Image is a screenshot of video footage of the "waterfall" at the Virginia Square station.

Drivers had to be rescued from cars, streets flooded, and the Virginia Square Metro station developed a waterfall on Monday. The record-breaking rain showed how powerful large amounts of water can be, and what it can do to infrastructure that isn’t built to handle it.

Rain fell inside the Virginia Square station for 20 minutes from 8:53 am to 9:13 am, according to Metro spokesperson Dan Stessel in a statement provided by Ian Jannetta. Stessel’s statement says, “During the period of heaviest rainfall, it appears that the drains were simply overwhelmed and the water found an alternate path.”

Stessel’s statement also says that Metro inspectors “found the drains clear, no obstructions,” and DC Metro Hero data shows trains single-tracked around the waterfall from 8:57 am to about 11:44 am.

I asked Jannetta for more information about prior water intrusion at this station as well as details about policies governing how Metro handles water in rail stations, but did not receive a response by the time this story was published. The water apparently entered the station by overwhelming the drains and piping meant to keep it outside.

Dome relief vents let hot air out and catch water

Metrorail stations below ground were built with vents at the top of their domes to allow hot air to escape up to ground level. The dome relief vent designs also include pipes designed to catch water that enters the vent at ground level. They then redirect the water around the outside of the station dome to where it can be pumped away.

The drains vent to the surface and are typically built behind curb storm drains. However, if those are blocked it’s even easier for water to find its way into a station. Built as part of a fairly standard Metrorail station design, the station relief vents and drainage are enough to handle a typical storm in the DC area. Monday’s rain was anything but typical, though.

The suspect dome relief vent is below one of the two grates on the sidewalk shown in this picture. Image by the author.

The dome relief vents above the area of leakage in Virginia Square are located at the bottom of a slight incline on one side and a minor hill on the other next to Fairfax Drive heading east-west between Ballston and Clarendon. The topology around them is relatively flat compared to some other Metrorail stations on inclined ground, and most nearby surfaces are impervious meaning they don’t absorb water.

A record 3.44 inches of rain fell at National Airport on Monday, breaking a previous record of 2.16 inches of rain in a single day from 1958. That 3.30 inches of rain fell within just the 9-10 am hour alone, according to Capital Weather Gang meteorologist Ian Livingston—and all that water has to go somewhere.

Metrorail dome relief vent drains each include six-inch pipes to redirect the water around the station. They’re large enough to drain an average 800 gallons per minute—a reasonably high water flow. However, the rain that fell in Virginia Square appears to have been more than enough to overwhelm this particular pipe.

Infrastructure built for what?

Building an underground rail station with vents at the top of their domes and fixed-diameter piping seem to have obvious design limitations that can be exceeded with a large enough amount of rain. Other factors that change over time, like more paved areas which lead to more water runoff, only compound the problem.

Monday's rain make for a soggy commute. Image by the author.

It’s not yet clear why Virginia Square was the only station to be impacted as badly as it was. The incident does at least show how much water is too much for what the system was built for. Metro says there were no debris blocking drainage, so the issue probably isn’t deferred maintenance as some have suggested.

“Much of our underground system is below the water table and is designed with the knowledge that water can and will come in,” Stessel’s statement says. A network of 58 drainage pumping stations throughout the system are “capable of removing more than 3 million gallons of water each day.”

The Washington Post reported there “appears to be no heavier one-hour rainfall” in Washington region records dating back to 1871, a time period about three times as long as Metrorail has been running. Such an abnormal storm was likely outside the design criteria considered when putting the stations in, and expending extra money to handle an amount of water that hadn’t been seen in almost 100 years presumably would not have been a high priority.

However, with the likelihood that climate change will cause more frequent severe storms in the future, strengthening the stations to be able to withstand more water may be a wise investment to make.

Metro Reasons is a regular breaking news, investigative reporting, and analysis column by Stephen Repetski about everything Metro. Please send tips to Metro Reasons.

Stephen Repetski is a Virginia native and has lived in the Fairfax area for over 20 years. He has a BS in Applied Networking and Systems Administration from Rochester Institute of Technology and works in Information Technology. Learning about, discussing, and analyzing transit (especially planes and trains) is a hobby he enjoys.