While this rain garden does a nice job of roughening surfaces and ponding water to slow the flow of stormwater, it is less effective at providing water quality than it could be. Here's why:

WATER QUALITY
The water quality storm is the size of a small, frequent storm that will scour pollutants from the surfaces it passes over. There's also an equivalent water quality storm that happens at the beginning of all storms larger than the water quailty storm. These small, dirty volumes are going to flow through the bottom of the facility. The sides of the facility won't have runoff on their surfaces until the storm has progressed beyond the size of the water quality storm. Scouring of pollutants early in the storm means that subsequent runoff is pretty clean.

Here's the catch. We get our water quality treatment in rain gardens by settling and filtering out total suspended solids (aka TSS and this system does a good job at that) and by decomposition of pollutants with microbes. A number of pollutants, such as hydrocarbons, are food to microbes and while they hang out in the soil, they multiply like crazy on the plant roots. But, this is a rock-lined channel with very few plants in the bottom of the facility where the water quality storm is going to flow.

Here's the good news. The facility is only treating roof runoff, which is much cleaner than runoff from vehicular areas. Regardless of the roofing material, though, we still have to contend with biologicals (bird poop), sediments and other pollutants that are carried on the air and settled on the roof during previous rain events.

NATIVE PLANTS
While there's a lot of controversy around native plants, Green Girl LDS' official position on it is that native plants alone should be used in stormwater facilities containing vegetation. These stormwater facilities almost always have a large storm overflow to a natural downstream waterway and the seeds and rhizomes of the plants in the facility are very easily conveyed downstream with the extra runoff. If the plants are native, this is a benefit to the downstream waterway. If the plants are invasive, they will reduce the habitat value. For instance, for native animals, native plants have been shown to provide more nutrition than non-native plants.

Notice the Yellow Flag Iris in the bottom of the facility. We know that this was intentionally planted here because it appears on the educational signage. This plant is on the Noxious Weed List of Oregon and the folks at Oregon Convention Center know this and

have dutifully been trying to get rid of it for years.

Here's how it happened. (I met with folks from the Oregon Convention Center (OCC)and this is what they told me.) The OCC expanded their building in the early 2000's. The City's new stormwater management manual required a vegetated stormwater facility, so with the best of intentions, they hired the best landscape architect they could to create something that would be multi-functional. At the time, the Yellow Flag Iris was on other noxious weed lists in the country, but not on Oregon's. The landscape architect and the City of Portland both felt that exclusively planting non-natives wasn't necessary and went ahead with some natives and some non-natives. The Yellow Flag Iris took over the bottom of the facility, pushing all other vegetation out. Some years after it was planted, Yellow Flag Iris was added to the noxious weed list.

The photograph you see is a result of the management at the Oregon Convention Center spending years trying to remove the the irises so they can plant something else in there. Yellow Flag Iris creates a dense mat with its rhizomes, which is one of the ways that it pushes and keeps out other plants. (It also compacts the soil as it grows making it a VERY unsuitable plant for facilities that must infiltrate!) Those rhizomes spread very easily and pretty quickly and breaking them and forgetting some pieces in the ground is a recipe for making more Yellow Flag Iris plants, so this plant is a challenge. The OCC has spent thousands of dollars each year in removal activities and continues to up their budget for more removal activities each year and are slowing "getting there".

To this day, many jurisdictions including Portland, are not very concerned about using non-native species in their stormwater facilities, but clearly, they should be.