The Study Of Holocene Palaeoflood Hydrology In The Jin-shan Gorges Of The Yellow River And In Baihe Reach Of The Upper Hanjiang River

The climatic change during the late Quaternary has been the focus of geomorphologists.The Holocene have a close relationship with the development of human society is the nearest period to modern time and the geological evidence is preserved relative completely.The middle Yellow River and the upper Hanjiang River is the birthplace of chinese ancient civilization.And floods have been affecting the process of chinese civilization from 'Dayu flood control'.The upper Hanjiang River,a major tributary of the Yangtze River in China,is noted for the national middle sources area of South to North Water Diversion Project and the important hydropower development zone.Extraordinary rainstorms and Holocene paleoflood often result from abnormal atmospheric circulations are closely related with intensified climatic variations.Study of floods has become more important incontrolling flood disasters,but also it has very important reference value for hydraulic engineering construction.Data series of existing historical floods and measured flood is far to meet the computing requirements of rare flood.The study of paleoflood hydrology makes extension of flood frequency curve change into interpolation in flood frequency analysis.So the study of Holocene paleoflood hydrology has important theoretical and practical significance.Based on detailed field investigation in the Jin-shan Gorges of the middle Yellow River,the lower Wudinghe,the upper Hanjiang River and the Danjiang River,we choose XBC profile,YHG profile,FJJ profile,MFT profile,SJGT profile,BJC profile,JJTZ profile,LZT profile that are 8 typical profiles of slackwater deposits as the research object.we have some experiments in the lab about palaeoflood SWD and modern flood SWD of 8 prodfiles such as magnetic susceptibility,grain size,the content of CaCO3 and loss-on-ignition.Through experiments results and combined sedimentary features in the field,we identify paleoflood slackwater deposit of the Jin-shan Gorges of the middle Yellow River,the lower Wudinghe,the upper Hanjiang River and the Danjiang River.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 and indoor topographic maps correction,cross sections of paleofloods are drawn and calculated.Finally we calculate peak discharges of every paleoflood in 8 profiles,using slop-area method and HEC-RAS hydraulic model.These results were further tested and proved to be reliable with reconstruction of the modern floods at the same reach with the same method.This article's main conclusions are as follow:(1)Analysis of the grain-size distribution indicates that these SWD consist of sandy silt and fine sand,sourced from the suspended sediment load of the floodwater.The lower magnetic susceptibility,higher CaCO3 content and lower loss-on-ignition indicate they are fresh deposit without weathering and pedogenic alteration.The results of field observation and grain-size and magnetic susceptibility indicate that these slackwater deposits are very typical.They have recorded extraordinary palaeoflood events on the Yellow River,the Wudinghe River,the Hanjiang River and the Danjiang River.(2)The grain size of the 2012 flood SWD in the Jin-shan Gorge of the Yellow River is dominated by sandy silt and silty sand.Grain-size indices shows that the difference of grain-size of 2012.7 flood SWD is very obvious,and with the longitudinal extension of the river,the grain-size of SWD are regular changes from the coarse to fine.Grain size distributions of modern flood SWD are related to the microgeomorphology environment and hydrodynamic conditions during flood periods.Compared with Wubu and Longmen gauge station,flood SWD is suspended sediment deposition at the end-point,not the averages at entire cross-section.Obviously the use of the annual average of entire cross-section suspended sediment grain size as reference of paleoflood SWD is unscientific.The deposition process and priciples are consistent between modern flood SWD and palaeoflood SWD.Therefore grain size characteristics of 2012 flood SWD have an important reference for the study palaeoflood sedimentology and hydrology in the Jin-shan gorges of the Yellow River.(3)Palaeoflood SWDs have recorded the extraordinary palaeoflood events which occurred 9000-8500 a BP,3200-2800 a BP,1900-1700 a BP as dated by the optically stimulated luminescence method in combination with pedo-stratigraphic correlations with the previously studied Holocene pedo-stratigraphy in the Weihe River drainage basin.The extraordinary floods documented in the Jin-shan Gorges of the middle Yellow River,the lower Wudinghe,the upper Hanjiang River and the Danjiang River occurred just in the same episode of climatic aridity and pedogenic regression that were forced by monsoonal shift.It is therefore inferred that the monsoonal shift and the resultant climatic variability at above-mentioned three periods are responsible for the occurrence of the intensive rainstorms and extraordinary floods in this semi-arid and sub-humid region.They are recognized as a response of regional hydrological system to global change.(4)Using palaeoflood slackwater depth figured out using SWD thickness and bulk of suspended sediment load of the:floodwater,therefore,the paleoflood stages of 8 studied profiles were determined.Firstly,the peak discharges of these palaeoflood events were calculated by using slop-area method.These results were further tested and proved to be reliable with reconstruction of the modern floods at the same reach with the same method.All of these calculation results are close to the instrumental records,so we conclude that this calculation method is suitable in the study area and the paleoflood reconstruction results are convinced.(5)Secondly,the peak discharges of these palaeoflood events of 8 studied profiles were estimated by using HEC-RAS one dimensional model.20?30 cross-sections were selected at each site in 2-3 km long river reach,and the roughness values consist of three segments,channel and bedrock riverbank.A sensitivity test performed on the model indicated that for a 25%variation in roughness values.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 less than 5%Relation between drainage area and peak discharge tested that the calculated peak discharges of palaeofloods using two methods were credible and reasonable.(6)Adding palaeoflood discharge data,together with gauged data and historical flood information,peak discharge-frequency curves of the Wubu gauge station on the Yellow River,Longmen gauge station on the Yellow River,Baijiachuan gauge station on the Wudinghe River,Baihe gauge station on the Hanjiang River in 10 000 year time scale were analyzed.The results prolonged flood data timeseries of the studied area from decades to millennia,improving the precision of flood frequency.Thus it is significant to provide a basic reference to engineering construction,water resources regulation,mitigate flood disasters,and soil and water conservation.(7)Innovations:Domestic initial the HEC-RAS one dimensional model in palaeoflood peak discharges had been used.The palaeoflood sedimentology and hydrology were firstly studied in the Jin-shan Gorges of the middle Yellow River,the lower Wudinghe,the upper Hanjiang River and the Danjiang River.The peak discharge-frequency curves of the Wubu gauge station on the Yellow River,Longmen gauge station on the Yellow River,Baijiachuan gauge station on the Wudinghe River,Baihe gauge station on the Hanjiang River in 10 000 year time scale were firstly analyzed.