A Study Of The Evolution Process And Morphometric Features Of Alluvial Fans In Northern Qilian Mountains Based On Numerical Modelling

Alluvial fans,as a common geomorphic unit,their morphology,development and evolution are affected by environmental factors such as climate,tectonic activity and geological settings of drainage basin.By using landscape evolution models,we simulate the formation and evolution process of alluvial fans,then analyze how climate and tectonic factors have effect on alluvial fans to find the mechanism behind.With modelling results,we explain the regional climate and tectonic information recorded by morphological and sedimentary features of alluvial fans,then discuss the relationship between morphological features of alluvial fans and external factors further.This study supplies new methods and thoughts for research work of surface processes in arid region.Firstly,we extract and analyze geomorphic parameters of alluvial fans in northern Qilian Mountains.Then,we construct a catchment—fan system model based on stream power model and diffusion equation.In this study,two small watersheds in the northern foothills of Qilian Mountains are chosen to process case study for numerical natural experiments.Finally,we analyzed the physical processes and mechanisms of the development and evolution of alluvial fans.And,we have discussed the response of morphological characteristics of alluvial fans to regional environmental changes.The main conclusions of this paper are as follows:1.A large number of alluvial fans were developed along the northern foot of Qilian Mountains.Based on the analysis result of these geomorphic parameters between alluvial fans and their drainage basins,we find that the alluvial fans often have larger area and lower gradient when the drainage basins have lager size and higher relief.Also,it is easy to produce wide fans.2.The height difference of the fans has a poor correlation with the drainage area,but the height difference has a strong correlation with some other morphometric characteristics of the drainage basins,such as mean slope,mean relief and mean local relief.In addition,the relations between the height difference and the fan radial length are significant.All of those relations may be due to the tectonic activities.Considering the slender topographical features of the area,the uplifting of the mountains restricts the development of the alluvial fans.3.With the landscape evolution model,we have done some numerical experiments.we found that the profile of the alluvial fan is more sensitive to the change of tectonic activity and is also affected by the precipitation:increment of the fault slip rate,leads to increases in the catchment-averaged erosion rates,sediment flux at catchment outlet,the height difference of the alluvial fan and fan slope,and the sediment flux at the fan toe.Increasing precipitation rate leads to an increase in the catchment-averaged erosion rates and the sediment flux at catchment outlet,but results in decreases in the height difference and slope of the fan,causes more material flowing out of the fan toe,the sediment flux at catchment outlet becomes larger.Under the continuous action of uplift and precipitation,the simulated profile of the alluvial fan gradually tends to reach a relatively stable state.4.The diffusion coefficient?in the model reflects the precipitation rate.The larger the value of?is,the more straight the profile of the alluvial fan.The coefficient k_p in stream power model contains the aspect of the basin relief.The higher the relief is,the larger the k_p is.In the simulation results,with the increase of the drainage basin area,the slope and the height difference of the alluvial fan decreased.In morphometric measurements,there was no apparent correlation between the fan height difference and the basin area.The results of the study help us to understand the implications of the morphological indicators of alluvial fans and build a quantitative relationship of how the catchment—fan geomorphology system response to the changes of climate and tectonic.It can provide an example for further application of numerical simulation methods in the study of alluvial fan and relative landscape evolution in Qilian Mountains.Meanwhile,it is of great significance to understand the development and evolution of the alluvial fan in advance,it is also an important way to study the history of environmental change in the arid region of Northwest China.