City reviews Northwest Drainage Study and authorizes SRF loan application for Lake Mitchell work

Mitchell — City staff and McLeod Engineering on Monday reviewed a Northwest Drainage Study that modeled stormwater flows across three northwest study areas and recommended options to reduce flooding in the Sanborn Boulevard basin and improve detention capacity feeding toward Lake Mitchell. The council subsequently approved a resolution authorizing application for state revolving fund (SRF) support for Lake Mitchell.

Ryan Murtha, senior project manager at McLeod Engineering, told the council the study split the area into three study zones and built a stormwater model of the entire drainage network. He said Study Area 1 (about 207 acres in Westwood Addition) was modeled in a fully developed condition and that “the detention ponds were capable of handling both of those design storm events” — a reference to 5-year and 100-year storms — though he cautioned that some pipes, inlets and junction boxes were modeled as over capacity and could cause localized flooding.

The study’s second area focused on the neighborhood east of Highway 37 Bypass, where Murtha presented two conceptual approaches to pull stormwater away from the Sanborn trunk system and into western storage. For that area he outlined four lateral drainage groupings (roughly Ninth–Eleventh, Thirteenth–Fourteenth, Fifteenth, and Sixteenth–Nineteenth) and described tradeoffs including cascading detention ponds adjacent to athletic fields and proximity to a middle-school activity center.

Murtha described Study Area 3’s flat topography and a modeled option to pipe water under Olman Street into the larger detention pond in Area 1. He said that, under the assumptions used (including limiting post-development discharge to pre-development rates per the city’s ordinance), Area 1’s pond could accept the additional routed flows for both the 5- and 100-year events.

Several residents and landowners urged more conservative assumptions. One resident said of the June floods, “the water, it was so close to going over 20 Third and spilling over into our neighborhood,” and warned basements in nearby neighborhoods had flooded. Longtime landowner Chuck Meschewski said the pond had exceeded the model’s red-line marker during past events and recommended adding more retention rather than only routing more water to an already-stressed pond.

Murtha acknowledged the tension between modeled design events and recent extreme storms, saying the study applied standard practice and the city’s 100-year design basis but that observed floods in recent storms had at times exceeded those events.

Following the presentation and public comments, council opened a public hearing on the Lake Mitchell SRF loan application and approved Resolution R2024-57 authorizing application for up to $16,815,900 and directing the mayor to sign. City staff said SRF loan terms were expected to be about 3.75% for 30 years and that the Board of Water and Natural Resources could forgive some or all principal depending on the final application and board action.

The council’s next steps, as discussed, include reviewing preliminary design or staging options, determining whether pond modifications (expanded surface area or additional ponds) are feasible, and budgeting future studies for areas not yet analyzed, including the cemetery/Augusta/Kemper area. The study also flags several local infrastructure elements — pipes, inlets and junctions — that the city might prioritize for repairs or targeted upsizing.