As part of an on-call contract with the Virginia Department of Conservation and Recreation (DCR), performed dam break flood inundation mapping for Lower North Dam #82 (Inventory No. 16507) located in Rockingham County, Virginia. Lower North Dam #82 is 110 feet tall and has a drainage area at 18.7 square miles. It impounds an approximate 21.3 acre reservoir at normal pool and was verified to be a High Hazard structure.
The project study area extended to a termination point approximately 38.7 river miles downstream of Lower North Dam #82. The resulting inundation zone impacted eleven bridges, nearly 2,260 occupied structures, and multiple roadways. In the middle of the inundation area the terrain created a split in the outflow when the water surface became higher than the southern ridge of the stream channel. The two flows then recombined several miles downstream. This was a unique challenge for F&R since the HEC-RAS model cannot process the second flow path. F&R solved this using a steady model for the region of the split flow.
F&R then conducted an Incremental Damage Assessment and determined the dam was not eligible for a reduction of the spillway capacity due to impacts of numerous downstream structures.