What is a bioretention system and how does it help control stormwater runoff?

Bioretention is a stormwater management practice that harnesses natural processes to treat and manage stormwater runoff from impervious surfaces such as rooftops, driveways, and parking lots. If you live in a city, you may have seen many of these types of systems. By utilizing a combination of vegetation, soil, and beneficial microorganisms, bioretention systems capture, filter, and break down pollutants before they can enter local waterways. But perhaps even more important is their ability to both decrease localized flooding and prevent sewer overflows (aka CSOs and SSOs).

Bioretention systems are among the most well-recognized forms of low impact development (LID), capable of fitting into various landscape styles and utilizing multiple stormwater treatment mechanisms. They are designed to promote infiltration and filtration, reducing both the volume and intensity of stormwater flow. As a result, bioretention improves water quality, but it’s also a great watershed protection technique to help mitigate the negative impacts of urbanization. Through the strategic use of plants and engineered soils, bioretention systems can help cities manage stormwater runoff, support healthy ecosystems, and provide a sustainable management practice for urban environments, which are almost always filled with too much concrete and not enough nature.

How bioretention areas are constructed

Bioretention areas are also sometimes called bioswales or bioretention cells (and sometimes rain gardens, although they are semantically different). Bioretention areas are essentially shallow engineered depressions that are designed to capture and infiltrate stormwater runoff from impervious surfaces like rooftops, driveways, and parking lots. Bioretention is often integrated into parking lot islands or medians to manage runoff from these areas.

The construction of bioretention areas involves several key steps, including site preparation, excavation, soil installation, soil compaction mitigation, and careful sequencing to ensure system functionality and longevity. In addition to promoting filtration and infiltration, and biological degradation to effectively treat stormwater, natural processes within the bioretention area help alleviate soil compaction and improve stormwater runoff quality. Drainage designers incorporate layers of engineered filter media, mulch, and vegetation – often native plants with deep roots – that are planted to maximize pollutant removal and water absorption, support healthy plant growth, improve soil structure, stabilize soil particles, and increase water storage capacity, thereby enhancing pollutant removal.

The importance of choosing the right soil

Proper bioretention design carefully considers site conditions such as soil composition, permeability, slope, ponding area, and drainage to optimize infiltration and treatment performance. Bioretention systems must be designed and tested under actual field conditions to ensure optimal performance in real-world environments.

The soil media is typically a carefully blended mix of sand, topsoil, and organic content (eg, compost), with an appropriate clay content and soil amendments to ensure both effective filtration and adequate infiltration rates. The specific composition of the filter media is tailored to the site’s conditions, including the types of impervious surfaces draining to the system, the expected volume of stormwater runoff, and the targeted level of pollutant removal.

Proper excavation techniques, such as raking and ripping, are critical during construction to prepare the site and avoid soil compaction, which can reduce infiltration rates. Pretreatment features like grassed filter strips or sediment forebays are often included to capture coarse sediment and debris before water enters the bioretention area, reducing clogging and extending system lifespan.

Plant selection for bioretention areas

Choosing the right plants is fundamental to the success of any bioretention system. Plant selection goes beyond aesthetics, and native plants are typically favored because they are well adapted to local soil and climate conditions. These plants also offer additional benefits, providing habitat for pollinators and enhancing the visual appeal of urban landscapes. Deep-rooted native grasses and wildflowers are especially valuable in bioretention areas, as their roots help improve soil structure, promote infiltration, and facilitate the uptake of water and nutrients.

When selecting plants, it’s important to consider factors like soil type, moisture availability, and sunlight exposure to ensure that the chosen species will thrive and contribute to pollutant removal. A diverse mix of plants can further enhance system resilience and performance, making the bioretention area a sustainable and attractive feature in the landscape.

How bioretention systems mitigate pollution

These systems are designed to manage the “first flush” of runoff, which carries the highest pollutant loads, things like heavy metals, pathogens, nutrients, and grease. It works by temporarily ponding water and allowing it to percolate through the filter media to improve water quality, where pollutants such as heavy metals, suspended solids, and excess nutrients are captured and broken down. Nutrient removal is especially important for water quality improvement and environmental compliance. These systems can also help remove phosphorus and nitrogen, which are often overused agriculturally and are thus present in a lot of stormwater runoff.

Bioretention systems are highly effective at removing heavy metals from stormwater runoff, thanks to a combination of physical, chemical, and biological processes. The bioretention media acts as a filter, adsorbing and retaining heavy metals such as lead, copper, and zinc. Plant roots and soil microorganisms further enhance this process by facilitating biological degradation and uptake of contaminants.

What they call these systems around the world

Bioretention systems are considered key watershed protection techniques in many technical manuals and environmental guidelines. Of course, these guidelines differ around the world and are even talked about very differently by water professionals in different countries:

• US: The Environmental Protection Agency (EPA) is generally considered the authority on how to implement bioretention systems in the US. These systems are often referred to as LiD (Low-impact Development) strategies, but most construction and drainage professionals in the US use the term BMPs (Best Management Practices) – the preferred EPA term – to talk about the actual structures themselves.
• Australia: Down under, they sometimes call them simply “biofilters” or refer to them with the acronym WSUDs (Water Sensitive Urban Design systems). The Government of Western Australia’s Department of Water has a good PDF that details how they should be constructed.
• UK: Perhaps the best term for these systems come from the UK: The term SuDS (Sustainable Design Systems) has a great way of making them sound both water-centric and more natural. They also sometimes call them “bioretention basins” or just “biobasins”. The UK government offers comprehensive regulations, although they may differ in Scotland, Ireland, and Wales.

