Spatiotemporal characteristics of discontinuous slope failures

Slope failures are important material supplies for debris flows, and field observations have indicated that failures are random and discontinuous. However, few studies focus on the nature of failures in succession. This study reports groups of field experiments of soil failures under artificial rainfall on slopes in two debris flow valleys, the Jiangjia Gully (JJG) in Yunnan Province, and the Niujuan Gully (NJG) in Sichuan Province, in southwest China (Fig.1).

Fig.1 Experimental sites of the study (upper, NJG; lower, JJG)

It is found that failures occur separately and intermittently on slopes; a slope process is composed of a failure sequence (Fig.2), which presents similar properties under different rainfalls and slope conditions: 1) the sequence is primarily random, with weak autocorrelation and small correlation to time progress and less dependence on rainfall; 2) the time interval between failures satisfies the exponential distribution, and the average interval decreasing with rainfall intensity, implying the frequency increases with rainfall intensity; 3) the magnitude of failure fluctuates up to three orders, from several to hundreds of volume unit (10^-3m³); and the distribution follows the power law, with total amount increasing with rainfall intensity.

Fig 2 Failure sequences under different rainfall intensities on the experimental slopes

We propose that these properties are ascribed to the spatial heterogeneity of soil, which can be described by two parameters, m and D_c, of the grain size distribution (GSD). The point-to-point variation of (m, D_c) leads to dramatic changes in the distribution of strength, infiltration, and pore water pressure generation, and finally results in the variety of failures across the slope.

Correspondingly, the discontinuous failures translate into separate debris flow surges in the tributaries, thereby providing a scenario for surge formation in the mainstream flow of the valley. It is suggested that surges in the mainstream channel result from cascading development of tributary surges, and that the spatiotemporal characteristics observed in mainstream surges are rooted in the sources of slope failures.