a report by Matt Covert, Green Downtown Program Manager
Metric: Impervious Surface
Downtown is filled with impervious surfaces like asphalt, roofs, and sidewalks. Stormwater runoff is generally channeled into storm sewers, which
eventually end up in water bodies. Sediment, pollutants, and organic matter that is carried out of urban areas can damage water quality throughout the watershed. In Downtown Madison, this problem is particularly acute, owing to the proximity of the lakes
and the high amount of impervious surface (figure at left). Combined with the intractable problem of farm runoff, stormwater drainage into the Yahara River watershed has negative implications for the health of the waters in the Madison area.
Metric: Underutilized Land and Infiltration Enhancement Potential
Crucial to the health of a watershed is the amount of impervious surface that forces runoff and pollutants into storm sewers. When downtowns redevelop, they have the opportunity to incorporate infiltration in areas where it would make the most difference. In Dane County, the Capital Area Regional Planning Commission analyzed a variety of soil, surface, and land use characteristics to determine which areas exhibit the largest difference between natural and engineered infiltration. When applied to a map of underutilized land in Downtown with infill potential, the greatest concentration of such land also has among the highest infiltration enhancement potential (green shaded area). The extent to which a neighborhood works with the city to prioritize reducing impervious surface in redevelopment will have profound impacts on neighborhood- or districtscale sustainability.
One method for intermediate-scale stormwater runoff mitigation is installing rain gardens, which involves planting a low-lying area with native vegetation (like grasses) to take up rainwater and slow its journey below the surface. Rain gardens can have positive impacts on downstream flood risk and urban heat island as well as pollutant load. However, they do require space. This may account for the heavy concentration of rain gardens targeted at the Lake Wingra watershed in the historic Regent neighborhood ((b), figure) and the near total lack of rain gardens in Downtown Madison (a).
Metric: Green Roofs
A different method for stormwater runoff mitigation is the use of green roofs. This term can refer to a wide variety of structural and landscaping changes to buildings’ rooftops that allow the buildings themselves to capture and slow water as it moves through the urban watershed. Some green roofs are heavily vegetated and accommodate multiple and diverse user groups (below: top), while others merely involve using gravel and some grasses to keep costs and structural stresses to a minimum (below: bottom). Like rain gardens, green roofs mitigate pollutants, flooding, and heat island, but they take up less two-dimensional space. As the map at bottom shows, Downtown Madison is home to a number of green roofs, especially northeast of the capitol. The UW-Madison also boasts a large number of green roofs, and the strong connection between university research and demonstration and neighborhood-scale implementation of green roofs is clear and compelling.