Engineering review finds Shawnee Twin Lake dam in fair-to-satisfactory condition; staff debate costs to make kayak launch ADA-compliant

Freese & Nichols engineers summarized a final report on the Shawnee Twin Lake dam and recommended a limited set of repairs and additional studies while saying they found no immediate dam-safety issues.

Colin Young, civil engineer with Freese & Nichols, told the City Commission that the firm conducted independent site inspections, topographic and geotechnical reviews and a spillway-capacity analysis; the report accounted for prior consultant work and peer review by the U.S. Army Corps of Engineers Tulsa District and the Oklahoma Water Resources Board (OWRB). "We didn't see any immediate dam safety concerns," Young said, adding that the team recommended targeted repairs and future studies rather than large-scale reconstruction.

Why it matters: Twin Lake is a municipal water-resource asset and the lake's dam is categorized under state dam safety rules. The report's recommendations affect both public safety planning (update of breach inundation mapping and the city's emergency action plan) and near-term maintenance budgets.

Key findings and recommendations:

- Condition ratings: The report used FEMA-adopted descriptors (satisfactory, fair, poor, unsatisfactory). The dam's crest and core components were generally rated satisfactory, while the upstream and downstream slopes received fair ratings because of localized sloughing, mismatched riprap and tree/vegetation encroachment on the downstream slope.

- Riprap and slope work: The upstream riprap is mismatched from prior spot repairs; the report recommends replacing or repairing riprap to protect the embankment from wave action. The downstream slope contained trees and heavy vegetation; the OWRB prohibits trees on dam embankments and within 30 feet of edges because uprooted root balls can compromise slope stability.

- Seepage and stability: Small amounts of seepage were noted at the lateral groins (normal for many earth embankments). Independent stability analyses using the available geotechnical data met OWRB-required factors of safety; earlier reports that suggested factors of safety below acceptable values were flagged as likely resulting from inconsistent drilling methods or overly conservative soil-strength inputs.

- Spillway capacity: As a grandfathered dam (constructed before 1973), Twin Lake is required to pass 50% of the probable maximum flood (PMF). The report found the existing spillway passes roughly 65% of the PMF, above the grandfathered requirement. By contrast, a new-build spillway would have to meet modern criteria (roughly 75% of PMF or more depending on classification).

- Next steps: The engineers recommended updating the dam-breach model using contemporary two-dimensional modeling so emergency action plans can show likely flood depths and velocities more accurately; they also recommended targeted repairs to upstream riprap, limited scour protection at the spillway and clearing vegetation on the downstream slope.

Cost and context: The team presented conceptual cost ranges for the recommended repairs and for optional, more extensive upgrades; staff did not present a definitive construction contract on the same agenda. Commissioners asked for clarification about differences between earlier geotechnical reports and the present peer review; the engineers explained that earlier studies used drilling and modeling approaches that produced unusually low strength parameters.

Quotes and concerns: "If that really were a slope failure, that would be alarming," the engineer said in response to a commissioner question about prior reports that implied very low factors of safety. Commission discussion also touched on whether prior pavement distress had been misclassified as slope failure and whether rehabilitation could proceed with focused repairs rather than full-scale reconstruction.

Kayak launch discussion: The meeting also revisited an adaptive kayak launch donated to the city by community donors. City staff reviewed the history: the kayak launch was donated and accepted, a 2021 Lake Advisory Committee recommended removing and storing the launch pending a master plan, and a 2024 master plan provided updated recommendations about location and access. Staff said making the launch fully ADA-compliant (parking, accessible walkways and a turn-around/unload area) in the current configuration could cost in the neighborhood of tens of thousands of dollars — a staff estimate given in meeting discussion was roughly $50,000–$60,000 for minimal ADA access from existing parking, while one previously proposed CIP design that included broader site improvements had been estimated materially higher (a figure discussed in prior budget deliberations was larger and prompted debate).

Commissioners asked whether the $50,000 launch gift had already been accepted (staff confirmed it had been) and whether the city would commit larger CIP dollars to complete the surrounding ADA amenities. Staff noted prior commission direction to budget for ADA-compliance work but said the current commission had not yet agreed on an exact scope or funding level. Several commissioners framed the choice as a budgeting priority question: complete minimal accessibility work now, or move ahead with a larger park-style investment that carries a higher price tag.

The engineering report and the kayak access options are expected to return to the commission with cost estimates and design options for a formal decision.