Green gooses: Rain gardens that don't work

Hardly anyone remembers the Spruce Goose. It was the largest airplane ever built--made entirely of wood, with eight engines and wings longer than a 747’s. It was completed too late for use in WW-II, and flew only once, to an altitude of 70 feet.

For some, it’s a symbol of technology that doesn’t work.

There’s plenty of incentive to make sure planes fly safely. But often, there’s no incentive to make sure rain gardens work.  Sometimes, owners don’t go out in the rain to to see if runoff even gets into the garden, before paying the contractor.

One reason why some rain gardens fail is that the runoff simply misses them. In most cases, this could be corrected with a few ridges of asphalt, to redirect the flow.

Metclafe’s Sentry

This is the most ineffective rain garden I've seen.  In this large parking lot, there are long rain gardens between rows of parking, with gaps in the curb to admit runoff.

Despite heavy rainfall, no runoff is entering this rain garden, because of improper grading.

During a heavy downpour, almost all the runoff simply failed to enter the gardens. This was partly because the gardens and their openings weren’t much lower than the gutter. And partly because there was nothing in the gutter to deflect the fast-moving gutter flow towards the garden (below). More photos.

Visitor’s Parking area at MG&E

In 2003, the drainage of this large parking lot became a DNR experiment to test how well a chamber with special filters could clean the runoff, before it escaped to Lake Monona.

They tested the runoff before it entered the filters, and after it came out, to see how well the filters worked.

Early on, the researchers noticed a lot less rainwater was coming out of the filter, compared to what fell on the lot. So they watched the parking area during rain, and "drew a red line" around the area that actually drained to the filters.

MG&E parking lot, site of the filter experiment.

The yellow line outlines the whole lot; the red line outlines the area that actually drains to the filter.

Although the parking area was 1.3 acres, they found that only .91 acres of the lot drained to the filters. The rest spilled out into the bordering streets, bypassing the filter. In other words, because the lot wasn't sloped correctly to capture all the water, only 70% of the water falling on the lot actually went to the filter. For the runoff that actually made it to the device, the filters worked well.

More photos during rain. More on the filter. Scientific report.

Sequoya Commons

I'm a big fan of this large rain garden. It's beautiful, and for the most part, very effective. But runoff coming down the north side of the lot misses the garden, and instead flows out the east entrance to the street (below).

Plugged openings

Other rain gardens fail because the openings become plugged with debris.

Clogged opening for a terrace rain garden near West High School.

Later designs by the City have larger openings.