The formation and breaching of a short-lived landslide dam at Hsiaolin Village, Taiwan - Part II: Simulation of debris flow with landslide dam breach

Typhoon Morakot (2009) caused serious damage in southern Taiwan due to intensive rainfall with long duration. The issue of greatest concern arising from the disasters brought about by this extreme event was the burying of the entire village of Hsiaolin by a massive debris flow and landslide. Based on seismological and near-surface magnetic data, this tragic scenario arose due to a combination of events, a massive landslide, the formation of a landslide dam, and the consequent debris flow when this dam was breached. The objective of this part of the study is to investigate the spatial and temporal characteristics of the debris flow induced by the landslide breach. The US National Weather Service BREACH model and the Federal Emergency Management Agency approved FLO-2D model are integrated to facilitate the investigation of this catastrophe. A series of simulations including a 2D rainfall-runoff simulation over the Cishan River basin, landslide dam breach routing, and 2D debris flow simulation around the Hsiaolin Village were conducted. Hydraulic calculations were performed to determine the equivalent top elevation of the landslide dam based on inflows computed from the 2D rainfall-runoff simulation in association with the Digital Terrain Model (DTM) and upstream constraint of the backwater inundation areas. The hydrograph of the upstream inflow which induced overtopping failure was provided by a 2D rainfall-runoff simulation using the FLO-2D model calibrated by comparison with the downstream discharge record. The longevity of the landslide dam was less than 1h, and it took only about 8 minutes to completely breach. The peak discharge rate of this massive landslide dam breach was 70,649m³/s. The dam break hydrograph was then used for upstream inflow to drive the FLO-2D debris flow simulation. The average sediment concentration by volume was 0.362. The simulated deposited sediment depth showed a reasonable match to the differences of DTMs before and after the disaster.