Preliminary Studies On Fluvial Processes Of Typical Alluvial Reaches In Yarlung Tsangpo River Basin

The longitudinal section of the Yarlung Tsangpo River(YTR)can be divided into alluvial river sections and canyon bedrock sections.Studying the longitudinal section helps to understand the river development process and explain the cause of the terrace.The YTR Grand Canyon is located in the eastern Himalayan tectonic syntaxis.It is a region with strong tectonic uplift and erosion.Under the action of earthquakes,glaciers and avalanches,it is easy to trigger secondary geological disasters such as collapse,landslides and mudslides.Causes siltation of the upstream channel.After the water storage in Zhikong Reservoir the Lhasa River in 2006,the sediment was intercepted and the clear water was drained to wash the downstream riverbed.The river bed is washed by water,which will affect the particle size distribution of the riverbed,resulting in changes in the river channel shape and ecological environment.Studying the riverbed particle size composition along the river bank is conducive to preventing natural disasters after the reservoir is built.Therefore,studying the riverbed evolution of the typical alluvial section of the Yarlung Tsangpo River Basin has reference value for disaster prevention and mitigation,reservoir construction and wetland protection.The results are as follows:(1)The best fitting form of the longitudinal section of most of the YTR is the exponential function.The concave curvature of the YTR,the Bodui Tsangpo and the Quzong Tsangpo cloth are 1.126,2.709 and 1.263,respectively,which are extreme concave curvature.The Niyang River and the Zep River are 0.218 and 0.134,respectively,which are low concave curvature;the rest of the rivers are moderately concave the YTR,Palung Tsangpo and Yigong Tsangpo appeared 6th,4th and 5th wide and narrow ridge-like forms respectively.The wide section was a braided section,the narrow section was a canyon section,and the latter was mainly tectonic activity.The product is also affected by collapse,landslides,mudslides,hail dams and so on(2)Taking Yigong Lake as an example,when the water depth of the barrier lake is increased from 4 m to 8 m,the single-wide sediment transport capacity is estimated to increase by about 3 times,but the sediment transport rate decays rapidly along the course,and the sediment enters the congestion.Severe siltation occurred in the lake to form more formed silt.At the same time,the water flow during the flood period washed away the original formed silt(such as sandbar)into irregular group sandbars,which made the riverbed shape continuously change.(3)Analysis of the UAV(unmanned aerial vehicle)data of five riverbeds in the upstream and downstream of the Zhikong Reservoir along the Lhasa River.The S1 of a riverbed upstream of the straight hole dam is not affected by the roughening of the river.The average fitting degree of the standard deviation ?R1=0.0347 is the smallest compared with the other four aerial survey points.The sediment particle size distribution is better than other aerial surveys.The point is scattered.From the river bed S2 at the downstream bend of the Zhikong Dam to the S5 at a riverbed downstream of the Maizhokunggar County,the roughening of the riverbed is gradually weakened.