Interpreting the influence of rainfall and reservoir infilling on a landslide

Abstract

This paper focuses on the potential influence of rainfall parameters and reservoir levels on displacement of a reservoir landslide. Displacements were monitored by a borehole inclinometer and four surface survey points. Monitoring data were obtained from 1985 to 2010, covering the periods before, during, and after impoundment of the reservoir. A simple stability analysis has been performed to understand the real influence of reservoir level changes on slope stability and to interpret the movement pattern during the impoundment period. Analyses of available data indicate that the overall trend of the displacement could be characterized by three distinct periods, corresponding to a slow creep phase before infilling, a sharp increase in velocity during the infilling, and another slow creep phase after infilling. Displacement rates show slight fluctuation in both “slow creep” episodes. Before impoundment, the displacement pattern was clearly controlled by rainfall, in which the displacement rate during the rainy period was twice that in the dry period. Displacement response to rainfall had a time lag of about 1–2 months. During the first 75 days of impoundment, displacement increased quickly with a linear trend, following the rising of the reservoir level with a rate of 2 m/day. Then, the displacement rate gradually decreased after the infilling of the reservoir until velocities slightly higher than that were measured before the infill. The result of the stability analysis coincides with this trend, indicating the complex effect of the impoundment: First, the decrease of stability of the landslide prevails over the increase in lateral support of the reservoir lake, while at a critical level, the increase in lateral support prevails over the decrease in stability. During the period of normal operations, an inverse relation between water level fluctuations and velocity fluctuations was observed, and rainfall had no significant effect on the sliding motion, with the exception that surface displacements showed a slight increase.