Evaluating the Effects of Spatial Resolution on 2D Pluvial Flood Modeling in Urban Built Environments

This study examines the impact of spatial resolution on urban pluvial flood modeling, emphasizing the role of high-resolution topographic data in flood inundation mapping. Using the physically-based H12 urban flood model, we simulated the December 6–8, 2015, pluvial flood event in a sub-watershed in Portland, Oregon. We compared the temporal evolution of inundation maps from a benchmark 1 m resolution model with coarser resolutions (2–50 m) and assessed accuracy using water depth error measures and grid-based inundation extent metrics. Our results indicate that accumulated inundated water volume increases significantly with coarser grid resolutions, leading to larger discrepancies in flood extent. Coarser grids generally overpredict inundation extents, except in the 2 m model. Accuracy metrics decline with resolution coarsening, with the hit rate (H) dropping below 0.7 and the critical success index (C) falling to 0.5 or lower beyond 7 m resolution. Multi-directional flow path analyses reveal that inundation extent expands with coarser resolutions, while computational efficiency improves. The primary source of accuracy degradation is the inability of coarse grids to capture key urban topographical details, such as road networks, which influence floodwater movement. While no single optimal resolution applies universally, grid resolutions must be fine enough to accurately represent major urban features critical to flood dynamics.