The continuous snake-line method as a potential indicator of warning levels for landslides and lahar disasters

Abstract

A combination of long-term rainfall index (LRI) and short-term rainfall index (SRI) is commonly used as an indicator for estimating whether a series of rainfalls is likely or unlikely to cause landslides or lahar flows. The LRI represents the condition of soil structure instability. The SRI represents the condition of soil surface instability. A critical line can be drawn separating zones of occurring and non-occurring landslides or lahar floods. An occurrence of series of rainfalls can be monitored by drawing a snake line representing the progression of LRI-SRI values. By selecting an appropriate definition of LRI and SRI indices, a continuous snake-line can be developed for monitoring warning conditions of progressing rainfall events. Several existing definitions of LRI vs. SRI such as cumulative rainfall vs. hourly rainfall, working rainfall vs. hourly rainfall, and working rainfall with long half-life vs. working rainfall with short half-life were reviewed in terms of their effectiveness in delivering warning and practicability in drawing and reading the snake-line. A procedure for accommodating snake-line drawings of several definitions was established. The procedure was tested by several serial rainfall events and has shown good capability in facilitating monitoring activities for landslides or flood warnings.