Seoul experienced serious flooding in 2010, 2011, and 2018. The torrential rain caused a series of flash floods and landslides, resulting in significant property damage and casualties. Accordingly, the Seoul Metropolitan Government selected 34 flood-prone areas based on the flood damage caused in 2010 and 2011, and invested approximately KRW 1,530 billion in the total project cost from March 2011 to December 2023 to increase the capacity of the drainage facilities, such as sewage and pumping stations. However, due to climate change, extreme weather events, such as flash floods and heavy rainfall that exceeded the designed capacity of drainage facilities, have increased in Seoul. It has become difficult to reduce the likelihood of flood events and damage with the Seoul’s flood control policy and management, which mainly increases the capacity of drainage facilities. Therefore, it has become important to understand the interaction between flood occurrence and regional characteristics because damage can vary depending on the spatial characteristics of watershed. In this study, flood risk in Seoul was evaluated by considering the spatial characteristics of watershed.

A total of 163 watersheds (drainage sectors) in Seoul were analysed with 52 indicators representing the physical, social, and environmental characteristics of watersheds. Although the area of Seoul is relatively small, it was analysed that the regional variation in watershed characteristics was large. We then classified each watershed characteristic into several types with similar characteristics through a hierarchical clustering method and examined the classification criteria through a Classification and Regression Tree (CART) method. Through the expert survey, each type of watershed characteristic was given a score according to the degree of impact on the flood occurrence and damage. These scores were applied to evaluate flood vulnerability, potential risk, and flood risk in Seoul based on the risk assessment concept in IPCC AR5. Flood vulnerability for each of the 163 watersheds was evaluated by the scores of watershed characteristic types related to indicators that could exacerbate or reduce flood damage; 33 indicators, such as lowland area ratio, impervious area ratio, green area, underground area, GRDP, population density, vulnerable population, rainwater tank capacity, flood pumping station capacity, and sewer pipe length, were considered. Potential risk for each of the 163 watersheds was evaluated by the scores of watershed characteristic types related to indicators of the likelihood of flood occurrence; 27 indicators, such as 1-day maximum precipitation, rainfall intensity, flood damage in the last 10 years, flood damage in the last 3 years, flooded areas in the last 10 years, flooded areas in the last 3 years, flood risk areas, and areas with a depth of 1 m or more in 100-year frequency flood, were considered. Flood risk was evaluated by the scores of flood vulnerability and potential risk for each of the 163 watersheds. Watersheds with high flood vulnerability and high potential risk corresponded to watersheds with high flood risk, and the 5 watersheds with the highest flood risk of the 163 watersheds in Seoul were analysed as Sanggye1, Mia, Jangwi, Changdong1, and Dongseon.

On analysing the rainfall characteristics of the previous 5 years (2010-2014) and recent 5 years (2015-2019) in Seoul, the spatial distribution of rainfall has changed significantly. This causes differences in the potential risk assessment depending on the setting of the analysis period for rainfall. As a result, it increases the ambiguity of the flood risk assessment. In addition, abrupt change in rainfall characteristics causes asymmetry between the spatial distribution of past flooded areas and current rainfall characteristics. This reduces the effectiveness of the Seoul’s flood control policies and management, which mainly manage previous flooded areas. Only 5 areas out of the 34 selected flood-prone areas based on flood damage in 2010 and 2011 were analysed as areas with high potential risk considering the current rainfall characteristics.

Therefore, it is necessary to increase the effectiveness of the Seoul's flood management policy & management and improve the adaptability to climate change through flood risk assessment based on the characteristics of watersheds that exacerbate flood damage rather than rainfall characteristics with high uncertainty.