Study On Critical Rainfall Of Mountain Flood Disaster In Typical Small Watershed Of Yiyuan County,Shandong Province

Critical rainfall is an index to determine whether a mountain torrent disaster occurs in a region,and it is a key technical problem of mountain torrent disaster prevention.On the basis of summarizing and analyzing the existing calculation methods of critical rainfall index at home and abroad,the critical rainfall of flash flood disaster in typical small watershed of Yiyuan County,Shandong Province was studied.Based on HEC-HMS,a typical watershed runoff model was established,and eight floods were used to calibrate and verify the parameters.On this basis,the model trial method was used to calculate the critical rainfall index,and the influence of reservoir regulation and rainfall time history distribution on the critical rainfall index was analyzed.The main conclusions are as follows.(1)Based on HEC HMS,a hydrological model of small watershed in Yiyuan County research area was established.The model parameters were determined and verified by using the measured rainfall and discharge data of eight sites in 1976-2015.The results show that the relative errors of peak discharge and runoff depth of calibrated and verified stations were less than 20%,the peak time was less than three hours,and the Nash coefficient of 80% stations was more than 0.7.The accuracy of the simulation results was good,and the values of parameters were reasonable,which was suitable for simulating the flood process of small watershed in the study area.(2)Three different initial water storage capacity of reservoirs were set up in the model to simulate the flood process of small watershed.In the model,three different initial storage volumes were set to simulate the flood process of small watershed.The results showed that when the initial storage capacity of the reservoir were half of the total storage capacity,one third of the total storage capacity and empty,the average peak discharge of the six flood simulations were 12.04%,20.19% and 25.2% respectively.It was proved that the less the initial storage of the reservoir,the greater the reduction rate.The maximum reduction degree of flood peak can reach 32.6%,and the maximum delay time can reach 2 h.(3)The critical rainfall values of disaster prevention objects under three reservoir regulation and storage schemes are calculated.The results show that the difference between the critical rainfall of Zhujiahzuang village with zero initial water storage and that without reservoir is between 8.6% and 41.6%.The critical rainfall difference of Caojiazhuang village is between 46.2% and 84.8%.When the initial water level is flood control limited water level,the difference between the critical rainfall of Zhujiahzuang village and that without reservoir is between 6.5% and 32.4%.The difference of Caojiazhuang village is between33.3%～72.8%.When the initial water level is the normal pool level,the difference between the critical rainfall of Zhujiahzuang village and that without reservoir is between 2.7%～% and29.6%.The difference of Caojiazhuang village is between 14.7%～67.7%It is proved that the initial storage capacity of the reservoir has an important influence on the process of runoff and confluence,and the regulation and storage function of the upstream reservoir should be considered when determining the critical rainfall of the disaster prevention object.(4)The influence of rain patterns at different rain peak positions on the critical rainfall of downstream disaster prevention objects is analyzed.The fuzzy recognition method is used to count the actual rainfall patterns in the study area,which are close to the first three types of rainfall patterns.According to the actual rainfall,the design rainstorm value is allocated by rainfall time history,and the critical rainfall value in the early warning period under the first three rain patterns and the design rain pattern is calculated by the model.The results show that under the condition of the same rainfall,the later the rain peak is,the greater the flood peak discharge is.The relative error between the critical rainfall obtained by the peak rainfall pattern and the design rainfall pattern is 1.2%～65.7%.The relative error range between the critical rainfall calculated by the peak rainfall pattern and the design rainfall pattern is-0.9%～33.1%.The relative error range of the critical rainfall value calculated by the rainfall pattern behind the rainfall peak and the design rainfall pattern is-22.9%～16.8%.Based on the analysis of the critical rainfall results of different rainfall patterns,the critical rainfall range of different warning periods is given,which provides a reference for the early warning of mountain torrents in gaocunhe small watershed.