| Since the holocene,the surface environment has been continuously evolving,accompanied by the occurrence of various natural disasters.The uncertainty,frequency and destructiveness of natural disasters have caused great loss to human production and life.Especially since entering modern society,the destruction of human production activities to the natural environment is far more than the carrying capacity of the environment itself,which leads to the increasing frequency of natural disasters.This requires us to study the modern natural environment through the study of the climate and environment in the past,this requires us to propose effective strategies for the changes of the modern natural environment through the study of past climate conditions.Loess,ice cores and deep-sea sediments are known as the three major information carriers of the global paleoclimate.In the study of the western loess,the various sediments contained in the loess paleosol sequence are the key to which we have opened the past.As far as our country is concerned,due to the diversity of topography and climate in China,climatic and hydrogeological disasters occur frequently,especially flood and mud flow disasters.By studying the sediments left by the prehistoric natural disasters,we can not only help us determine the type of disaster events,but also help us to have a further understanding of the rise and fall of the prehistoric natural environment and the different periods of human civilization,and provide important reference for future flood hazard assessment and environmental archaeological research.This paper takes the Taohe River basin in the upper reaches of the Yellow River and the Lajia Ruins in the Minhe section as research objects,and uses the related theories and methods of hydrology,geomorphology,sedimentology,climatology,and chronology to determine the Holocene palaeoflood and ancient mudslide events,and carries out a detailed experimental study and comparative analysis of the two.On the one hand,this paper restores the flood peak flow of the palaeoflood,reconstructs the palaeoflood in the Holocene of Taohe River Basin,enriches the relative lack of hydrological data in the basin and reveals the climate and environment background at that time.At the same time,this paper identifies the mudflow disaster through the analysis of the red mudflow deposits found in the Minhe section in the upper reaches of the Yellow River.and compared them with palaeoflood sediments,revealing the differences between the two sediments and the differences between two kinds of sediments and two natural disasters.The conclusions of this paper are as follows:(1)through a detailed observation of the macroscopic characteristics of the river section,and the previous criteria of the palaeoflood slackwater deposit(SWD)in the the Yellow River basin for many years,A group of soil profiles with approximately 70cm thickness that are suspected of palaeoflood slackwater deposit were found at Xiabucun site(XBC)in Lintao section of the Taohe River basin in the upper reaches of the Yellow River.After indoor physical and chemical experiment,the sediments of this layer were indeed deposited during the flood-producing in the high water level and stagnant environment.The field observation can also be found that the upper part of the study section is a three layer of continuous palaeoflood slackwater deposit,this group of sediments are clearly stratified but there is no other gap between them,so it can be judged to be a three palaeoflood events.The discovery of these evidences once again enriched the data of palaeoflood events in the upper reaches of the Yellow River.(2)In traditional palaeoflood research methods,"Palaeoflood SWD Top Elevation Method" and "Pining Point Elevation Method" are usually used to reconstruct the palaeoflood,but both methods have their own inadequacies.On the one hand,the former method neglects the sedimentary water depth,and the latter's required pinch-out point is difficult to preserve.Therefore,this paper adopts the method of "The relation between thickness and sediment concentration" to the location of XBC study the recovery of the flood peak water level and uses HEC-RAS to restore the flood peak flow of the palaeoflood.The results are as follows:the water level of SWD1-SWD3 is 1759.75 m,1756.82 m and 1756.43 m respectively,and their corresponding peak flows are 6 700 m3s-1 and 5100 m3 s-1 and 4 865 m3s-1 respectively.At the same time,we use the same method to restore the flood peak flow based on the actual field observation of the flood elevations in 2012,the error of the result and the measured flow rate is only-2.9%.So the result is more accurate.(3)Flood disasters and heavy rain and mudflow disasters are actually responses to the climate and environment at that time.In this paper,the OSL dating results for XBC site show that the palaeo-flood event occurred at around 300-480 a BP.According to the characteristics of environmental evolution in different stages of the Holocene,palaeoloods usually occur during the turning point of climate change.This palaeoflood event exactly corresponds to the stage of severe climate fluctuations in the Little Ice Age in the Ming and Qing Dynasties.In this stage,the climate is warm and cold,and floods and droughts occur from time to time.Combining previous research on palaeoflood,it is found that almost all palaeoflood events are accompanied by climate fluctuations,so it can be said that the occurrence of palaeoflood events is also a response to global climate change.(4)Compare the nature difference between SWD and mudslide sediments.In the vicinity of the Lajia Ruins at the Guanting section in the upper reaches of the Yellow River,we found a typical layer of red muddy stream sediment(RC)in a group of loess-paleosol profiles.Based on the extensive research done by previous researchers,after detailed field investigations and physical and chemical experiments,this paper determined that these three layers of red mudslides(RC)were derived from Tertiary red clay in the Dahongshan area of the northern part of the site,among them,there are also FFD deposits.Through comparative analysis,it is found that the red clay layer in the formation is distinctly different from the SWD of palaeoflood.Indicating that the destruction of the Lajia Ruins is caused by the mountain flood mudflow,not the Yellow River flood,and this disaster includes at least three mudflows and a mountain flood event.(5)Based on the stratigraphic framework of the Shanglajia Section(SLJ-E),we can determine that in the middle of the Holocene,there were three large-scale mudflow events in the area of the Yellow River and in the Lajia Ruins area of the Guanting section,which meant that the destruction of the Lajia Ruins was largely caused by the three mudflow disasters.Although the occurrence of these extreme disaster events is inseparable from the Holocene climate and environmental changes,the destruction of the environment by early human activities is one of the important reasons.This also warns us to attach importance to the environmental change in the future development,and to the the evolution of human-earth relationships. |
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