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Marion officials, consultants present stormwater master plan findings; 73 resident comments helped prioritize work
Summary
City staff and HDR consultants reviewed condition assessments, hydrologic modeling and a prioritization approach for Marion’s stormwater system, identifying 100 problem areas and 30 priority projects; the plan will focus field inspections on the top 10 detention basins and ask council to rank prioritization criteria.
Marion City stormwater coordinator Steve Cooper and HDR consultant Steph Dane Hoge gave council a progress update on the city’s stormwater master plan on July 15, outlining public input, condition-assessment steps, hydrologic and hydraulic (H&H) modeling results and the approach staff will use to prioritize improvements.
The presentation said staff and HDR received 73 distinct public comments so far, spread throughout the city. The consultants used the comment map, city staff input and model results to evaluate about 65 city‑owned detention basins and to prioritize the top 10 for field inspections, with the next 10 noted as secondary priorities.
The consultants described the modeling approach, built in InfoWorks ICM, that combines primary infrastructure (inlets, manholes, pipes, outfalls and basins), road and terrain surface flow, and more than 1,000 delineated subcatchments. Modeling used standard industry ‘‘design storms’’ — a 10‑year, 24‑hour event (about 4.5 inches) and a 100‑year, 24‑hour event (about 7.5 inches) — and results were compared to SUDAS (Iowa Statewide Urban Design and Specifications) criteria the city has adopted.
Hoge said the model flags pipe surcharge (pipes running full) and surface ponding in one‑foot depth increments. For the 10‑year event, ponding on streets or private property is a deficiency; for the 100‑year event, any ponding that reaches structures or private property is a deficiency. The model results split Marion into three major watersheds — West Indian Creek, East Indian Creek and Waanatee Creek — and identified most of the higher‑priority problems in central and south East Indian Creek, where older infrastructure predates current design standards.
HDR proposed that the master plan develop projects and cost estimates for the top 30 problem areas (4 projects in West Indian Creek, 19 in East Indian Creek and 7 in Waanatee Creek). The presentation noted a range of potential solutions: localized upsizing of storm sewer in small areas; system‑wide responses in older neighborhoods, including combinations of detention, upsize, or conversion to open channels; and how choices would vary by location constraints and constructability.
To guide how staff selects which solutions to pursue first, HDR proposed a multi‑criteria decision analysis and asked council to rank eight prioritization criteria: amenity‑rich public space, ecological enhancement, existing infrastructure risk (reducing structures impacted), water quality, capital cost, ease of constructability, annual operations and maintenance cost, and social/economic impacts. Consultants said the ranking will be combined across councilmembers and used to screen alternatives and identify project types to study in more detail.
Councilmembers asked about the structure of the ranking (one councilmember suggested scoring attributes per project rather than an ordered priority), validation of the model using the comment map, and how staff will report back to commenters. HDR said comments were used to validate model results and to refine prioritization; the comment map will remain open through the end of the year and the final plan will document comments and map the projects assigned to those areas.
Staff said field inspections will focus on the top 10 basins to document scour, sedimentation, vegetation and outlet conditions and that additional basins may be inspected later depending on findings.
