In 2016 August, Otofuke River, located in Tokachi prefecture of Hokkaido, faced active bank erosion leading into a significant migration in the path of the channel due to the four majour flooding events withing a time span of as short as 14 days. In order to understand the mechanism of river meandering of steep-slope rivers under the unsteady-flow conditions during floods, field investigation and numerical analysis were conducted. Results from field research showed that bank erosion seemed to be caused by lateral channel migration which was occured after peak-flow discharge, since the maximum water level traces were lower than the hight of both embankments. Additionally, numerical analysis indicated that sediment accumulation becomes active when the flow rate decreases and it deflects flow path toward to the lateral direction.

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 Models enable to reproduce an estimate river discharge considering the influence of flood inundation and flood inundation decrease of particular places in a basin by calculating from rain-runoff to flood inundation integrally. Furthermore, the influence of flood control by dam on the river water level and the inundation can be assessed quantitatively by reflecting dam operation into such models. In this study, the dam program for the Rainfall-Runoff-Inundation (RRI) model was developed, and then the effect of flood control by dams in Kinugawa River in 2015.09 Kanto-Tohoku Heavy Rainfall was analyzed. The effect of dams was quantified using the scenario calculation when there was no dam upper reaches. This study confirmed the effect of flood control by dams in upper Kinugawa River from the result that the peak river water level was decreased approximately 1 meter, and the area of inundation above floor level was decreased by approximately 20% by flood control.

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 Recently, the serious inundation damage due to local heavy rain has increased. This study treated the inundation damage due to heavy rain in Kasugai city, Aichi prefecture, JAPAN that occurred on September 20, 2011. In this study, the analysis model for inundation due to heavy rainfall was developed and the validity of analysis model was shown by comparison between the analysis results and the observed inundated area, the analysis results of Aichi Prefecture. Moreover, it was shown clearly the inundation water behaviour of left or right bank side area of jizou river basin by the examination with visualization techneque of transport of rainwater. By the inverse analysis of the movement of rainwater, the inundation prossess was investigated in detail considering the hydraulic phenomenon of the sewer system.

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 During floods, it is quite easy for automobiles to be swept away by water flow. In this study, the risk of this happening to a vehicle during a flood situation was examined by a laboratory experiment. Horizontal hydrodynamic forces on a model car were measured in a straight channel. Especially, effects of the angle between main flow direction and a vehicle (θ) on drag force coefficients were examined with two dimensionless parameters, relative water depth and Froude number. In addition, the hydrodynamic force on a prototype car was predicted using the experimental results, and the risk to automobile traffic in a flood situation was estimated as a function of θ, flow velocity and relative water depth. As a result of this study, it is made clear that driving a car during a flood situation is dangerous.

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 As a flood inundation analysis model for urban area, the Building Porosity (BP) model, which approximates the influence of the building by using the porosity and resistance coefficient, is effective. By using this model, it is possible to conduct an accurate inundation simulation even if the floodplains have a huge area. In this study, we focused on the method of setting the porosity, and conducted basic flooding experiments in which building groups have different porosities and layouts. In addition, we carried out the reproduction simulation with two models; the Averaged Building Porosity (BP-A) model which gives uniform distribution of the building porosity and the Localized Building Porosity (BP-L) model which gives local porosity for each calculation meshes. As a result it is clarified that the BP-L model shows higher reproducibility than the BP-A model, and the calculation accuracy with BP-L model greatly depends on the size of calculation mesh in the building group.

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 In this study, prediction of urban inundation which is caused by torrential rain of the 100mm/h was conducted. And the investigation of large-scale inundation due to the collapse of river levee of the Arakawa river was also carried out by using the same prediction technique in the eastern area from the Sumidagawa river in 23 wards of Tokyo. As a result, each process of inundation was revealed, and the weak area against such natural severe events. Such area locates the place whose ground level is lower than the sea level of Tokyo bay, and the damage may be much serious compared with the western area of this target area.

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 We evaluated the mitigation effects on pluvial inundation by means of hypothetical on-site storage facilities, using a simulation model for pluvial inundation. The storage capacity of each on-site facilities was determined from the potential bottom area and depth, depending on the land use; individual houses, apartment house, office, school, and playing ground. As the evaluation index, the amount of damage was adopted instead of water depth or inundated area. As the results, it is found that the inundation damage can be significantly decreased by enhancement of the storage capacity of individual houses, occupying the most of the study area. The amount of damage enables to represent the mitigation effects by only one quantitative value, which would be useful for optimal installation of on-site storage facilities.

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 Recently serious torrential rain event occurs frequently, whose intensity is much larger than the designed value for drainage network and urban rivers. To take proper countermeasure, reliable risk information based on more accurate numerical computation are required. In this study, the followings were confirmed by conducting numerical computation of urban area of western area from the Sumida river in 23 wards of Tokyo. The first is the effectiveness of existing rainwater countermeasures, and the second is the possibility to find the sign of inundation on underpass of road in drainage pipe as reverse flow to road.

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