Characteristics And Controlling Factors Of Alluvial Fan Slopes In The Northern Piedmont Of The Qilian Shan

Geomorphic characters of alluvial fans are one of the special aspects of landforms that received abundant concerns in geomorphology, while alluvial fan slopes are the key characteristics in describing the morphology of alluvial fans. The controls on alluvial fan slopes have been studied by empirical investigations, lab simulations and numerical modelling, however, the dynamic processes in forming fan slopes are far more understood. There are also significant conflicts in interpreting the process in controlling the fan slopes, and much more work should be done before fully understanding the mechanics.We selected alluvial fans developed in the northern piedmont of the Qilian Shan, NE margin of the Tibetan Plateau, to study the characteristics and controlling factors of fan slopes. Alluvial fan data were obtained by field investigations and analysis on DEM data. Obtained data include alluvial fan areas, fan slopes, drainage areas, particle sizes, hydraulic radiuses, mean local reliefs, Melton’s ruggedness number, etc. Based on these data, we analyzed how the environmental factors affect alluvial slope and the characters of longitudinal profiles of alluvial fans.Along the northern piedmont of the Qilian Shan, results of alluvial fan data extracting and field measurements yield that following characteristics for the alluvial slopes and depositions:1) Comparing the morphological characteristics of alluvial fan in different regions in Qilian Shan, we found that there is an exponential relationship between alluvial slopes and drainage areas. The relationship is different according to different areas. Furthermore, the longitudinal profiles of alluvial fans are also different in different areas, such as the profiles are relatively straight (slopes change slowly from the apex to the toe) in the eastern and western of Qilian Shan, and are greatly concave up (slopes change fast from the apex to the toe) in the Yumu Shan and Longshou Shan.2) The alluvial fan particle sizes are mainly related to the lithologic characteristics of the drainage basin, such as the grain sizes are relatively large in granite basins, while they are relatively small in basins with sedimentary and metamorphic rocks. From fan apexes to toes, the particle sizes decrease and sand contributions gradually increase with all the fans, while the particle size decreases faster in granite basins than that in the basins with sedimentary and metamorphic rocks.3) An positive linear relationship between hydraulic radiuses and median particle sizes is found with the fans we studied. There is no obvious relationship between the median particle sizes and alluvial slopes. Thus we propose that the particle sizes are mainly controlling the changes of hydraulics radiuses of alluvial fan channels, and have little influence on alluvial slopes.By analyzing on the geomorphic and hydraulic characteristics of alluvial fans, and by analyzing on the computing results of physical equations of sediment transporting, we conclude that:1) The hydraulic radius is mainly controlled by the grain size in the fan channel.2) The concave up longitudinal profiles of alluvial fan channels are formed by the change of sediment flux on fan channels.3) The differences on concavities of alluvial fan longitudinal channels are mainly related to the relief and precipitation of the drainage basins.