Wei River Middle Reaches The Ancient Flood Viscous Flow Sediment Characteristics Research

The Middle reaches of Weihe River extends from Baoji gorge to steel cable bridge of Xianyang, and flows though Baoji city, Yaglin and Xianyang city. Since ancient time, this region had frequent flood disasters. But there were only a few tens of years gauged flood data, which can not meet the need to estimae flood frequency and guarantee the accuracy of flood control standard for hydraulic engineering in both sides of Weihe River. So it was necessary to study the palaeoflood events in the middle reaches of Weihe.By field investigation, the Holocene palaeoflood slackwater deposits(SWD) in Holocene loess-paleosols profile was found in middle reaches of Weihe River. The profile in Nansi village which was not affected by human race was selected as object of study and fifty six samples were successive collected. The grain size, the magnetic susceptibility, content of CaCO3, loss-on-ignition, hygroscopic moisture, content of chemical elements and the surface micro-structure of quartz grains were analyzed in the laboratory, the results showed that the deposits Characteristics of SWD were different from those of loess and palaeosol, and this SWD is identified as the most typical Holocene palaeoflood deposits. According to stratigraphic configuration and OSL dating for slackwater deposits and Loess-paleosol samples, the age of the extreme palaeoflood is fixed between3200a B.P. and2800aB.P. The conclusions maily are as follows:(1) Acording to history material, there are135times floods counted from1532to1949and there is one time in there point two years. Basis of mathematics analysis, the breaking of floods has apparent sections. There are more floods in warm climate. unstable climate and destroying ecology environment strongly by humen, and there are few floods in cold climate.(2) By field investigation along the Weihe River, the Holocene palaeoflood slackwater deposits(SWD) was being first level river terrace, which have hummocky bedding, densification structure, muddy reddish-brown being wide difference with loess-paleosols deposits. (3) Experimental results show that:the magnetic susceptibility and frequency magnetic susceptibility of SWD lower than loess-paleosols, the SWD is silty clay, median size and average particle size are lower; sorting coefficient of SWD is lower and the sorting property is better, all results show that the SWD is typical palaeoflood deposits.(4) The analysis of the chemical elements show that the contents of Fe2O3、Al2O3、 Ba、Cr、Cu、V、Rb in palaeoflood slackwater deposits were higher than that in paleosol and modern loess formation, and the content of SiO2、Zr in palaeoflood slackwater deposits were lower than that in paleosol and modern loess formation. The chemical elements characteristics of SWD also indicate SWD is fine particle slackwater deposits which were formed in weak hydrodynamic condition. Sr、P、CaCO3、CaO leaching from morden loess were enrichment deposited in SWD, which shows SWD have better sticky.(5) Though observing and counting surface micro-structure characteristics of quartz grains, The triangular marks and v-shaped pit characteristics of SWD are obvious, which indicate that the SWD are deposited in running water force environment. Though analyzing rounded degree of quartz grains, modern loess(Lo) and paleosol (So) lead to secondary edge-secondary circle, Malan loess(L1) and transition layer(Lt) lead to cusped edge-secondary edge, rounded degree of quartz grains from SWD is relatively disperse, the proportion of secondary edge-secondary circle、secondary edge and cusped edge-secondary edge are relatively balanced, which show that source of quartz grain is relatively disperse and the ridges of quartz grain are less changed because of weak hydrodynamic condition.(6) Though contrasting slackwater deposits characteristics from upper reaches, slackwater deposits characteristics of Weihe River are mostly silty clay. Magnetic susceptibility and clay/silt ratio of upper reaches are highest, and next are slackwater deposits in middle reaches and lower reaches. Md and content of coarse silt in lower reaches of Weihe Rivers are biggest and next are slackwater in deposits middle reache and upper reaches. This characteristics of particle size distribution and characteristics of the magnetic susceptibility values have important relationships with climate.(7) According to stratigraphic configuration and OSL dating for slackwater deposits and Loess-paleosol samples, the age of the extreme palaeoflood is fixed between3200a B.P. and2800a B.P., which was just the transition period from the mid-Holocene Climatic Optimum toward late Holocene dry conditions in the Weihe Basin. the unstable climatic system brought out the abrupt precipitation and caused this extreme palaeoflood. Tianshui Gorges and Baojixia Gorges in the upper Weihe River, Xianyang section in the middle reaches of Weihe, Lintong section in the lower Weihe River all recorded successive palaeoflood.(8) Because of broad bed, small gradient and northward moving of River bed, the river cross section changed great, which can lead to big error for flow calculation, favorite hydrological model can not be used to calculate flow calculation of palaeoflood recored in NSC. According to flux process of run off and the source of catastrophic flood in the Weihe River, and calculated discharge of GCZ profile in the upper Weihe River, the flow calculation of palaeoflood recored in NSC are speculated to be at least22560～25960m3/s.