371 / 2024-02-29 22:37:40

Influence of debris flow events on sediment transport in the main river under a changing climate: an example of the Tianmo Gully, Southeast Tibet Plateau, China

debris flow; sediment transport; climate change; Southeast Tibet Plateau

Abstract: Parlung Tsangpo basin is located in Southeast Tibet, China, characterized as deep valleys and steep river channels with abundant precipitation. This is one of the basins that are most sensitive to climate change in the Tibet Plateau. Natural hazards like collapses, landslides, debris flows are frequent in the tributaries along the main river due to river incision, earthquake, heavy rainfall or rapid heating in this basin. These natural hazards cause intense episodic sediment supplies to the main river and hence, affect the sediment transport significant in the main river. However, limited research has been done on the influence of debris flow events in tributaries on the main river near the confluences in Parlung Tsangpo, specially under climate change.

To fill this knowledge gap, the Tianmo Gully was chosen as a representative of the tributaries in the Parlung Tsangpo and this study aims to reveal the impacts of historic debris flow events in the Tianmo Gully on the sediment transport processes in the Parlung Tsangpo River under climate change in the past two decades. The relationship between the volume and velocity of debris flow and climatic conditions was firstly established based on historical data. Based on identifying the governing physical processes when a debris flow enters the main river channel and the sediment supply is transported downstream in the main river, a physically-based model on the sediment transport in the main river was built and validated by field measurement. We then used the model to evaluate the sediment transport in the main river following the historic debris flow events with various scales and climatic conditions.

The results show that the sediment transport in the main river was enhanced immediately a debris flow entered the main channel for most debris flow events. This is the result of the increase of local flow velocity and transport capacity at the confluence owing to narrowing of the main channel and increase of the riverbed. Extreme climate events such as heavy rainfall and rapid heating would significantly increase the scale and frequency of debris flow events, which eventually lead to greater fluctuations in the sediment transport processes of the main river.