Mechanisms of debris-flow volume growth by eroding highly-consolidated landslide dams

Abstract

Erosion of landslide dams by a natural debris flow as it travels down a sloping channel can dramatically increase flow size and destructive potential. Most research on the erosion of landslide dams focuses on those newly formed due to recent earthquakes or heavy rainfall. In this study, the debris-flow erosion of so-called highly-consolidated landslide dams (HCLDs) that are typically remnants of ancient earthquakes or rainfall events is investigated by using satellite image interpretation, field measurements, and mechanism analysis. Several HCLDs were identified in two catchments in Zhouqu and Ganluo counties in Gansu and Sichuan province, China, where two high-magnitude debris-flow events occurred in 2010 and 2020, respectively. The existence of HCLDs resulting in wide and narrow reaches alternatively alters the boundary conditions of debris flows and significantly affects the flow erosion and deposition processes. It is observed that the flow discharge increased notably after passing through or during narrowing sections of the channels and the estimated discharge amplification coefficients are approximately 3.3 and 2.7, respectively. The two cases demonstrate that rapid erosion through narrowing sections and subsequent headward erosion play a key role in the debris-flow volume growth by eroding HCLDs. Additionally, other mechanisms such as deposition of erodible material upstream of the dam, localized vortexes in converging and diverging sections, and collisional stresses within the debris flows can further contribute to the erosion of HCLDs and the amplification of debris-flow magnitude. Recognizing these hazardous effects of HCLDs on increasing debris-flow volume is helpful for making non-structural and engineering countermeasures against similar debris flow disasters.