Here’s an interesting article about an infiltration basin that has failed in its infiltration function and “transitioned” into a wetland. Interestingly, the researchers determined that it still performs a stormwater management function, while also performing ecological functions.

Ecological assessment of a transitioned stormwater infiltration basin

Infiltration basins are stormwater control measures (SCMs) widely employed for urban stormwater management. A transitioned infiltration basin is a failed infiltration basin that has gradually transformed into a wetland- or wetpond-like practice. The transitioned basin was found to effectively control the storm runoff flows and volumes, and improve the discharge water quality, thereby reducing the downstream hydrologic and pollutant loads on most occasions. Qualitative assessment of the site showed presence of wetland and non-wetland vegetation, small animals, and some potential for cultural benefit. The ecological evaluation demonstrated that runoff management and habitat provision in a sub-urban setting enhance the overall functionality of this new type of SCM ecosystem. A functionality assessment guide was developed for assessing infiltration basins considered to have failed. The Level-1 assessment includes visual criteria such as hydrophytic vegetation, hydric soils, hydrologic regime modification, and design check. The rapid assessment plans developed in this study can be applied to determine the ecological and stormwater management functions and benefits of failed/transitioning/transitioned basins, and may be adapted for other similar SCMs.

A lot of us engineers assume that green infrastructure will have a useful service life and then eventually fail. This is in keeping with the idea of infrastructure, which needs constant maintenance to keep it from wearing out, or else eventually wears out and has to be replaced. But green infrastructure is supposed to be a designed ecosystem. Ecosystems can change over time but they don’t exactly wear out, in fact their functions tend to stay stabilize or increase over time. So if we really understand an ecosystem thoroughly and are able to design it, we should be able to anticipate and even control these changes. An example would be planting deep rooted, self-mulching plants that keep the soil of the infiltration basin loose and permeable for the long term. But even if there is a limit to that, you could let it gradually transition to a forested and/or wetland condition in a controlled way over time.

So here’s an idea I have to build streetside rain gardens on the cheap. Take a typical sad, compacted tree pit where a tree recently died or was removed (sadly, very, very common here in Philadelphia). Remove a foot or so of soil, or at least down to a few inches below street level. Throw in a handful of seeds like clover, daikon radish, prairie grasses, horseradish, or anything else aggressive, deep-rooted, perennial or self-seeding. Throw in an acorn or other tree seed (why not pick something edible). Now wait a year or two for all this to grow and begin to loosen up the soil and create some organic matter. When the plants have established themselves, go back and cut a hole in the curb to let water in. Gradually, the tree will grow and shade out the smaller plants. With this system, you get a functioning ecosystem in a few years with maybe $5 worth of seeds, and a lot of patience. If it doesn’t grow, you can afford to throw in another $5 worth of seeds.