Reconstructing the palaeoflood events from slackwater deposits in the upper reaches of Hanjiang River, China

Through investigation in the upper reaches of Hanjiang River, China, an aeolian loess-soil profile with four palaeoflood slackwater deposit (SWD) interbeds was found on the cliffed riverbanks in Yunxian county reach of Hubei province. Based on field observation, and laboratory analysis including magnetic susceptibility and particle-size distribution, the four palaeoflood SWDs were differentiated from aeolian loess and soil by sedimentary criteria and analytical results, and recorded four episodes of palaeoflood events in the upper reaches of Hanjiang River. By using stratigraphic correlation and OSL dating, the four palaeofloods were dated to 12,600-12,400 a B.P., 11,600-11,400 a B.P., 5500-5000 a B.P. and 1000-900 a B.P., respectively. According to the calculated maximum palaeoflood stage, the maximum peak discharges of the palaeofloods events were reconstructed by using the HEC-RAS one-dimensional model. The same hydrological method was used to calculate the peak discharge of the 2010 flood in the same reach. The error between the calculated and gauged discharges was 4.9%, showing that the reconstructed four palaeoflood discharges were reliable. The flood peak discharge-frequency relationship at 10,000-year timescale was established by a combination of the gauged flood, historical flood and palaeoflood hydrological data in the upper reaches of the Hanjiang River, assessing reliability of peak discharge data in flood design for the water conservancy construction and flood mitigation.