| Fluvial geomorphology not only records the history of tectonic activity shaping topography,but also reflects the geomorphologic evolution process controlled by climate change.In view of the river aggradation-degradation process,previous studies have been made in recent decades,explained the evolution of fluvial geomorphology from tectonic activity difference,climate change,stratigraphic lithology difference and other aspects.The change of sediment and discharge caused by the change of these external factors is an important direct reason that impact on the river evolution processes.The research on this item gradually turns from qualitative description to quantitative description.As we all known,when the mountains river out of the mountains,the alluvial fan will be built in the piedmont.Many studies have found that the alluvial fans have multiple aggradation-degradation processes in the study area,which are related to the aggradation-degradation of the rivers on the fans.The aggradation-degradation process of an alluvial fan is mainly affected by the change of the ratio of discharge and sediment-supply at the top of the fan,the first question is: how much the ratio of discharge and sediment-supply change will lead to the change of the processes of aggradation-equilibrium-degradation? What is the relationship between the ratio of discharge and sediment-supply change and aggradation-degradation process? How to analyze these changes through the morphological characteristics of the alluvial fan? Focus on these problems,we plan to build numerical simulation model of the ratio of discharge and sediment-supply change on the piedmont alluvial fan and channel profile,and verify and analyze the actual morphological characteristics of the alluvial fan.Trying to obtain the quantitative relationship between the alluvial fan channel profile and the ratio of discharge and sediment-supply change.We select 4 alluvial fans in the piedmont of Qilian Mountains(Maying river,Fengle river,Hongshuiba river,Beida river),many times of aggradation-degradation process happened on these alluvial fans since the late Pleistocene.The thickness of deposition and incised depth can reach more than 200 meters,the modern fan surface represents the deposition surface before 15 ka,the deep incised channel represents the recent rapid incised process.This phenomenon indicates that the river deposition-incision is extremely intense in this area,which is closely related to the changes of the upstream sediments and water supply.This typical river process provides us with ideal research object and verification material for numerical simulation.We use the four alluvial fan rivers at the northern piedmont of the western Qilian Mountains as the study area,we investigated the characteristics of regional river terraces,alluvial fans and modern river channels in the field,and analyzed the history of their accumulation and cutting based on the formation ages of geomorphic surface obtained by others.Differential GPS and laser rangefinder were used to measure the profile shape of these fans and the incised channels.Based on the basic hydraulic geometrical formulation and hydraulic transport equation,a 1-D numerical model for the profile evolution of alluvial fan channel is established in combination with the actual situation in the study area.The numerical solution of the model was obtained by finite difference method,and the code was written in MATLAB.Through the simulation analysis and reconstruction of the actual river process,the main conclusions are as follows:(1)The ratio of water flux and sediment flux,referred to as the water-sand ratio,can be obtained by dividing the total discharge and sediment load on the upstream input fan by the channel width.Under the condition of constant channel width and sediment size,the river with a given water-sand ratio will evolve into an equilibrium channel profile with a constant slope,which related to the water-sand ratio.At the middle moment of accumulation,the disequilibrium profile presents a concave shape with a decreasing slope.At the middle moment of the cutting,the disequilibrium profile is in the form of convex upward along the slope.Different water-sand ratio results in different profile morphological characteristics and the time to reach the equilibrium.The larger the variation water-sand ratio is,and the shorter the time will be,the larger the absolute value of water flux and sediment flux will do the same effect.(2)When the water-sand ratio remains unchanged and the sediment size remains unchanged,the channel width increases along the river,river will form an equilibrium channel profile with a gradient gradually increasing downstream and slightly convex.As the channel width decreases,the river will form an equilibrium channel profile with the gradient gradually decreasing to the downstream and slightly concave.If the sediment size decreases gradually from upstream to downstream,the river will adjust to form a slightly concave equilibrium channel profile.(3)We used the measured data as the initial equilibrium channel profile,water-sand ratio changed from 40% to 50%,can be in close to the chronological results in this area,the rivers incised and accumulation rate consistent with our measurement results in the field work,show that in Qilian Mountains,by the end of the last glacial period to the Holocene,the climate change caused by river runoff variation must be at least more than 50%.(4)Compared with other rivers in this area,the erosion rate in the upper reach of Fengle river may change significantly with climate change,resulting in more sediments discharging into downstream alluvial fan,caused a different water-sand ratio change at the same time.Fengle river is still in the disequilibrium and continues to aggrade,slightly incised,caused a concave shape in the profile of the alluvial fan and channel. |
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