New subdivisions built post 1980's, have separated sanitary sewer systems with lower exposure to inflows and infiltration that surcharge older systems, major/minor drainage system design to keep overland flow on right of ways, away from buildings, or safely conveyed to valleys along easements.
Flood risks exist across a range of spatial scales from large river flood plain to local infrastructure floor drain. Spatial analysis of overland flow paths has correlated the risk of surface flooding along flow paths and their spread with observed basement back-up flooding. The map below illustrates how overland flow paths extend beyond regulated valley limits onto development areas, and shows how clusters of sanitary sewer back-up often lie within the overland flow spread.
