Qian River Basin In The Ancient Flood Sedimentology And Hydrology Characteristic Research

Qianhe basin locates in the periphery of the summer monsoon. It is controlled by the monsoon climate and also is sensitive to climate change. With a high annual precipitation rate, it is a flood disaster-prone area. Since the ancient time, flood disaster has become a huge danger to the life of people and safe of property, as well as influenced the development of the area. The study on palaeofloods is helpful to the reconstruct the flood sequence, and plays an important part in the forecast of climate change, Hydraulic engineering construction, flood control and Up to now, the study on palaeofloods of the Qianhe basin was relatively rare. Therefore, the study on palaeoflood sedimentology and hydrological research is of great importance in this area.Through the detailed field study of the downstream of Qianhe basin, we got many sedimental samples from QH-LHC profile, which contented several slackwater deposits and paleostage indicators. First of all, they were anayzed in the sedimentologic laboratage in Shannxi Normal Univerdity, including particle-size distribution, magnetic susceptibility, percentage composition of calcium carbonate, loss on ignition and quartz super-micro-state examination, to determine the depth and thickness of the paleofiood slackwater deposit layers and their depositional age. Then the water level and peak discharges of the extreme floods with ten millennia-years return period were reconstructed by hydrological methods, extending the time scale of floods from decades to millennia. Finally, we also analyzed and discussed the climate background of extreme floods extreme in Holocene. The main results are as follows:(1) The drought and flood disaster happened frequently in the Qianhe basin from1470to2009, and had obvious features of phasicrule and periodic law, which was close correlation with the the drought and flood disaster in the northwest of China, especially Guanzhong plain. According to the results of11-years moving average and accumulative anomaly, it is concluded that the drought and flood disaster of Qianhe basin could be divided into three stages. At the basis of three stages, we could further dividethem into seven drought-partial or flood-partial periods, In addition, by wavelet analysis, it was found that the540years’drought and flood disaster of Qianhe basin changed obviously in law, and associated with the action of thesun closely.(2) Both the eyewitness account and in-house laboratory investigation showed that the paleoflood slackwater deposit were obviously different from loess and palaeosol. Accordingly, we determined the deep of Depth of SWD1and SWD2, which were295-250cm and220-165cm respectively. In addition, through the systemic era identification, the era of Holocence Loess Plateau stratigraphic sections were determined. The period of lagest flood which were record by slackwater deposits was between6000-5000a B.P.(3) Model than the drop method was used to calculate each section stagnant flood peak discharge by sediment, the peak discharge of SWD1was10870m3/s and the peak discharge of SWD2was10380m3/s. Through the the relationship of peak discharge and drainage area, the palaeoflood discharge was tested reliable. Palaeofloods were used in combination with historical flood date and the gauged record; we drew the rating curves of peak discharges and the frequency, which the time was since ten millennia years.(4) Using palaeoflood discharge data of Holocence extreme flood, together with annual maximum peak discharge and historical flood information from Qianyang precipitation station, applying procedure of the Pearson curves in Hydrology, peak discharge-frequency curves of Qianhe River have been established. This date provided an important basic for Hydraulic enhineering construction, flood control and water conservancy in Qianhe basin.(5) We analyzed and discussed the climate background of extreme floods in Holocene. Conclusion showed that the global climate was in violent fluctuation periodvolatile between6000and5000a B.P.. In the Qianhe river basin, the climate changed from warm and wet to cold and dry, and extreme floods respond instantaneously to the Holocence extreme climatic event.