A stormwater bumpout is a vegetated curb extension that protrudes into the street either mid-block or at an intersection, creating a new curb some distance from the existing curb. A bumpout is composed of a layer of stone that is topped with soil and plants. An inlet or curb-cut directs runoff into the bumpout structure where it can be stored, infiltrated, and taken up by the plants (evapotranspiration). Excess runoff is permitted to leave the system and flow to an existing inlet. The vegetation of the bumpout will be short enough to allow for open sight lines of traffic. Aside from managing stormwater, bumpouts also help with traffic-calming, and when located at crosswalks, they provide a pedestrian safety benefit by reducing the street crossing distance.
Stormwater Bumpouts at Queen Lane
Philadelphia's first stormwater bumpouts on Queen Lane in East Falls help to reduce runoff and prevent combined sewer overflows into our rivers and streams. Runoff from the street is diverted into these landscaped curb extensions, where it infiltrates into the soil instead of entering our storm sewers.
Each bumpout is custom designed on a site-by-site basis; the six Queen Lane structures are each 8 feet deep and range in length from 24 feet to 80 feet (the bumpout pictured above measures 8' by 60'). Each bumpout is planted with a mix of native grasses, perennials and trees, and the entire system manages the first inch of runoff from an acre of drainage area. That means these bumpouts manage between 800,000 and 900,000 gallons of runoff each year.
Philadelphia Water Department