Wiley|Wilson prepared dam break inundation zone maps required by the Virginia Dam Safety Regulations for four dams owned by the City of Portsmouth. Dam break inundation zone mapping is prepared for four flood event conditions: the sunny day dam break, the probable maximum flood (PMF) with dam break, and the spillway design flood (SDF) with and without dam break. Flood modeling computer programs and GIS were utilized to calculate water surface elevations in the river section downstream of the dam in order to determine impacts to roads, bridges and structures resulting from the four flood event conditions.

The dam break inundation zone study identified an insufficient spillway capacity for three out of four dams, creating overtopping flows for the SDF storm event. Overtopping flows will cause an erosion failure of an earthen dam. A preliminary engineering report was prepared to study modification alternatives to determine feasible options for safely passing the SDF. Typical modification alternatives included increasing spillway capacity, raising the embankment crest, and overtopping protection to prevent erosion.