Bioretention areas (also sometimes called rain gardens) are a very useful tool in the toolbox of stormwater treatment BMPs. They provide a high level of pollutant removal and runoff volume reduction, and, if properly designed, require little maintenance.
Bioretention was developed to mimic the hydrology of a natural forest. It consists of a shallow (typically 4-8 inches) basin with 3 inches of mulch and a variety of selected plants on the surface that can accommodate periodic flooding and drought. Under the mulch is a layer of engineered soil mix (typically 12-36 inches). The basin collects surface runoff and filters out nitrogen and phosphorus (nutrients for plants, but pollutants in stormwater) as water passes through the engineered soil mix. Water is absorbed by surrounding soils or is collected by an underdrain. The plants convert the nitrogen and phosphorus collected in the engineered soil mix into woody plant matter and leaves.
If designed and built correctly, the water in the bioretention basin will drain completely in 20-40 minutes after a rainfall and be a beautiful naturally planted area. If not, the basin holds water for longer periods of time, the plants will die, and it becomes a maintenance issue for the owner and possibly a subject of regulatory non-compliance.
I’ve been designing bioretention for 15 years and have learned a great deal from following my projects over time and checking bioretention basins designed by others wherever I see them. Here are my top three lessons learned:
1. It is one tool in a big stormwater toolbox
Often, other designers suggest a “rain garden” at the very beginning of a project. I can appreciate their enthusiasm for stormwater, but bioretention is only one tool in a growing toolbox. Currently in Virginia, there are over 32 types of approved BMPs, counting subtypes. Selecting bioretention at the start of the project is like being a contractor whose only tool is a screwdriver.
At the start of a project, it is crucial to understand the site and the stormwater regulatory requirements. Only then, can you design the best stormwater approach for the site that meets the regulatory requirements cost effectively from both a construction and long-term maintenance standpoint.