The Holocene Palaeoflood Events in the Yunxi Reach in the Upper Reaches of Hanjiang River

Through field investigation in the upper reaches of Hanjiang River,palaeoflood slackwater deposits(SWD)were found in the bedrock gorges in the Yunxi reach.Based on field observation,laboratory analysis including grain-size distribution and magnetic susceptibility,and comparison with modern flood deposits,this SWD is identified as the most typical Holocene palaeoflood deposits in the Hanjiang River basin.By using stratigraphic correlation and OSL dating method,the palaeoflood events were dated to 3200-2800 aBP at the turn from the middle to late Holocene.According to the palaeoflood peak stages and hydraulic parameters,the peak discharges of the palaeoflood were reconstructed to 48830-51710 m3/s by using the slope-area method.At the same time,the flood peak discharges of 1983,2005 and 2010 extraordinary floods were reconstructed with the same method and hydraulic parameters in the same cross-section.The error between the reconstructed and gauged discharges is 1.99%-4.21%.This shows that the reconstructed palaeoflood peak discharges are reliable.The flood peak discharge-frequency relationship at 10000-year time scale is established by a combination of the data of gauged flood,historical flood and palaeoflood hydrology.