THE RELATIONSHIP BETWEEN SEDIMENTARY STRUCTURES AND HYDRODYNAMICS IN THE MIDDLE REACH OF THE LUAN RIVER,HEPEI PROVINCE
Gao Jian Yan Qinshang Xu Shiyuan ( Deparmens of Geography,East china Normal University, Shanghai )
The relationship between sedimentary structures of fluvial deposits and hydrodynamics was investigated in the middle reach of the Luan River, Hepei Province by relating sedimentary structures observed in dry seasons to the hydrologic data recorded during the tlood stage. It seems that such method is fairly satisfactory to meet the need for quantitatively analysing the hydrodynamics of primary sedimentary structures. Various bed forms have ripples, sandwaves and dunes widely distributing in the study sections, while plane beds formed in flood stages are either covered or reworked by low-flow regime bedforms or destroyed by other external agents. Six distinct stratification types have been recognized in the examined trenches. With decreasing hydrodynamic force, they appear in the following order from bottom upward: parallel stratification, large-scale trough cross-stratification, tabular cross-stratification, small-scale climbing and trough cross-stratification and horizontal stratification. They were the products of the transport of the corresponding bed forms at a certain Froude number: ( 1 ) parallel stratification was produced by the plane bed transport at Froude number above 0.35; ( 2 ) large-scale trough cross-stratification was the products of dune migration at Froude number between 0.12 and 0.4;(3) tabular cross- stratification was formed by sandwave migration; and( 4 )sm-all-scale climbing and trough cross-stratification were the products of ripple migration and formed at the Froude number below 0.12.Horizontal stratification was the product of low regime flood plane deposits. As compared with the flume experiment results of other authors, the above mentioned values of Froude number appear to be relatively lower, owing to the differences in depth, grain size and other boundary factors between flume and river channel. In a vertical section, the following variations in the sedimentary fabric can also be observed from bottom upward: sorting of sediments grows better and worse in turn; grain size becomes finer; lamina frequency and its true dip angle turn less; the width and height of trough cross-stratification get smaller.