Milwaukie reviews stormwater system plan update after modeling, data and permit changes

Milwaukie city staff and the project consultant told the City Council at an Aug. 19 work session that the stormwater system plan update is behind schedule after the team encountered missing and incompatible data while updating hydraulic models.

The plan update matters because it will guide capital projects, operations and permit compliance for drainage that discharges to Johnson Creek, Kellogg Creek and the Willamette River. Peter Passarelli, a city staff member, said the team has resumed modeling after data work and cautioned that additional surveying and data collection will be needed before the plan is finalized.

Passarelli said, “this plan, we're actually a little bit behind schedule, in terms of getting getting the work done.” He and consultant Scott Durham described two causes: newer model versions require more consistent elevation and structure data, and extending the model to smaller pipe diameters exposed gaps in the city GIS. Durham, vice president with Marsh Consulting, said the team initially tried to reuse older models and then rebuilt them when incompatibilities prevented reliable results.

The consultants said small elevation errors for inlets and manholes can magnify across a gravity storm system and noted that comprehensive surveying can be expensive; one consultant recounted a previous program in another jurisdiction that spent on the order of $1 million for field verification of sewer and storm infrastructure. The Milwaukie team said it expects to prioritize targeted field surveys rather than citywide resurveys, focusing on spots the model flags as vulnerable.

Regulatory drivers are also steering recommendations. Durham identified three permit sets the update will address: the MS4 and PS permit (including TMDL provisions), increased monitoring and temperature requirements, and the UIC (Underground Injection Control) rules that govern dry wells. The team told the council that the city submitted a UIC application in April 2024; preliminary review is pending and the assessment found no current compliance issues because groundwater depths are well below dry‑well bottoms in most locations.

Staff identified a handful of neighborhoods and facilities as likely candidates for capital work: a low area near the industrial park, a constrained reach by the Dave Green wetlands, the longstanding Apple–Plum–Hemlock flooding area, and a short section along the railroad at Highland Station. The consultants said some projects listed in the 2014 master plan were completed, others remain, and a few earlier projects may now be unnecessary after streetwork altered local drainage.

Climate projections also informed hydraulic runs. The team applied regional downscaled climate models to multi‑day design storms and reported that future storm intensity and back‑to‑back events could push system peaks to levels near historical 100‑year peaks. Staff said the modeling will not automatically recommend upsizing pipes for every location but will help prioritize where resiliency investments are most urgent.

On operations and maintenance, presenters said the city already captures much asset condition data in Cityworks but lacks consistent inventories for privately owned rain gardens and some green infrastructure. Staff are collecting those inventories now; the fall staff report will include recommendations on routine maintenance staffing and rehabilitation budgets. The presentation noted opportunities to pair rain‑garden maintenance with targeted tree plantings to capture both water quality and urban‑forest benefits.

Standards and permitting practice will be reviewed for administrative fit. The consultants recommended comparing the city’s requirements against manuals used by Portland and Clackamas County, and considering a locally tailored manual if other jurisdictions’ rules are administratively burdensome for Milwaukie. Staff flagged operations and maintenance documentation and ownership/maintenance agreements as areas that can create downstream workload if requirements are tightened without administrative capacity.

Timeline: staff said the model is running, climate‑vulnerability work will be wrapped up in the fall, and the draft plan and capital program recommendations are expected to be developed ahead of a final plan targeted for 2026.

Councilors asked clarifying questions during the discussion about dry‑well siting, the influence of Kellogg Dam removal on temperature goals, cost tradeoffs for surveying, and whether adoption of new standards would increase permit compliance workloads. No formal action was taken; staff said they will return with prioritized project recommendations and a maintenance‑cost estimate in the fall.