Discussion of landslide self-organized criticality and the initiation of debris flow

The study contains descriptions of landslides and debris flows in a small upland catchment in Taiwan between 1986 and 2004. This catchment, the Chushui Creek, is situated near the epicentre of the 1999 M7·6 Chi-Chi earthquake. This is an area with high background erosion rate. The Chi-Chi earthquake caused a sharp increase in the rate of mass wasting in the epicentral area, and the data presented in this study illustrates the geomorphic change associated with the earthquake. The measurements of the geometry of the trunk stream of the Chushui catchment also show its change due to the impact of a strong typhoon in 1996. Two channel reaches that were affected by separate debris flows during this event were identified. Each reach has three sections: scour, transfer and deposition. Cross profiles of these sections show a systematic change from V-shaped bedrock channel in the scour areas to flat-floored channel in the transfer and deposition areas. Debris flows also occurred on other occasions in this channel, and their frequency has increased since the 1999 earthquake. In addition, this study contains precise geographic and statistical descriptions of the landslides triggered by three typhoons and an earthquake that affected the catchment since 1996. The total landslide area is measured for different time intervals, and a marked increase in landslide incidence is found after the 1999 earthquake. Frequency-area plots of the 126 landslides in the catchment indicate a power-law scaling with an exponent of about -1·5 of these quantities, as has been observed in other studies. We propose that a link exists between landslide frequency-area distributions and initialization of debris flow in the catchment. It may be classified as a self-organized criticality process with a critical frequency-area distribution for the landslide population. In this case, the landslide frequency-area distribution is useful in quantifying the severity of the trigger and the contribution of lanlides to debris flow.