Quantifying Flood Hazards in a Neighborhood Near You: Off-the-Grid Adventures in 2D Modeling

Protecting people and property from flood hazards is a primary concern for stormwater modelers. Flood hazard assessments are meant to characterize the likelihood and magnitude of a damaging flood event. Traditional assessments use one-dimensional hydraulic models, identifying flood severity at selected points within the 1D model domain. With two-dimensional hydraulic models, it is now possible to quantify flood hazards over selected areas within the 2D model domain, greatly enhancing the effectiveness of flood management, mapping, and insurance programs.

Compared to traditional 1D modeling, there are important considerations and decisions that must be made when 2D models are used to represent surface flooding in urban areas that feature storm sewers. First, the level of detail must be increased in the representation of the minor drainage system (i.e., by including the catchbasins/inlets along with their corresponding lead/lateral pipes that connect to the sewer trunk mains). Further, a key decision about the spatial level of detail in the major drainage system must be made (i.e., the appropriate resolution of the subcatchment size in the hydrologic model as well as the cell size of the surface mesh in the hydraulic model). The spatial resolution of the hydrologic model is often described as either lumped or distributed. Lumped hydrology is used in traditional 1D models where subcatchment sizes typically exceed 1 hectare and span multiple surface cover types. Distributed hydrology features much smaller subcatchment sizes that are more commensurate with the resolution of the surface mesh in the 2D model. The popular phrase “rain-on-grid” is a form of distributed hydrology modeling in which rainfall is applied directly onto the 2D surface mesh, generating runoff at the same spatial resolution of the 2D hydraulic model.

This presentation explores the various ways of representing surface flooding in an urban area at a neighborhood scale. It illustrates how spatial resolution, minor vs major system loading, and rain-on-grid assumptions can impact the results of flood hazard assessments.

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