Chronology And Simulation Of Historical Debris Flow At Eyu,Mengshan,Shandong Province

Debris flow is a kind of geological disaster with the characteristics of mass,strong destructive power and short-term outbreak,especially the modern debris flow ditch close to residential areas has always been paid attention by the government.There are still small debris flow gullies in Mengshan Mountain,Shandong Province,and there are also a large number of flood records in historical materials.There are many boulder levees accumulated at the exits of Mengshan Mountain.After the analysis and research of predecessors,combined with gravel data,dating and historical documents,it was found that they were debris flow boulder levees.On this basis,determining its recurrence period can provide necessary information for the safety of residents’production and life.Based on the collection and collation of actual measurement data,local historical materials,and geological and geomorphological investigation of field debris flow ditches,this paper makes a more comprehensive judgment on the cause of boulder levees in Mengshan,and uses multi-index dating method and local historical data to judge the formation age of boulder levees.Finally,after the numerical simulation of modern debris flow,it is confirmed that the simulation parameters are applicable to Mengshan area,and then the specific numerical simulation of historical debris flow is carried out.Achieved the following results:（1）Through the measurement and identification of the gravel fabric,gravel shape and sedimentary structure of the Eyukou boulder levee,Baiyunyu boulder levee and Baziyu boulder levee.It is found that above-mentioned boulder levees in Mengshan has typical debris flow sedimentary structures,such as gravel group structure,support-superimposed structure,large-angle shingle structure,etc.Based on the gravel fabric and gravel shape data,the isodensity map,a-axis trend map,and gravel C₄₀ value are formed.According to the characteristics of the gravel tending to the upstream,the gravel inclination angle is large,and the roundness of the gravel is mostly sub-angular and sub-round.The gravel b/a value,c/b value,sorting coefficient and other indicators are determined to be the cause of debris flow,and most of them are dilute debris flow or water flow.（2）A variety of dating methods were used to estimate the age of the Eyukou boulder levee.The previous OSL（Optically stimulated luminescence）and ¹⁴C dating data of the Eyukou boulder levee and the Baziyu boulder levee indicate that there have been many flash floods and debris flows in the Mengshan area.Use a camera to take lightness photographs of the Eyu debris flow scratches and the surrounding rock carvings of the same type,and use PS（Photoshop）to read the lightness values to form a lightness-age curve to obtain the unknown age of the debris flow scratches.Finally,based on historical data and previous dating data,it was determined that the Eyukou boulder levee was roughly formed between 1647 and 1688.（3）Carry out numerical simulation on the historical debris flow in Eyu,and obtain the recurrence period of the debris flow that caused the boulder levee in Eyukou.By comparing the measured shape data of the boulder levee,and comparing the simulated mud depth and flow velocity results,it is shown that it may be a 2-hour or 24-hour precipitation that occurs once in 300 years or 500 years,or it may be a 2-hour precipitation that occurs once in 1000 years,thus forming Eyukou boulder levee.This paper can play a certain reference value for the parameter selection of numerical simulation of debris flow in eastern China,and also for the prevention and control of low-frequency debris flow ditches in Mengshan area of Shandong.