Estimation of the Relative Severity of Floods in Small Ungauged Catchments for Preliminary Observations on Flash Flood Preparedness: A Case Study in Korea

Abstract

An increase in the occurrence of sudden local flooding of great volume and short duration has caused significant danger and loss of life and property in Korea as well as many other parts of the World. Since such floods usually accompanied by rapid runoff and debris flow rise quite quickly with little or no advance warning to prevent flood damage, this study presents a new flash flood indexing methodology to promptly provide preliminary observations regarding emergency preparedness and response to flash flood disasters in small ungauged catchments. Flood runoff hydrographs are generated from a rainfall-runoff model for the annual maximum rainfall series of long-term observed data in the two selected small ungauged catchments. The relative flood severity factors quantifying characteristics of flood runoff hydrographs are standardized by the highest recorded maximum value, and then averaged to obtain the flash flood index only for flash flood events in each study catchment. It is expected that the regression equations between the proposed flash flood index and rainfall characteristics can provide the basis database of the preliminary information for forecasting the local flood severity in order to facilitate flash flood preparedness in small ungauged catchments.

A method of flood severity assessment for predicting local flood hazards in small ungauged catchments

“…Compared with the results of the flash flood index FI by Kim and Choi (2012), the FHI proposed in this study shows much stronger relation to rainfall characteristics for the two study basins. Kim and Choi (2012) showed that the flash flood index FI had a high relation to the 3-h maximum rainfall depth R 3h with the coefficient of determination R 2 of 0.753 for the Oui-mi River basin, and 4-h maximum rainfall depth R 4h with the coefficient of determination R 2 of 0.800 for the Mae-gok River basin, respectively.…”

“…Kim and Choi (2012) showed that the flash flood index FI had a high relation to the 3-h maximum rainfall depth R 3h with the coefficient of determination R 2 of 0.753 for the Oui-mi River basin, and 4-h maximum rainfall depth R 4h with the coefficient of determination R 2 of 0.800 for the Mae-gok River basin, respectively. The flash flood index FI was averaged of the two flood severity factors such as the flood magnitude ratio M and the rising curve gradient K in Kim and Choi (2012), while this study takes one more flood severity factor of the rising average runoff R into account for the newly proposed FHI.…”

“…Kim and Choi (2012) showed that the flash flood index FI had a high relation to the 3-h maximum rainfall depth R 3h with the coefficient of determination R 2 of 0.753 for the Oui-mi River basin, and 4-h maximum rainfall depth R 4h with the coefficient of determination R 2 of 0.800 for the Mae-gok River basin, respectively. The flash flood index FI was averaged of the two flood severity factors such as the flood magnitude ratio M and the rising curve gradient K in Kim and Choi (2012), while this study takes one more flood severity factor of the rising average runoff R into account for the newly proposed FHI. In addition, this study defines the rising curve gradient K from the starting time to the peak flow occurrence time for a flood event, while it was obtained by measuring a relatively moderate gradient near an inflection point of the peak discharge above the bankfull Bold value indicates the best-fit regression curve discharge for a hydrograph in Kim and Choi (2012).…”

“…The flash flood index FI was averaged of the two flood severity factors such as the flood magnitude ratio M and the rising curve gradient K in Kim and Choi (2012), while this study takes one more flood severity factor of the rising average runoff R into account for the newly proposed FHI. In addition, this study defines the rising curve gradient K from the starting time to the peak flow occurrence time for a flood event, while it was obtained by measuring a relatively moderate gradient near an inflection point of the peak discharge above the bankfull Bold value indicates the best-fit regression curve discharge for a hydrograph in Kim and Choi (2012). Note that these different features of flood severity factors may lead to the relatively shorter duration of the rainfall (R 3h for FI and R 2h for FHI) in the highest relation with the FHI at the hilly region of the Oui-mi River basin, but they do not affect the best-fit rainfall duration result (R 4h for both FI and FHI) in the Mae-gok River basin, a relatively flat watershed.…”

A New Flood Index for Use in Evaluation of Local Flood Severity: A Case Study of Small Ungauged Catchments in Korea1

“…Kim and Kim (2008) estimated the flash flood index for investigating the relative severity of flash floods in the Han River basin with 101 flood events, and quantified the flash flood severity for some flood events caused by heavy rainfall in July 2006. Although previous studies computed the flash flood index directly from the observed flood hydrographs, Kim and Choi (2011, 2012) made initial attempts to quantify the severity of floods that occurred in small catchments of the Korean Peninsula where usually runoff observations are not available.…”