We are very pleased to publish the Special Issue on NIED Frontier Research on Science and Technology for Disaster Risk Reduction and Resilience 2020. There are nine papers in this issue.

The first two papers concern hazard and risk information systems: Sano et al. constructed a real-time risk information map for flood and landslide disasters, and Hirashima et al. created an alert system for snow removal from rooftops. These systems are already in use on the NIED website. The next three papers are case studies of recent storm disasters in Japan and the United States: Cui et al. analyzed the time variation in the distribution of damage reports in the headquarters for heavy-rainfall disaster control in Fukuoka, Shakti et al. studied flood disasters caused by Typhoon Hagibis (2019), and Iizuka and Sakai conducted a meteorological analysis of Hurricane Harvey (2017). Regarding volcanic disasters, Tanada and Nakamura reported the results of an electromagnetic survey of Mt. Nasudake.

This special issue also includes three papers on large-scale model experimentation: Danjo and Ishizawa studied the rainfall infiltration process using NIED’s Large-Scale Rainfall Simulator, Kawamata and Nakazawa conducted experiments concerning liquefaction, and Nakazawa et al. reported the results of experiments on seismic retrofits for road embankments. The experiments used E-Defense, the world’s largest three-dimensional shaking table.

We hope this issue will provide useful information for all readers studying natural disasters.

It is important to discern in real time the risk level of rain-related disasters such as floods and landslides in order to maintain readiness against heavy-rainfall disasters and to decide on the suitable response measures. In this study, we developed an information processing technology that employs hazard information which indicates the risk of inundation or landslides. It also presents indices of social vulnerability and applies the spatial resolution functions of a geographic information system (GIS) to extract in real time the highly exposed and vulnerable areas that are faced with an increased risk of flooding or landslides. The technology’s validity was verified using a case study – namely, the heavy rainfall that accompanied a pressure front in August, 2019. The results show that, with respect to flood risks, we were able to extract in real time specific areas where flooding may be taking place, thus demonstrating the possibility of applying the technology to decide priorities in disaster response measures. Future issues are related to information dissemination, including the specific labeling and expressions that are easy for the user to understand as well as improving the user interface so as to facilitate delivery of relevant risk information in real time.

This study developed a snow load alert system, known as the “YukioroSignal”; this system aims to provide a widespread area for assessing snow load distribution and the information necessary for aiding house roof snow removal decisions in snowy areas of Japan. The system was released in January 2018 in Niigata Prefecture, Japan, and later, it was expanded to Yamagata and Toyama prefectures in January 2019. The YukioroSignal contains two elements: the “Quasi-Real-Time Snow Depth Monitoring System,” which collects snow depth data, and the numerical model known as SNOWPACK, which can calculate the snow water equivalent (SWE). The snow load per unit area is estimated to be equivalent to SWE. Based on the house damage risk level, snow load distribution was indicated by colors following the ISO 22324. The system can also calculate post-snow removal snow loads. The calculated snow load was validated by using the data collected through snow pillows. The simulated snow load had a root mean square error (RMSE) of 21.3%, which was relative to the observed snow load. With regard to residential areas during the snow accumulation period, the RMSE was 13.2%. YukioroSignal received more than 56,000 pageviews in the snowheavy 2018 period and 26,000 pageviews in the less snow-heavy 2019 period.

After a natural disaster occurs, the production and sharing of damage reports are extremely important for a disaster response site. However, one of the problems is that the data shared by the damage reports cannot clearly indicate when the damage situation could be grasped because such data change day by day. Accordingly, in this study, the data of the damage reports of the Headquarters for Disaster Control are treated as unequally spaced time series data to evaluate the changing conditions of the data quantitatively. For this purpose, a case is examined for the Headquarters for Disaster Control of Fukuoka Prefecture at the time of the Northern Kyushu Heavy Rainfall event in July 2017. As a result of the examination, it is indicated that the quantitative evaluation would be possible for 1) analysis on timing when the data of the damage reports are updated, 2) analysis on the characteristics of time series of the report data, and 3) visualization of the progress of the damage report service.

The frequency of severe flood events has been increasing recently in Japan. One of the latest events occurred in October 2019 and caused extensive damage in several river basins, especially in the central and northern regions of the country. In this study, we selected the Hitachi region (Hitachi-Omiya and Hitachi-Ota) within the Kuji River Basin which underwent considerable flooding due to the failure of embankments at two locations in the region. Maximum-possible flood inundation maps were generated using survey-based data and hydrological modeling for the Hitachi region. These maps incorporated the flood scenarios (embankment failures). All the generated products were compared with the reference flood mapping, i.e., Sentinel-1 data and Geospatial Information Authority of Japan (GSI) data for that region. It was observed that generated flood inundation mapping product based on the survey-data yielded results similar to those obtained with GSI data for the Hitachi region. Although each flood mapping product has advantages and disadvantages, they can be a good reference for the proper management and mitigation of flood disaster in the future. The rapid development of flood inundation mapping products that consider varying flood scenarios is an important part of flood mitigation strategies.

