Abstract:

Four Holocene palaeoflood slackwater deposits (SWD) were found at Shangjiahe reach in the Yun County of Hubei province in the upper reaches of the Hanjiang River. In the laboratory, the grainsize distribution, magnetic susceptibility and lossonignition were analyzed and compared with modern flood SWD, and the results indicated that four palaeoflood SWDs were typical and recorded four palaeoflood events in the upper reaches of Hanjiang River. The HECRAS software was developed at the Hydrologic Engineering Center (HEC), which is a division of the Institute for Water Resources (IWR), U.S. Army Corps of Engineers. HECRAS is an integrated system of software, designed for interactive use in a multitasking, multiuser network environment. The HECRAS system contains four onedimensional river analysis components for steady flow water surface profile computations, unsteady flow simulation, movable boundary sediment transport computations and water quality analysis. A key element is that all four components use a common geometric data representation and common geometric and hydraulic computation routines. In addition to the four river analysis components, the system contains several hydraulic design features that can be invoked once the basic water surface profiles are computed. The system is comprised of a graphical user interface (GUI), separate hydraulic analysis components, data storage and management capabilities, graphics and reporting facilities. The maximum peak discharges of the four palaeoflood events were calculated from 45 550 m3/s to 61 750 m3/s using HECRAS model. These calculated discharges were checked by the reconstructed palaeoflood peak discharges by the slopearea method, and the errors were 1.58%-3.09%. At the same time, the flood peak discharge of July 18, 2010 extraordinary flood was also reconstructed with the same model and hydrographical parameters, and the error between the calculated and gauged discharges was about 4.91%. This result showed that it was reasonable to calculate palaeoflood peak discharges and choose hydrological parameters, and it was simple and easily operable to use HECRAS model to reconstruct palaeoflood peak discharges. Moreover the study results provided a new method on the study of palaeoflood hydrology. Thus, the results effectively extended the upper reaches of the Hanjiang River flood hydrology data sequence and provided a reliable reference for engineering construction, flood control and disaster alleviation in upper reaches of Hanjiang River