HAZARD ASSESSMENT AND TOPOGRAPHIC CONTROL ON POST-SEISMIC DEBRIS FLOWS: INSIGHTS FROM THE 2015 GORKHA EARTHQUAKE

Postseismic debris flows are an important component of the cascading hazard chain following large earthquakes, propagating destruction and loss of life many kilometers downstream from the steep hillslopes where coseismic landslides occur. Debris flow hazard assessment in the wake of a large earthquake is extremely time-sensitive, as landslide material in steep, colluvial channels may be mobilized during the next significant rainfall after the earthquake. Therefore, improving our understanding of when and where postseismic debris flows are most likely to take place, and developing a rapidly applicable model for hazard analysis, is critical for protecting vulnerable lives and allocating disaster response resources. A major gap in understanding of debris flow processes and the hazards they pose is the lack of large datasets of debris flows associated with a single earthquake. We compile an inventory of debris flows associated with the 2015 Gorkha earthquake in the Nepal Himalaya to field test prior experimental studies on debris flow initiation and develop a new model for hazard assessment. We are able to identify and distinguish two distinct mechanisms for debris flow initiation: 1) co-seismic landslide debris deposited in steep channels is remobilized during monsoon rains; and 2) post-seismic landslides occur in saturated conditions and become fluidized and channelized. Debris flows initiating by these different mechanisms appear to occur in channels with distinct geometry and may have different runout characteristics. We find that debris flows initiating by mechanism 1 initiate in channels with a similar threshold slope to that identified in flume experiments. These debris flows occurred almost exclusively during the 2015 monsoon, indicating that unstable landslide debris is largely flushed out from steep, debris-flow susceptible channels during the first rainy season. Identifying channel segments which exceed the threshold slope for debris flow formation and which are within a certain distance of a co-seismic landslide offers the potential to develop a swiftly applicable model for assessing hazards posed by post-seismic debris flows, based only on a landslide map and topography data.