Stormwater BMP/LID Design and Retrofits—Fairfax County, VA

Case Study Under:  Engineering Solutions         Location:  Fairfax County - Virginia
The Case Study

Stormwater BMP/LID Design and Retrofits for Fairfax, VA

The Situation

Fairfax County is located in the suburbs of Washington, DC and has a population of approximately 1 million people.  Much of the County has been developed over the past 40 years, which presents challenges in treating stormwater runoff pollution discharges as required by the County’s NPDES MS4 Phase I permit.  Fairfax County hired GKY to design new stormwater infrastructure as well as existing infrastructure retrofits to reduce pollution discharges associated with stormwater runoff.

The Solution

GKY has designed several new stormwater BMPs (including a regional pond) as well as numerous retrofits to existing County stormwater infrastructure.  In one specific example, officials at Fairfax County tasked GKY with retrofitting a high school with SWM/BMP/LID treatment devices.  In addition to providing water quality treatment and stormwater control, the proposed facilities were designed to serve as teaching tools for the students at the high school.  A wide variety of BMP treatment devices and stormwater detention facilities were designed for the project.  Emphasis was placed on using Low Impact Development techniques including porous pavement, infiltration trenches, rain gardens and tree conservation.  GKY designed a bioretention facility to treat stormwater runoff from one of the parking lots, multiple tree box filter units to treat runoff along the perimeter of a ring road, an infiltration trench to treat runoff from the main entrance, porous pavement to replace impervious asphalt in the overflow parking lot and several underground carbon filter devices to treat rooftop runoff.  In addition, GKY designed an enhanced extended-detention basin to replace the old, undersized dry pond and preserved many of the existing old growth trees around the school by creating a conservation easement which ensures that the trees cannot be cut down for a future parking lot expansion or new athletic fields.