As rainfalls exceeding the designed level have increased, so has damage associated with pluvial flooding. Typhoon Hagibis, which swept Japan in 2019, left 140 municipalities in 15 prefectures scarred from flooding. The sewage networks damaged by the typhoon affected civic life by paralyzing urban functions, raising concerns in urban flood risk and management. Increases in heavy rainfall events associated with global climate change are expected to increase damage from pluvial flooding, thereby necessitating reviews of current urban flood risk management for the purpose of making further improvements against future threats. As we enter an era of frequent urban flooding, it is vitally important that we prepare for urban flood risk management by sharing scientific and technological knowledge among academics, private companies, and administrators.

In this context, the current issue is a compilation of contemporary research studies in academia, technological advances in private companies, and practical applications in public administrations in Japan. The works include: the application of urban flood modeling in safe evacuation strategies, the assessment of economic loss, and the impact of climate change; state of the art technologies for urban flood management with the Internet of Things (IoT) and Internet Communication Technology (ICT), Unmanned Aerial Vehicles (UAV), and the next generation of weather radars; and best practices for flood countermeasures, based on knowledge and experience from historical flooding and applied in prefectural governments and local municipalities.

We are grateful to all the authors and reviewers who contributed to this special issue, and we hope that it may internationally enhance knowledge-exchange in preparation for growing urban flood risks.

The series of annual flood disasters that struck Japan in recent years pose challenges to urban stormwater management. Japan has been implementing nation-wide hydrometeorological observation through a dense network of rain gauges. Since the recent decade, ground radars have been deployed to observe heavy rainfall with high spatiotemporal resolution as a countermeasure. While commercial software is popular in designing stormwater drainage systems, several integrated urban flood models have been developed domestically and are applicable in stormwater management. A paradigm shift with the rise of Internet of Things (IoT) provides an inexperienced opportunity in hydrological observation, and has been implemented for monitoring sewer network conditions. Despite this broad scope of research works and technological innovations, such advancement is not internationally recognized yet. The present study aims to review the development and role of science and technology in stormwater management in Japan, focusing specifically on rainfall observation, integrated urban flood modelling, and emerging technologies for stormwater monitoring. In addition, the possible future direction of stormwater management is envisioned. Considering the series of record-breaking rainfall events that struck Japan, we will have to face more severe challenges in urban flood management alongside the impact of global climate change. As compared to structural measures, which are subject to budgetary constraints, the relative importance of non-structural measures is increasing; therefore, effective application of numerical modeling techniques is required. A common weakness of the urban flood modeling framework is the limited availability of observations in sewer networks, which can be relaxed by emerging IoT based observations. The fusion of IoT based observations with an integrated urban flood modeling technique appears to the emerging technology for stormwater management.

In urban areas of Japan, there are numerous underground spaces, such as subways and shopping malls. These areas hold the possibility of being inundated not only above the ground but also underground when intense rainfall causes pluvial flooding. For this reason, it is necessary to take measures to anticipate urban floods. In this study, an analysis of inundation on the ground surface and underground spaces was performed, and the risk of underground inundation was evaluated using the index of safe evacuation. In addition, as one of the countermeasures against underground inundation, the effect of flood boards on inundation prevention was examined. Furthermore, the factors for improving the success rate of evacuation are discussed by simulating the evacuation of users from an underground mall during inundation. From the results, the water inflow to underground spaces was larger as rainfall was greater, and the reduction effect of flood boards on underground inundation was only temporary if the inflow to the underground mall was large. In addition, the success rate of evacuation depended on the timing of evacuation because the flooded water spread broadly and rapidly across the underground mall. These results show that users in an underground mall should move with a quick response to flash flood from high intensity rainfall and would be useful in creating a safe and prompt evacuation plan.

