[A-CG42] Water and sediment dynamics from land to coastal zones

[ACG42-P02] Breaching flood impact analysis of landslide dams in the Yarlung Tsangpo River Basin

*Danyi Shen^1,2, Zhenming Shi^1,2, Ming Peng^1,2 (1.Key Laboratory of Geotechnical & Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China, 2.Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China)

Keywords:Landslide dam, breaching flood, peak discharge, overlapping effects, flood attenuation

Breaching floods of landslide dams occur frequently in mountainous area, and pose great threats to people and infrastructures downstream. Evidences show dam-break floods resulted in extremely catastrophic in the Yarlung Tsangpo River due to its high relief, narrow river gorges and numerous tributaries. On 9 April 2000, a large landslide occurred at the Zhamu Creek and blocked the Yigong Tsangpo River (a tributary of the Yarlung Tsangpo River). On 17 October 2018, another large landslide blocked the main course of the Yarlung Tsangpo River near Gyalha. The breaching of the Yigong and Gyalha landslide dams caused mega floods with peak discharge of 124,000 m³/s and 32,000 m³/s, respectively. Previous researches mainly focused on breaching floods of a single dam or cascade dams in the same river. However, the impact of breaching floods caused by simultaneous dam-break of mainstream and its tributaries on downstream are significantly different from that of a single dam-break flood.

This study presents works on flood routing caused by failures of the Yigong and Gyalha landslide dams in the Yarlung Tsangpo River and its tributaries, amplified floods by considering overlapping effects on the basis of these two landslide dams, and possible measures taken to mitigate breaching floods downstream. The erosion-based physical model is applied to predict flood hydrographs at dam site, and the one-dimensional hydrodynamic model is used to simulate the flood routing along the river and its tributaries.

Based on results of simulation, the peak discharge at dam sites and corresponding outflow rate along rivers agree well with field observations. The attenuation of peak outflow rate and flood propagation time along the river is decreased with the increase of peak discharge at dam site. The peak outflow rate is amplified when breaching floods of the mainstream and its tributaries are overlapping. Both flood peaks and inundated area downstream can be reduced by changing discharge magnitude and desynchronizing peak flow in different rivers. The work can serve as basis to predict overlapping floods of landslide dams and manage flood risks in the Yarlung Tsangpo River and its tributaries.