Flood Events Recorded By High-Resolution Fluvial And Lacustrine Deposits In The Middle And Lower Reaches Of The Yangtze River And Their Properties

The study of ancient flood includes the reconstruction of ancient flood sequence and the reconstruction of ancient flood intensity.It is of great significance to disaster reduction and water conservancy project construction,and it is also a hot spot in the study of global change.Based on several high-resolution fluvial and lacustrine sedimentary sequences in the middle and lower reaches of the Yangtze River,this paper uses ²¹⁰Pb-¹³⁷Cs radioisotope test,SCP and AMS-¹⁴C test to establish a profile chronology framework;uses laser grain size test to analyzes the grain size characteristics of the sedimentary sequence and identifies the response of grain size index to flood;selects sand content and average grain size,and then combined with historical documents,the flood events recorded in each section are reconstructed.Through filtering and power spectrum analysis,the properties of flood sequence is discussed;the flood event sequence and regional temperature change,ENSO,sunspot activity and solar radiation intensity,Indian summer monsoon,East Asian summer monsoon and volcanic activity are compared,and the internal relations between them are discussed.The results are as follows:1.Based on the dating results and the judgment of sedimentary events,the sections?boreholes?of XRD,GJ,YZR?upper 8.4m?,HMT and DCL were deposited in 1775,1882,1788,1832 and 1660,respectively.The deposition rate of floodplain is between2.38cm/a and 7.23cm/a,while that of lake is relatively slow,ranging from 0.43cm/a to0.56cm/a.2.Through the analysis of the grain size of each section?borehole?,it is found that the grain size of each section is different,the distribution curve of grain size frequency and the cumulative curve of grain size frequency are sensitive to the hydrodynamic response;there are relatively more rolling components and jumping components in flood period,there are obvious coarse-grained peaks in lake deposition above 100?m,and there are obvious peaks in flood plain deposition above 300-500?m or 800?m.The average grain size is coarser and the content of sand is higher than that in non-flood period.are relatively more rolling components and jumping components in flood period.Samples of grain size frequency in no-flood period are obvious coarse-grained peaks in lake deposition above 8-20?m and 0.1-1?m.The projection area is also different in granularity parameter and granularity group scatter point graph.3.Based on the grain size characteristics of flood period and non-flood period and historical documents,the flood sequence of each profile is distinguished.43,18,46,30and 25 flood-event years have been identified by the sections of XRD,GJ,YZR,HMT and DCL respectively.It shows that the frequency of flood occurrence is between 7.25%and 22.05%.4.The frequency and power spectrum analysis of the flood sequence identified by each section shows that:from 1775 to 1860 and 1925 to now,it is the frequent period of flood;from 1860 to 1925,it is the less frequent period of flood;there are high frequency periods of about 2-8a and 11a and low frequency periods of 14-16a,18-22a,30-34a,42a,47a,58a,62a,68a and 147.67a in each section,respectively,QBO,The period of NSO and sunspot activity is close.5.The comparison of flood event sequence with temperature change,ENSO,sunspot activity,solar radiation intensity,Indian summer monsoon,East Asian summer monsoon and volcanic activity shows that:In the long-term scale?70-80a?,floods are more likely to occur in the period of warm air temperature;in the short-term scale?10a?,floods are more likely to occur in the period of low air temperature.Floods are more likely to occur after El Nino and El Nino phenomena;and On a long-term scale?70-80a?,floods occur frequently when the solar radiation is strong and the sunspot activity is intense.Flood events occur more frequently when the East Asian summer monsoon is weak,and its relationship with the Indian summer monsoon is not significant.Within two years after a strong volcanic eruption,there are often more serious floods in the middle and lower reaches of the Yangtze River.The above results make clear the regularity and climate background of regional flood occurrence,expand the connotation of ancient flood research,and provide a reference for regional flood control and disaster reduction.