Since the Edo period, flood control forests have been planted along the Kuji River in Ibaraki Prefecture, where they remain to the present day. However, in recent years, flood control related issues have emerged because of inadequate management of bamboo in these forests. For this study, two scenarios for flood control forest density were created. Then, using Typhoon No. 19 in 2019 as a case study, flood inundation simulations were conducted for quantitative demonstration of flood control forest effects on flood flows. Additionally, we conducted a detailed analysis of Onuki district, Hitachi-Omiya City, where high risks of flooding were found for both scenarios. The simulation results showed that high bamboo density of flood control forests led to increased flood damage. Results also show that high bamboo density of flood control forests prolonged inundation in flood areas and led to long-term isolation of the Iwakami area in the Onuki district.

　Mountain streams with complex topography, particularly those characterized by step–pool structures, exhibit highly variable flow fields, making it difficult to apply conventional hydraulic models. This study evaluates a method for reproducing high-resolution flow dynamics in natural mountain streams by integrating field observations with numerical simulations. A 12 m reach of the upper Aono River, Minami-Izu, Japan, was surveyed at 10 cm cross-stream and 20 cm longitudinal intervals, yielding approximately 1,000 measurement points of bed and water surface elevation across 60 cross-sections. A high-resolution 3D topographic dataset was constructed by integrating above-water terrain data obtained from a smartphone-based LiDAR scan. Using this dataset, two-dimensional hydraulic simulations were conducted with the iRIC Nays2DH model. Calibration of the roughness parameter based on observed discharge and water surface elevation reproduced the measured water depth distribution with RMSE of 3.3 cm, capturing key flow characteristics such as rapid water surface drops at steps and deceleration zones within pools. The model performance was considerably worse after the survey resolution was reduced to one-half and one-quarter, with RMSE increasing respectively to 4.2 cm and 5.9 cm. These results demonstrate the effectiveness of combining high-density leveling surveys with smartphone LiDAR measurements. Moreover, the findings highlight the applicability of this approach for hydraulic simulations in complex mountain stream environments.

　Rain gardens have recently gained attention in “River Basin Disaster Resilience and Sustainability by All” projects promoted by Japan’s Ministry of Land, Infrastructure, Transport and Tourism. For this study, our organization, Rain City Support, collaborated with residents and municipal staff of Musashino City to install a small but highly visible rain garden at the Musashino Eco Resort facility entrance. This garden is linked hydraulically to a rainwater tank: when the tank reaches full capacity, overflow is directed into the rain garden, thereby allowing the two to function in tandem to achieve runoff control. This report describes a year-long monitoring study conducted for quantitative evaluation of this rain garden's performance. During 1 June 2024 – 31 May 2025, continuous monitoring was conducted to evaluate runoff control, infiltration, soil moisture retention, and tank collection efficiency. Results indicate that 71.2% of roof runoff was retained. Also, 54.5 m³ of rainwater infiltrated into the ground over the year, as estimated for the rain garden area and its corresponding catchment area. The rain garden surface soil had a lower frequency of high pF values (drier conditions) compared to other areas, indicating greater moisture retention. This moisture supported plant growth throughout the year. Our findings quantitatively demonstrate the role of rain gardens, when integrated with rainwater tanks, in contributing to both flood mitigation and the maintenance of planting substrate moisture.

　In 2024, northern Thailand experienced a series of widespread heavy rainfall events, leading to severe flooding and considerable damage to urban infrastructure across multiple provinces. Mae Sai City, located in the northernmost part of Chiang Rai province, underwent recurrent flooding that disrupted urban infrastructure and residents' livelihoods to a considerable degree. In November 2024, the authors conducted a field survey and interviewed the city's mayor to document the extent of the flood damage systematically and to identify challenges associated with recovery and reconstruction. Analyses of hydrometeorological data revealed that precipitation reached approximately 1.8 times the climatological average during August–October, confirming precipitation as a major factor exacerbating the flood damage. Widespread land-use changes in the upstream basin were also documented, implicating these changes as a background factor amplifying the damage because of reduced water retention capacity. The survey revealed that numerous levees within the city had failed and confirmed the location of many residential structures within the river corridor, which exacerbated the flood effects. In response, a comprehensive recovery and reconstruction plan has been formulated. This plan includes construction of new levees, upgrading of drainage infrastructure, and removal of buildings from the river corridor. This case study specifically examined this flood event driven by the interplay of natural and anthropogenic factors, highlighting challenges of flood management in transboundary ungauged basins.