INVESTIGATING THE COUPLING BETWEEN STORMWATER AND FLOOD ROUTING MODELS Quantifying the Role of Building Roofs in Urban Flood Dynamics

Pluvial floods occur in small urban catchments when intense rainfall exceeds the stormwater drainage capacity. These events have been happening more frequently and with increasing impacts due to ongoing urbanization and the intensification of sub-daily precipitation extremes. Modeling these fast-evolving phenomena at very high spatial resolutions (1-5 m) is crucial to understand their dynamics and mitigate the impacts. In this work, we focus on better quantifying the role of building rooftops in the rainfall-runoff transformation involved in pluvial flooding. We adopt a modeling framework based on the LISFLOOD-FP rain-on-grid hydrodynamic model forced with gridded inputs of net precipitation, which accounts for the roof effects. A significant challenge in incorporating roofs lies in accurately representing how the distinct effects of construction type, slope, surface material, and drainage systems affect rainfall losses, runoff delays, and, ultimately, water depths on the streets. We evaluate two methodologies inspired by recent research. The first involves categorizing buildings based on roof typology—such as flat or sloping roofs—and calculating the volume of water flowing from roofs to streets using variable runoff coefficients. In the second method, the distribution of downspouts is considered to account for the spatial variability and time delay in flood water discharged from the roofs. We test these methods in an urban catchment in New York City, where a wealth of high-resolution terrain, infrastructure, and building data is publicly available, along with distributed observations of street flooding. This work provides useful insights into the role of roof runoff in urban flooding, contributing to improved flood management strategies in highly urbanized areas like New York City facing similar challenges of increased pluvial flooding due to climate change.