Evaluation of flash flood severity in Korea using the modified flash flood index (MFFI)

This paper was initially motivated by a flash flood index (FFI) study performed by Bhaskar et al. and others that explained flash floods using a runoff perspective by extracting a rising curve gradient, the flood magnitude ratio, and flood response time variables from runoff hydrograph. However, the runoff data are not sufficient to estimate a robust flood magnitude ratio in Korea. In addition, the existing model is not able to consider high rainfall intensity, which is an important flash flood characteristic in Korea. Flood magnitude ratio variables, thus, were replaced by rainfall intensity variables. Then, the results were compared with the existing FFI. The spatial distribution of the flood event in the Han River basin that occurred in July 2006 showed that a modified FFI, which reflects rainfall characteristics, outperformed in representing damaged (flash flood occurrence) areas in the past. Furthermore, the FFI for three different time windows (early, middle, and late periods) for the Han River basin during the localised intensive rainfall in July 2006 were analysed. Analysis of spatiotemporal distribution showed that sudden intensity was pronounced in the Seoul area during the first 10 days of July. During the second 10 days of July, a probable high-intensity flash flood appeared in the Gangwon-do area. Meanwhile, an onsite survey of the Gangwon-do area showed that the actual occurrences of a high-intensity flash flood were largely in agreement with the predictions made by the model.