Engineer: Lake Mitchell spillway falls short of state 50% PMF standard; variance and emergency‑planning steps proposed

An engineering update presented to the Mitchell City Council found Lake Mitchell’s spillway would not meet the state’s 50% probable maximum flood (PMF) pass criterion and instead reaches roughly 23% of PMF under the study’s conditions, a consultant said.

Connor Kelly, a water resources engineer with Houston Engineering, told the council the firm used updated soils, elevation data, 70 years of USGS gauge records on Fire Steel Creek, modern land‑cover inputs and a two‑dimensional rain‑on‑grid model to reassess inflow design flood conditions and spillway hydraulics. He said the spillway’s unusual horseshoe ogee chute and legacy design made it essential to use computational fluid dynamics and consequence mapping to predict performance under extreme loadings.

The study updated breach maps and evacuation modeling, showing that extreme inflows could force flows both downstream and more than 10 miles upstream on the James River in worst‑case scenarios. Using FEMA guidance (P‑94) for an incremental consequence analysis, the consultants concluded that although the structure does not meet the 50% PMF criterion, a risk‑based variance petition is defensible because structural upgrades needed to meet the criterion would not meaningfully reduce downstream life‑safety consequences, given the low probability of the event.

City staff said the state’s chief engineer has reviewed the study and recommended approval; a formal hearing before the Water and Natural Resources Board is scheduled in early May. If the board grants a variance, the city still must maintain an updated emergency preparedness plan and meet any qualifying conditions tied to the variance.

Council members asked about the study’s assumptions, the rainfall intensities used, and the timeline for permitting and any lake drawdown. Kelly said the probable maximum precipitation inputs came from a regional study extending from Nebraska into the Mitchell area and include extreme intensities (for planning purposes, very large but physically plausible events). He said the team expects a public comment period and a board hearing in early May and that permitting and Army Corps approvals remain steps before bidding seasonal drawdown and dredging work; staff estimated bidding would occur after the lake season with a best‑case construction bid window in September–October.

The council did not vote on policy changes during the update; the presentation was recorded as informational and the next procedural step is the state variance hearing and any conditions that follow.