Study on the activities of the massive debris flows and sediment transport characteristics in the Grand Bend of the Yarlung Zangbo River Gorge, Xizang

The Grand bend of the Yarlung Zangbo River, located at the boundary between the middle and lower reaches of the basin, is one of the most developed areas for debris flow activities in China. This study focuses on the Paiba gully and Qulu Valley on the south bank of the entrance section of the Yarlung Zangbo River’s grand bend. We collected and sorted out the historical optical satellite image data of the region in the past 50 years. By combining multi-period remote sensing image comparative interpretation and field investigation, we carried out the research on the characteristics of the development and distribution of debris flow sources, the characteristics of sediment transport, and the historical evolution of debris flow activities. The results show that from 1980 to 2023, the annual average sediment volume of the Paiba gully entering the main river was about 1900 m³. Before the construction of a retaining dam in 2016, the annual average sediment volume of the Qulu Valley entering the main river channel was about 700 m³. Both Paiba gully and Qulu Valley experienced large-scale debris flow around 1980. Over the next 40 years, Paiba gully was mainly characterized by floods and small-scale debris flow, while Qulu Valley experienced multiple debris flow events of varying scales. In recent years, the number and area of material sources in Paiba gully have increased relatively slowly, and the susceptibility of debris flows is moderate. In contract, Qulu Valley has seen significant increases in the quantity and area of channel and slope material sources, leading to high debris flow susceptibility. Sources developed in the Qulu Valley, and the quantity and area of these material sources have increased significantly in recent years. The susceptibility of debris flows is high, and the previously built control projects are functioning well and can effectively reduce the risk of debris flows.