Stormwater 101

How those big stormwater tanks being buried in downtown Spokane work

Spokane has committed to spend $144 million on tanks designed to capture water running into combined sewer systems during periods of heavy rain and snowmelt, preventing it from flowing into the Spokane River untreated. That money includes construction of three massive tanks downtown, all of which operate with the same basic goal: buying the system time before runoff is sent to the river.

The park that will help clean the river

There’s nothing complicated about the massive chambers of concrete being buried downtown, capable of storing millions of gallons of water. “If the Romans cared about it, they would have done it,” said Marcia Davis, a principal engineer with the city who’s helped design the tanks over the past decade.

The Overlook Park project

What sits on top of the stormwater tank is determined by city planners and the representatives of the neighborhood where they’ve been built. The city plans to build a public greenspace over the tank that’s going in at First and Adams downtown, and a climbing wall feature is being proposed over the Bosch lot tank that has been under construction since last year.

How the tanks work

Though the tanks come in different shapes and sizes, here’s essentially how they work: During heavy rains or snowmelt, water captured in combined sewage pipes that would otherwise be diverted by weir to the river is routed instead to the tank. Untreated water first fills what’s known as a “flush chamber” near the entrance to the tank. As the water level rises, it spills over a wall in the flush chamber and fills the rest of the tank.

How much is too much?

A permit from the Department of Ecology allowing the city to discharge untreated water into the river will limit such events to one per pipe, per year. Brook Beeler, a spokeswoman for the Ecology Department in Spokane, said the department follows the Environmental Protection Agency’s policy on combined sewer outflows, which allow the one per site, per year standard.

Managing stormwater

In 2017, the city started construction on some of its largest tanks to date, including two downtown that will be able to hold more than four million gallons of runoff combined when they’re complete.