藏东南泥石流沟纵剖面演化的最小功模式

西藏东南地区的泥石流可分为雨水、冰雪融水和冰湖溃决等3种成因类型,其沟谷纵剖面演化均遵循最小功原理,即通过调整坡降使流速增大,表现为单位流体的全程流速平均值ū与纵剖面形态指数N正相关:ū∝f(N),对雨水型泥石流沟,流速函数f(N)={1/3-2/(N+1)(N+2)(N+3)]}^1/2;对融水型泥石流沟,f(N)={2/3-2/(N+1)(N+3)]}^1/2;对溃决型泥石流沟,f(N)=NP(N+1)]^2/3。在泥石流沟谷演化进程中,与ū值由小变大相应,N值由小变大,沟谷纵剖面从上凸抛物线形变为下凹抛物线形,泥石流沟谷地貌演化阶段按泥石流孕育阶段、发展阶段、旺盛阶段、衰减阶段的顺序演替。以藏东南典型泥石流沟作为实例,检验了上述沟谷地貌演化的模式与规律。

Model of Minimum Energy Dissipation in Evolution of Valley Longitudinal Profile of Debris Flow in Southeast Tibet Area

The debris flow in southeast Tibet area can divided into three genetic types: a rainwater type, a ice-snow melt-water type and a glacial-lake burst type. Theory of minimum energy dissipation is observed in longitudinal profile evolution of the debris flows. The theory is increase of velocity with adjustment of longitudinal gradient in valley. It is expression of the theory that average value (ū) of velocity along the valley relates to a forming index (N) of the valley longitudinal profile: ū∞f(N). The f(N) is a velocity function. For debris flow valleys of rainwater, melt-water and burst types, the f(N) are respectively {1/3-2/(N+1)(N+2)(N+3)]}^1/2,{2/3-2/(N+1)(N+3)]}^1/2 and N/(N+1)]^2/3. In process of evolution of the valley, increase of the value N with increase of the value ū,and shape of the valley longitudinal profile develop to a concave parabola pattern from a convex one, and morphological stages of the debris flow are a brow stage, a developing stage, a exuberant stage and declined stage, and stable stage of the basin in order. As examples with model debris flow valleys in southeast Tibet area, above model and regular of morphological evolution in the valley are examined.