The Study Of Holocene Paleoflood Hydrology From Wubao To Yanchang In The Middle Reach Of The Yellow River

The Holocene was the quickly development and prosperity period of the mankind's civilization,and also the great change period of the global environment.The Middle Yellow River is the birthplace of chinese ancient civilization.And floods have been affecting the process of chinese civilization from 'Dayu flood control'.Study of floods has become more important in controlling 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.Residents living alongside rivers,urban and rural constructions and their security depend on the hydrological information and the interpretation of prediction.However,the complexity of hydrological phenomena and the deficiency of hydrological information restrict the accuracy of the flooding frequency forecast.Paleoflood study extends flood recurrence to several thousand years and can be used to determine the recurrence interval of large outliers in flood series,so it is helpful in frequency analysis.However,there is not any paleoflood hydrology study of Jin-Shan Canyon in the middle reaches of the Yellow River.Deep studies on slackwater deposit from Wubao to Yanchang,identifying and obtaining the paleoflood information recored in several profiles,ensuring the amount,frequency,time and scale of the extreme Holocene paleofloods,has important practical significance in testing water and flood control projects,flood-designing of traffic engineering and improving the reliablity of engineering.Based on detailed field investigation in valley of the Yellow River from Wubao to Yanchang,the Yanhe River,and the Wudinghe River in northern Shaanxi,we choose XHG profile,XBC profile,YHG profile,GJC-a profile,GJC-b profile,BJC profilethat are 6 typical profiles of slackwater deposits as the research object.First we have some experiments in the lab about 6 prodfiles,environmental proxies such as magnetic susceptibility,grain size,loss on ignition,CaCO3 content,and>0.1mm percentage content.Second we have data analyses in grain size distribution,grain size parameters,probability cumulative curves,triangular diagram,scatter diagram and Sahu discriminant function.Through experiments results and combined sedimentary features in the wild,we identify paleoflood slackwater deposit of the Yellow River.According to the thicknesse of slackwater deposit in 6 profiles in Wubao-Yanchang,we calculate paleoflood water levels.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 6 profiles,using area-gradient ratio.This article's main conclusions are as follow:1.The analysis of field investigation,chronological measurement,and ancient flow direction of riverbed gravel and the results of the holocene stratum of nearly profiles showed that the paleoflood in the study area occurred in the middle and late holocene period,between 3200-3 000 a B.P.,and the major reason would be the variation of the climate.2.In the wild,the color of slackwater deposit layers is significantly different from loess and paleosol,and there are horizontal bedding and wavy bedding in slackwater deposit layers,and rust spot of slackwater deposit layers is clearly visible.The magnetic susceptibility value of slackwater deposit is lowest in whole profile,and loss on ignition in slackwater deposit layers has larger fluctuation,and CaCO3 content of slackwater deposit is higher than loess and paleosol.Because of different sedimentary environment,the grain size characteristics of slackwater deposit and loess deposit have big difference.Slackwater deposit form under standing water environment,and its grain size is fine.Fine sand content of slackwater deposit is less,but silt content is higher than loess and paleosol.From grain size parameters,the median diameter and the average diameter of slackwater deposit are similar to loess and paleosol.Standard deviation(a)and sorting coefficient(S0)of slackwater deposit are smaller than loess and paleosol,which shows that the sorting of slackwater deposit is better than loess and paleosol.Slackwater deposit of the Yellow River is sandwiched in slope deposit layers,and it is dense and tight,like a plate.Most slackwater deposit was found in back baies of litter tributary of the Yellow River.Analysis of indoor grain size characteristics shows that coarser sediment and fine sand of slackwater deposit of the Yellow River is main,and that skewness(SK)of slackwater deposit is positive skewness and very skewness,and that kurtosis(Kg)of slackwater deposit is sharp and very sharp.Sorting coefficient(S0)of slackwater deposit is smaller than 1.4,which shows that the sorting of lackwater deposit is well-sorted.The probability cumulative curves of grain size of slackwater deposit is two-segment,which explains that the mode of transport of slackwater deposit is saltation and suspension.The scatter diagram of slackwater deposit of the Yellow River shows that the average diameter of slackwater deposit of the Yellow River is bigger than loess and paleosol,and that the sorting of loess and paleosol is better,and that skewness of slackwater deposit of the Yellow River is bigger than slackwater deposit.3.Both eyewitness accounts and the analysis of proxy environmental indicators of the profiles show that all of the deposits in the study area can be identified into the classical slackwater deposits,and the silt particles(during0.063-0.016mm)take part in a high proportion,and all of these slackwater deposits are well-sorted.They show obvious difference with loess and palaeosol in the same profiles.In the spatial distribution,from southwest to northeast the coarse particles content of slackwater deposits increased gradually.4.Based on field measurements and indoor topographic maps correction,cross sections of paleofloods are drawn and calculated.When water surface slope and roughness coefficient are determined,we calculate peak discharges of every paleoflood in 6profiles,using area-gradient ratio.The peak discharge of XHG profile is 40610 m3/s.The peak discharge of XBC profile is 39600 m3/s.The peak discharges of YHG-N profile are 27320m3/s?39280m3/s.The peak discharges of YHG-S profile are 24240m3/s?39080m3/s.The peak discharges of GJC-a profile are 12740m3/s?13910m3/s.The peak discharges of GJC-b profile are 6840m3/s?8310m3/s.The peak discharges of BJC profile are 17820m3/s.So,it showed that area-gradient ratio is reasonable.5.We also reconstructed three modern floods in the study area by the same calculation method.The peak discharge at GJC-a section in the Yanhe River is 9 800 m3/s,July 6th,1977,and the peak discharge at BJC section in the Wudinghe River is 4930 m3/s,July 18th 1966,with the error range between 1.7-2.0%.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.6.Combine the peak discharge of the Holocene extreme flood to the modern flood and the history flood,we get four new flood frequency curves.Curves of wubao,and Ganguyi hydrologic station get lower than the original ones,and curve of BJC get higer than the original one.7.Other researchers reconstructed the flood stage of the paleoflood according to the top of depositional termination,considering the unique features in the rivers in the Loess Plateau,we combined the sedimental thickness when we recalculated the flood stage.And our results show that,accurate results can be drawn by this way.We reconstructed the Holocene extreme flood events of some major rivers,and our research result is the first study in this area.And the flood rating curves were established to improving the reliability to the engineering construction of flood mitigation,water conservancy and hydropower in the study area.This research results are not only practical to the flood mitigation,water resources exploitation and eco-environmental construction,but also fill in the blank of the palaeoflood hydrological study in Northern Shannxi,which are of great scientific significance to the comprehensive improvement of the eco-environment of the basin.