Recreational trails provide opportunities to enjoy Dane County’s many scenic areas, however, their construction may impact water quality and be subject to county stormwater standards. Depending on characteristics of the trail and surrounding land area, there are stormwater treatment options that provide flexibility when planning work on a trail. The prescribed options discussed in this guidance will assist municipal or state trail designers with meeting County stormwater performance standards. This guidance applies only to new impervious area directly resulting from a trail installation – not to other impervious features such as parking lots or restrooms.

Trail projects that create 20,000 square feet or more of new impervious surface require a County stormwater management permit. Any paved or graveled surface is considered impervious. If the site (e.g., park) is already covered by a stormwater permit, any additional impervious area requires a revision to that permit. Redevelopment does not apply to recreational areas.

The County has determined that municipal and state trails typically have low pollutant loads associated with post-construction storm water runoff if designed, constructed, and maintained such that runoff is directed to a pervious area to facilitate adequate filtration and infiltration of runoff. This guidance provides requirements for trails that, if followed, can be assumed to meet Dane County stormwater performance standards, including peak rate, TSS, and infiltration.

The requirements are:

1. The construction of the trail is associated with a State or municipal project.
2. The impervious source area (i.e., recreational trail) is:
   1. No wider than 12 feet, including graveled shoulder.
   2. Intended for pedestrian and bicycle use, with vehicular access restricted to maintenance, safety, and other limited operational needs.
3. The pervious area receiving the trail runoff is predominantly:
   1. Covered with dense, native vegetation in good condition.
   2. At a slope not exceeding 20 percent.
   3. At least 20 feet long.
4. The pervious area must receive runoff as sheet flow from the trail.
5. The trail subbasin impervious area is 10% or less of the overall area.

Trail sections of the site that meet the above conditions are assumed to meet all Dane County stormwater performances standards. The stormwater plan must include a site plan that clearly indicates which trail segments meet the above criteria and the pervious treatment areas. If sections of the trail cannot meet these conditions, it must be shown that those sections are meeting standards by using approved methodology and modeling.

Design Considerations for Post-Construction Performance Standards for Public Trails near Waterways or Wetlands