In Japan, flood disasters caused by record-breaking heavy rainfall frequently cause significant damages. It is also great concern that heavy rainfall may increase and occur more frequently due to global warming. In July 2013, a heavy rainfall event caused record-flooding of the Kakehashi River in Ishikawa Prefecture. In this study, pseudo global warming (PGW) experiments were implemented for the heavy rainfall in 1998 and 2013 around the Kakehashi River basin. Based on the results of PGW simulations, rainfall with different return periods were generated. Runoff analyses and inundation simulations were carried out by forcings with multiple return periods, and the results were used to estimate the economic losses due to flood inundation. Expected values of the economic losses were calculated using two methods for multiple return periods. Differences between the two expected values indicates the importance of the weighting method for the result of each return period. In addition, variations of spatial distribution of houses and firms in a city (i.e., urban structure) were simulated using a computable urban economics (CUE) model for the area of middle-lower reach of the Kakehashi River basin to examine its impact on economic loss due to flooding. In the simulation using the CUE model, a more severe flood inundation risk and an additional insurance burden for general households were added, and possible variations of urban structure were estimated around the lower part of the Kakehashi River basin. Under the more severe risk condition, relocation proceeded from higher risk areas to safer areas, and possible economic losses decreased in the target area. This result indicates that proper recognition of risk can reduce flood damages. On the other hand, there were small variations in economic losses under the condition with the additional flood insurance burden.

Heavy rain and river flooding due to Typhoon No. 19 in October 2019 led to overflow and a dike breach on the left bank of the Chikuma River that caused large-scale inundation damage in Nagano City, Japan. To devise countermeasures, an inundation analysis model is an important tool. In this study, an inundation analysis model was developed to examine the inundation water behavior. The calculated inundation water depth and inundation area showed good agreement with the observed inundation water depth and the inundated area, confirming the validity of the analysis model. In addition, temporal changes of the inundation state were calculated considering the drainage process. However, the sewerage system, waterway, and drainage pump car were not taken into consideration in this analysis, and future issues for model improvement were also revealed. In addition, an analysis model with a 2 m grid was developed in the dike breach site, and the inundation water flow on roads and the fluid force around houses were obtained after taking into consideration the effect of houses. In paticular, the calculated value of the specific force exerted on damaged houses was very high. Moreover, it was proposed that house hazard should be evaluated while taking into consideration the loss of houses around the dike.

Climate change is believed to have increased the intensity and frequency of heavy rainfall, and also to have caused sea level rises over this century and beyond. There is widespread concern that small-island nations are particularly vulnerable to increasing risk of inland flood due to such climate change. Understanding the impact of climate change on flood hazard is of great importance for these countries for the development of better protection and adaptation strategies. This study conducted a case study focusing on the impact of climate change on flood hazard at Faleolo International Airport (FIA), Samoa. FIA is a typical small islands airport, located on the lowland along the coast fronted by a fringing reef. Annual maximum daily rainfalls for different return periods were first estimated for the present and future climate around FIA. The estimated rainfalls were input as the forcing of a two-dimensional flood inundation model to investigate the flooding behavior and effectiveness of probable drainage systems. Results showed that a part of the runway can be inundated under heavy rainfall. Construction of drainage pipes significantly contributes to reducing the flood hazard level. Sensitivity analysis showed that the astronomical tide level has relatively little influence on the performance of the drainage system, while the combination of sea level rise and the sea level anomaly induced by stormy waves on the fringing reef could have non-negligible impacts on the drainage system. Location of the drainage pipe is also important to effectively mitigate flooding. The time-concentration of torrential rainfall may also significantly impact the overall performance of the drainage system.

