Committee says city-installed terrace rain gardens aren’t cost-effective; suggests private incentives

The presenter told the Middleton water-resources committee that installing terrace rain gardens citywide would be cost-prohibitive and unlikely to meet the city's TMDL phosphorus-removal goals. The presenter said the meeting figures showed a very high cost per pound of phosphorus removed — stated in the presentation as "2,500 to $31.50" per pound — compared with $50 to $60 per pound under an adaptive-management approach, and that a single terrace rain garden removes only "1 tenth of a pound." The presenter said the city would need about 1,250 pounds removed to meet goals and that would require roughly 12,000 terrace rain gardens, which "would be a little bit, not feasible."

The presenter also raised maintenance concerns: many commercial infiltration devices are taken offline for winter under maintenance agreements and, citing the DNR technical standard for bioretention devices, staff said the city has a poor success rate getting private commercial devices taken offline and then returned to service. Phil, a staff member, added that private homeowners sometimes install terrace rain gardens to address sidewalk ponding, but that such installations require continuous maintenance to prevent grass or other impediments from blocking flow. "The salt rate and runoff, that's not a deal breaker," the presenter said, "I think the real deal breaker was cost."

Given the cost and limited per-device benefit, the presenter recommended against municipal installation and proposed exploring a private incentive rain-garden program that would deliver similar public-education and habitat benefits while placing maintenance responsibility with homeowners. Committee members said they would continue the discussion at a future meeting and staff noted outreach and neighborhood advocacy can substantially affect voluntary participation rates.