Alternatives to Retention Ponds at Zoo Interchange

There are a number of alternatives to surface retention/detention ponds which do not use up acres of precious parkland, nor attract waterfowl and their fecal bacteria, nor expose passersby to toxins. And these alternatives pose little or no liability, and are less expensive to maintain.

A combination of these options would probably work best, depending upon the site.

1. In-line storage: a larger-diameter-than-needed sewer pipe running down the median or center of the highway, or on the shoulder. It acts similar to a miniature “deep tunnel”. For example, the sewer pipe would be 60 inches in diameter rather than a required 24 inches. This system can be combined with catchment tunnels and basins. This alternative would likely work well following the newly widened sections of 894, HW 100 and I-94 as alternatives to the ponds planned at County Grounds, Underwood Creek and Honey Creek, respectively. Similar systems are used in Portland with much success.

2. Bio-swales: or vegetative infiltration swales, are miniature sliver ponds. The use of native plants and grasses encourages wildlife, but not waterfowl or the breeding of bacteria. By filtering pollutants, ground contamination is limited. The swales are usually tiered, with intermittent curves and berms as catch basins. They act similar to retention ponds, but are much smaller and narrower, and are spread over a larger distance or area. For example, they would be five to eight feet wide, and spread over three to five miles, rather than cover up to five acres in one constrained area. Their smaller size allows for less expensive maintenance with smaller equipment. Bio-swales would work well amidst right-of-way areas, such as within the planned circular ramps and diamond interchange spaces. They could also be used alongside narrow green strips next to the County Grounds. A similar system is used by Illinois Dept. of Transportation along I-94 just south of the Wisconsin border.

3. Bio-Infiltration Fields: essentially a very large rain garden. The area is back-filled with engineered soil, covered with a mulch layer and planted with a diversity of woody and herbaceous vegetation. Stormwater percolates through the mulch and engineered soil, where it is treated by a variety of physical, chemical and biological processes, before infiltrating into the native soil. Maintenance requires periodic replacement of topsoil and plants. Bio-infiltration fields are planned for stormwater management at Innovation Park on the Country Grounds. They may be a suitable match for land near the Monarch Trail, perhaps in combination with bio-swales and/or in-line storage.

4. Underground Storage: a cistern that acts like a pond underground. It can be either a closed system or an open, porous system which percolates into the ground. The parking lot at State Fair Park alongside I-94 will need to be dug up for reconstruction. It would be a workable site for underground storage as an alternative to a surface pond at Honey Creek Parkway. Similarly, the Sheriff's parking lot west of 894 will be dug up, and could also be a site for underground storage instead of the South Berm of the Monarch Trail. There are underground stormwater storage sites at MMSD and some retail parking lots in southeastern Wisconsin.

Dianne Dagelen, Conservation Chair