Floodwaters Affecting Flow Through Rapidan Dam
The recent rainfall has Rapidan homeowners wondering when floodwaters will recede and hopeful that the dam could help in the process but according to county engineers it will take some time.
Currently the Blue Earth River, which runs through the Rapidan Dam is experiencing very high flow rates, about 21,300 cubic feet per second, fairly close to the flood event of 2010 when 27,000 hammered through the hydroelectric facility.
"We're in the similar neighborhood albeit, not quite there yet. Following the 2010 flight events as part of the mitigation and reporting for the dam to ensure the safety of its operation the county was required to identify the maximum permissible flood event for Rapidan Dam and we identified that the spillway capacity was about 42,000 cubic feet per second. So as we identified that maximum capacity of the existing spillway we were required to meet it while going into a potential failure mode analysis study," Blue Earth County Engineer Ryan Thilges said.
That study identified concerns for the dam, significantly if the timber needle gates were ever pulled it would create a major concern for the highly erodible sandstone bedrock which would compromise the integrity of the dam and could result in collapse.
"Those timber needle gates were required by the Federal Regulatory Committee per this study to be removed and replaced with reinforced concrete. In order to pass the maximum flood event through the five remaining steel tainter gates. We came up with a solution where we were able to reconfigure the pulley location to raise the gates to a higher elevation, still being able to pass that approximate 42,000 ft.³ per second flow," Thilges added.
Unfortunately this isn't the best news for those who live upstream from the dam. One homeowner on the Blue Earth River has 25 acres underwater while another at the confluence of the Watonwan and Blue Earth Rivers has water creeping into his basement. The residents hoped the gates could be lowered to allow more water to flow through but that is not a possibility following last year's reinforcements.
"The county's hands are fairly tied again because the operation of the dam, the installation of the concrete walls in the former timber needle gates it wasn't optional, it was a requirement per the federal oversight," Thilges said.
Thilges welcomes anyone with concerns to continue the conversation while homeowners are expectant the floodwaters will recede in the next week hopefully with no further damage to their properties.
--KEYC News 12