During Tuesday’s huge thunderstorm, the Cleveland Park Metro station flooded so badly that Metro ended up closing it for nearly two hours. Why was the flooding so severe?

The storm that swept across the region yesterday afternoon brought over an inch of rain to many areas in a very short amount of time. Here’s how crazy things got at Cleveland Park:

Metro spokesperson Sherri Ly told the Washington Post that the reason the Cleveland Park station got hit so bad is that it “…is prone to flooding because it is at the bottom of a hill.”

But that’s not quite true: many areas east of the Metro station are farther downhill.

Greater Greater Washington contributor Matt Johnson says it’s less about the topography of the area than it is about the amount of impervious surface (like roads, sidewalks, and parking lots).

Next to the Cleveland Park Metro, there’s a parking lot to the east and buildings to the west. All of the water that falls flows into Connecticut Avenue (from downspouts from the buildings, and down the driveway aprons from the parking lot)— it has to go somewhere.

Near the intersections, some of that water may flow down toward Rock Creek, but there’s a fixed amount of drainage volume that can be accommodated that way. The rest of the water gets collected by catch basins along the curb of Connecticut Avenue. Those also have a fixed capacity. Once the rainfall exceeds the ability of the street to drain itself downhill and into the catch basins, the water level will start to rise.

Once the water level rises above the level of the curb (between 4 & 6” above the pavement surface), it starts to spread onto the sidewalk, and will flow downhill, including into anything at a lower elevation than the sidewalk (like a subway station).

Then the water will begin to flow into the grates in the sidewalk that lead to underground vaults that hold things like transformers and Metro vent shafts. Additionally, water will trickle down the escalator and stairway into the station.

Metro has placed sandbags around the grates in the sidewalk near the Cleveland Park station to help keep water out. That works often enough, but not during Tuesday’s storm.

“The level of water on the sidewalk had probably reached several inches high during the peak of the rain event, demonstrating how overwhelmed the catch basins were,” says Matt.

“In other words, the water volume exceeded DDOT’s ability to handle the runoff, so it began to flow into the Metro.”

So is there any solution to keeping this from happening again? The sandbags help some, but there are some other options. In New York, MTA has raised the level of some street grates. And at the South Ferry subway station, which is in danger of tidal flooding during storms, MTA has added a few stairs that go up, before going down, at the entrance to the station.

NYC’s South Ferry subway stop. Image from Wikipedia.

Kelli Raboy works as a federal contractor supporting research on vehicle automation and communications. She loves all things cities, public transit, and rail. She lives in Navy Yard.