A Flash Flood Disaster Warning In Small Watershed Based On The Critical Rainfall

The research of the flash flood disaster warning and forecasting in small watershed can provide technical basis and solving measures for prevention and treatment of flash flood disaster, and reduce casualties and economic losses caused by flash floods. Because of the large area and poor vegetation condition in Gansu Province, flash flood disasters occurred frequently(especially the debris flow hazards in mountainous areas) which threaten the safety of life and property of the people, and restrict the development of the society. So far there are few studies on mountain flood disasters in Chengguan District of Lanzhou City, Gansu Province. In this paper, the flash flood disasters warning index in Chengguan District of Lanzhou city is quantitatively studied. On this basis, analyzing the current situation of flood control capacity and dividing the danger area can provide the theoretical basis for the flood disaster prediction of the area, personnel transfer and resettlement and the management of flash flood disaster.In this paper, the study area is Chengguan District of Lanzhou city. The research objects are the three flash flood ditches in Chengguan District, which is Yuergou Ditch, Nannigou Ditch and Dashagou Ditch. Firstly,the flood characteristics of three flash flood ditches such as the design flood discharge and flood volume, flood duration, flood hydrograph is quantitatively studied through the methods of rational formula, instantaneous unit line and “tieyiyuan” method. Secondly, inverse-water level method is applied to calculate the critical rainfall of three flash flood ditches. Finally, the prepare for transfer and immediate transfer index of the flood disaster are determined by the critical rainfall, and analysis three channel current flood prevention capacity, the division of mountainous flood disasters occurred in the danger zone, the high risk area and the extremely high risk area. The results show as follow:(1) The design floods of small watershed which is lacked of measured data of flood in Gansu Province are computed by the methods of rational formula, instantaneous unit line and “tieyiyuan” method. The results show that these three methods have some adaptability to data deficient area the design flood of the calculation data. Combined with the small watershed area, this paper determined the calculation results using the "succession" method. The design flood discharge of hundred years once of Yuergou ditch, Lannigou ditch, Dashagou ditch are 104m3/s, 184 m3/s, 432 m3/s; the duration of flood are 1.2h, 2.3h, 3.0h.(2) The critical rainfall at different time periods is calculated by using inverse-water level method. The one hour critical rainfall of Yuergou ditch, Lannigou ditch, Dashagou ditch are 38.4mm,42.6mm,26.2mm. The results show that the method is not dependent on the rainfall data and the survey data of flash flood disaster and strong practicability and popularization significance for the calculation of critical rainfall of mountain torrent disasters of small watershed in no data region.(3) Because the rainfall information is convenient for observation, the index is clear, the practicability is strong, and the warning is convenient, in this paper the warning index of flash flood disaster is determined based on the critical rainfall. The ready to transfer rainfall index of three ditch is 26.3mm,34.4mm,22 mm. When an hour of rainfall of these three ditches reach to 38.4mm, 42.6mm, 26.2mm, personnel who are near the ditch should be immediately transferred.(4) Based on the survey and analysis of calculation results of Yuergou ditch, Lannigou ditch, Dashagou ditch, flood control capacity of these three ditch are 25 years, 43 years, 13 years. There are no residents and enterprises and institutions in extremely high risk area and high risk area of Yuergou ditch and Lannigou ditch. Residents are distributed in the danger area and a higher range, the threat level of flash floods is not high. There are no residents and enterprises and institutions in extremely high risk area of Dashagou ditch, but there are residents in high risk area and danger area of Dashagou ditch. the threat level of flash floods is high.