泥石流是山区的主要灾害类型，其发生条件和暴发规律尚在探索之中，泥石流过程本质上并不是纯随机事件，地形坡度，暴雨强度和风化岩土物质积累的组合决定了泥石流的规模，人们推测其暴发具有周期性，但并没有得到直接的证据。作者从山坡松散物质积累条件和泥石流暴发的概率分布出发，在考虑其发生规模的前提下，论证了泥石流暴发风险具有周期性，并且给出了其风险度及暴大风险重现期的估算公式，理论研究和调查结果表明，泥石流间隔期越长，其发长的规模将越大，这在制定山区防灾对策中应该特别注意。

Debris flow is the common disaster in mountain region. The valley slope, storm rainfall and amassed sand-rock materials in a watershed may influence the type of debris flow. The bursting of debris flow is not a pure random event. Field investigations make us infer that the debris flow bursts periodically, but the direct evidence has not been found yet. The author proposes a risk definition of debris flow based upon the accumulation conditions of loose material in channel and probabilistic distribution of breaking out debris flow. According to this definition, the destroying risk of debris flow is of the quasi-periodicity. A formula of this risk estimation is derived. The analysis of relative factors reveals the relationship of frequency and size of debris flow. For a debris flow creek, the longer the time interval between two occurrences of debris flows is, the bigger the bursting event may be.