This paper focuses on finding efficient solutions for the design of a highly permeable pile spur (or slit type) dike field used in morphologically dynamic alluvial rivers. To test the suitability of different arrangements of this type of permeable pile spur dike field, laboratory experiments were conducted, and a three-dimensional multiphase numerical model was developed and applied, based on the experimental conditions. Three different angles to the approach flow and two types of individual pile position arrangements were tested. The results show that by using a series of slit-type spurs, the approach velocity of the flow can be considerably reduced within the spur dike zone. Using different sets of angles and installation positions, this type of permeable spur dike can be used more efficiently than traditional dikes. Notably, this type of spur dike can reduce the longitudinal velocity, turbulence intensity, and bed shear stress in the near-bank area. Additionally, the deflection of the permeable spur produces more transverse flow to the opposite bank. Arranging the piles in staggered grid positions among different spurs in a spur dike field improves functionality in terms of creating a quasi-uniform turbulence zone while simultaneously reducing the bed shear stress. Finally, the efficacy of the slit-type permeable spur dike field as a solution to the riverbank erosion problem is numerically tested in a reach of a braided river, the Brahmaputra–Jamuna River, and a comparison is made with a conventional spur dike field. The results indicate that the proposed structure ensures the smooth passing of flow compared with that for the conventional impermeable spur structure by producing a lower level of scouring (low bed shear stress) and flow intensification.

A two-dimensional debris and mud flow model that considers both laminar and turbulence flow was developed. Subsequently, the model was applied to a debris flow that occurred in Asaminami, Hiroshima, Japan in August 2014. The applicability of the model and the debris flow characteristics are discussed. The calculated horizontal distribution of sediment deposited in the Asaminami residential area was in good agreement with the horizontal distribution of the deposited large rocks and driftwood. This result indicates that the fine material in the downstream area was transported by water flow resulting from heavy rain that occurred after the debris flow. The scale of the initial debris flow was small; however, it increased with time, because eroded bed material and water were entrained to it. Therefore, it is important to reproduce the development process of debris flows to predict the amount of sediment produced, the deepest flow depth, the maximum flow velocity, and the inundation area. The averaged velocity of the simulated debris flow was about 9 m/s, and the velocity at the entrance to the residential area was about 8 m/s. This kind of information can be used to design sediment deposition dams. The travel time of the simulated debris flow from the upstream end of the main channel to the entrance of the residential area was 96 s. This kind of information can be used for making evacuation plans. Valley bed steps can suppress the deepest flow depth which is very important for the design of check dams; therefore, the high-resolution elevation data and fine numerical grids that reproduce step shapes are required to accurately calculate the deepest flow depth and maximum flow velocity.

2018年7月に広島市安芸区矢野東の梅河団地で発生した複数の土石流を対象に，土石流の数値シミュレーションを実施し，土石流の流動特性を検討するとともに，設定されている土砂災害警戒区域（イエローゾーン）や土砂災害特別警戒区域（レッドゾーン）の評価を行った．宅地域における家屋に作用する最大応力の大きい領域は，破壊した家屋の分布と良く一致していた．土砂災害特別警戒区域（レッドゾーン）以外でも，家屋が破壊するおそれのある箇所が存在すること示した．また，複数の斜面崩壊の発生タイミングや斜面崩壊数の違いにより，宅地域に流入した土石流の氾濫域や流動深の空間分布を検討した．避難計画等を検討する上では，宅地に流入する土石流の規模やタイミングを考慮すること，複数の渓流からの土砂流出量を考慮することの重要性を示した．

On August 9–10, 2021, the Koakagawa Bridge in Mutsu, Aomori Prefecture, Japan, collapsed owing to the flood caused by a tropical cyclone. In this study, the cause of the collapse was examined. The characteristics of flood inundation with bypassing flows and countermeasures for bank erosion due to channel blockage by driftwood were investigated based on field surveys and numerical simulations. The scouring of the bed around the bridge pier was found to be the cause leading to bridge failure. Channel blockage owing to driftwood would have caused flood inundation and damage to the left bank, the top of the bank is bare of sediment, by forming a bypassing flow. However, the surface of the right bank on which the bypassing flow with high velocity formed was covered with concrete; therefore, bank erosion did not occur. These results indicate that to prevent bank erosion caused by bypassing flows during floods, it is important to protect banks downstream of the bridge using non‐erosive materials. Only a small area, where bypassing flows form and non‐dimensional shear stress is larger than 0.05, is sufficient for protection. It is also important to predict bed degradation characteristics around piers and banks, especially when the bed material is small.

土砂災害危険情報サービスにおける土砂災害雨量指数（XRAIN実況雨量による土壌雨量指数第1タンクと60分累加雨量の積算値）は，その閾値を超過すると土砂災害が発生する可能性がある．その捕捉率は約80%である．ある地域の土砂災害雨量指数が閾値を超過する回数が多いと言うことは土砂災害の発生確率も高いと想定される．本検討では，閾値を超過する回数を長期間累積した結果が，その地域のリスク評価指標として妥当かどうか検討した．面的な雨量データとしては，長期間データが存在する解析雨量を用いて，2006年から2021年までの15年分を計算し，閾値超過回数を累積した．その結果，数値のばらつきが大きかったため，単位時間あたりに事象が発生する確率としてポアソン分布を用いて計算した．その結果，九州ほぼ全域，太平洋岸の四国，紀伊半島付近の確率が大きく，これらの地域で日常的な強雨により土砂災害の発生確率が大きいことを確認した．