Green infrastructure is any stormwater practice that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters. Practices that meet this requirement include, but are not limited to, the following: Rain Garden, Bioretention Device, Infiltration Basin, Vegetated Swale, Permeable Pavement, Green Roof, planter connected to roof drainage, rain barrel and rainwater harvesting/reuse.

Dane County requires that for redevelopment with proposed impervious surface area greater than 80% of existing, the first 0.5 inches of runoff from impervious surfaces must be captured using green infrastructure. Surfaces designed to generate no runoff from a 0.5 rainfall and are approved by the local approval authority may be considered pervious when determining if this requirement applies to the site.  

Depending on the chosen practice, there are different ways to model that the first 0.5 inches of runoff is being treated. For simple systems that store the full volume of runoff, the calculation is a simple volume check. For more complex systems that have water being actively routed through them, some additional modeling is required. Details on how different practices must be modeled is described below.

A cistern or rain barrel should be sized to capture the volume associated with the first 0.5 inches of runoff across the area that drains to the cistern or rain barrel. Applicant must have a plan detailing how the collected volume will be used on site for a beneficial use (such as watering landscaped areas) within 24 hours after rain subsides. Reuse requirements must be documented in the stormwater maintenance agreement.

1. Volume Method

Show that the ponding volume in the device (above the soil surface and below the lowest outflow device) is at least the treatment volume. This approach is conservative because it ensures that if there is suddenly 0.5 inches of runoff and there is no time for the water to infiltrate, it will be captured.

2. HydroCAD

Using TR-55, a 0.7-inch rainfall produces 0.5 inches of runoff from an impervious surface (CN=98), as seen in Figure 1. To show that the green infrastructure is able to capture this runoff, a 0.7-inch 24-hour MSE4 design storm (Figure 2) must be modeled with no surface discharges from the practice (Figure 3). For the most common green infrastructure practices, such as bioretention and permeable pavement, this means all captured water must leave via the underdrain and/or by infiltrating into the native soils. For practices using engineered soil, underground storage should not be incorporated into the model, because of the relatively slow rate at which water flows through the soil.

Figure 1: 0.7-inch rainfall to 0.5 inches of runoff

Figure 2: 0.5-inch 24-hour design storm

Figure 3: Output showing no surface overflow

3. WinSLAMM

Run the WinSLAMM model from 4/6/81 to 4/7/81 (Figure 1) and show that no surface discharges occur from the green infrastructure practice (Figure 2). A 0.7-inch storm occurs during this period, resulting in 0.5 inches of runoff.

Figure 1: Dates for modeling 0.5 inches of runoff

Figure 2: Output showing no surface runoff

It must be shown that the volume of voids within the Green Roof’s growth media is equal to 0.5 inches over the contributing area.

It must be shown that the volume of voids in the pavement system is equal to 0.5 inches over the contributing area. The Permeable Pavement must be designed per the DNR technical standard and specified run-on ratio maximums.

All water routed through the green infrastructure must be modeled, including runoff from non-redeveloped surfaces.

You may trade “like”, or dirtier, impervious surfaces that are not being redeveloped, if constructed prior to 2001.

For sites that are a mix of new development and redevelopment, treating the full site to new development standards is acceptable. If this approach is used, an infiltration exemption cannot be claimed.

Green Infrastructure in the Great Lakes

EPA Green Infrastructure Homepage