| The palaeoflood hydrology is mainly dedicated to the palaeoflood reconstruction, our scholars have made some research achievements in the palaeoflood hydrology of the Yangtze River, the Yellow River, Haihe river and Huaihe river, but calculating the peak discharge in the palaeoflood reconstruction mainly adopts the level-discharge method and slope-area method. HEC-RAS model is one of the more advanced and mature hydrological models abroad to reconstruct palaeoflood peak discharge, our courtry applys ralely HEC-RAS modelto study of the paleoflood hydrology.HanjiangRiver is a tributary of the Yangtze River, it is controlled bythe monsoon climate and also is sensitive to climate change, with a high annualprecipitation rate.Since ancient time,this regionhad frequent flood disasters. However, there were only a few tens of years or even decades gauged flood data, which can not meet the requirement to estimate the flood frequency and guarantee the accuracy of flood control standard of HanjiangRiver. So it is very necessary to study the paleoflood events in the upper reach of the HanjiangRiver.This paper uses HEC-RAS model coupled HEC-GeoRAS extension module to study the Holocene paleoflood slackwater deposits found bedrock gorges inyunxian county in the upper reaches of the HanjiangRiver. Using palaeoflood slackwater depth figured out using SWD thickness and bulk of suspended sediment load of the floodwater, therefore, the paleoflood stage was determined.Based on field measurements,indoor topographic maps correction anddetermining hydrological parameters. Finally we calculatepaleofloodpeak discharges using HEC-RAS hydraulic model and verify the reliability of the model results through several aspects.The results prolonged flood data timeseries of the studied areaand establish peak discharge-frequency curves of Baihe gauge station on the HanjiangRiver.This article’s main conclusions are as follow:(1) Through field investigation in the upper reaches of the Hanjiang River, four palaeoflood slack-water deposits (SWD) were found in Loess-soil profile on the first terrace of frontier of Shang Jia Hein Yun county of Hubei province. Palaeo flood SWDs have recorded the extraordinary palaeoflood events which occurred 12,600～12,400 a B.P.,11,600～11,400 a B.P.,5500～5000 a B.P.,1000～900 a B.P.respectivelyas dated by the optically stimulated luminescence method in combination with pedo-stratigraphic correlations with the previously studied Holocene pedo-stratigraphy in the Weihe River andin the upper reaches of the Hanjiang River.(2) 14 cross-sections were selected at SJH deposition sites reachin 2-3 km long river reach. The roughness values consisting of the channel and bedrock riverbankrespectively were 0.028,0.05, the typical values of contraction coefficient and expansion coefficient were 0.3 and 0.1,energy slopewas1.1‰. Using palaeoflood slackwater depth figured out using SWD thickness and bulk of suspended sediment load of the floodwater, therefore, the flood peak water level of the 4 palaeoflood SWD records was determined. Firstly, the palaeoflood watersurface line was calculated by using HEC-RAS modelcombined withthe paleoflood stages.(3) The peak discharges of palaeoflood were estimated by using HEC-RAS one dimensional modelcombined with hydrological parameters andthepaleoflood stages.The flood peak dischargeswere reconstructed to between 45,350～62,390 m3/s.The results were compared with the results calculated by using slope-area method, the error less than 5%,so we conclude that HEC-RASmodel is suitable in the study area and the paleoflood reconstruction results are convinced.(4) A sensitivity test performed onpeak discharges estimated by using the HEC-RAS model and slope-area method.Assumed thatother hydrologic parameters constant, a 25% variation in roughness values,the errors of palaeoflood discharges were-20%-33% using the slope-area method and -15.70%～7.56% using HEC-RAS model to reconstruct palaeoflood discharges, respectively. The error is much smaller than that obtained by using the slope-area methods.This indicated that HEC-RAS model could effectively reduce the influence of roughness coefficient to reconstructe palaeoflood discharges, and the palaeoflood discharges calculation was more reliable.(5) To verify the results calculated HEC-RAS model through several ways. First, the flood peak discharges of July 18,2010 extraordinary flood was also reconstructed in the same reach using the HEC-RAS model and the same hydraulic parameters, the error between the calculated and gauged discharges is about 2.80%, less than 5%.Secondly, Relation between peak discharge and catchment area tested that the calculated peak discharges of palaeofloods were reasonable and credible.(6) Taking advantage of palaeoflood discharge data together with gauged data and historical flood information, peak discharge-frequency curve of the Baihe gauge station on the HanjiangRiver in ten thousand year time scale was drew.The results prolonged flood data timeseries of the studied area, improving the conventional procedure involves statistical extrapolations from short gauged hydro logical records documenting 30～40 year records of the estimation of the discharge of very large, rare flood andimproving the precision of flood frequency. Thus it is significant to provide a basic reference to hydraulic engineer ing construction, traffic engineering construction, water resources regulation and mitigate flood disastersin the upper HanjiangRiver.(7)We analyzed and discussed the climate background of ertreme floods. In the last glacial period is a transitional period in a gradually warming climate, climate change events occur frequently. The firstand the second flood SWD1 and SWD2 occurred respectively in 12,600～12400 a B.P. and 11,600～11,400 a B.P., the two flood events correspond to Younger Dryas seriously deteriorationperiod; In the 6500～5000 a B.P. period, national ranges appear extreme climate events, frequent flood disaster, so in the 5500～5000 a B.P.period, this time occurred third big ancient flood SWD3; InNorthern Song dynasty culture period (1000～900 a B.P.), drought and flood disasters frequently occurred, this time happened the fourth flood SWD4. |
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