Research On Critical Rainfall Of A Flash Flood Disaster Based On Concentration Model Of GIUH

The critical rainfall is an important parameter for the early warning and prevention system of mountain torrent disaster. Therefore, the calculation precision of the critical rainfall is very important to reduce the casualties and property losses. In the absence of measured hydrological data, confluence is always calculated by rational formula method for most small watershed. But,this method represents more experience, and flood process is generalized by only five points. In this process, the calculation of watershed confluence is only related to the watershed area, the river length and the gradient. For those reasons, there are some errors comparing with actual process. In order to solve the problem of calculation accuracy of critical rainfall, this paper tries to establish the model of topography geomorphic instantaneous unit hydrograph based on landform. This model is used to calculate the critical rainfall which is both accurate and conformed to the actual situation.Based on the DEM, Chenjiahe river tributary located in Long county of Baoji is used as a case study, the geomorphic parameters were obtained by using ArcGIS10.0 in hydrological analysis and calculation: its Horton's bifurcation ratio is 4.20, the length ratio is 2.66, the area ratio is 3.11; Combined with the hydrodynamic parameters, the model of instantaneous unit hydrograph is established. The relationship between average river width and net rainfall intensity is obtained by using the measured parameters of three flood processes: B=0.407Ln(ir)+1.006.Then, the confluence model is used to simulate the other two flood processes, and its results are compared with the measured results. The results show that all aspects of the flood process are in good agreement. The local rainstorm process needs to be designed. Through the analysis and calculation of the runoff and convergence of this process, we can get the flood process at the early warning object. Under the assumption of rainfall and runoff at the same frequency, critical rainfall and prepared transfer rainfall of basin warning objects for three typical soil moistures Pa=20mm(drought) and Pa=50mm (General) and Pa=80mm (wet) are calculated by using ditch data measured at Chenjiahe with the Stage/Discharge Inversion method.After hydraulic calculation of section, according to the relationship between water level and discharge curve, the characteristic flow of the river basin is 364m3/s. In the P?Qmp curve obtained by the instantaneous unit hydrograph method, the disaster frequency is 1.75%, and then the critical rainfall corresponding to the values of 1?3?6?12?24h is calculated to be 52?90?104?125?152mm. By comparing the calculated results with the results of the previous inference formula, it is found that the result of the inference formula method is conservative. Because flood process is generalized by only five points, the calculation of the transfer rainfall can only be interpolated,which inevitably lead to a certain gap between the calculation result and the actual value. For this reason, it will be early to make the transfer of personnel and lose the meaning of early warning when we are early warning. Therefore, the results of the instantaneous unit hydrograph method based on topography and geomorphology are more accurate and more consistent with the local situation.