| Flash torrent occur frequently in the southwest of China and cause serious losses to these areas.In addition to limited engineering measures,non-engineering measures are important part of flash torrent disaster prevention and control.Establishing effective early warning mechanism is the main plan to prevention and control of flash torrents.The key link of the mechanism is to define the quota of early warning.With the continuous development of practical application and the improvement of meteorological forecast quality,the critical rainfall is the most important early warning index of flash torrents.The rationality of the critical rainfall is extremely important to the early warning of flash torrents.Therefore,how to determine the critical rainfall of flash torrents is an important point in the research of prevention and control of flash torrent.In this paper,the flash torrents and geological features of northeast Yunnan province were investigated.On the basis of full investigation,the HEC-HMS,a mature hydrological model,was used to simulated the flash torrent events that had occurred in the Dawenxi-River,a typical basin in this area.The purposes of the simulation are to explore the relationship between rainfall and flash torrent,and to calculate the critical rainfall based on the relationship.The main results of this paper are as follows:(1)According to the investigation of flash torrents and geological features in Zhaotong,a municipal administrative region in Yunnan Province,it can be found that,due to the specific climate and geological combination,flash torrents in this area are serious.The flash torrents in this area were mainly distributed in summer and autumn,and the proportion of flash torrents that occur in June to September account for up to86%.In terms of spatial distribution,flash torrents in this area were concentrated in the central and eastern regions represented by Yiliang county and Zhenxiong County,more than 141 flash torrents occurred in the two counties,accounting for 62.4%of the recorded number of flash torrents,while the other 9 counties only account for 37.6%of the total record.Dawenxi-River Basin is a typical flash torrent basin in northeast Yunnan.The flash torrent disasters occurred frequently and in large scale,the maximum peak discharge was as high as 1050m~3/s.According to six years of hydrological and meteorological observation data,it can be found that the rainfall in the Dawenxi-River Basin is concentrated in summer and autumn,which corresponds to the fact that flash torrents in this basin mostly occur in summer and autumn.It is proved that heavy precipitation is the main inducing factor of flash torrents in this basin.In addition,the number of occasional extreme weather events in the basin is increasing,which will bring more pressure to the prevention and control of flash torrents in this basin.(2)Based on DEM digital and remote sensing digital in Northeast Yunnan,the Dawenxi-River digital basin was constructed by Arc GIS platform and HEC-Geo HMS module.The digital basin including 23 sub-basins,which provides basic information for subsequent calculation and simulation of the HEC-HMS hydrological model.The flash torrents occurred in Dawenxi-River Basin from 2013 to 2018 were selected as the simulation object.By using the SCS Curve Number(CN)Loss Method and SCS Unit Hydrograph Method and Monthly Constant Base Flow Method and Muskingum Method in the HEC-HMS hydrological model,four stages of runoff generation,confluence,base flow river course evolution are simulated respectively,and 12 flash torrents events(large discharge events)with selected are simulated at calibration stage.In the first simulation,the SCS Curve Number(CN)based on the previous soil moisture and soil hydrological grouping of each sub-basin and the land use data in August 2016,and the flood routing parameters of Muskingum river section calculated according to the standard method.The CN was set as:forest land,70,shrub and grassland,65,cultivated land,78,building,98,bare soil,88.According to the actual situation of sub-basin underlying surface,the CN value of sub-basin is obtained.At the same time the x of Muskingum channel evolution method is 0.5.The results show that,based on the above calculation methods and parameters,the HEC-HMS hydrological model has a poor simulation effect for the 12 torrents events in the Dawenxi-River Basin except for4 of the torrents events that met the expected standard.The simulated value is generally much smaller than the actual observed value,but the simulated peak present time has a high degree of agreement with the measured data.(3)Use the parameter optimization function of the model to optimize and simulate 12flash torrents at calibration stage.The simulation results are optimized through several trial calculations.When the optimized simulation results are consistent with the observation results,the model stops.At this time,relevant participants change occur on the basis of the original parameters,which are more in line with the comprehensive characteristics of the basin.Through optimization,it can be determined that when the soil moisture condition in the early stage is relatively dry,the CN should be increased to 1.3 times that of the original value,and the initial loss value should be reduced to0.8 of the original value.When the soil moisture condition in the early stage is in a general state,the CN should be increased to 1.13 times of the original value,and the initial loss value should be reduced to 0.9 of the original value.When the soil moisture condition is wet in the early stage,the CN value and initial loss value shouldn’t be changed.After adjusting the parameters,the simulation results of all 4 floods in the verification period are qualified,which proves that the HEC-HMS hydrological model can be used in the simulation of flash torrents in this area,and can provide effective support for the early warning and management of flash torrents.(4)Based on the HEC-HMS effective simulation of recent flash torrents in Dawenxi-River Basin,a more accurate relationship between rainfall and flood is obtained,which provides a basis for deriving more suitable early warning indicators for the basin.The survey data shows that the confluence time of the basin is concentrated in 4-8 hours.Therefore,the early warning period of the basin is determined to be 1h,2h,3h,and 6h.According to the field survey,it is determined that the disaster water level in the Shuanghekou reach of the Dawenxi River Basin is 407.5m,and the disaster flow rate is 515m~3/s calculated according to the characteristic parameters of the section.The early warning index was calculated by using the trial algorithm,and the results were adjusted according to the actual situation of the river basin.Finally,the critical rainfall of flash torrent at Shuanghekou reach of the Dawenxi-River Basin is determined. |
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