Spatiotemporal characteristics of discontinuous soil failures on debris flow source slopes

Abstract

Slope failures represent important supplies of material for debris flows, and field observations have indicated that failures across a slope are random and discontinuous. However, few studies have focused on the nature of successive failures. This study conducted field experiments on soil failure under artificial rainfall on slopes in two typical debris flow valleys, and established the random nature of the failure sequence that comprises a slope process. It was found that failures occur separately and intermittently on slopes. Furthermore, failure sequences under different rainfall and slope conditions have certain characteristics in common: 1) a failure sequence that comprises primarily random uncorrelated individual failures is independent of rainfall conditions; 2) the time interval between failures satisfies an exponential distribution, and the average length of the interval decreases with increasing rainfall intensity; and 3) the magnitude of failure fluctuates by up to three orders, from several to hundreds of volume units (10−3 m3), and the distribution follows a power law whereby the total amount increases with increasing rainfall intensity. We propose that these properties are ascribed to the spatial heterogeneity of the soil, which can be described by the grain size distribution (GSD). The variation of GSD parameters across a slope determines the randomness, intermittency, and fluctuation of shallow failures. These findings support the development of scenarios for the occurrence of intermittent debris flow surges supplied by random failure sequences.