The existence of impervious areas is one of the most distinguishing characteristics of urban catchments. They decrease infiltration and increase direct runoff in urban catchments. The recent introduction of green infrastructure in urban catchments for the purpose of sustainable development has contributed to the decrease in directly connected impervious areas (DCIA) by isolating existing impervious areas, and consequently, has also contributed to flood risk mitigation. This study coupled the width function-based instantaneous hydrograph (WFIUH), which is able to handle the spatial distribution of the impervious areas, with the concept of the DCIA to assess the impact of decreasing DCIA on the shape of direct runoff hydrographs. Using several scenarios for typical green infrastructure and the corresponding changes of DCIA for a test catchment in Seoul, South Korea, this study evaluated the effect of green infrastructure on the shape of the resulting direct runoff hydrographs and reducing peak flows. The results showed that the changes in the DCIA immediately affect the shape of the direct runoff hydrograph, and decrease peak flows by up to 12% depending on spatial implementation scenarios in the test catchment. This study demonstrates the importance of the DCIA concept for the evaluation of green infrastructures in urban catchments, enabling quantitative assessment of the spatial distribution of impervious areas, and also changes to the DCIA by various types of green infrastructure. The results of this study also suggest that more effective and well-planned green infrastructures could be introduced in urban environments for the purpose of flood risk management.

Drainage networks are essential compartments in an urban infrastructure system for the efficient collection and prompt drainage of flood water. In addition to the advances in numerical techniques on urban flood simulation, the topological characteristics of urban drainage networks and their impacts on flooding have not been investigated thoroughly despite their importance. This study evaluated the urban drainage networks in Seoul, South Korea, in terms of the network configuration and its implication for peak flows and flood mitigation. Gibbs’ model was used to analyze the network configuration of 31 urban catchments with various slope ranges. The results showed that urban drainage networks can be less efficient than river in nature in terms of the drainage time, which is counter-intuitive. On the other hand, the analysis showed that efficient networks have risks of flood concentration and, hence, increase potential flood risks. This study showed that efficient networks tend to have higher peak flows at the outlet and vice versa. Therefore, an alternative drainage network layout, which is less efficient and more sinuous, was introduced and it resulted in reduced peak flows and flooding. This result shows managing a proper drainage network layout can contribute to flood mitigation in urban catchments.

The recently observed hydrologic extremes are unlike what has been experienced so far. Both the magnitude and frequency of extremes are important indicators that determine the flood safety design criteria. Therefore, how are design criteria updated faced with these extremes? Both a sudden increase of design rainfall by the inclusion of these extremes and complete ignorance are inappropriate. In this study, the changes in extremes were examined and an alternative way to estimate the design rainfall amounts was developed using the data from 60 stations in South Korea. The minimum density power divergence estimator (MDPDE) with the optimal value of a tuning parameter, α, was suggested as an alternative estimator instead of the maximum likelihood estimator (MLE); its performance was evaluated using the Gumbel (GUM) and the generalized extreme value (GEV) distribution. The results revealed an increase in both the frequency and magnitude of extreme events over the last two decades, which imply that the extremes are already occurring. The performance of the MDPDE was evaluated. The results revealed decreased and adjusted values of the design rainfall compared to MLE. On the other hand, the MDPDE of the GEV distribution with a positive shape parameter, ξ, does not show its advantage conditionally because the GEV distribution has a heavier right tail than the GUM distribution (ξ = 0). In contrast, the results showed the high sensitivity of the MLE to the extremes compared to MDPDE.

According to the Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report, the amount of precipitation in South Korea would increase regardless of the reduction of Greenhouse Gas (GHG) emissions. However, at the same time, it is expected that the temporal and spatial rainfall variation would also increase. Due to the impact from typhoons, 90% of the annual precipitation in Korea occurs in July, August, and September. Moreover, the Representative Concentration Pathways (RCP) scenario projected that the average precipitation in this period is expected to increase markedly, especially over the next 100 years. These predictions imply an increased variability of available water resources. In this study, we assessed a RBSN (rain barrel sharing network) as an efficient way to respond to the future climate change projections under the RCP scenarios when compared to the historical data. We proposed an evaluation procedure for the reliability, resilience, and vulnerability of RBSN based on a storage-reliability-yield (SRY) relationship. The result shows that the reliability and resiliency of a RBSN will improve but be more vulnerable compared to the results from the historical rainfall data. However, even in the climate change condition, the results showed that a RBSN still contributes to reduce vulnerability. The results of this study imply that a RBSN is an effective and alternative measure that can deal with the impacts of climate change in the future.

This study introduces the profile method as a simple and less expensive approach for estimating the surface energy balance of green roofs, addressing the limitations of costly monitoring systems based on measurements at two vertical points. Four separate experiment buildings were constructed to minimize temperature disturbances: concrete, highly reflective painted, short bamboo, and grass-roofed. This setup allowed the evaluation of the thermal performance of each roof type without interference from connected building structures. The flux profile method was used to estimate sensible and latent heat fluxes using temperature, atmospheric pressure, and wind speed measurements at two elevations and demonstrated its potential applicability. The results showed that the sensible heat flux was highest (103.81 W/m²) for the concrete roof and that the latent heat flux was highest (53.28 W/m²) for the short bamboo roof. These results indicated the reliability of the method in estimating fluxes across all roof types, where the Nash–Sutcliffe efficiency was 0.90 on average. Furthermore, sensitivity analysis showed that the optimal values of albedo and surface roughness for each roof type were within reasonable physical ranges, providing additional validation for the flux profile method. The surface energy balance analysis of green roofs indicates that the profile method could serve as an effective tool for quantitatively evaluating the advantages of green roofs, especially in reducing urban heat island effects and lowering building energy consumption.

Stochastic channel network modeling is an informative tool to replicate river networks for the purpose of understanding the variability of geometry and distinguishing observed patterns in networks. In contrast, the application of stochastic network models to artificial or urban drainage networks is not common despite their practical implications for engineering purposes. Gibbs’ model is a useful tool to investigate the network characteristics of drainage networks and also has an advantage to produce alternative networks with the same network characteristics due to its stochastic nature as a network model. This study utilized Gibbs’ model to estimate the network configuration of urban drainage networks in Seoul, South Korea, with an increased number of flow directions from four (N, E, S, W) to eight (N, NE, E, SE, S, SW, W, NW), which enables improved accuracy. Based on that, the network configuration affects the hydrological response directly, the results of this study imply a new design criterion which concerns the connection of upstream and downstream subcatchments with different network configurations to mitigate downstream flooding. Additionally, in order to evaluate the model’s usefulness to be employed to estimate the hydrologic responses of actual drainage networks, the width function-based IUH (WFIUH) was applied to a highly urbanized and pipe-networked catchment of the Shinweol Watershed in Seoul, South Korea.