The Virginia Department of Transportation (VDOT) approved StormChambers for One Loudoun, a large mixed-use development in Ashburn, Loudoun County, VA. It was one of the first applications where VDOT runoff water—combined with private runoff water—went to this type of system, notes Steve Pandish, director of water resources for the engineering firm of William H. Gordon Associates in Chantilly, VA.
The StormChamber system was designed to capture and detain runoff and sediment from roads and other impervious surfaces. The system at One Loudoun consists of 595 StormChambers and several StormChamber SedimenTraps, most of which are used for extended detention. They also are used for conveyance in place of reinforced concrete pipe, taking the drainage from upstream areas. In addition to detention, driving factors included lower cost, peak flow attenuation, and water-quality enhancement. Additionally, unlike the reinforced concrete pipe, the open-bottom chambers helped to maintain base flow to adjacent wetlands.
In addition to being significantly less expensive to purchase and install for conveyance, StormChambers provide significant environmental benefits, even over perforated pipe. The open-bottom chambers provide more than 1,000 times the open area in direct contact with the surrounding stone and soil than does perforated pipe. In pipe, water follows the course of least resistance and tends to flow through the pipe more readily than through the pipe’s small perforations. About 80% of the flow-through occupies approximately the bottom 20% of its area, causing the perforations in this portion of the pipe to clog in a relatively short period of time and reducing infiltration. This does not occur with plastic chambers due to their open bottoms and the significantly larger perforations along the sides.
Peak flow attenuation was achieved with flow over the stone base directly under the chambers as well as the partially closed lower portion of the “downstream” end wall of each chamber, which also offers extended retention time and helps to facilitate infiltration.
The biomat that forms on the stone and soil under the chambers is responsible for most of the water-quality enhancement. There are microorganisms naturally occurring in the soil that will metabolize just about anything. The populations of most of these microorganisms are very low. When a food source is made available, the numbers of microorganisms that metabolize it increase exponentially until they eliminate it, and then their populations die back to preexisting levels. This is similar to how septic drain fields function.
The US Navy received a prestigious environmental award for inadvertently applying this natural science to eliminate hydrocarbon contamination in groundwater (in a process commonly referred to as “bioremediation” or “natural attenuation”). The Beaufort, SC, naval base monitored the boundaries of a hydrocarbon-contaminated groundwater plume. Before they proceeded with the prescribed procedure of digging up the contaminated soil, treating it, and replacing it, they discovered the plume began to retreat. They found that the microorganisms in the soil that metabolized hydrocarbons quickly multiplied and eventually eliminated it, metabolizing the hydrocarbons into carbon dioxide and water. Tens of millions of dollars was saved and the soils and wildlife habitats were left undisturbed.
Base flow is that component of the groundwater that slowly seeps into a body of water. About 50% of the flow in a stream is provided by base flow during normal flow periods. During dry periods, as much as 100% of the flow is contributed by base flow. As the base flow component of the groundwater enters and moves through the soil, it becomes cooled and cleaned up. The latter occurs because of the natural filtration process of the soil and the wide variety of naturally occurring microorganisms.
Installed in the summer of 2008 by the contracting firm of William A. Hazel, the One Loudoun system went online in 2015 after VDOT ensured all upstream areas were stabilized, which required the use of a temporary bypass structure that has since been deactivated, notes Pandish.
The client had sought an underground solution, Pandish points out. “We were considering a dry extended detention pond, but at that location, it was going to be adjacent to a community center and the client didn’t want an ugly dry pond there,” he says.
In researching options, Pandish favored the use of StormChambers. His firm’s concerns focused on ease of maintenance and hydraulic ability to pass a 10-year storm, says Pandish.
“We added more than the recommended amount of StormChamber SedimenTraps,” he says. “We did a flood routing through the system to validate the hydraulic capacity.”
Additionally, although his firm did not consider the infiltration ability of the system, “we liked how it would provide some infiltration into the surrounding soils because of the nearby wetlands that we wanted to feed,” notes Pandish.
“The system sits on a stone base, so it has some inherent infiltration in it. I also like that it can be cleaned by air vacuuming via inspection/cleanout ports instead of using water. Anytime you have to clean something with water, then you have to have a truck that collects the water and the sediments in it. Depending on where you can dump that, it can get much more costly, as opposed to going out with a vacuum truck.”