In a typical Kansas City rainstorm, more than 5,000 cubic feet of water falls onto the 18Broadway site and the paved surfaces that surround it—totaling nearly a million gallons a year. In the initial downpour of a rainstorm (called the "first flush"), pollutants deposited on paved and other surfaces are dislodged and drawn into the flow of rain water runoff.
Biofiltration is a natural way to reduce pollutants in storm water before run off flows into the storm sewer systems and ultimately to lakes, rivers or streams. The 18Broadway project uses a series of storm water management systems to capture, purify and reuse this storm water, diverting it from Kansas City's storm sewer system. Rain gardens and swales have significant filtering capabilities. As storm water flows through the swales, solid particles settle out; plants in the swale act as a natural filter to help remove contaminants in the water.
The biofiltration systems at 18Broadway are designed to retain 100 percent of "first flush" rainfalls. Clean water is then directed to a 40,000-gallon underground cistern storage system, where it is UV–treated and pumped back out as needed for irrigation of 18Broadway's community gardens.
Stormwater runoff from the street flows downhill until it reaches the planter. A 12-inch-wide trench drain channels the runoff into the planter. Stormwater is allowed to pond to a depth of 7 inches before infiltrating through the soil at a rate of approximately 4 inches per hour. During large storm events, water may enter at a faster rate than it can infiltrate. In that case, the additional water is piped into the rain gardens or flows to the next consecutive planter. The primary purpose of the stormwater planter is to capture and reduce the flow of water and clean the water that does not infiltrate so that it enters the storm drain system clean.
A swale is a vegetated filter, also referred to as a biofiltration swale or bio-swale. As stormwater flows through the swale, solid particles settle out. Plants in the swale act as a natural filter to help remove contaminates in the water. Swales are planted with erosion-resistant and flood-tolerant grasses that have the capacity to retain pollutants and slow stormwater.
A rain garden is a shallow filtration basin strategically placed to catch runoff from roads, parking lots, driveways and roofs. Planted with deep-rooted vegitation, rain gardens facilitate evaporation, filter pollutants, and allows water to be slowly absorbed by the soil. This bio-retention concept is based on the hydrologic function of forest habitat, in which the forest produces a spongy litter layer that soaks up water and allows it to slowly penetrate the soil layer.
The alley swale, similar to the 18th Street swale, is a linear bio-retention system. This porous depressed box (or trapezoidal ditch) will soak up 12 inches of water within three days. The alley swale will have the ability to hold some water and slow the flow from the 18th Street swale and the alley runoff as water flows toward the lowermost rain garden.
Curb bump-out swales are a bio-filtering extension of a curb into a street, driving lane or parking area. They preform a multi-function service of filtering and slowing stormwater runoff, calming and directing traffic patterns, and beautifying streetscapes with lush vegetation. Bump-outs are one of the green stormwater strategies called for in the Kansas City Overflow Control Program.
The Broadway Court at the northwest corner of the site is a plaza that provides the perfect vantage point to view the conservation principles of 18Broadway. While providing a place of rest, the plaza is also the release location of a rain garden recharge fountain. From the fountain a small flow of water, supplied from the runoff sequestering cistern storage system, will help ensure that the rain garden along Broadway will remain green and lush during dry spells. The fountain is controlled to shut down after dark, so the energy consumption of the pumps can be used for site lighting. The energy consumption of the entire site is offset by a grid-tied photovoltaic array to maintain a net-zero energy footprint over a year's timeframe.