In August 2017, Hurricane Harvey brought an unprecedented amount of rainfall and catastrophic flooding to the Houston metropolitan area, as it stalled near the coast of Texas for several days after weakening to a tropical storm intensity. The present study examines the relationship between tropical cyclone rainfall totals over Texas and the track, residence time, rainfall intensity, and rainfall area coverage of past tropical cyclones that approached Texas after 1979. The most significant factor affecting rainfall totals over Texas is whether a tropical cyclone makes landfall on the central coast of Texas and travel inland. Another significant factor is the length of time a tropical cyclone resides near Texas. Rainfall intensity also contributes in part to rainfall totals over Texas, whereas contribution of rainfall area coverage is not significant. The track of a tropical cyclone traveling near Texas is controlled by the steering winds over Texas, while its residence time near Texas is related partly to the meandering of the subtropical jets. Rainfall rate depends on the intensity of tropical cyclone. No significant relationship between rainfall intensity and environmental moisture in the lower atmosphere is found in the present analysis. Furthermore, the extreme rainfall totals over Texas induced by Harvey can be attributed to the combined effect of extreme long-term stalling of Harvey near the central coast of Texas and the higher rainfall rate.

A time domain electromagnetic survey (TDEM method) was conducted to investigate the resistivity structure of the crater, fumarole, and hot spring area of the Nasudake (Chausudake) volcano. The findings of this survey are as follows: (1) Under the crater area, a thin low-resistivity layer (approximately 50 m) was found on the surface, and lens-shaped high-resistivity areas continued to a depth of 800 m below it. The lens-shaped high-resistivity areas are believed to correspond to a thermal volcanic gas region. (2) From the east-west direction survey line crossing the foot of the Nasudake, two or three horizontal resistivity layer structures, which are considered to be caused by the geological structure and surface water, were observed.

The infiltration of rainfall into a slope surface may affect slope stability; thus, it is important to understand the amount of rainfall infiltration (hereafter referred to as the “infiltration capacity”) for a slope surface layer when evaluating slope stability. This research focuses on slope gradient, a factor affecting the infiltration capacity, and performs two types of water-spraying experiments using pit sand under the same conditions but with different slope gradients. In the first experiment, the surface flow rate and soil loss were measured using an earth-tank model with a horizontal distance of 0.5 m, depth of 0.1 m, and width of 0.2 m to form slope gradients of 2°, 20°, and 40° to clarify the effect of slope gradient on the infiltration capacity. In the second experiment, a water-spraying experiment that closely simulated natural rainfall was performed at a large-scale rainfall facility owned by the National Research Institute for Earth Science and Disaster Resilience (NIED), Japan. This experiment used an earth-tank model with a horizontal distance of 1.21 m, depth of 0.5 m, and width of 0.5 m to form slope gradients of 2°, 10°, 20°, 30°, and 40° with the aim of proposing a quantitative evaluation method for the relationship between the slope gradient and infiltration capacity. The results showed that the soil loss and infiltration capacity increased as the slope gradient increased in the case of the pit sand used in the experiments. This was confirmed to be due to the fact that an increased gradient allowed grains with diameters of <50 μm in the slope surface layer to flow out easily, thereby increasing the infiltration capacity. In addition, the relationship between the rainfall intensity and infiltration capacity revealed that the infiltration capacity varied depending on the rainfall intensity and slope gradient, which is unlike the relationship for constant values such as the permeability coefficient. Moreover, the research findings indicated a strong, positive linear relationship (R² = 0.98) between the slope gradient and fitting factor I_c. Therefore, the relationship between rainfall intensity and the infiltration capacity could be expressed using the fitting factor I_c. This suggests the possibility of quantitatively evaluating the relationships between rainfall intensity, the infiltration slope gradient, and the infiltration capacity.

Various studies have examined soil liquefaction and the resultant structure damage. The 1995 Southern Hyogo Prefecture Earthquake, a near-field earthquake, caused significant damage when the ground was liquified due to the rapidly increased pore water pressure in several cycles of major motions. Therefore, the effect of pore water movement during earthquakes has been assumed to be limited, and liquefaction has mainly been evaluated in undrained conditions. Additionally, the ground and building settlement or inclination caused by liquefaction are deemed to result from pore water drainage after earthquakes. Meanwhile, in the 2011 Tohoku Earthquake, off the Pacific Coast, a subduction-zone earthquake, long-duration motions were observed for over 300 s with frequent aftershocks. Long-duration motions with frequent aftershocks are also anticipated in a future Nankai Trough Earthquake. The effect of pore water movement not only after but during an earthquake should be considered in cases where pore water pressure gradually increases in long-duration motion. The movement of pore water during and after an earthquake typically results in simultaneous dissipation and buildup of water pressure, as well as volumetric changes associated with settlement and lateral spreading. Such effects must reasonably be considered in liquefaction evaluation and building damage prediction. This research focuses on pore water seepage into the unsaturated surface layer caused by the movement of pore water. Seepage experiments were performed based on parameters such as height of test ground, ground surface permeability, and liquefaction duration. In the tests, water pressure when the saturated ground below the groundwater level is fully liquified was applied to the bottom of the specimen representing an unsaturated surface layer. Seepage behaviors into the unsaturated surface layer were then evaluated based on the experiment data. The results show that the water level rises due to pore water seepage from the liquefied ground into the unsaturated surface layer right above the liquefied ground. For this reason, a ground shallower than the original groundwater level can be liquified.

