A geoprocessing routine was used during the development of a traditional SWMM model to aid in defining the magnitude of occasional basement backups and surface flooding which is often found in older urban areas served by combined sewer systems. Detailed stage-storage relationships (including both basement and surface flooding storage) were developed utilizing: standard GIS tools, a geospatial sewer network, building footprint boundaries, and a high-resolution digital elevation model. In addition to the accurate storage volumes now being included in SWMM, surface routing of flood water was also simulated. SWMM peak HGL results were then post-processed in GIS to determine the exact location within each SWMM shed where basements or surface locations experienced flooding. This approach, when compared to a more traditional SWMM routine, resulted in more accurate flood volume and depth calculations. Additionally, this routine produced a more accurate representation of flooded streets and buildings within the study area.