The relationship between the characteristics of rainfall and disasters of debris flow in Beijing area is analyzed in this paper. Topographically induced convergence and upward motion of air are one of the most importam mechanism that may cause and strengthen storm rainfall in the western and northern mountain area of Beijing. Most disasters of Beijing debris flow occurred on both sides of the rainfall detention line due to the storm clouds detained by the mountains in the last decades. It is related to the topographically nocturnal storms that the debris flow often occurs during the night in Beijing. The local thermal circulation causes violent cyclonic convergence that motivates nocturnal storm rainfall in the mountain area. The rainfall intensity triggering the debris flow is the 3-day-precipitation analogous to the value at recurrence rate of once in fifty years. The storm rainfall with the intensity of 50 mm per hour may trigger debris flow, when the water content of the loose earth reaches saturation. It is found that the 3-day- rainfall over 300 mm and 1-hour-rainfall over 50 mm are the critical warning intensity of rainfall in Beijing area.
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