Developing Multiple Lines of Evidence to Decrease Drainage-to-Surface Area Ratio for Effective Stormwater Control Sizing Using Bioretention

Bioretention units were constructed at the US Environmental Protection Agency’s Edison Environmental Center to evaluate drainage-to-surface runoff ratio for sizing of bioretention stormwater controls. Three sizes of hydraulically isolated bioretention units were tested in duplicate with changes in aspect ratio of length from inlet wall by doubling successive length from smallest (3.7 m) to largest (14.9 m) while width remained the same (7.1 m). The watershed areas were nominally the same, resulting in watershed-to-surface area ratios of $5.5:1$ for largest duplicate units, $11:1$ for the middle units, and $22:1$ for the smallest. Each unit was instrumented for continuous monitoring with water content reflectometers (WCRs) and thermistors with data collected since November 2009. The bioretention units were filled with planting media initially comprising 90% sand and 10% sphagnum peat moss by volume and approximately 99% and 1%, respectively, by weight. These units were then planted between May and November of 2010 with a variety of native grasses, perennials, shrubs, and trees that were tolerant to inundation, drought and salt. In late 2012, a survey of the shrubs planted in these bioretention units was performed. The published results of the combined analyses of moisture content, rainfall, and size of shrubs indicated that the smaller units had superior shrub growth due to the more frequent saturation of the root zone as measured by WCR, while the plants in the largest units, particularly away from front wall where runoff entered, potentially relied on direct rainfall only. Starting in 2017, additional monitoring was performed in these units, including chemistry analysis by loss on ignition and total phosphorus of the engineered planting media and an additional survey of the plants. As in the previous study, plants did better in the medium ($11:1$) and small ($22:1$) bioretention units than in the largest units ($5.5:1$), and there was greater buildup of organic matter and phosphorus in the smaller units. One species of grass that dominated the two largest bioretention units away from the inlet was drought tolerant, which indicated that plants in these units relied on rainfall rather than stormwater runoff. Oversized units did not completely use the stromwater control volume, and many of the other original plantings grew slower or were less widespread in comparison to plantings in that smaller units that flooded more frequently and achieved greater growth.

Defining the size of stormwater controls can be difficult because there are often multiple objectives imposed on the final design of these structures, including safety and flooding. Results presented here would indicate that if the objective is to create a bioretention area with healthy vegetation, undersized controls may be acceptable because undersized infiltrating controls will have healthier plantings and infiltrate throughout the storm. For municipalities, this means that rights of way previously thought to be too small to use for infiltrative stormwater controls may be converted to such a purpose. This does not free municipalities from stormwater systems that address flooding and safety design objectives, but demonstrates that increasing plantings in the municipal right of way could help to address stormwater as well as other objectives, like greenhouse gas emissions, urban heat island reduction, and clean air. Distributed bioretention controls that capture part or all the runoff of the smaller, most frequent rainfall events should be incorporated throughout municipalities and into their overall stormwater control systems. If clogging by runoff is a concern, roof runoff may be more appropriate for bioretention, or other measures such as sediment capture or increased maintenance may need to be performed.