Using Bioretention to Treat More than Five Acres

One of the most important aspects of water resources engineering is designing stormwater management (SWM) ponds for quality and quantity control of stormwater runoff. As developments grow, new pavement and clearing of forested areas increases the amount of nutrients in stormwater runoff and the total volume. Stormwater quality typically refers to the treatment of nutrients within runoff, like phosphorus and nitrogen, which can be detrimental to natural water sources. Stormwater quantity refers to the amount of runoff that will enter a system and need to be detained to a lower flow rate.

Defining a Bioretention

A bioretention is a type of best management practice (BMP) that can be used for water quality and quantity. Bioretention practices have a shallow ponding depth of 6 inches to 12 inches and operate by filtering runoff through the filter bed, which consists of sand, soil, and organic material. There are many different scales of design of bioretention: micro-bioretention for small developments such as a single-family residential development, urban bioretention for densely populated urban areas with small areas for BMPs, and bioretention basins for larger areas of treatment. The focus of this article is on bioretention basins.

By following and applying creativity to the stormwater specifications, there are limitless options to optimizing stormwater management facility design.” Annie Tkacik

Bioretention basins are useful because they provide a high total phosphorus (TP) removal efficiency in comparison to other types of stormwater management facilities. In Virginia, for example, the TP removal for a Level 2 bioretention pond is 90%, and from a water quality treatment perspective, is the best option for a high pollutant removal efficiency. However, the Virginia Clearinghouse states that the maximum drainage area to a bioretention is five acres with a maximum impervious cover of 2.5 acres, which is less advantageous from a water quantity perspective.

So, what is the best option if you want to design a bioretention basin with a drainage area over five acres?

Understanding the Caveats of a Unique Design

Bioretention basins can be designed to handle more than five acres and a maximum 2.5 acres of impervious cover, with a few caveats. First, the Virginia Clearinghouse states that if hydraulic considerations are addressed to manage large drainage areas, then there are case-by-case instances to adjust recommended maximums. This means that exceptions can be made in constrained areas. Second, bioretention basins can be expanded to provide quantity control with a detention cell and take in larger drainage areas. In Figure 9.6 of Specification No. 9 in the Clearinghouse, a detailed bioretention facility is shown in the upper shelf of a detention pond. Using this option, a drainage area greater than five acres may be used for the stormwater facility and treated for quality and quantity, as long as the other design criteria are followed.

The Virginia Department of Environmental Quality has accepted bioretention basins with more than 10-acre to 40-acre drainage areas, but it’s not as well known to the design community. By splitting up the quality and quantity components, you can design for quality by only claiming treatment for five acres of drainage area and 2.5 acres of impervious area, while detaining the entire drainage area with an added detention cell. Understanding the design standards and requirements will allow more flexibility for unique cases.

Outside of bioretention basins, there are other opportunities to use components of different stormwater management facilities together. For example, Level 2 wet ponds are required to have a wetland cell, which incorporates design elements from the constructed wetland with a wet pond. By following and applying creativity to the stormwater specifications, there are limitless options to optimizing stormwater management facility design.