The annual number of deaths resulting from landslides and debris flow in Japan over the past 40 years has shown a decreasing tendency as corresponding countermeasures are implemented. The countermeasures constituted of structural and non-structural methods are introduced, focusing on their phenomenological function to clear up roles of sediment research on them. In addition, governing equations which are employed currently in numerical predictions are shown, including formulas for bed shear stress and sediment erosion and deposition in order to illustrate differences between the flow of water and sediment mixture in steep areas and the flow of usually observed floods in rivers. It is recommended that equations which are able to distinguish rapid changes of flow characteristics due to internal solid type friction, sediment erosion and deposition and the corresponding changes of bed elevation should be applied in the numerical computations.