City staff and consultants presented proposed revisions to Moline’s stormwater ordinance aimed at updating design standards and adding best management practices to better address changing rainfall patterns and protect the river.

The changes being recommended would adopt the Illinois State Water Survey’s Bulletin 75 rainfall data, lower the additional impervious-area threshold for Class 2 drainage permits from 1 acre to one-third acre, require design using a critical storm duration rather than a fixed 24-hour event, add a "first-inch" volume-control requirement for impervious surfaces, and require velocity-control measures so post-development runoff does not exceed pre-development velocities.

"This updated rainfall data will allow the city to better plan for the current runoff," said Scott Desplinter, a consultant with CMT, referring to Bulletin 75, published in 2020 by the Illinois State Water Survey. Desplinter explained the bulletin brings rainfall monitoring forward to 2017 and reflects more frequent and intense storms.

Desplinter said the one-third-acre threshold would bring smaller but still significant expansions — such as parking-lot additions — under engineering review. "A class 2 drainage permit requires an engineering analysis, stormwater detention, and other best management practices," he said, adding that recent redevelopment patterns across Illinois have prompted other communities to lower their thresholds.

City staff and CMT also recommended using a site-specific critical storm duration to size detention and outlet structures rather than relying solely on a default 24-hour duration. "It's important that we understand which duration controls or creates the largest runoff so that we can design our detention ponds and our outlet structures correctly," Desplinter said.

On treatment and source-control measures, Desplinter recommended nonstructural practices such as rain gardens, infiltration trenches and filter strips and said the ordinance should include volume-control systems designed to capture the first inch of runoff from impervious surfaces because that initial runoff typically carries the highest pollutant load. Examples discussed included porous pavement, bioretention and dry wells.

City staff noted two ongoing incentives and assistance programs that developers and property owners can use: stormwater utility credits (a credit manual is part of the updated ordinance) and the Moline Stormwater Assistance Program, which the staff said has been in place about a year to help individual residents with drainage improvements.

Council members asked about cost and constructability. "There are lots of different ways. Maybe you take a piece here, a piece here," said Andy (staff), describing options such as underground storage, bioswales or upsized pipes. Charlie (Public Works staff) added that the goal is to give developers options rather than a single prescriptive solution.

No ordinance vote occurred at the meeting; staff said the revised ordinance language will be brought forward for formal consideration and that final wording would mirror model language used by the Illinois Department of Natural Resources and be designed to satisfy the city's NPDES permit requirements.

The council directed staff to return with ordinance language and implementation details, including how credits and the assistance program would work in practice.