Oshkosh staff weigh tougher stormwater code to meet TMDL targets; council seeks cost and development impact data

Oshkosh City staff and a Brown and Caldwell consultant reviewed proposed updates to the city’s stormwater code in a workshop focused on how the municipal separate storm sewer system (MS4) permit and the Upper Fox‑Wolf watershed total maximum daily load (TMDL) affect local requirements.

Consultant Chuck Boehm, Brown and Caldwell’s client services director for Wisconsin, told the workshop the state DNR expects municipalities covered by MS4 permits to “show continued progress” toward TMDL reductions but that the current MS4 language does not set a single near‑term compliance deadline. “We’re not going to be required to meet those numbers tomorrow or 5 years or 10 years or maybe even 50 years,” Boehm said, describing how DNR has approached stormwater compliance differently than point‑source permits.

Staff said the city is considering several ordinance scenarios that would change the pollutant‑reduction standards developers must meet during site plan review. Under current Chapter 14 language, Oshkosh requires reductions of about 40% total suspended solids (TSS) for redevelopment and 80% for new development; the proposed scenarios add numeric phosphorus (total phosphorus, TP) targets and, in some cases, raise TSS targets in certain drainage basins. Two options the team said it favors are a basin‑specific approach (scenario 3) that asks for higher reductions in the Sawyer Creek reach and a citywide approach (scenario 5) that applies the same TSS/TP targets across Oshkosh.

Why it matters: the TMDL adopted by the Wisconsin DNR identifies large percentage reductions — in some reaches an 85.6% TP reduction — that are technically demanding and, staff said, can require practices and maintenance regimes more akin to wastewater treatment if applied broadly. Staff emphasized that model‑based planning (using tools such as WinSLAMM) and a mix of on‑site and regional practices would be the path to demonstrate compliance. Boehm and staff also noted the DNR’s Urban Nonpoint Source (planning) grant program helped fund the citywide analysis.

What the workshop examined: staff presented five policy scenarios and applied them to five representative sites (three redevelopment, two new development) to test whether common stormwater practices — wet detention ponds, sand filters, permeable pavement, biofilters and improved inlet controls — would meet the different targets. Results were mixed: some existing and proposed wet detention designs could meet higher TSS and TP levels, while other sites would require larger footprints, different practices or added maintenance.

Costs and maintenance: the team provided capital, routine maintenance and periodic (non‑routine) maintenance estimates for the evaluated practices and annualized those costs over a 50‑year life. Staff said capital cost differences between options exist but are often smaller than anticipated once ongoing maintenance and periodic renewal are included. They warned that certain practices (for example, permeable pavement) have recurring vacuum maintenance and that snow‑management and winter sanding practices can affect suitability and life‑cycle cost. Staff also noted that some municipal units (for example, large regional wet ponds) may be more cost‑effective at achieving watershed reductions than many small on‑site practices.

Policy tradeoffs and development impacts: several council members raised concerns about development competitiveness and the city’s stormwater fee level. Council members noted Oshkosh already charges higher stormwater fees than many peer communities and asked how new ordinance targets would affect projects now under review and future investment. Staff agreed to score large projects already in the pipeline against the recommended scenario so the council could see project‑level impacts before adopting code changes.

Other points raised: participants discussed coordination with Winnebago County conservation programs and other watershed partners (including the university and neighboring towns), the role of pollutant trading and multi‑discharger variance funding in meeting watershed goals, and scientific uncertainty about credits for some practices (staff noted biofilters currently receive no TP credit in state guidance in some circumstances because monitoring has shown variable phosphorus performance).

What was decided and next steps: no formal ordinance changes or votes took place. Staff requested direction from the full council and said their current leaning is toward the middle path (scenario 3 or scenario 5) but emphasized further analysis is needed. Staff will (a) model several large developments already under review against the proposed standards, (b) refine cost and maintenance estimates, and (c) bring draft ordinance language and implementation details back to the council and stakeholders for review and public outreach. The workshop closed with staff and the consultant asking council members for guidance on which scenario to take to subsequent stakeholder meetings.

Ending: The workshop was informational rather than final. Staff said they will return with project‑level modeling and clearer ordinance language — including how administrative exceptions and appeals would be handled — before asking the council to adopt any Chapter 14 changes.