There exists many road embankments in Japan which are not earthquake resistant. For example, a road embankment collapsed at Okuradani IC in Hyogo Prefecture during the Great Hanshin-Awaji Earthquake of 1995. In 2009, a road embankment along the Tomei Expressway collapsed during an earthquake with epicenter in Suruga Bay. Road failure makes relief activity and transportation of goods difficult, causing social damage. Furthermore, recovery of damaged embankments takes much time and cost. Accordingly, it is important to conduct research on methods of construction which would help build embankments inexpensively and swiftly. Against this background, a full-scale experiment was conducted at E-Defense to confirm the validity of a method of construction that uses flexible container bag to pack soil for quick embankment recovery. Generally, flexible container bags are easy to handle, and ensure and maintain the earthquake resistance performance of embankments after the completion of recovery work, taking the longer life time of the reinforced structure into consideration. In the experiment, two kinds of reinforced structures with flexible container bags stacked differently were placed at either toe of the slope of an embankment of height 4 m, and shake tests were performed three times to compare the effectiveness of both reinforced structures. For both kinds of structures, the flexible container bags were stacked in two tiers and compressed from top and bottom using compression plates to make the structures rigid. One of the structures was one-tier type where the flexible container bags were stacked in series and the other was two-tier type where the flexible container bags were stacked along the side of the embankment. In the case with the target acceleration of sine wave of 376 Gal, crack occurred on the reinforced structure of one-tier type, but the embankment collapsed a little near the top of the slope. There was little displacement in both reinforced structures, hence, it is judged that the deformation would not impair the functionality of the road. As for the seismic performance, it can be said that the two-tier type would be slightly superior to one-tier type, however, this assumption cannot be evaluated decisively under the present circumstances. For practical use in future, form, size, workability, and economy of embankment should be examined for designing and construction which takes the specification of the structure into consideration.

In recent years, participatory bosai (disaster prevention) map creation activities have been gaining ground for the effective promotion of community-based disaster management. The participation of school-going children and local residents is a key feature of this map creation activity. Engagement is important for promoting ownership and the effective use of bosai maps. However, there still remains a “just make and complete” problem, even when a bosai map is successfully created in a participatory manner. In order to solve this issue, it is important to focus not only on the map, but also on the preparation process and the period after its completion. This study conceptualizes the entire process of bosai mapmaking as a “bosai map cycle.” The research was implemented in the manner of action research to deal with the practical issues we faced during school disaster education of bosai mapmaking, and is aimed at overcoming potential issues by activating the “bosai map cycle.” Consequently, diverse people were involved in bosai mapmaking, including local residents who were not previously involved in the process. It is important to carry out bosai map creation activities as a cycle of pre- and post-creation.

People with special needs are at higher risk during a disaster than those without because of delayed disaster evacuation behaviors. Therefore, one of the top priorities in the field of disaster risk reduction is implementing evacuation support for the people with special needs. However, assistance is often limited, especially in areas with declining and aging populations. In addition, past evacuation activities for people with special needs have tended to focus on the barriers they face and assistance they need during evacuation rather than their utilizable capabilities. Therefore, this study considers evacuation drills that utilize the capabilities of people with special needs. An “indoor evacuation drill” was developed and the evacuation behaviors of residents with special needs were analyzed. An indoor evacuation drill is defined as an evacuation activity that participants carry out within their own homes – for example, evacuating from the bedroom to an exit. In coastal areas, such a drill helps residents prepare to evacuate their homes in case of a tsunami, while in mountainous regions, it helps them prepare for evacuation to the upper floors in case of a landslide. The study participants were residents of Hamamachi ward (a coastal area) and Kumai ward (a mountainous area) in Kuroshio town, Kōchi Prefecture, Japan. The results indicate that an indoor evacuation drill conducted in the participants’ living area, such as the entrance or second floor of their home, is easier to implement than usual disaster evacuation drills, and helps people with special needs regain autonomy in disaster risk reduction activities. Moreover, the participation rate of the target population in local evacuation drills increased after participating in the indoor evacuation drills. Existing evacuation drills often overlook people with special needs, and delays in disaster prevention for this population are often associated with their lack of interest in related activities. However, the results of this study suggest that disaster prevention activities themselves sometimes overlook the challenges faced by people with special needs and prevent them from participating.