Assessment of local outburst flood risk from successive landslides: Case study of Baige landslide-dammed lake, upper Jinsha river, eastern Tibet

Two large landslides in October and November 2018 sequentially dammed the Jinsha river at Baige village, eastern Tibet, China. Subsequently, breaching of each dam induced massive outburst floods that posed a severe threat to downstream cities and infrastructure. Field investigation indicates volumes of the first and second landslide dam are about 24.5 x 10(6) m(3) and 8.53 x 10(6) m(3), respectively. However, the peak discharge of the flood generated from the second landslide (3.1 x 10(4) m(3)/s) was significantly larger than that from the first (1.0 x 10(4) m(3)/s). The second peak discharge would have been 5.15 x 10(4) m(3)/s if the dam breached naturally. In this study, we developed two geometry parameters: effective dam height He (elevation difference between over-topping level and river bottom) and a narrowing number distribution Nr(H) (measures the degree to which a river section is occupied by landslide materials to elevation H) which represents river section narrowing effects of successive landslides. Using numerical simulations, we show that He dominates peak discharge, and it has a linear relationship with peak discharge with slope of 1009.4 m(2)/s. Furthermore, the dam includes two sub-areas: a higher part (SB-1) and a lower part (SB-2). Two floods only eroded SB-2. However, breaching of SB-1 and breached SB-2 still narrowed the river. The narrowing effects of the first and second breached dam on the river channel are around 0.35 and 0.5, respectively. Spillway and landslide runup deposits increased the local flood risk by narrowing and lifting the local river section; the first landslide promoted the second landslide, which occupied the same area and forms a dam with high He value. Consequently, a more catastrophic flood could be triggered by a small subsequent landslide. Spatial superposition phenomenon of successive landslides increased the local flood risk. This research quantitatively analyzed the influence of the geometry of dam induced by successive landslides on the process of outburst floods and the risk.

Automated summary

The study demonstrates that successive landslides pose a threat to river channels, potentially triggering more severe floods. Analysis of two flooding events shows that the effective dam height significantly influences peak discharge of floods, exhibiting a linear relationship. Additionally, the narrowing effects of river sections during landslide processes also increase flood risk.