| Summary: | The July 2020 debris flow in Japan caused enormous damage, and briefing sessions on disaster prevention have prompted demands for detailed explanations and predictions of such phenomena in high-risk areas. It is necessary to obtain four-dimensional risk information, which considers temporal changes in disaster risk, rather than limiting the analysis to conventionally static information. In this study, we developed a method for setting the boundary conditions necessary for debris flow prediction via a four-dimensional hazard map using various types of digital information. To understand the effects of hydrograph characteristics from the upstream, flow discharge was analysed under different flow conditions, such as topography-driven riverbed shear stress, using a one-dimensional numerical model that considers water and sediment flow. Our results suggested that characteristics of the upstream inflow hydrograph affect flood runoff processes downstream; therefore, we developed a separate downstream flood hazard grade index that uses characteristics of the upstream inflow hydrograph as input.
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