In the past, flash floods were characterized by rainfall and limited effort was made to examine them from a runoff perspective. For example, the question of how flash floods differ from other floods remains to be investigated. In order to satisfy this need, runoff hydrographs of 30 flood events from four watersheds in eastern Kentucky are selected and analyzed. In flash flood situations, certain portions of the hydrograph, such as the rising limb, time to peak, and magnitude of the peak, are important. To recognize this, runoff hydrograph characteristics such as the rising curve gradient, K, flood magnitude ratio, M, and flood response time, TP, are evaluated directly from observed floods. Using these characteristics, relative severity factors RK, RM, and RT are determined and summed to obtain a flash flood index, RF. Results from this study show that a reported flash flood event of May 18, 1995, has values RK = 3, RM = 16, and RT = 6 and a flash flood index, RF = 25. Overall, half of the flood events have flash flood indices greater than or equal to 15.
