Prediction of debris flow for warning and evacuation

Abstract

The occurrence condition of debris flow due to rainfall is given by solving the equations for flow on a slope. The solution shows that a debris flow will occur on a slope when the accumulated rainfall within the time of concentration exceeds a certain value determined by the properties of the slope. To estimate this critical value, the system analysis technique would be commendable. In this study, a procedure to find the critical rainfall from the rainfall data with and without debris flows is proposed. Reliability of this method is verified by applying to the debris flows in Unzen Volcano which recently began to erupt.

Discharge of debris flow in a stream is obtained by solving the equation of continuity using the kinematic wave theory and assuming the cross sectional area to be a function of discharge. The computed hydrographs agree well with the ones observed at the rivers in Sakurajima and Unzen Volcanoes. It is found from the derived equation that the runoff intensity of debris flow is in proportion to the rainfall intensity and accumulated rainfall, jointly. This gives a theoretical basis to the conventional method which has been widely used.