Stormwater management is a key factor in every land development project. During the development of the land, the natural cycle of water is altered and disturbed. Rainfall that would normally infiltrate the soil can now run along the surface and in a different direction than before. The developed conditions of the site can also increase the amount of pavement, building structures, and other surfaces that are impervious to stormwater. We recently talked about stormwater runoff and its effects here. To mitigate these effects, engineers develop stormwater management methods. One such method is the construction of bioretention areas. Read on to learn how bioretention areas work to mitigate stormwater issues and the pros and cons of choosing this method.

How Do Bioretention Areas Work? Bioretention areas are shallow basins designed to infiltrate water into the soil and slow down the release of stormwater from the project site. These basins can be designed with specific engineered soils to enhance filtration and are required to be planted with some type of vegetation or landscaping. Bioretention areas also typically include a ponding area and organic or mulch planting media. The ponding area has a maximum drain time of 24 hours, and the usual strategy is to provide a depth of 9-12”. Finally, a bioretention area will achieve an 85% Total Suspended Solids (TSS) pollutant removal and provide water quality volume as well.

Designed to treat up to a maximum drainage area of five acres, smaller land development projects can utilize bioretention to achieve their total required stormwater volume. Larger projects can use multiple bioretention areas or pair them with other stormwater practices.

Diagram of a bioretention area designed by the Foresite Group Greenspace team.

Pros and Cons of Bioretention Areas As with anything, there are pros and cons to every type of stormwater practice, and an engineer must weigh the options against a site’s needs. As mentioned, these areas are commonly used in smaller projects since they can serve as the standalone stormwater method. Urban land development projects will sometimes use bioretention areas to add greenspace and natural features to a site in addition to accomplishing their stormwater mitigation goals.

Some disadvantages of bioretention areas to consider include cost, high slopes, and the maintenance of the engineered soils. Between the engineered soils, landscaping and vegetation, and the long-term maintenance costs, the total fee can make some projects look for less expensive stormwater options. Another challenge to using a bioretention area comes when a site has higher/steeper slopes. The recommended average slope for a bioretention area is 5%, but they can be designed up to a maximum of 20% slope. Sites with higher slopes than this will need to consider other options.

Our unique Greenspace team is home to both civil engineers and landscape architects. Bioretention areas are a great example of ways we work together to provide a unique product to our clients. While our civil engineers achieve the stormwater requirements, our landscape architects can get creative with the landscaping and plantings within the bioretention area. Not all stormwater management practices offer this opportunity for both disciplines to showcase our designs in collaboration.

A bioretention area is ideal for small project sites where there is a need for more green and natural space. The area can remove suspended solids and achieve required water quality and stormwater values as well. While the cost can sometimes be steep, it is often worth it to brighten up a small site with soothing natural features that only hint at their stormwater management functionality.

About Foresite Group

Foresite Group is a multidisciplinary engineering, planning, and consulting firm providing services to public and private sector clients nationwide. Our team’s collaborative process results in creative products and services that help our clients achieve their goals. Our team takes pride in enhancing and developing the cities and communities where we live, work, and raise our families.