The Flood Risk Grading And Threshold Determining Of Mogao Grottoes

The Mogao Grottoes is located in arid inland surrounded by wilderness gobi desert. The climate of this region is extremely dry due to the facts as follow:rainfall is sparse, evaporation is intense, besides the sandstorm happens quite frequently. Theoretically, this kind of climate is benefits the preservation of these frescos. However, the Mogao Grottoes is under threat of flood quite often, which could be proved by the fact that the frescos is suffering from flaking and efflorescence. The Mogao Grottoes is within the Daquan River basin, the upper reaches of Daquan River flows through the Qilian Mountain, where the altitude is quite high and the rainfall is rich. And with the snow melt water supply, abundant of water flows into Daquan River every year especially during summer, from June to September, when heavy rains happen a lot. The damage of flood in Daquan River puts Mogao Grottoes under huge threat. Therefore, the study in this paper work, which is focus on the flood risk prediction of Mogao Grottoes area, is quite essential and meaningful. In this paper, the flood risk has been classified into four grades according to the destructiveness of the flood. They are normal risk, great risk, significant risk and catastrophic risk. Then, threshold respective to the four grade risks have been calculated as the theoretical and technical support to the subsequent work of building up the flood early warning system for Mogao Grottoes.When determining the flood risk threshold values, this paper adopts two methods, one kind is according to the Daquan River upstream of the simulated rainfall experiment, analysis of rainfall infiltration and runoff characteristics and relations, respectively test data was used to simulate the rainfall infiltration and runoff yield equation, and then calculate the risk corresponding to different rainfall intensity and time value. Used in runoff intensity fitting calculation formula method, the infiltration rate of fitting formula method and Green-Ampt formula are used to be calculate the threshold, and than getting the final average to map. Another method to determine the threshold is basin drop the convergence of the aquatic reasoning formula method, change the method to calculate threshold of Mogao Grottoes is designed according to have Daquan River cross-section flood process line, through the peak flow to calculate the rainfall intensity and time of the flood, through calculation, it is concluded that a normal flood risk (10～20 years) threshold of Mogao Grottoes is:t=-0.15R+4.26, the threshold of peak discharge is between 147.25～186.71 m3/s; A great flood risk (20～50 years) threshold is:t=-0.15R+4.35, the threshold of peak discharge is between 186.71～232.54 m3/s; A significant flood risk (50～100 years) threshold is:t=-0.12R+ 4.11, the threshold of peak discharge is between 232.54-344.37 m3/s; A catastrophic flood risk (≥ 100 years) threshold is:t=-0.16R+4.82, the threshold of peak discharge is above 344.37m3/s.