Effect of Building Representation on Pluvial Flood Risk in Urban Areas: The Podoniftis Basin, Greece

Marcos Julien Alexopoulos^1*Panayiotis Dimitriadis^1*Emela Mitsi¹Demetris Koutsoyiannis¹Denis Istrati^1,2Theano Iliopoulou¹

• ¹National Technical University of Athens, Athens, Greece
• ²Technische Universitat Braunschweig, Brunswick, Germany

Urban flood modeling requires accurately representing building structures, as they influence flow paths, flood depths, and hydraulic dynamics in urban basins. This study assesses the effectiveness of three commonly used building representation techniques—Stubby Building (SB), Building Block (BB), and Building Resistance (BR)—within a rain-on-grid framework in the urbanized Podoniftis basin, Attica, Greece. Using a 2 m resolution digital elevation model, buildings are represented with BB using 5 m and 12 m building footprint elevations, SB using a 0.5 m elevation, and BR using increased Manning's roughness coefficients of 1 and 10 over building footprints. The different methods substantially influence simulated flood extent, water depth, and velocity fields. The BB method yields the highest mean depths (0.62 m) but tends to concentrate high-water depths adjacent to buildings in densely built-up areas. The BR method produces a larger flooded area with the lowest mean velocity (0.27 m/s), reflecting higher hydraulic resistance and a more distributed water storage. The SB method provides an intermediate behaviour between the three approaches, while not explicitly resolving runoff generation and redistribution associated with building ingress and drainage systems. A Flood Hazard Rating assessment further shows that building representation affects the spatial pattern of hazard classes. Overall, the findings highlight that building representation is a key modelling choice in rain-on-grid applications and underline the need for more refined techniques that explicitly incorporate drainage networks and building ingress processes for urban flood hazard assessment.