Urban drainage networks have been developed to increase the efficiency of a system in terms of drainage time so far.Typically, a drainage system is designed to drain water from developed areas promptly as much as possible during floods.In this regard, an artificial drainage system have been considered to be more efficient compared to river networks in nature.This study examined artificial drainage networks and the results showed urban drainage networks can be less efficient in terms of network configuration compared with river networks, which is counter-intuitive.The case study of 31 catchments in Seoul, South Korea shows that they have wide range of efficiency in terms of network configuration and consequently, drainage time.On the other hand, the analysis showed that efficient networks have risks of the 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 study also demonstrates that efficient drainage networks are more sensitive to spatial and temporal rainfall variation such as rainstorm movement.Peak flows increase more than two times greater in effective drainage networks compared with inefficient and highly sinuous drainage networks.Combining these results, this study shows that the layout of a drainage network is an important factor in terms of efficient drainage and also safety in urban catchments.The layout of the drainage network can be an alternative non-structural measures that mitigate potential flood risks and it is crucial for the sustainability of urban environments.