Science Inventory

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

Citation:

Oconnor, T. Developing Multiple Lines of Evidence to Decrease Drainage-to-Surface Area Ratio for Effective Stormwater Control Sizing Using Bioretention. Journal of Sustainable Water in the Built Environment. American Society of Civil Engineers (ASCE), New York, NY, 9(1):04022019, (2023). https://doi.org/10.1061/JSWBAY.0001005

Impact/Purpose:

This paper presents data that indicates that oversized green infrastructure stormwater design is inefficient, and designs of smaller controls that use a a larger watershed to surface area ratio remaove carbon and phosphorus and have healthier plants. The paper details a recent plant survey and soil sampling in bioretention cells at the Edison Environmental Center and relates to an earlier study in the same bioretention cells. Proper sizing of green infrastructure design will have improved function of green infrastructure as an source of greenery and pollutant removal mechanism to protect receiving waters. Results should be of interest to EPA program or regional partners, the general public and academics, local communities and municipalities.

Description:

Bioinfiltration units were constructed at USEPA’s Edison Environmental Center to evaluate watershed to surface runoff ratio for sizing of green infrastructure controls. Three sizes of hydraulically isolated 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; typical guidance is for a ratio of ~10:1. Each unit was instrumented for continuous monitoring with water content reflectometers (WCR) and thermistors with data collected since November 2009. The bioinfiltration units were filled with planting media initially comprised of 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 June 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 shrubs planted in these bioretention units was performed. The published results of the combined analyses of moisture content, rainfall and size of shrubs indicated 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 soil chemistry analysis, i.e., loss on ignition and total phosphorous, and plant surveying. As in the previous study, plants did better in the medium (11:1) and small (22:1) bioinfiltration units compared to largest units (5.5:1), and there was greater buildup of carbon and phosphorous. One species of grass that dominated the two largest bioinfiltration units away from the inlet was drought tolerant which was indicative that plants in these units relied on rainfall rather than stormwater runoff. Oversized units do not completely use the control volume and many original plantings were compromised in comparison to plantings in that smaller units that flooded more frequently and achieved greater growth.