories: (i) “B-flow” that is controlled by backwater from downstream of the weir, and (ii) “C-flow” that is controlled by a critical hydraulic depth appearing at the weir exit. Theoretical solutions for water surface profile and discharge rating curve were obtained for the six flow regimes and were validated in comparison with experimental data.

 Sediment inflows originating from mountain streams have caused railway disaster. In the past, there had been accidents where trains ran over some sand formations and were derailed. It takes a lot of time to evaluate risks posed by mountain streams, because there are a huge number of mountain streams along the railway, and those basin areas are outside the railway sites. And, there is a possibility that variations in investigators’ evaluations are possible because mountain streams are influenced by various factors, such as topography and devastation condition of the riverbed or the riverside slope. Therefore, creating a method that can efficiently and effectively evaluate the risks posed by mountain streams to railways has become an important task. This paper proposes a method for evaluating the risk of small sediment inflows from mountain streams in the vicinity of railways, using data gathered from simple surveys. Specifically, it shows a scoring table for evaluating the risks posed by mountain streams. The table was created using statistical analysis based on data involving disaster and non-disaster mountain streams.

 Energy loss in a manhole as a junction of storm sewers or combined sewers is an important factor for more accurate calculation of inundation. The authors formulated energy loss coefficients in two-way and three-way circular drop manholes where one or two inflow pipes are connected to an outflow pipe. Since there are few studies on the four-way manhole where three inflow pipes are connected to an outflow pipe like one at an intersection, we conducted experiments on energy loss in a four-way circular manhole to which three inflow pipes and an outflow pipe with several types of inner diameter of the pipes and flow rate. We successfully obtained energy loss characteristics in the manhole through the experiments. Furthermore, we developed formulas for the energy loss coefficients on the manhole, considering structural elements and hydraulic factors on pipe and manhole. The results calculated with the formulae were in fair agreement with the measured values.

 This study investigated sediment release from the bottom outlet of a dam using 3D numerical simulations of turbidity current and bed deformation. A numerical model was developed by customizing the opensource computational fluid dynamics library OpenFOAM. The erosion of cohesive sediment was modeled based on previous experimental results. The numerical model was validated using field measurements of sediment release. The field data obtained after the sediment release operation indicated the existence of a scour hole expanding laterally in front of the entrance of the bottom outlet. The numerical simulations accurately reproduced this scour hole. The concentration of turbidity (suspended solids, SS) was also quantitatively accurately simulated. The contribution of the scouring process to SS was evaluated based on the results of the simulations. The numerical simulations revealed that in some time intervals, sediment is temporarily efficiently eroded during sediment release. The developed numerical model and findings of this study should be useful for optimizing the operation of sediment release gates to mitigate reservoir sedimentation by cohesive sediments.

 Some rainfall disasters in railways are caused by sediment inflows, which originate from mountain streams far from the railway. Thus, it is necessary to calculate the rain that falls in mountain stream basins, as these data can be used to regulate train operations during rainfall. At present, the railway rain gauges installed to control train operations are too far apart to monitor localized areas of heavy rain. For this reason, the Japan Railway Company is considering using analyzed rainfall data to control train operations during heavy rainfall. This study clarifies the size of mountain stream basin areas and the distance between the railroad tracks and the farthest side of a basin based on the previous sediment inflow disasters to explore rainfall distance dependency. As these results show, the distance between the railway and the farthest part of the basin is approximately 1.3 km. At this distance, there is a high correlation between the amount of rain that falls in the basin and in the mesh directly above the railroad tracks. The findings show that it is sufficient to analyze the rainfall directly above the maintenance object.

 This paper presents the inverse analysis of boundary conditions and parameters of river flow. The adjoint variable method is adopted for data assimilation for weather forecasting and is found to improve forecasting accuracy. The adjoint equation and sensitivity were derived for one-dimensional unsteady flow, a numerical simulation method was illustrated, and the applicability of the method to an actual river was verified. Data assimilation using multi-point water gauges successfully estimated discharge at any point and the accuracy changed with the number of water gauges. In the case of a river channel network, the data assimilation results also showed high accuracy. Furthermore, the forecasting simulation using the assimilation results as initial values showed highly accurate predicted water levels up to two hours in advance. The data assimilation method was then applied for channel shape optimization. In the optimization of the channel shape, considering two cases of riverbed excavation and channel widening where the river water level was below the levee height, the inverse analysis was successfully applied to determine the optimized channel shape via one-time simulation.

 Since the Great East Japan Earthquake, the mound-type tsunami evacuation facility has become crucial because evacuees to this type of facility survived the disaster caused by the tsunami. Based on the examined results in Fukuroi City, Shizuoka Prefecture, a method for designing mound-type tsunami evacuation facility has been developed. In this design method, the inundation depth estimated from the tsunami simulation was applied to examine the crest height and the stability of the facility. A high-resolution analysis based on the tsunami simulation was implemented to model the surrounding houses as terrain. We believe that runup will rise in the tsunami evacuation facility, judging from the positional relationship between the tsunami evacuation facility and the surrounding houses, which cause flow inhibition. This paper reports the examination of the separation distance between tsunami evacuation facility and the surrounding houses.

