Back-analysis of the 2000 Yigong dam breach flood morphodynamics: Challenges and promises

Abstract

Landslide Dam-Break Outburst Floods (LDBOF) are among the most devastating natural hazards, significantly altering downstream river morphologies. The challenge of collecting field data, compounded by the inaccessibility of such events and the risk of equipment loss, hampers our understanding and capability to issue timely warnings. The 2000 Yigong event in China is one of the most significant recorded modern LDBOF occurrences, yet it suffered from sparse data collection. In our study, we have compiled a dataset that includes open-source elevation data, literature-based records, satellite-derived flood inundation extents, and direct field observations. We utilized a hydro-morphodynamic model to simulate the 2000 Yigong event. Comparisons of the simulated lake emptying, dam breaching, flood inundation, bank erosion, and channel infilling with field observations and similar studies indicate that our results are reasonable. Our analysis of the bed evolution during the event revealed it could be divided into two stages influenced by the flow dynamics and primarily by the sediment supply from the dam breach. In the initial stage, the breach development was transport-limited, resulting in intense erosion of dam material, which caused widespread aggradation in the downstream river channel over a distance of around 12 km. In the latter stage, the flow became sediment-supply-limited, redistributing the previously deposited sediments in the downstream reach. This process involved eroding sediment in the broader valley near the dam and depositing it downstream, creating an aggradation area. The aggradation ceased when the river reached a narrow valley, where the transported sediment load balanced with transport capacity, halting further aggradation downstream. The local valley characteristics significantly influenced flow hydraulics and subsequent bed evolution. Furthermore, our findings contribute to interpreting field observations, including forming new channels, deposition on bars, and the varying textures of bar surfaces from coarse near the dam to finer further downstream. We advocate for expanded data collection to advance the study of LDBOF events and enhance hazard mitigation efforts.