Debris flows in Lebai gully along the Yarlung Tsangpo River in Tibet: characterization, causes, and dynamic prediction of potential debris flows

摘要

In the past 20 years, Lebai gully (Linzhi, Tibet, China) has been affected by several debris flow events. These debris flow events caused the blockage of three-quarters of the river channel at the mouth of the Lebai gully, threatening thousands of acres of cultivated land, houses, and residents' property in the upper reaches of the Yarlung Tsangpo River. However, few investigations have been carried out on the debris flows in the Lebai gully, and the triggering mechanism of Lebai debris flows remains elusive. In this paper, we presented a detailed demonstration of the geological environment and geomorpho-logical features of the Lebai Basin, and a detailed geohazard history of the Lebai gully was collected and presented based on interviews with local villagers. To provide information on the lithological, hydrological, and geotechnical characteristics of the terrain in Lebai gully, the electrical resistivity tomography (ERT) method was carried out to research the subsurface characteristics of debris flow fans. The ERT results illustrated the two-layered vertical strata of the debris fan, and the shallow layer of relatively low resistivity, which has thicknesses of 5-18 m, could be described as Quaternary colluvial deposits, while the deep layer of relatively high resistivity could be considered Precambrian bedrock. Taking a relatively large-scale debris flow on August 20, 2003, as an example, a detailed discussion was carried out on the possible factors triggering this event, and it was found that the Lebai debris flows were closely related to pre-event rainfall, air temperature, and antecedent earthquakes. Moreover, with the intention of evaluating the future risk of river blockage and helping to make rational decisions on developing suitable mitigation measures, the unstable deposits found in field investigations were generalized to geometric models according to their geometric characteristics, and the volumes of these unstable materials in each location were estimated using digital elevation models and geometric spatial analysis methods. A dynamic model named RAMMS was used to predict these unstable materials' dynamic processes and final deposit characteristics. The simulation results revealed a great possibility that the Yarlung Tsangpo River will be blocked at the mouth of Lebai gully in the future, and it is necessary to take further mitigation measures. The research in this paper contributes to a better understanding of the debris flows in the Lebai Basin and provides meaningful and valuable information for debris flow hazard mitigation in the future.