Consultants recommend targeted BMPs and a stormwater utility after Cottage Grove stormwater study

Strand Associates project manager Dan Johnstone summarized a stormwater quality management plan update and the accompanying stormwater utility feasibility work for a joint meeting of the Village of Cottage Grove Utility Commission and the Public Works, Properties & Sustainability Committee on June 10, 2026. The plan assesses compliance with state and watershed requirements and outlines alternatives to reach required pollutant reductions.

The consultants said the village’s municipal separate storm sewer (MS4) permit requires a baseline 20% total suspended solids (TSS) reduction, while the Rock River Basin TMDL imposes more stringent targets (about 62.2% TSS and 54% total phosphorus). Johnstone said modeled conditions show Reach 83 is in compliance and Reach 66 has improved but remains short of full compliance. The 400‑page study was funded with a WDNR grant and had a project cost of about $165,000, split roughly 50/50 between state and local shares.

To close the gap, Strand analyzed three compliance approaches: constructing and modifying best management practices (BMPs) within the village, participating in watershed adaptive management through the Yahara WINs program, and water‑quality trading. Johnstone said the recommended path is a hybrid — a smaller set of cost‑effective BMP investments plus ongoing Yahara WINs participation — described in the study as Alternative 2.

Johnstone described common BMP work such as converting existing dry detention basins to wet detention basins and routine pond maintenance. He said five of the 10 reviewed wet detention basins need dredging under DNR guidance and that maintenance work across basins is substantial. On the feasibility of a fall leaf‑collection credit, Johnstone said the village could gain a projected 41.5 pounds per year of TP reduction but that implementing the required ordinance, street‑sweeping frequency increases and two vacuum trucks would cost an estimated $1 million up front and about $85,000 per year to operate. “That would yield a TP credit, but it was not included in our recommended alternative,” he said.

The presentation also summarized the Yahara WINs watershed adaptive‑management buy‑in administered through MMSD and Dane County. Johnstone said the original buy‑in had been roughly $12,000 per year, rose slightly in 2019, and now the updated modeling reduces the estimated buy‑in for Cottage Grove to about $9,000 per year.

Strand provided a long‑range implementation plan spanning 2026 to 2055 with schedules for BMP design, property acquisition needs, estimated costs for three alternatives and a grant‑seeking strategy. Their recommendation to the village was to proceed with Alternative 2, update stormwater ordinances to align with county standards where appropriate, budget for design and construction, and establish a stormwater utility to cover capital and maintenance costs.

During questions, Johnstone and staff clarified that the village remains bound to permit requirements even though the Department of Natural Resources (DNR) has been slow to reissue some permits; they confirmed Yahara WINs credits apply village‑wide. Staff said the feasibility study will next quantify likely per‑parcel utility rates, credit policies for on‑site BMPs, and detailed cost estimates. The feasibility report is expected within the next month or two, and staff said a follow‑up joint meeting will present those results and include public outreach.

Next steps: staff and Strand will finalize the feasibility report, present detailed rate modeling and credit policy options to the committee and the public, and return with an implementation schedule and funding recommendations.