The importance of preventing damage from pluvial flooding has been increasing under global climate change. The discovery of premonitory symptoms of pluvial flooding enables effective evacuation and inundation prevention activities. However, apparatuses that automatically detect this in real time are not widespread. There are difficulties in the cost of installing them and the agreements made by the parties concerned, especially in cities. To solve this problem, we devised an apparatus to be installed inside a catch basin that detects its water level. The water level in the catch basin may indicate a sign of pluvial flooding, and the number of people involved in operating the catch basin is smaller than that of facilities on the ground. In order to reduce the cost of installation and operation, we adopted Low Power Wide Area (LPWA), which is a communication method that enables wireless transmission of detected information over long distances for a long time using batteries. So far, for catch basins, a wireless transmission experiment was conducted using LoRa, which is part of LPWA. However, Sigfox, which uses the same frequency as LoRa but has a different wireless system, has not been verified. In this study, the reliability of wireless communication was assessed by apparatuses using LoRa and Sigfox side by side in each catch basin in two places in a densely populated city. The number of experiment days and transmissions differed depending on the apparatus, with the number of days ranging from 97–151 and the number of transmissions from 2328–3748. The reliability in the experiment ranged from 99.97–99.53%. The experimental results showed that wireless transmission was possible with high reliability using either the LoRa or Sigfox system from inside these catch basins. This study expands the options for communication infrastructure that can be used for apparatuses that detect premonitory symptoms of pluvial flooding. This will enable a reduction in installation costs and will expand the range of areas of potential installation.

Integrated flood models have been previously developed to simulate diverse inundation situations and combined with models of storm surges and river floods. However, drainage systems, ground elevation, and surface information of human settlements have only been digitized in large cities. Digitization of surface information is essential for developing a drainage system model for pluvial flood analysis. Occasionally, suburban drainage areas exhibit various complex land-use conditions, including residential and green areas, agricultural land with drainage, and irrigation channels. Herein, UAV photogrammetry was applied to obtain high-resolution geospatial data associated with small-scale flood-prone areas whose elevation, land-use, and waterway networks have not been elucidated sufficiently. The resolution of elevation and land-use data ranged from 2.61–5.22 cm/mesh. The measurement accuracy of the width and depth of the open channels was high, and the errors were mostly within 10%. A drainage system model was developed using data on open channel, elevation, and land-use.

Compared to pumping stations, pump gates have small sites and are constructed for a low cost over a short period of time. A new type of horizontal axial submersible pump that is optimal for pump gates has recently been developed. Since the new type of pump can be operated at full speed regardless of water level, it is possible to conduct standby operations in preparation for stormwater runoff inflows. In a simulation comparing the conventional pumps and the new type of pump, there was no serious inundation condition in the new pump up to 70 mm/h, which is 140% of the design’s rainfall intensity. As a result, it was clarified that standby operations can keep the peak water level low and maximize the water storage capacity of the drainage channel.

The frequency of localized short-term torrential rains that exceed the planned rainfall is increasing along with inundation damage due to inland flooding. Stepwise inundation measures utilizing existing stock and disaster prevention/mitigation for excessive rainfall are required. In this study, we describe the results of empirical research using a statistical model constructed based on rainfall and water level observation data as a highly accurate water level prediction method suitable for real-time prediction. This is aimed at application in flood control activities and operation support of pump facilities. By comparing and verifying the prediction accuracy between the water level prediction model and the statistical model by Convolutional Neural Network (CNN), which is generally used as an image recognition technology, the usefulness of the statistical model was confirmed. Further improvement in accuracy and widespread use of these water level prediction models are expected.

This study was conducted to develop and evaluate the prediction accuracy and effectiveness of “ICT operation support system for urban flood control facilities,” which is installed in Eba catchment area in Hiroshima city, Japan. This system consists of real-time facilities monitoring technology, rainfall data from eXtended RAdar Information Network (XRAIN), and Real-time flood prediction technology. High prediction accuracy is crucial for effective control facility management using ICT operation support system. In this study, ICT operation support system was installed for effective operation of stormwater pump. The prediction accuracy and effectiveness of this system were evaluated based on the past rainfall record from XRAIN. The system takes only three minutes to distribute the urban flood prediction information, and it has been operated stably during the operation. Flood risk reduction effect can be better expected in case of central concentrated rainfall pattern.

