Johnson County moves to designate HO Canary watershed an impact drainage area, lowers future release-rate standard

Johnson County commissioners voted Nov. 3 to proceed with study and designation of the HO Canary Watershed as an impact drainage area and to apply a more restrictive post‑development release rate for future development within the mapped area.

Greg Campbell, Johnson County surveyor, told the board the county completed reassessments in 2024 and has been dividing regulated drains into prioritized segments for reconstruction; staff and consultants recommended using designation under the county’s comprehensive stormwater management ordinance to require more stringent controls on future development while long‑term reconstruction is planned. "We were successful in those reassessment hearings. We're now ready to move on to the actual study of the drains and to bring those drains to physical update," Campbell said.

Dave Eichenberger of Burke Engineering presented technical findings and a recommended change to unit release rates. He described the HO Canary regulated drain as an open ditch beginning south of Whiteland Road west of Interstate 65, flowing south through Franklin into Youngs Creek. Eichenberger said the watershed shifted from primarily agricultural land to intense development beginning about 2018, reducing the effective capacity of the outlet. He said the engineering study completed in 2024–early 2025 recommends that future developments in the upper portion of the watershed limit post‑development release for the 100‑year storm to a maximum of 0.2 CFS per acre (the county’s current unit release rate for the 100‑year event is 0.3 CFS per acre; the 10‑year unit rate would remain at 0.1 CFS per acre). The recommendation is not retroactive to existing development.

Commissioners asked how the lower release rate would be achieved for new projects. Eichenberger said meeting a 0.2 CFS per acre limit could require larger detention volumes, more restrictive outlet structures (for example, smaller outlet pipes), or deeper storage; he noted that detention ponds generally reduce peak discharge even though the total stormwater volume entering the system increases with development. A commissioner raised concerns that detention ponds can be at or near normal pool much of the time and questioned public‑safety risks; staff explained that standard calculations count only storage above the normal pool and that the purpose is to reduce peak discharge to levels downstream channels can handle.

Campbell said the county will meet individually with commissioners in coming months to explain mapping and next steps and that Youngs Creek — currently not a regulated drain but subject to increased development runoff — will be brought forward for consideration as a regulated drain. He estimated full reconstruction of all regulated watersheds would cost roughly $500 million to $800 million and framed the restrictive release rate as a bridging tool until capital work can proceed.

After discussion, a commissioner moved to proceed with the study/designation; the motion was seconded and carried on a recorded "aye" vote.

Ending: Staff will return with follow‑up briefings and a map of the exact impact drainage area boundaries; the board directed staff to coordinate with municipalities and developers on implementation details and to bring Youngs Creek forward for future action.