 Although tidal flats with good natural environment spread on both banksides in the lower reach of the Yoshii River, the tidal flat area considerably decreased owing to the river bank and dam construction works. The Otogo tidal flat is the remaining tidal area in the Yoshii river estuary. This study proposes a candidate site for tidal flat creation in an estuary depending on numerical analysis results. First, physical environment of the Otogo tidal flat was clarified by examining the observed salinity data and numerically obtained salinity and hydraulic data. Second, a promising site for creating a new tidal flat, which has a physical environment similar to that of the Otogo tidal flat was investigated and proposed on the left bank side of 4-4.6 km from the river mouth, which is approximately 2 km upstream of the Otogo tidal flat. Finally, some numerical experiments were conducted assuming the sediment replenishment and construction of new groynes. The results show that the extracted site has a similar sustainability with the Otogo tidal flat against the flood flow with several years return period.

 Air entrainment phenomena often occur in hydraulic structures such as the head-tank spillway of hydro power plants and urban stormwater systems. When the air–water mixture flow is discharged to the submerged downstream tank, both shock and noise occur owing to the instantaneous release of elongated bubbles. In other words, the air hammer phenomenon occurs. This study considered two-phase air–water slug flow with air hammer phenomena in a horizontal circular pipe with an internal diameter of 100 mm, and investigated the influence of the computational grid size on the length and velocity of the elongated bubbles, the energy loss, and the pressure fluctuation to contribute to the quantitative evaluation of the air hammer phenomenon using computational fluid dynamics (CFD). The numerical results were validated by experimental results.

 An emulator was developed for a runoff inundation model predicting the range of inundation using a machine-learning method based on the results of a rainfall-runoff inundation model experiment. The 1500-year equivalent of the current reproduction results of large-scale ensemble climate experiment data was used as input. The model was validated using the Omono River basin, which has frequently flooded in the past, as an example. It was found that the machine-learning method could reproduce the experimental results of the model with approximately 80%–90% accuracy for the major inundation areas near the river channel. Comparing the results of inputting the actual observed rainfall during the July 2017 floods into the emulator obtained by machine learning shows that the prediction results were comparable to those of the experiments using the rainfall-runoff inundation model. However, the accuracy of the reproduction varied depending on the machine-learning method.

 This paper proposes fragility curves for asset damage based on questionnaire replies from companies that were affected by floods after the heavy rain event of July 2018 in Japan. 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 in the Manual for Economic Evaluation of Flood Control Investments. These results may reflect the progress of the flood control and the Business Continuity Plan (BCP) of companies; thus, it is important to verify the validity of the evaluation model while continuously updating it.

 We developed and validated artificial neural networks (ANNs) to map the streamflow characteristics in the most upstream basins throughout Japan. The ANNs output mean annual runoff height (QMEAN) and percentiles of daily streamflow, including nine different groups, by inputting basin characteristics, including climate, land use, soils, geology, and topography. The generalization performances of the ANNs showed R² = 0.70 in the QMEAN and R² = 0.20 – 0.74 in the streamflow percentiles. We succeeded in mapping the streamflow characteristics in the most upstream basins throughout Japan, which reflected the rainfall and snowfall characteristics in the country. The streamflow characteristic maps revealed that developing run-of-river hydropower stations in heavy snowfall areas, such as the Tohoku and Hokuriku regions facing the Sea of Japan, is suitable.

 Seepage analysis models have been estimated based on field measurement data using the merging particle filter, which is a data assimilation method. In previous studies, one-dimensional seepage analysis models were assumed, and unsaturated seepage behaviors were simulated by setting the free drainage boundary condition at the bottom of the one-dimensional models. During heavy rain, however, owing to the rising groundwater level at the field slope, the simulation setting of the free drainage boundary condition could not reproduce the field measurement data. In this study, the authors propose a new method for estimating the seepage analysis model, including the boundary condition at the bottom of the one-dimensional model using the merging particle filter. The applicability of the proposed method is also discussed. Using the proposed method, the estimated seepage analysis model could reproduce the measurement data of not only numerical experiments but also the field slope. Therefore, the ability of the proposed method to simulate unsaturated conditions under saturated conditions was verified.

 Corrosion due to insufficient grout filling can result in a sudden fracture of PC steel tendons. When a vertically tightened PC tendon in PC girders is ruptured, all the accumulated strain energy is suddenly released. The ruptured PC bar will severely damage cover concrete and asphalt pavement, and PC bars may protrude out of the structure, which may cause severe accidents. In the current study, the effects of 15 mm cover concrete and two-layered asphalt pavement system on preventing protrusion of PC bars were investigated. Numerical simulation of rupture and protrusion of PC bars were studied using the Applied Element Method. The numerical simulations were verified based on the experimental results. In the process of verification, many influential parameters, such as the effects of contact stiffness between elements, fracture energy of concrete, mesh sensitivity, time interval sensitivity, material properties, and strain rate effect, were investigated. This study found that AEM numerical simulation with appropriate modeling showed good agreement with the experimental results, which exhibited the effectiveness of the asphalt pavement system with appropriate material and thickness preventing protrusion of the PC bar tendon.

