Abstract: Palaeoflood hydrological studies were carried out in the upper reaches of the Hanjiang River. Typical palaeoflood slackwater deposits of the Holocene period were identified at several sites along the banks of the Wufeng reach in Yunxian County. They have recorded four episodes of extraordinary palaeoflood events (12,600-12,400 a BP, 4200-4000 a BP, 3200-2800 a BP and 1900-1700 a BP). The palaeoflood peak stages were estimated with the elevations and the deposition depths of the SWD. The palaeoflood peak discharges were estimated by using the HEC-RAS one dimensional model running within an ArcGIS environment. The results indicated that these palaeoflood discharges were between 42,220 and 63,400 m³/s. A sensitivity test performed on the model indicated that for a 25% variation in roughness values, an error between -10.6% and 6.3% was introduced into the results of peak discharge. The error is much smaller than that obtained by using the slope-area methods. Based on the flood stage indicators, the peak discharges of modern floods were also reconstructed in the same reach using the HEC-RAS model and the same hydraulic parameters. The error between the reconstructed and gauged peak flood discharges is between -3.9% and 1.0%. These showed that the palaeoflood peak discharges reconstructed by using the HEC-RAS model are reliable. The reconstructed peak discharges of the palaeoflood are much larger than those of the gauged largest floods in the upper reaches of the Hanjiang River. The flood data series of the river are therefore extended to a 10,000-year time-scale. This result provides reference in hydrological engineering, water resource management and flood mitigation on the river. And it is also very helpful in understanding the relationships between extreme flood events and global climate change.

Key words: palaeoflood hydrology, HEC-RAS hydraulic model, Holocene, Hanjiang River