Jun 10, 2015

Protecting the Promenade

Wayzata, Minn., redevelopment utilizes three-pronged storm water solution to protect Lake Minnetonka

Wayzata, Minn., redevelopment utilizes three-pronged storm water solution to protect Lake Minnetonka

Located about 12 miles west of downtown Minneapolis, the “Promenade of Wayzata” is a 14-acre, three-phase mixed-use redevelopment on the east edge of downtown Wayzata, Minn. The project replaces an old mall with six distinctive blocks focused on the pedestrian.

A signature goal of the project was to mimic historic predevelopment storm water conditions from when the site was a wetland, thereby far exceeding regulations that required matching existing conditions. This would demand a significant infiltration component to reduce volume. This proved difficult due to the limited land space, the poor soils and a labyrinth of foundation piles and other utilities. Three primary systems were designed to overcome these constraints, each roughly the size of a football field: an infiltration basin, a filtration basin and a pond.

The infiltration basin is located below the street, where it turns 90 degrees between three buildings. Termed the “Engel Street Infiltration Basin,” the system is the most significant of the three primary storm water systems and consists of a large infiltration area and attached detention vault. Excavated through the urban fill and swamp deposits to native clean sands, the infiltration area measures roughly 200 ft wide by 300 ft long with a depth ranging from 14 to 31 ft deep—a big hole.

The infiltration basin was equipped with 1,000 linear ft of 48-in.-diameter perforated aluminized steel Type 2 corrugated metal pipe (CMP) from Contech Engineered Solutions. The maximum feasible infiltration area came up short for storage capacity to reach project goals. Therefore, a 40‐ft-by-80‐ft precast vault is connected to the infiltration pipe network to provide additional storage capacity. Runoff from about 5 acres of exclusively building roofs is directed to the detention vault and CMP, which distributes the runoff to the clean sands of the infiltration basin, thereby recharging the groundwater. A full 2.4‐in. runoff event is fully infiltrated by the system; this translates to no runoff to Lake Minnetonka for 99% of all annual rainfall for 5 acres of mostly impervious surfaces. The bulk of larger, very infrequent storms are also infiltrated while the surficial portions of these storms (the cleanest portions) overflows to risers in the vault. These risers drain to discharge lines routed to an under-building pond, where this clean portion is further cleaned and managed before discharging to Lake Minnetonka.

The second of the primary storm water systems is an under-street filtration system that detains and filters runoff from about 5 acres of building roofs and boulevards. Termed the “Mill Street Filtration Basin,” this basin features 40 ft by 220 ft of similar construction as the smaller detention vault connected to the Engel Street infiltration basin; however, the floor consists of fabric and engineered fill to do the filtering. Two Contech Vortechs units pre-treat runoff entering this system; these units combine swirl concentration and flow controls into a shallow treatment unit that traps and retains trash, debris, sediment and hydrocarbons. Capturing sediment in the Vortechs units, where maintenance is relatively easy, was preferred versus the larger, less accessible filtration basin and pond.