Technical Note: Influence of building representation in flood hydrodynamic modelling: the case of the 2021 Ahr valley flood

Abstract. The increasing flood risk in urban areas, driven by rising urbanization and climate change, underscores the need for accurate representation of buildings and urban features in flood hydrodynamic models. This study investigates the impact of different building representation techniques on flood hydrodynamic and impact modeling, using the 2021 flood event in the Ahr Valley, Germany. Three methods—Building Block (BB), Building Hole (BH), and Building Resistance (BR)—are applied across varying model resolutions to assess their influence on flood extent, water depths, and flow velocities.

Our findings reveal that building representation can affect both flood extent and flow dynamics. The Building Block and Building Hole approaches generally lead to larger flooded areas with deeper water and higher velocities, while increased resistance or omitting buildings results in smaller flood extents, shallower water, and slower flow. Additionally, we show a strong link between building representation and model resolution. At coarser resolutions, the Building Hole method produces the most accurate flood extents, while the increased resistance method performs better at finer scales. Our findings show that at coarser resolutions, the choice of building representation is more critical, with larger differences in flood extent across setups. We show that while all methods produce acceptable flood extents, variations in water depths and velocities highlight the importance of choosing the right building representation for accurate flood impact calculations. These variations are vital for flood impact assessments, especially in dense urban areas. Our results emphasize the importance of selecting appropriate building representation methods based on model resolution to enhance urban flood modeling and impact assessment accuracy.