Effects of earth dam obstruction-collapse on dynamic motion of long-runout mudflow from 2023 Gansu Ms 6.2 earthquake in China

In 2023, an Ms 6.2 earthquake in northwestern China initiated a cascading disaster chain of loess liquefaction, landslide, mudflow and subsequent dam breach, causing more than 20 fatalities. The mudflow travelled an exceptionally long runout distance of approximately 3 km along a slope gradient below 3°. A road crossing the gully temporarily intercepted the mudflow and acted as a dam before it breached. Thus far, the influence of this dam’s blockage and collapse on the mudflow movement remains unclear. Based on field investigations and satellite imagery, we analyzed the landslide-mudflow evolution and simulated its transport and deposition processes using the Massflow software. Results based on different simulation scenarios revealed that the dam breach increased the mudflow peak velocity by approximately 30%. However, this velocity amplification effect attenuated rapidly downstream, and was mainly confined to the transport zone. The water input from a pond (~ 1,000 m²) upstream of the dam may have played a more important role in causing the long runout of the mudflow by increasing its water content. Earth roads are ubiquitous in the fragmented terrain of the Loess Plateau. This study demonstrates that roads lacking effective drainage facilities can exacerbate geological disasters such as debris flow, especially under seismic conditions.