The Evolution Trend Of Debris Flow Basins In The Upper Minjiang River Under The Diversion Of River Valley Geomorphology

The upper Minjiang River are located in the transition zone from Sichuan Basin to Qinghai-Tibet Plateau.With high-steep mountains,strong neotectonic movements and frequent earthquakes,the debris flow disasters in the upper Minjiang River are remarkable.Debris flow is one of the manifestations of basin evolution mechanism,and its activity intensity is closely related to evolution.At the same time,the evolution of basin is progressive layer by layer,and it has obvious characteristics of valley geomorphology differentiation.Currently,the studies on the evolution of debris flow basins are mostly from the perspective of geomorphological evolution,lack of the description of the mechanism of debris flow disaster,and less attention to the progressive evolution relationship between the hierarchical watershed systems.Therefore,from the perspective of disaster,this thesis selects the small basins in the upper reaches of Minjiang River as the research object.Constructing the hierarchical basin system and debris flow disaster pregnancy system,evaluating the susceptibility of debris flow,establishing the evaluation model of evolution stage,quantitatively dividing the evolution stage of small basins.exploring the characteristics of debris flow activity in different evolution stages and the background of valley development,the evolution trends of small basins under diversion of river valley geomorphology are obtained.The main results are as follows:(1)The system indexes and debris flow distribution of debris flow disaster pregnancy system are summarized.In the development system,the debris flow mainly occurs in the small basin where Hi 0.4~ 0.6,the Gi 0.3~ 0.5,the basin area 1~10 km2,the shape coefficient 0.2~0.8,the erosion transfer coefficient 0.4~1,and the gully slope 100‰~500‰.In the physical system,when the rock hard-coefficient <3,debris flow rate is the highest,in the fault distance <10 km or seismic intensity 7~11 area debris flow distribution is more.In the environmental system,debris flow is distributed in dry-hot valley with less annual rainfall,Dujiangyan-Wenchuan rainstorm center zone and ndvi>0.5 vegetation area;with the increase of land use index,the incidence of debris flow increases gradually.(2)The extremely high-prone areas are mainly distributed on both sides of the main stream and tributaries of the upper reaches of the Minjiang River.The high-prone and middleprone areas are scattered in the study area.The low-prone areas are mainly distributed in the western and northern alpine canyon transition zone to the plateau.Among the three systems,the developmental system is the most important factor,the contribution to debris flow is 0.531,and the contribution of environmental system and physical system is 0.278,0.191,respectively.(3)The comprehensive incidence rate of debris flow in maturity stage is the highest,which in old satge is the lowest.The order of small basins evolution stage according to the gestation ability of debris flow is as follows: maturity stage > early maturity stage > infancy stage > late maturity stage > old stage.(4)The erosion capacity of rivers is different.The main stream of upper Minjiang River decreases gradually from downstream to upstream(from south to north),and in tributary rivers,According to the order of erosion ability from strong to weak,Shou River > Yuzi River > Ziguao River > Heishui River > Xiaoxing gully.The distribution of small basin evolution stage has very strong valley differentiation characteristics.On a whole,the main stages of small basin evolution in each valley coincide with the erosion background of the river.(5)The evolution trends of small basins in Songping gully,Mengtun River,Bipeng gully and Chibusu gully are from infancy stage to early maturity stage.The evolution trends of small basins in Caoba River,main stream of middle Minjiang River,Shou River,main stream of Yuzi River and main stream of Zagunao River are from early maturity stage to maturity stage.The evolution trends of small basins in Mouni gully,main stream of north Minjiang River,Daxing gully,Xiaoheishi River,Heishui River Source and Zagunao River Source are from late maturity stage to maturity stage.The evolution trends of small basins in Yangdong River,Maoergai River and Xiaoxing gully are from old stage to late maturity stage.The small basins maintain early maturity stage,which are located in Zheng River,Sanlong basin and main stream of Heishui River.The small basins maintain maturity stage,which are located in main stream of south Minjiang River and Baisha River.Combined with the evolution trend of small basins and the debris flow activity in each evolution stage,the general development trend of debris flow activity in each second-class valley can be preliminarily inferred.