While plastics are classified by size as microplastics (25 mm), research in rivers has centered on microplastics, followed by macroplastics, with limited research on mesoplastics (research gap). This study aims to clarify the concentration, shape, and polymer composition of microplastics and mesoplastics in Japanese river water. We conducted field surveys for microplastics and mesoplastics in 147 rivers and at 185 measurement stations. The novelty of this study is in the use of a large number of field data to minimize the effect of the spatial difference in the microplastics and mesoplastics on the data analysis. Microplastics and mesoplastics were found at 183 (99 %) and 136 (74 %) stations, respectively. The difference between the concentration of microplastics and mesoplastics increased significantly with the increase in the concentration of microplastics, showing that the concentration of both microplastics and mesoplastics should be monitored to prevent an underestimation of plastic pollution in rivers with the appropriate sampling. A 2-stage size classification with microplastics (5 mm) is not suitable because the mesoplastics may be substantially overlooked. The regression slopes between microplastics and mesoplastics concentrations significant decrease in variance with increasing data number, suggesting the necessity of the large number of samples used in this study. The predominant shapes and polymer types of microplastics and mesoplastics were found to be fragment and fiber and polyethylene and polyethylene terephthalate (PET), respectively, which were affected appreciably by many fiber clusters. The fiber and PET ratios were dominant at stations with small population densities and urban ratios and can be attributed to atmospheric deposition and the underdeveloped rate of wastewater treatment plants (WWTPs). Therefore, it is necessary to monitor the dynamics and fate of fiber clusters inside and outside the basin.

In recent years, large-scale heavy rainfall disasters have occurred frequently in several parts of the world. Therefore, a quantitative approach to understanding how buildings are damaged during floods is necessary to develop appropriate flood-resistant technologies. In flood inundation simulations for the quantitative evaluation of a building’s resistance to flooding, a subgrid model is necessary to appropriately evaluate the resistance of buildings smaller than the grid size at a medium grid resolution. In this study, a new subgrid (SG) 3D inundation model is constructed to evaluate the fluid force acting on buildings and assess the damage to individual buildings during flood inundation. The proposed method does not increase the computational load. The model is incorporated into a 2D and 3D hybrid model with high computational efficiency to construct a 3D river and inundation flow model. Its validity and effectiveness are evaluated through comparisons with field observations and the conventional equivalent roughness model. Considering horizontal and vertical velocity distributions, the proposed model showed statistically significant improvements in performance in terms of building loss indices such as velocity and fluid force. These results suggest that the SG model can effectively evaluate the fluid force acting on buildings, including the vertical distribution of flow velocities.

Paddy field dams are basin-level flood control measures that promote rainwater storage; however, a general runoff model cannot adequately describe the water balance in paddy fields. This study develops a subgrid model for evaluating paddy water balance considering land use on a computational grid. Subgrid models can account for the storage effect of paddy field dams without disregarding the general grid-based distributed rainfall–runoff model framework. To investigate the effect of current paddy field storage and the introduction of paddy field dams on reducing peak flood discharge, rainfall–runoff analysis was conducted using the proposed model in the Kashima River basin, which flows into Lake Inba-numa in Chiba Prefecture, Japan. The computational results indicated that the rainwater storage effect of current paddy fields reduces the peak river discharge, suggesting that the drainage process of the paddy field should be incorporated into runoff models. Furthermore, the storage effect of paddy fields became more pronounced as the height of the drainage pipe in the paddy field dam increased. The calculated results quantitatively show the flood control effect of paddy field storage over the entire basin; thus, the proposed subgrid model may be a useful tool for promoting basin-level flood control measures.

千葉県における流域治水対策として，広域に分布する谷津地形を活用した洪水調節が挙げられる．本研究では，一宮川流域を対象とした流域治水モデルの構築を行うべく，格子解像度の異なる水文地形データを用いた流出解析を行い，谷津を有する河川流域の計算精度を検証した．その結果，150m解像度のピーク流量は30m解像度の64%となり，計算結果が大きく異なるという結果が得られた．これは，格子解像度が粗いと谷津特有の局所的な標高の高低差が平滑化されることにより，流量フラックスを過小評価し，結果として河道への流出量を過小評価することが示された．以上より，谷津を有する流域での数値モデリングにおいて，地形データを高解像度で反映する必要があることを示した．

気候変動に伴い発生が予想されるスーパー台風により洪水と暴風の同時生起リスクが懸念されるが，河川洪水流の水理に対する風応力影響に関する知見は皆無に等しい．本研究では，暴風条件下における河川洪水流の水理特性に関する基礎的知見を得ることを目的とする．そのため，長さ30kmの一様断面を有する矩形開水路における洪水流に対して，定常・一様な風向・風速を変えた数値実験を実施した．その結果，順風（逆風）時では，風応力作用後，速やかに主流方向流速が増加（減少）し，極大値を経てから減少（増加）し，定常状態へ移行していた．また，風応力に対して，流速の方が水深（水位）よりも応答が早く，これには運動量の鉛直混合を示す鉛直渦動粘性係数による拡散距離が関係していることが示唆された．