Managing stormwater is going to be an increasingly difficult task in Wisconsin. Climate change is causing wetter conditions generally, and more frequent, intense storms. This is particularly true in urban areas where “impervious surfaces” like pavement, concrete and rooftops prevent stormwater from soaking into the ground. All this water runs into nearby waterways and stormwater drains, carrying with it trash and pollution like grease and oil from streets. In heavier rainfalls, all this volume can overwhelm the stormwater system, causing basement backups and even combined sewer overflows.
To meet this challenge, the Milwaukee Metropolitan Sewerage District has set an ambitious goal of capturing the first half inch of all rainfall hitting impervious surfaces with green stormwater infrastructure. Green stormwater infrastructure (GSI) mimics many natural processes to capture and store stormwater and then slowly release it over time. Examples of public GSI are large retention basins, bioswales established in or along roadways, and permeable roads and alleys. Examples of private GSI include rain barrels connected to gutter downspouts and rain gardens that absorb water during storms.
At Clean Wisconsin, as part of the Resilient Communities Program’s GSI outreach efforts, we use stormwater modeling at the neighborhood level to contextualize the challenge of capturing the first half inch of rainfall. The modeling helps us understand what it would take for a neighborhood to reach the goal. For example, how many homes would need rain barrels or rain gardens? What combination of public and private GSI is needed to reach our stormwater capture goal? By presenting this data, we show how collective action is critical to success. This year we are working with the Sherman Park and Amani Neighborhoods.
Figure 1: The existing GSI in the Lincoln Creek neighborhood.
To visualize what a half-inch rainfall looks like, imagine a moderate rainfall lasting a couple of hours or a heavy rain lasting 30-45 minutes. Milwaukee typically receives 2-4 inches of rain per month and has had 26 days with more than ½ inch of rain so far this year.
When this rainfall hits impervious surfaces, runoff adds up quickly. For example, a typical single-family home will generate about 400 gallons of runoff during a half-inch rainfall.
Extending this out to an entire neighborhood, the Amani neighborhood is about 1 square mile and 56% of this area is covered with impervious surface. During a half-inch rainfall, this will generate over 4 million gallons of stormwater runoff. The Sherman Park Neighborhood, at 3.2 square miles with a similar percentage of impervious surface, will generate over 14 million gallons of stormwater in a half-inch rainfall.
A million gallons is about 1½ Olympic-sized swimming pools, so these neighborhoods will need to capture 6 and 21 pools’ worth of stormwater, respectively, to capture the first half inch of runoff.
Residential GSI has an important role in complementing larger public GSI installations to meet this goal. A typical residential rain barrel holds about 50 gallons, while a 50-square good rain garden can capture about 350 gallons. Pairing these two on a single-family lot can capture the equivalent of runoff from the roof and reduce stress on public GSI.
The goal of capturing the first half inch of stormwater is challenging, but with everyone contributing it’s possible to make a meaningful difference. Understanding what this looks like in your own neighborhood helps to build support for installations, both public and private, and a sense of community pride and ownership in tackling this important challenge.
So far, Clean Wisconsin’s green stormwater infrastructure includes:
- 3,885 square feet of rain gardens
- 738 Rain Barrels
- 39 StormGUARDens
Together, our green stormwater infrastructure projects capture 60,744 gallons of water per storm.
