Evaluation of the causes of inundation in a repeatedly flooded zone in the city of Cheongju, Korea, using a 1D/2D model

Currently, unprecedented levels of damage arising from major weather events have been experienced in a number of major cities worldwide. Furthermore, the frequency and the scale of these disasters appear to be increasing and this is viewed by some as tangible proof of climate change. In the urbanized areas sewer overflows and resulting inundation are attributed to the conversion of previous surfaces into impervious surfaces, resulting in increased volumes of runoff which exceed the capacity of sewer systems and in particular combined sewer systems. In this study, the characteristics of sewer overflows and inundation have been analyzed in a repeatedly flooded zone in the city of Cheongju in Korea. This included an assessment of inundation in a 50-year storm event with total rainfall of 165 mm. A detailed XP-SWMM 2D model was assembled and run to simulate the interaction of the sewerage system overflows and surface inundation to determine if inundation is due to hydraulic capacity limitations in the sewers or limitations in surface inlet capacities or a combination of both. Calibration was undertaken using observation at three locations (PT #1, PT #2, PT #3) within the study area. In the case of the subsurface flow calibration, R² value of 0.91 and 0.78 respectively were achieved at PT #1 and PT #2. Extremely good agreement between observed and predicted surface flow depths was achieved also at PT #1 and PT #2. However, at PT #3 the predicted flow depth was 4 cm lower than the observed depth, which was attributed to the impact of buildings on the local flow distribution. Areas subject to flooding were classified as either Type A (due to insufficient hydraulic capacity of a sewer), Type B (which is an area without flooding notwithstanding insufficient hydraulic capacity of a sewer) or Type C (due to inlet limitations, i.e. there is hydraulic capacity in a sewer which is not utilized). In the total flooded zone, 24% was classified as Type A (10.2 ha) and 25% was classified as Type C (2.61 ha). It was concluded that greater attention should be paid to the area classified as Type B, which covers 50.5% (5.15 ha) of the total flooded zone.