From the Aspen Daily News (Curtis Wackerle):
The “Urban Runoff Management Plan” lays out minimum standards and recommended practices for development and redevelopment projects. The manual is part of the city’s program to remove sediments and pollutants from stormwater runoff before the runoff reaches local rivers. Sediments and pollutants are often added to runoff as it crosses pavement, rooftops and other hard surfaces…
City policy outlined in the manual operates on a sliding scale: The more impervious — impenetrable by water — surface area on a site, the more water that has to be detained on site in a vault, settling pond, rain garden or by some other method. On-site detention allows sediments and pollutants in runoff to settle before the water flows into the city’s stormwater collection system. Green space on a site — either grass, vegetation of green roofs, all of which reduce actual runoff — offsets the amount of water that has to be detained, under the new guidelines. The manual also recommends pervious or modular paving, which employs large blocks with gaps in between that allow water to infiltrate the ground.
Previous city stormwater guidelines were focused on large rain events, Barker said. The new manual shifts the focus to smaller rain events, which make up the majority of rain in Aspen, and are most troublesome from a runoff perspective since the first bit of rain collects the most pollutants. The project also loosens rules for projects that result in less than 1,000 square feet of new impervious area. Before, any project with more than 200 square feet of disturbance had to hire a professional engineer to do a grading study. That requirement has now been eliminated for projects with less than 1,000 square feet of disturbance.