Despite the challenges of social distancing due to COVID-19, dam safety remains a risk management practice for the U.S. Army Corps of Engineers-Omaha District. Recently a dam safety inspection team conducted its periodic inspection of the Salt Creek Dam Site 2, Olive Creek Dam south of Lincoln, Nebraska.
“It consists of daily and monthly inspections by project personnel with instrumentation data collection and interpretation,” explained Ross Cullin, dam safety program manager. “Annual inspections performed by project and District dam safety personnel. Periodic inspections performed every five years. Periodic inspections include a thorough review of all components of the project and are attended by project personnel, technical staff from various disciplines within the Omaha District, division technical experts, and government officials from outside USACE. Special Inspections performed during high water events, seismic events, and other unusual conditions. Site specific surveillance and emergency action planning for each dam within the District.”
This was Omaha District’s first project inspection since the pandemic began.
“Because of the size of this project, we have historically had an average of 14 professionals performing the scheduled inspections,” said Kristle P. Beaudet, dam safety engineer. “But to adhere to the health guidelines of COVID-19, we limited the inspection to a staff of ten.”
The District routinely provides periodic inspections of the project’s flood risk management, navigation, water supply, hydropower, environmental stewardship, fish and wildlife conservation and recreation. The inspections ensure that the structure safely passes the Probable Maximum Flood without overtopping the embankment, while following health guidelines.
“We drove separately instead of carpooling as we have in the past. During our onsite team huddle to discuss findings we wore masks, maintained physical distancing, and frequently used hand sanitizer,” Beaudet said “The inspection took 3.5 hours, instead of the planned 3.”
The PMF is estimated using probable maximum precipitation estimates developed by the National Weather Service. If a flood event happens, it could lead to a hazard to life conditions, and property damage. Inspections are carried out to determine if the structure can withhold the updated PMF without the embankment being overtopped.
In order to complete a successful inspection, the site safety team uses an annual inspection checklist that provides remarks and findings from the previous year’s inspections, and things to look out for. Some areas of deficiencies the team is looking for include erosion, sedimentation, or depressions in the crest, upstream slope, downstream slope, abutment contacts, and intake structure.
For example in 1975 the Corps recorded a visual depression on the upstream slope from due to an old channel that’s now a service road. As noted in the Salt Creek Site 2, Olive Creek Dam Periodic Inspection No. 11 Pre-Inspection Brochure; “All Salt Creek reservoirs were designed to contain a minimum routed flood equivalent to 5.0 inches runoff from the controlled drainage area, between the conservation pool level and below the spillway crest. Favorable topography for cut and fill balance permitted reservoir sizing to provide for 8.5 inches of runoff storage before spillway crest. The drainage area for Olive Creek Dam is approximately 7.8 square miles.”
According to the Tributary Reservoir Regulation Activities Annual report for 2018, the Omaha District had substantial flood damages prevented during the runoff season, which typically begins March 1 account for $23,215,500 in 2018, and since this project’s initial construction in 1964, the dams have prevented a total of $208,768,700 in total damage control.