We developed a system that combines urban area rainfall radar (small X-band, dual-polarization radar), short-term rainfall prediction model, and real-time runoff analysis technology, and the demonstration study was conducted on the drainage districts in Fukui City and Toyama City. We demonstrated the effectiveness of the flood damage, by providing the real-time information on rainfall prediction, water level in sewerage pipes, and inland flood prediction to the operators of drainage pump of stormwater storage pipes, and residents in flood-prone areas. During the study for about two years, it was confirmed that the accuracy of the radar rainfall observation was comparable to that of the X-band dual-polarization Doppler weather radar managed by the Ministry of Land, Infrastructure, Transport and Tourism. In the operation of the drainage pump for the Tsukimiminori Stormwater Storage Pipe in Fukui City, we were able to secure the storage capacity for the next rainfall based on the forecast information by maximizing the drainage capacity of the discharge destination. In addition, it was also confirmed that the residents themselves could secure the lead time for setting up water-stop sandbags and moving their vehicles to higher ground.

A dual-polarization, phased array weather radar, also known as the multi-parameter phased array weather radar (MP-PAWR), was developed by the Japanese Cross-ministerial Strategic Innovation Promotion (SIP) Program. Since this weather radar has been made into an active phased array, three-dimensional observation of weather phenomena can be realized at high speed by means of electrical scanning in the elevation direction and mechanical scanning in the azimuth direction. This is expected to shed light on hydrological processes in river basins, such as those of urban rivers, and improve prediction accuracy. In this study, river water levels in urban areas were estimated from vertically integrated liquid (VIL) Nowcast water content results, a meteorological forecasting method based on the three-dimensional observation MP-PAWR data, using a synthesized rational formula. A runoff analysis for urban basins was carried out using the rainfall forecast results based on MP-PAWR observational data. Since it is known that this formula can be used to deliver a rapid response time for runoff phenomena in the basin, it is possible to fully exploit the features of the MP-PAWR. This study shows how MP-PAWR is used in a series of hydrological processes. In this paper, we report the results of a basic study on water level predictions based on MP-PAWR observational data and also present future prospects for the use of this technology.

All over Japan, damage has occurred from flood from inland waters. Because the measures against urban inundation should be promoted swiftly and economically, the Ministry of Land, Infrastructure, Transport and Tourism has implemented policies to push forward efficient and effective mitigation measures. Such comprehensive policies combine tangible and intangible measures and select prioritized areas for implementation. In June 2020, forecasting methods for probable maximum rainfall were announced, taking heavy rain and climate change into consideration. Furthermore, because a sewer system itself can be damaged from heavy rain, measures have been adopted to make a water-resistant sewerage system. In this manner, various policies in Japan address measures against urban inundation.

One of the roles of sewerage is to effectively discharge stormwater and protect urban areas from inland flooding, but there are several challenges to overcome. Urban development is progressing in Tokyo, and the amount of stormwater flowing into the sewers is increasing. In order to respond to the increase in the frequency of heavy rainfall in recent years, it is necessary to improve the capacity of stormwater pumps. The Bureau of Sewerage Tokyo Metropolitan Government has been working on the technological development of higher-performance stormwater pumps since the 1950s, when initial-type stormwater pumps were installed. The technological development history of stormwater pumps in Tokyo can be broadly divided into four periods. The first period spanned approximately 30 years when initial-type pumps, which start to operate after stormwater flows into the pump station, were active. In the second period, which spans approximately 20 years from 1980, two types of pumps were developed; a non-water supply pump that could operate reliably even in an earthquake, and a standby operation pump that could respond to sudden stormwater inflow during torrential rains. The third period spans approximately 15 years from 1999, when the waterless standby pump, which integrated the features of the two pumps developed in the second period, was developed and introduced. The fourth period is the era of pumps with higher performances than third period pumps developed after 2015. Currently, these stormwater pumps are operated together with a rainfall radar system that accurately grasps the rainfall situation and protects the citizens of Tokyo from inland flooding.

