Historical Evolution Analysis Of The Yellow River Delta Based On Typical Tail Channel Sedimentary Column And Shoreline Change

The Yellow River Delta is one of the three major estuarine deltas in China.In recent100 years,affected by human activities and climate change,the incoming water and sediment conditions have changed dramatically,resulting in frequent River Diversion at the tail of the Yellow River and violent shoreline evolution of the estuarine delta.Since the Yellow River was changed into the Bohai Sea in 1855,the river channel at the tail of the Yellow River has been changed for 10 times.The sediment preserved in the river channel is a valuable historical record of environmental changes and human activities in the Yellow River Basin.From a long time scale,this study explored the vertical distribution changes of particle size,heavy metals and nutrient elements in the columnar sediments of Diaokouhe,Qingshuigou tail channel and the current channel Qingbacha of the Yellow River in recent60 years,and established the sedimentary records of the tail channel sedimentary column.On a short time scale,based on remote sensing image data,the influence of incoming water and sediment process on shoreline evolution in recent 20 years is analyzed.The results of grain size study of sedimentary column show that the median particle size of Diaokouhe changes from 2.61 to 65.63μm.The median particle size of Qingshuigou varies from 2.67 to 84.91μm.The median particle size of Qingbacha varies from 33.76 to61.1μm.While the median particle size fluctuates with depth,it also increases downward and decreases slightly upward.The content of coarse silt and fine sand in each sedimentary column increases with the increase of depth,and the total component content exceeds 65%,and the component content of coarse sand is the lowest.The probability distribution curve of each sedimentary column is mainly bimodal,the kurtosis value of the main peak increases with the increase of depth,the skewness is mainly positive,and the particle size of the sedimentary column is fine,which belongs to typical fluvial sedimentation.The research results of heavy metal content in the sediment column shows that the content of heavy metals in the sedimentary column is enriched downward with increase of depth.The average contents of eight kinds of Cr,Co,Ni,Cu,Zn,As,Cd and Pb in the sediment column of Diaokouhe are 16.33,4.59,11.91,8.01,48.92,5.23,0.18 and 25.09mg/kg respectively,and the average contents of eight kinds of heavy metals in the sediment column of Qingshuigou are 15.43,4.01,11.07,7.51,47.68,4.48,0.32 and 40.90 mg/kg respectively,The average contents of eight heavy metals in the sedimentary column of Qingbacha were 21.84,4.16,11.65,6.76,28.64,3.92,0.18 and 24.82 mg/kg respectively.Comparing the average content of heavy metals in each sedimentary column,it is found that except Cr,Cd and Pb,the content of the other five heavy metals in Diaokouhe sample is the first in the three sedimentary columns,but the difference is small,and the difference range is within 2 mg/kg.The content of Cd and Pb in Qingshuigou is much higher than that in the other two samples.The content of Cd is 1.7 times that in the other two samples,and the content of Pb is 1.6 times that in the other two samples,the content of Cr in the sedimentary column of Qingbacha is the highest.The content of heavy metals in each sedimentary column,E_rⁱ value is less than 40 and RI value is less than 150,which belong to slight harm.The I_geo value of Cr,Co,Ni,Cu,Zn and As in each deposition column is less than zero.According to the I_geo classification standard,these six kinds of heavy metals belong to the degree of no pollution.In the three sedimentary columns,the pollution degree of Cd is relatively high,with mild,moderate and moderate pollution.Pb is slightly and moderately polluted in Diaokouhe and Qingshuigou sedimentary columns.The pollution risks of these two heavy metals should be paid enough attention.The research results of carbon,nitrogen and phosphorus content in three sedimentary columns show that:The average contents of total carbon in Diaokouhe,Qingshuigou and qingbacha are 9629,8788 and 11332 mg/kg respectively,the average contents of total nitrogen are 180,146 and 164 mg/kg respectively,and the average contents of total phosphorus are 573,601 and 820μg/g respectively.The average nitrogen content of Diaokouhe sedimentary column is the highest,which is related to the existence of farmland around the sampling site.The average content of total carbon and total phosphorus in the sedimentary column of Qingbacha is the highest,which is related to the material exchange between the overlying water and the sedimentary column.The content of carbon,nitrogen and phosphorus decreased with the increase of depth.The C/N values of each depth in the three sedimentary columns are greater than 20,and the input characteristics of terrigenous organic matter are obvious.Through the comprehensive pollution index evaluation,it is found that the nitrogen pollution degree in each sedimentary column is clean,and the phosphorus pollution is slightly serious.The phosphorus pollution at each depth of Diaokouhe is light pollution,and the comprehensive pollution index is clean.The phosphorus pollution at the depth of 100～200 cm in Qingshuigou is medium pollution,and the other depths are light pollution,the comprehensive pollution index is clean.The phosphorus pollution at the depth of 0～100 cm in Qingbacha is strong,which is medium pollution,and the comprehensive pollution index is light pollution.Enough attention should be paid to the phosphorus pollution in each sedimentary column.Based on 25 satellite remote sensing images taken from 1999 to 2019,used the mean high tide line to calculate the accretion and erosion of the Yellow River delta under different runoff and sediment transport scenarios.Water-Sediment regulation scheme（WSRS）have effectively reversed the erosion of the Yellow River delta that had operated from 2002 to2019.Runoff and sediment transport during the WSRS period are positively correlated,and both are negatively correlated with erosion area;correlations between variables during the non-WSRS period are not significant.During the WSRS,a sediment and runoff transport ratio greater than 10 kg/m3results in overall accretion,and a value lower than this results in erosion.In general,during the WSRS period,runoff,sediment transport,and the sediment–runoff ratio jointly control the changes in the accretion area of the river delta.Our research provides a scientific basis for the sustainable development of the Yellow River delta and WSRS scheduling for the Xiaolangdi Reservoir.