Study On Flood Events Recorded In The Deposits Of Lake Dongping During The Past 100 Years

The problem of flood disasters in the lower Yellow River basin has been around for a long time.Especially under the background of global warming and the strengthening of monsoons since modern times,the problem of flooding in the middle and lower reaches of the Yellow River has become increasingly prominent.Therefore,an in-depth exploration of the causes and pattern of flood variations in the lower reaches of the Yellow River is of great significance for flood control and disaster reduction.This dissertation takes the old lake area of Dongping Lake in the lower reaches of the Yellow River as the research object,drilling sediments cores for the analyses.Also modern sediments of the rivers(Yellow River and Dawen River)discharging the Dongping Lake were collected.Through laboratory analyese,information on the grain size and chemical elements of the sediments is obtained,combined with dating technology,the Yellow River and Dawen River flood events recorded in the sediments of the Dongping Lake area in the past 100 years are distinguished,and the floods recorded in historical documents and measured floods are compared.The main conclusions are as follows:(1)By analyzing the composition of the grain size components of the major rivers(Yellow River,Dawen River)in Dongping Lake and establishing the contribution of end-members with different grain sizes to the Dongping Lake area through the comparison of end-member analysis models,it can be seen that the substances brought by the Yellow River into Dongping Lake has a larger particle size,while the materials of Dawen River material is poorly sorted,and the particle size is too fine.The Yellow River flood source material retrieved from the Dongping Lake sedimentary record is more,while the Dawen River material reflection is relatively insufficient.(2)The different indicators of Dongping Lake flood mainly lie in the granularity.The indicating effect of chemical elements on the Yellow River flood is more obvious,while the indicating effect on the Dawen River is insufficient.It may be that the differences of different elements in the two different river environments are reduced.Compared with the particle size,the effect is not good.Comparing the sedimentary records with historical flood events,it can be seen that the recognition rate of the Yellow River flood sedimentary records is relatively high,while the recognition rate of the Dawen River flood is relatively insufficient.Sedimentary records can identify multiple floods of the Yellow River,including 1855,1857,1863,1868,1877,1931,1933,1953-1954,1957-1958,1963-1964,1973-1974,1982,1991-1996,but only identified the Dawen River floods in 1855,1931-1933,and 1954.(3)Human activities will have an impact on the flood deposit records in the Dongping Lake area,which are mainly manifested in the artificial construction of river and lake dams,lake sluices,and the control of the flooding of the Yellow River and Dawen River into the lake.In addition,the weakening of the runoff generation capacity of the Dawen River under the influence of human activities may also cause the Dawen River flood event recorded by the Dongping Lake core to be insignificant.The frequency of floods in the Dongping Lake area in the warm period of the 20 th century was about 1.2 times per 10 years,and the cold period in the 19 th century was about 1time per 10 years,and the warm period was higher than the cold period.On an interdecadal scale,La Nina events are more frequent when solar activity is weakened,and during periods when the East Asian summer monsoon is weaker,there is more rainfall in the middle and lower reaches of the Yellow River.Floods in Dongping Lake in the 20 th century generally followed the two La Nina in 1920-1940 and 1940-1960 The average state(cold phase)occurs with high frequency,which is similar to the flood evolution characteristics in the middle and lower reaches of the Yellow River.