Typhoon Hagibis, which hit Japan directly on October 12, 2019, caused great damage, including the flooding of rivers, across various parts of Japan. The Tama River, which flows north of Kawasaki City, also experienced flooding which exceeded the designed high water level; although it did not cause fluvial flooding, river water flowed into the urban areas through the sewerage system, causing unprecedented inundation damage. This damage was reproduced with the inland flood simulation model. Furthermore, we performed simulations in which the water level, precipitation, and sluice gate operation of the Tama River differed from actual conditions, and compared them with the actual damage. Based on these results, we examined methods for reducing inundation damage, such as improving the operation method of sluice gates, and confirmed their effects.

Continuing urbanization has reduced permeable land areas, causing rainwater runoff volumes to rise, which not only increases the drainage burden placed on rivers and sewerage system, but also increases flood damage risk with the increases in torrential rains brought about by climate change. As physical flood control infrastructure has high costs and requires long planning and construction times, more effective utilization of existing infrastructure is required in addition to new construction. Moreover, as urbanization continues, the use of privately owned land for flood control as well as the limited publicly owned land available is increasingly important. This article reports the results and efficacy of a project to create a rainwater management network by connecting multiple pumping stations with the city’s main rainwater pipeline in coordination with river development in a major river drainage basin region, while rebuilding aging pumping stations for continued use, as well as the effectiveness of rainwater storage facilities in underground locations beneath privately owned buildings.

The purpose of this study is to understand the fluidity of survivors’ narratives and to clarify the changes such narratives undergo after time has elapsed. Nineteen survivors of the landslide disaster that occurred on August 20, 2014 in Hiroshima City were interviewed twice-three years after the disaster and five years after the disaster-and the changes in the content of their narratives were analyzed. In addition, by analyzing the titles of newspaper articles that were published within one month of the disaster, the characteristics of narrative transformation were quantitatively identified. The narratives of disaster victims that were once accepted as “dominant stories” become “personalized” as “alternative stories” with the elapse of time, even though they are told by the same person. Comparisons of two interviews conducted at different times show that the word “evacuation” undergoes a significant change in context over time elapse, while the word “disaster” appears in a new context in the fifth-year interview. While social or community groups are the bearers of “socialized” memories, individuals are the bearers of “personalized” memories which are expected to continue to change as time elapses. It is necessary to examine what should be shared in order to utilize disaster victims’ memories to prevent and mitigate disasters, and how to such memories should be shared.

Focusing on green infrastructure (GI), which utilizes nature’s diverse resources, we developed urban flood control measures on three small-scale private tracts in Tokyo and Fukuoka in Japan, experiencing high rainfall. In addition, we implemented these measures and verified the possibility of introduction. Using a target rainfall of 100 mm/h and previous rainfall data, we set our goal of reducing runoff from each site below the capacity of a public sewage pipe. Implementation was conducted by assessing the soil infiltration rate and developing and installing rain gardens and storage layers using crushed stones. These measures satisfied the initially set goals, drastically reducing runoff at all three sites. The target installation cost was set at 100,000 yen per cubic meter of runoff reduction. The target costs were met in the two Fukuoka sites but not at the Tokyo site. The key reasons were the high costs of removing non-permeable surfaces or improving the soil of compacted surfaces, which called for a process to balance the runoff reduction and cost to determine the most effective plan for implementing GI in urban areas. The development and implementation processes were conducted in collaboration with the house owners and concerned parties; the workshops produced constructive ideas being unconstrained by conventional thinking. Visitors highly appreciated ideas related to using water because the techniques were derived from the Japanese culture of lifestyle. Thus, introducing attractive and effective GI may be possible through collaboration. Additionally, sharing experiences led to the formation of new community ties, supporting post-implementation site maintenance.