Emergency Response at a High Hazard Dam Lasting Weeks

ABSTRACT ONLY - Virginia Department of Conservation and Recreation's Division of Dam Safety and Floodplain Management (DDSFM) is the lead entity responsible to provide for proper and safe design, construction, operation and maintenance of dams to protect public safety in the Commonwealth. DDSFM partners with the VDEM and local county emergency managers during a dam incident. DDSFM also is responsible for issuing O&M Certificates and Alteration Permits.
On January 5, 2022 DDSFM received a call from the owner's engineer (GKY and Associates (GKY)) notifying us of a Stage III Emergency (potential dam failure via piping) and a virtual meeting with all concerned agencies was immediately convened. In addition to 5 residences and another dam, a state highway was located downstream of Lake Monroe Dam in King George County. Given that a whirlpool had been detected near the riser structure, the working hypothesis was that the outfall pipe had ruptured near the riser. The riser structure and outfall were made of corrugated metal pipe with a 60'• riser transitioning to a 48'• outfall pipe about 160 feet long. The pool was dropping and the outfall pipe was flowing about 2/3rds full. The dam was under a Conditional Certificate to install a low-level drawdown device, but work had yet to commence on that project. As the event continued, the flow eventually filled the outfall pipe.
GKY had done the dam break inundation zone modeling for the projects and undertook new inundation studies for several lower than normal pool elevations and determined that, although the water level was below normal pool, the five residences remained at risk. After a follow-up discussion with law enforcement, emergency management, owner, owner's engineer and DCR it was decided to evacuate the residences below the dam and put the sheriff in an emergency response mode to close the highway should a dam break occur.
GKY mobilized engineers to inspect the dam including a geotechnical engineer. Meanwhile the owner contracted to rent two pumps and a siphon to drawdown the lake as low as possible. Fortunately a clay lip near the riser held and once the water level fell below that level no more significant water entered the outfall. Eventually the water level was lowered more than 23 feet and a temporary emergency action plan for the distressed condition was developed enabling authorities to cancel the evacuation order. Meanwhile GKY is proceeding with remediation and plans and specifications are expected in February.