Stormwater Retention Project

I always heard that the Pacific Northwest gets a lot of rain. Having lived in southern Louisiana for 25 years, I was a little skeptical. So when I moved here last year, to discover that all of the yards were brown, but the flowers and trees were flourishing, I became interested in the annual rain pattern. It doesn’t rain here in the summer. No thunderstorms twice a week. Nothing.

Water isn’t particularly expensive here, but I don’t like using clean tap water on plants. It turns out that most of Tacoma’s water comes from glaciers on Mount Rainier (Tahoma to natives). They melt naturally in the summer, so we aren’t using groundwater and have no control over their rate of melting.

We inherited a number of young fruit trees, which need a lot of water. Seeing how nice many of our neighbor’s yards were, we decided to collect plants. I watched water flowing out of the downspout during spring rains, flowing down the driveway into the street, and directly into Puget Sound. We installed a couple of rain barrels to collect some of it; you would be surprised how fast they fill during the light rains (compared to the Gulf coast) I’ve experienced here in Tacoma.

It turns out that Pierce County has a stormwater retention program, to prevent pollution of Puget Sound, and one can get free consultation on home projects that reduce storm runoff. There is even a competitive proposal system to get limited financial assistance. We jumped at the opportunity to contribute to the continued health of the place we’ve already fallen in love with.

The back yard was a basketball court at some point, complete with 3-4 inches of concrete. The previous owners removed the concrete (thank god!), but left some irregularly cut tree trunks. We were thinking about planting a garden in this area, but now we were part of something bigger than our back yard. We could get assistance on the whole project, from collecting rainwater to distributing it, and keeping it out of the pristine waters of Puget Sound.

But there were some engineering hurdles to clear…

We had to demonstrate that the soil under our back yard would absorb a reasonable amount of rainwater, which necessitated digging a hole and filling it with water. That sounds easy enough. The catch is that the soil in the hole has to be saturated before the test can be conducted. To assure saturation for all soil types, this means keeping a two-foot deep hole full of water for three hours. I had to keep the garden hose running at half-volume, tweaking it constantly, to meet this standard.

I spent the morning babysitting a one-foot-diameter hole with a hose running constantly.

But I kept the hole full for three hours. Finally, I could conduct the test. I filled the saturated soil to 12 inches and waited, but not for long! As I already knew, it drained in 17 minutes, as measured by a stopwatch. We have well-drained soil (mostly sand with some clay and rock cobbles).

While my garden hose kept the ground saturated, I kept myself busy, removing 4 inches of mulch filling the 4-inch deep depression left by the basketball court. I’m going to keep the material on-site as much as possible, to avoid hauling it up the slope seen to the right in this photo, where it would (please, god, no!) be spread to form a slight hump in my back yard.

You would be surprised how fast soil is used to fill a 4-inch deep area the size of the remaining mulch seen in this photo.

I can’t wait to begin digging the pond. I’ll see you then.