*2.1. Study Area and Input Rainfall Data*

The Samsung-dong area is divided into Samsung 1-dong and Samsung 2-dong, and is located in the Seoul metropolitan area, which contains 239 drainage basins. Samsung-dong has a population of 44,031, with an area of 3.18 km2, and there are 342 manholes and 359 conduit links from the urban drainage system in the study area. Rainwater from both areas is pumped to the Tancheon river (Figure 1). The ratio of the impervious area in Seoul is as high as 54.4% according to the management report of the National Institute of Environmental Sciences of Korea, 2014. Samsung-dong, which is a part of Gangnamgu, Seoul, consists of relatively low land and has a complex drainage system. Figure 2 showed the actual rainfall on 21 September 2010 which was used for the verification of the EPA-SWMM and FLO-2D model and extreme rainfall scenario using the Huff time distribution method which were used for analysis. An inundation trace map shows the extent of flooding from rainfall on 21 September 2010 (Figure 3) [22].

**Figure 1.** Location of the study area in the Samsung-dong and flood risk zone.

For more accurate research on the impact of extreme rainfall on severe flooding in the city, the latest precipitation data from an automated synoptic observing system (ASOS) were obtained from the South Korea Meteorological Administration. The actual rainfall on 21 September 2010 was investigated, and the Huff method produced data for different duration periods of extreme rainfall; the total extreme rainfall event data included 432 different periods of extreme rainfall event data, such as 10-year, 50-year, 80-year, and 100-year periods. The Huff curve characterizes the temporal distribution of rainfall depth over an area and is widely utilized as an input to rainfall–runoff models for drainage design [23,24]. Most urban floods occur within 6 h, and the duration of extreme rainfall is divided into 1-h, 2-h, and 3-h timespans [25]. Table 1 shows the total, maximum and minimum rainfall during five extreme rainfall periods. Figure 2 shows that the actual total rainfall was 278 mm, minimum rainfall was 1 mm and maximum rainfall was 19 mm on 21 September 2010, with representative changes over 6 h of extreme rainfall event data.

**Figure 2.** (**a**) Actual rainfall sample on 21 September 2010 and (**b**) 6-h extreme Huff rainfall sample.

(**a**) (**b**) **Figure 3.** Verification of two-dimensional flood analysis results.


**Table 1.** 1, 2, and 3-h total rainfall during different return periods.
