| The Yellow River is one of the most frequently flooding rivers in China.The floods caused by frequent channel migrations and diversions have resulted in a huge loss of life and production.Therefore,understanding the hydrological processes and extreme climate-hydrological events of the Yellow River is the scientific basis of the effective utilization of water resources,water energy and flood control.The Hetao section of the Yellow River is located in the transition area between china humid region and arid region.The YR flows through the desert area,in which a large volume of sands were transported into the river and hence resulted in a complicated water and sand relationship.In recent years,channel silting has caused an increasing river bed of the Yellow River,which has led lots of potential flood disasters in this region.In this study,sedimentological analysis and the Optically Stimulated Luminescence(OSL)dating of three 20-m boreholes within the Yellow River floodplain enabled us to reconstruct the palaeo-flood records since the Late Quaternary.Moreover,we discuss the relationship between the palaeo-floods and climate change.Our results show that the three cores(DKZ01,DKZ02,DKZ03)are dominated by silts,with the silt content are 63.12%,57.74% and 40.97%,respectively.The frequency curves of the sediments are characterized by monomodal type and the probability accumulation curve is mostly "jump-suspension" two-stage.The vertical variation of sediment particle size is the response to hydrodynamic conditions,which could be used to reconstruct regional paleohydrological conditions.Based on the study of the relationship between water and sediment in the Ningxia-Inner Monglia section of the Yellow River,it is concluded that the sediment particles of 2-16 ?m in the boreholes are mainly transported as bedload Which is controlled by the upstream sediment supply.While the sediment particles of 16-80 ?m are transported mainly as suspended load that increased with the water velocity and discharge.Therefore,in this study,we select the U ratio(16-80 ?m/2-16 ?m)to represent the intensity index of the palaeo-flood.when the U ratio is higher,it implies a higher hydrodynamic condition and also a larger magnitude of floods.Based on the Pearson correlation analysis,it was found that the correlation coefficient between ln(Zr/Rb),hence we could deduce that ln(Zr/Rb)can be used as a representative indicator of particle size to indicate the intensity of the palaeo-flood,and then the high-resolution palaeo-flood records can be established.In addition,combined with the OSL dating,we reconstructe the palaeo-flood sequences of the YR during the last 22 ka in the front of the Langshan Mountain,and found that there were three high-frequency and four low-frequency periods in the paleo-flood event,Among them,the flood intensity is larger and the duration is longer in the period of 14-16 ka,7.5-11.5 ka and 1.2-3 ka.In the period of 22-16 ka,14-11.5 ka,7.5-3 ka,and 1.2 ka to present,the intensity of the palaeo-flood is less and the duration is shorter.According to the comparison and analysis of regional climatic data,it is found that the paleo-flood event is the response of the hydrological process to the extreme events of the climate,that means palaeo-floods generally occur in the climate transition periods.During the period of 16-14 ka,the regional climate is unstable and the climate change is fast and large,which may be the main cause of frequent flooding.During the period of 11.5-7.5 ka,the global temperature gradually increased and the East Asian monsoon gradually increased.The precipitation and effective humidity in the prey-front area of the monsoon fringe increased,and the flood activity was more frequent than before,Increased strength.In 3-1.2 ka,the instability of the climate system and the enhancement of human activities were the main causes of frequent floods. |
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