County approves updated emergency plan for Marendall Dam after engineering briefing

The Yankton County Commission on April 21 approved an updated emergency preparedness plan for Marendall Dam, a state-owned high-hazard dam, following a presentation by Ryan Tobin, engineering supervisor with the Department of Game, Fish and Parks.

Tobin told the commission the plan mainly updates contact lists, calling trees and inundation mapping required by a state repair order after prior flooding and stilling-basin damage. "Marendall Dam has had that plan in place here, but it's been a while since we've updated it," he said, adding the Department of Natural Resources issued a repair order and the plan is a first step toward longer-term repairs.

Commissioners pressed Tobin about historic seepage at the dam toe and how it is monitored. Tobin said seepage is common in earth embankment dams and crews monitor both quantity and clarity: if the discharge becomes cloudy and carries material, that triggers further action. "We monitor that to make sure that the water that is coming out is clear," he said, and added field inspections and annual checks by Game, Fish and Parks staff are standard.

Resident Diane Hobden, listed on the county's population-at-risk listing, asked how residents on the inundation maps would be notified. Tobin and county staff said the maps used for the plan come from a US Army Corps of Engineers Silver Jackets project and FEMA's DSS modeling; the maps flag census-identified residences that would be inundated under modeled failure scenarios. Tobin acknowledged that notification to those residents would be appropriate as copies are finalized: "We can reach out to DNR and see what the appropriate vehicle is and do we reach out to each one of those as this is approved," he said.

Commissioners also confirmed the county has no direct financial liability for repairs; Game, Fish and Parks staff indicated state and DNR processes would lead on funding and repair implementation. The plan requires co-signatures from DNR and the Department of Game, Fish and Parks before distribution.

After discussion, Commissioner Ryan moved and Wanda seconded approval of the plan as presented; the commission voted to approve it. County staff said they would distribute signed copies and keep the public informed of planned repairs and funding needs.

The commission noted follow-up steps will include continued monitoring of seepage, coordination with DNR on required fixes, and outreach to residents identified in the inundation mapping.