Palaeoflood History Of The Upper Yellow River Recorded In The High Alluvial Flat Sediments Of The Guide Basin

This research will contribute to understanding the Holocene environmental change in north-eastern Tibet Plateau,as well as providing references to local hazard prevention,hydraulic engineering,and sustainable development,by reconstructing the palaeoflood history in the Guide Basin,where the upper Yellow River wriggles.During a field trip in June 2008,220 high-resolution sedimentary samples were collected from two profiles GD1 and GD2,both of which were located at the Yellow River's high alluvial flat near Heyin Town,Guide County.The samples' sedimentological properties including panicle size and percentage of calcium carbonate were determined later for distinguishing flood events recorded in the sedimentary layers.The following conclusions have been made.(1) Nineteen flood events have been recognized.Both the profile GD1 and GD2,which formed as a result of modern flood sedimentation,represent nineteen different flood periods.Coarse silt and very fine sand are the most important compositions in profile GD1,accounting for 43.68%and 17.33% respectively,whereas clay for 12.37%.Similarly,coarse silt,very fine sand and clay constitute the body of the particles of the profile GD2.(2) A semi-quantitative model entitled Normal Alluvial Flat Particle-Size Model has been created for estimating flood dynamics.The following hypotheses are proposed,â‘ there are no notable lateral migration or alluvial slope change;â‘¡flood erosion is rare at the high position of alluvial flats;â‘¢all sedimentary layers are approximately horizontal.The first step is to constrtict an ideal particle-size sequence by considering that each flood is of similar(/same) discharge.Then experimental particle-size data and that of the above-mentioned ideal sequence will be compared to estimate the flood dynamics' relative differences. (3) The particle-size distribution in both the profile GD1 and GD2 varies according to the depth of different layers,and therefore is probable to apply the proposed model.The order of flood dynamics recorded in different layers of GD1 is arranged as:19^th＞7^th＞8^th＞9^th＞12^th＞18^th＞6^th＞5^5th＞17^th＞13^th＞2^nd＞3^rd＞11^th＞1^st＞10^th＞15^th＞16^th＞14^th＞4^th,while similar obvious dynamic changes exist in the profile GD2.(4) Statistical relation between proxies of percentage of calcium carbonate and particle-size has been constructed.As it is found that lower content of CaCO₃ exist in coarser layers,from which high statistical negative correlation is calculated,inferences that calcite mineral may concentrate in finer sediments in our research area and percentage of calcium carbonate could be used as a valuable proxy for analyzing flood periods have been made.However,more experiments should be designed in the future to find out how this proxy reveals flood discharge.(5) The formation time of the profiles has been estimated.Consulting references concerning the flood period of upper Yellow River,we have deduced that the alluvial flat formed as early as 330a B.P.under a hypothesis that main sedimentary dynamic is a kind of quasi-30-year flood.However, chronology should be further established through more reliable ¹⁴C or luminescence dating.