Stormwater system allows development to proceed

In 1938, Frank Lloyd Wright first proposed an ambitious plan to construct a convention center on the shore of Lake Monona in Madison, Wis. Fifty-five years later, construction finally began on the $61.1 million project.

The project is being built on top of a former landfill and will extend 90 feet into Lake Monona. The impact of the structure on the waterfront area was closely scrutinized, and addressing environmental concerns became a major undertaking for Arnold and O’Sheridan, Madison, Wis., consulting engineers on the project. Treating the stormwater runoff from impervious areas adjacent to the site and immediately surrounding the center before discharge into the lake was one of the environmental concerns.

A high volume of vehicle traffic is anticipated in the urban area surrounding the convention center including heavy equipment during the construction phase, convention center guests, employees, delivery trucks and maintenance vehicles after the center opens.

Consequently, there were concerns about runoff of petroleum products, sand, metals and floatable debris common to parking areas that would wash into Lake Monona.

The use of traditional wet ponds for stormwater treatment was dismissed because of the layers of unstable household debris in the subsurface soils that would be exposed by pond excavations.

The city, therefore, opted for the Vortechs system, developed by Vortechnics, Portland, Maine, which uses a tangential inlet that channels stormwater from the site’s storm drains into a circular grit chamber, causing the water to follow a vortex-like flow path. This swirling action directs sediment into a stable pile in the center of the grit chamber. Carefully designed flow controls cause oily contaminants and floatables to rise to the surface where they are trapped within the tank.

As strong lakeside storms gain momentum, the swirling action is increased, and removal of sediments and oils will remain constant — substantially reducing the pollutant level of discharges into the lake.

Two systems designed to remove more than 80 percent of sediments and oils at flow rates of up to 17.0 CFS, or 7,600 gpm, were installed. Peak 25-year flows of 45 CFS and 60 CFS are anticipated at the two system locations.

It is estimated the units will be bypassed on an average of two times each year, and they will capture and store a combined volume of 20 cubic yards of sediment, and as the sediments and oils accumulate within the system, they are removed by pumping the contaminants out with a vacuum truck.