Evaluating the post-earthquake landslides sediment supply capacity for debris flows

Abstract

Earthquake-triggered landslides provide sufficient loose material for the formation of subsequent debris flow events in earthquake-affected catchments. The landslides sediment supply capacity (SSC) for debris flows plays an important role in post-earthquake sediment transportation, which is influenced by landslide location during different post-earthquake periods. In this study, the logarithmic slope–area diagram was used to distinguish hillslope and channel processes in catchments affected by the Wenchuan earthquake, adopting a threshold between hillslopes and channels set at 1 km2. In comparison with the existing structural sediment delivery ratio (SDR), it was found that the SSC assessed using the SDR showed a decreasing trend of power exponent as distance increased, effectively eliminating the impact of earthquake-triggered sediment supply. To efficiently assess the SSC of post-earthquake landslides for debris flows, we proposed a novel dimensionless index, namely Mean Landslide Sediment Delivery Ratio (MLSDR), by considering the impact of earthquakes and eliminating the impact of catchment area. It was found that, the SSC of coseismic landslides assessed by the MLSDR in catchments affected by the Wenchuan earthquake could better reflect seismic impact, with higher values concentrated along the seismogenic fault of the earthquake, thereby highlighting the satisfactory applicability of the method in the Wenchuan earthquake-affected area. Moreover, it was found that the proposed index had a reasonable corresponding relationship with debris flow development in Wenchuan earthquake-affected catchments, mainly for values of 0.2 ≤ MLSDRout ≤ 2.25 and 0.3 ≤ MLSDRchannel ≤ 4.5, which could help identify potential debris flow development. The controlling factors of long-term changes in coseismic and post-earthquake SSC in the Wenchuan earthquake-affected region were assessed. It was found that SSC was controlled by earthquakes during first three years and subsequently controlled by precipitation and grain composition. This study will strengthen the understandings of the long-term evolution of post-earthquake debris flow.