 This study presents the development and application of an autonomous program for road surface inspection using Laser Crack Measurement System (LCMS) images acquired by a road surface property measurement vehicle. The program automatically detects road lane line marks and makes grid mesh of analyzed parts. It applies a robust color space transformation and Hough transformation algorithms for the detection of analysis area between two lane lines. The program utilizes a deep convolutional neural network (CNN) to automatically detect crack, non-crack, and patch parts on a high-resolution road surface image. Transfer learning and fine-tuning parameters were employed during CNN model training for the image classification. The model accuracy for the classification of three classes was 97.9%. The precision for each class, crack, non-crack and patch classes were 99.7%, 97.9%, and 95.5%, respectively. The study reveals the novelty of transfer learning application, and the importance of data preparation in CNN model training for road surface image inspection. The program is automated for image importation, detection of the analysis area, analysis visualization, and summary of the crack and patch parts. It significantly improves road inspection efficiency. Compared to the inspection time of a professional engineer in 1-km road surface images with a crack ratio of 20%, the program shortens inspection time by 98% for lane line detection and marking grid mesh, and road damage judgment by 82%. Total inspection time of 1-km road is shortened by 50%. This study opens the potential application of AI on road inspection.

 In the New Austrian Tunneling Method (NATM) of tunnel design and construction, it is important to know the actual compressive strength of lining concrete at early ages, which determines the time of demolding and its future durability. The degree of hydration reaction of in-place concrete is regarded as a useful reference to estimate the development of the strength of concrete. Based on the concept of On-Site Visualization, this paper presents a new approach to monitoring the hardening progress of fresh concrete/mortar using plastic optical fiber (POF) sensors. The principle of sensing is illustrated conceptually. The fundamental experiments were carried out to investigate the change in the visible light intensity reflected from the sensing plane of POF sensor during the hydration of concrete/mortar at early ages. The typical influence factors were also studied. The hydration process of concrete/mortar at early ages was well observed by the POF sensors. The direction of POF sensor installation had a significant effect on the pattern of light intensity change. When the sensor was installed pointing downwards, the light intensity dropped at first but increased rapidly in a few hours, which indicated the phase evolution when the free water disappeared in the hardening concrete/mortar. While the sensor was set up pointing upwards, the light intensity dropped in the first stage and then grew up slowly with time. Furthermore, the hydration process of concrete was characterized by introducing the nominal refractive index (NRI), which could be calculated from the measured light intensity. Thus, the hydration of concrete could be divided into three stages approximately according to the curve of NRI of concrete (when the sensor was installed pointing upwards), which coincided with the common knowledge of the concrete hydration process. Finally, the future work is discussed briefly.

 The design of support structures for Japanese mountain tunnels is difficult owing to the limitations in investigating techniques and the complexity of the geological structures involved. In general, during excavation, the size of a support structure is modified to best suit the geological conditions. As such, engineers must carefully observe the rock mass conditions to understand their mechanical and hydraulic properties as accurately as possible, and to predict their deformation behavior and hydraulic structure. In recent years, artificial intelligence (AI) has been adopted in various fields. In this study, a deep neural network (DNN) was applied to evaluate a mountain tunnel’s rock mass. The input was a photo of the excavation surface (face) of the mountain tunnel, and the output was the rock mass properties such as degree of weathering, alteration, and fracture. Based on past excavation records, the DNN was tested using supervised learning, and the results showed that the AI judgments were consistent with the engineers’ judgments, having a 73%–97% accurate answer rate. Therefore, practically applying the method of rock mass evaluation using AI was determined as being feasible. Furthermore, to allow ease in its field-based application, a cloud computer system using a tablet computer device was used to enable evaluations, creating a system that contributed to increased productivity.

 The volume of radioactive cesium-contaminated soil being removed during the cleanup process after the explosion of Fukushima nuclear power plant 1 is estimated to be 13 million m³. Soil contaminated with more than 20,000 Bq/kg of radioactivity is targeted for decontamination by the particle-size separation method to pass the criteria for reuse in Japan. Because more than 80% of the targeted soil is composed of fine silt and clay derived from farmland, a system was developed to increase the amount of recyclable soil by a combination of mud crushing and fine-particle separation with the classification point set lower than about 20 μm. Mud crushing is one of the key technologies for effective decontamination because clay forms aggregates due to the presence of organics such as humus. Four types of mud-crushing equipment were examined on site, followed by pebble milling or ultrasonic dispersion, both of which showed promising results. Using the selected equipment, the proposed soil-size separation was carried out. Overall, the proposed system was proven effective by the results of radioactivity reduction ratio and volume of soil for recycling. Furthermore, lab-scale pebble mill tests using a pot-type wet mill were carried out to confirm the basic characteristics of mud crushing for organic aggregates.