Characteristics Of Sediment Grain Size And The Influence Factors In Alluvial Rivers

Fluvial sediment transport and the grain size variations of the sediment is one core problem in fluvial geomorphology and sedimentology.In recent 30 years,a large number of flume experiments and numerical modelings have been applied to the research,promoted the deeper development of the research.In addition,some researchs have taken the tendency of fluvial sediment grain size changing as a evidence,to deduce the changes of paleoclimate and paleostructure,which provides a new method to reveal the environmental change.However,due to the difficulty with measuring hydraulic parameters in ancient sedimentary strata,researchs on fluvial sediment transport and grain size variations still concentrating on modern rivers.A series of alluvial rivers developed in the north pediment of the Qilian Shan,which located in the northeastern margin of the Tibetan plateau,and the region has its distinguishing features in climate,structure,relief,and lithology,these geographical conditions is good for the research.In this study,we take 10 alluvial channels on alluvial fans,which developed in the north pediment of the Qilian Shan,as an example,to examine the river hydraulic parameters and sediment grain size which were collected in alluvial fan channels,on the basis of detailed illustrates of the theory about fluvial sediment transport and sorting,we use these theories to analyse and discuss the change characteristics of sediment grain size and the influence factors of 10 alluvial fan channels.According to our analyzing and discussions,we can conclude:(1)Our analysis indicates that the selective deposition is the dominant factor in cotrolling downstream fining of sediment grain size in these alluvial rivers which developed in the north pediment of the Qilian Shan,and the abrasion plays only a minor role in the process.(2)The downstream fining rate of fluvial sediment grain size is proportional to the sorting effects,and increase along with the increasing sorting effects.(3)The differences in flow dynamic between these alluvial rivers causes thedifferences in sorting effects.The sorting effects is strongly When the ratio of bed shear stress to critical shear stress is relatively small,and become weaker when it substantially increases.(4)The lithology of the upstream basin exerts an significant influence on the range of grain size in sediment supply,and the downstream fining rate of sediment grain size increase along with the enlarging the range of grain size in sediment supply.(5)The relief and climate of the upstream basin exert an influence on the condition of deposition in alluvial channels by influence the discharges of sediment and water,and the downstream fining rate of sediment grain size increase along with the increasing deposition rate.(6)The downstream fining rate of fluvial sediment grain sizes is inversely proportional to river lengths,but this relationship can only establish when river lengths differ greatly,and vice versa.(7)The gravel-sand transition in the Fengle River is relevant to a segregation in the threshhold Shields stress between gravel and sand,but in the Liyuan River,it is caused by the difference in deposition rate between two reachs.