Assessing the Impact of Building Indoor-Inundation on Flood Depth and Extent at City Scale: A Novel 2-D Coupled Hydrodynamic and Building Inundation Model for Urban Flood Modeling

Hydrodynamic models solving various forms of the shallow water equations are commonly used to assess flow depths and velocities during floods. Recent advances have extended these models to incorporate elements of the catchment such as sewer systems, culverts, bridges, and weirs. However, many flood models still represent buildings as raised elevations or with increased roughness coefficients, blocking water from entering building areas. This can lead to an overestimation of flood depths and extents, as these storage areas are not considered. Until now, the cumulative effects of flow into buildings on city-scale inundation have not been simulated using physically based models. This study develops and implements a building indoor-inundation model, comparing it with simulations that neglect this effect (storage and retention inside buildings) to quantify the differences in flood depth and extent.

A hydrodynamic model was coupled with the indoor-inundation model to estimate flow into and out of buildings (through windows, doors, etc.). This model incorporates building geometry, including walls, doors, and stairwells, to determine flow throughout the building. It also accounts for the transition between pressurized and non-pressurized flow, allowing water to move from lower to upper floors. The building indoor-inundation model was coupled with the 2-D diffusive wave model P-DWave, which simulates surface flood inundation outside the buildings. Water is exchanged between the two models at run-time in a bi-directional manner, with water flowing both into and out of the buildings.

Flood simulations were conducted for the city of Baiersdorf in northern Bavaria, which experienced flash floods in 2007 caused by heavy rainfall, resulting in over 86 million euros in damages. The event was recreated using the dual-drainage model PD-Wave/SWMM to simulate interactions between overland flow and the sewer system. Three test cases were modeled: buildings represented with increased roughness coefficients, buildings raised above the floodplain, and buildings modeled using the presented hydrodynamic approach. The results show that modeling the flow into and out of buildings has a moderate to significant impact on both flood depths and extents, highlighting the importance of including building inundation in urban flood modeling.