Components of a bioretention system

A bioretention system is composed of several essential layers and features that work together to maximize its effectiveness. At the surface, a vegetation layer with native plants serves to stabilize soil particles, absorb water and remove nutrient, and support healthy plant growth.

Beneath the vegetation, the soil media layer acts as a critical filter, trapping pollutants and providing the nutrients necessary for robust plant development. This layer must be carefully engineered to balance drainage and nutrient retention, ensuring optimal conditions for both filtration and plant health.

Below the soil media, a drainage layer, typically featuring a perforated underdrain, facilitates the movement of water through the system, preventing erosion and maintaining proper moisture levels. To further enhance the system’s performance, materials such as mulch and compost can be incorporated, helping to retain moisture, suppress weeds, and boost the breakdown of pollutants.

Together, these components create a bioretention system that is both effective and adaptable to a variety of site conditions.

Fundamental components

• Inlets which may be curb openings (eg, modified curbs, spillways), pipes, road or side inlet catchbasins, trench drains, which may already be pre-fabricated and modular.
• A surface ponding area defined by landscaped side slopes or hardscape structures and the invert elevation of the overflow outlet structure
• A filter bed containing filter media. This can be soil but often includes materials that filter water better than soil and don’t lead to soil compaction.
• A filter bed surface cover layer (eg, mulch and stone)
• Plants of various sorts, either native to the area, ones that are especially good at filtering out pollutants, or ones that attract insects and pollinators.
• An overflow outlet to limit surface ponding and safely convey excess flow to a downstream storm sewer – or on to the next stormwater control system if you link them together.

Optional components

• An underdrain to redistribute or remove excess water and access structures or standpipes for periodic inspection and flushing
• An internal water storage reservoir composed of a reservoir aggregate layer, which may include embedded void-forming structures to minimize depth and conserve aggregate, and organic material derived from untreated wood (aids in dissolved nitrogen removal)
• Monitoring wells installed to the base and screened in the underdrain aggregate to verify and track drainage time
• Filter media additives intended to enhance retention of nutrients, metals, petroleum hydrocarbons and/or bacteria
• An impervious liner can be installed if the bioretention area is designed as a stormwater planter and is in close proximity to buildings or in a tight space in an urbanized environment or if located near pollution hotspots/contaminated soils, or in areas with higher than normal water tables.

How drainage designers create bioretention systems

If you’re ambitious, you can make one of these yourself at home, but in the professional world they must be designed with a heightened level of accuracy because they often are required to follow strict local, regional, or national regulatory guidelines. Typically, drainage designers use software like InfoDrainage to digitally model them well before construction, which provides a way to predict with extreme accuracy what will happen under both normal everyday operation and in extreme unexpected circumstances like a 100-year-flood.

The best drainage design software includes:

1. Standards: Includes universally accepted rainfall standards that are specific to regions or countries
2. Interoperability: Has the ability to work with software and systems that are typically used across the entire building project lifecycle
3. Reporting: Includes comprehensive reporting outputs that can be submitted directly to regulators, for either approval or as a historical record of work completed.

InfoDrainage excels at all three of these and is considered by many in the industry to be the gold standard for drainage design, particularly since it can be directly integrated into Civil 3D with a toolbar ribbon.

How do they do it? Read our customer story that dives into how VHB simplified and streamlined their workflows by adopting InfoDrainage:

• Tailwater conditions: How they took the movement of the tides into account when designing a waterside stadium.
• University goes green: Accomplishing ambitious green infrastructure goals on a landscape with low existing elevations and a complex stormwater challenges.
• Highway design: When a road ages, so does its drainage. When it comes time to refresh or widen the road, drainage engineers sometimes look at how localized flooding has shaped the area to determine if they can preserve a “natural” inflow that has emerged over time.

Read customer story

Daisy-chaining sustainable drainage systems together

Bioretention systems are highly versatile and can be seamlessly integrated with other stormwater management practices to create a comprehensive approach to urban water management. For example, combining bioretention areas with permeable pavement or green roofs can further reduce stormwater runoff and enhance water quality.

This integrated strategy supports watershed protection by minimizing the impact of impervious surfaces (usually concrete) and promoting low impact development throughout urban areas. By incorporating bioretention systems into broader stormwater management plans, communities can reduce reliance on traditional infrastructure, improve environmental outcomes, and foster sustainable development practices that benefit both people and the environment.

Bioretention system inspection and verification

Ongoing inspection and verification are essential to ensure that bioretention systems continue to deliver effective stormwater management and water quality benefits. Routine inspections should include visual assessments of the bioretention cell, checking inlet and outlet structures for blockages, and evaluating the health of the vegetation. Verification goes a step further by monitoring the system’s ability to remove pollutants, reduce stormwater runoff, and support healthy plant growth over time.

It’s important for drainage designers to establishing clear inspection and verification protocols during the design phase – and follow them throughout the system’s lifespan. This helps ensure that a bioretention system remains a reliable tool for environmental protection and stormwater management.

Advantages & disadvantages of a bioretention system

Cost and maintenance: Do not set it and forget it

The cost of bioretention system installation and maintenance can vary significantly depending on factors such as land price, permit fees, equipment, and the type and amount of amendments used, making budget considerations important during project planning.

These are not “set it and forget it” applications. Maintenance is essential to sustain their long-term functionality and prevent issues like clogging and weed invasion. Maintenance should include:

• Regular inspection
• Litter/debris removal
• Replacement of mulch layer
• Vegetation management
• Soil spiking and scarifying

They will last even longer if you take pretreatment measures, such as adding grassed filter strips or sediment forebays. This can reduce the amount of sediment and debris entering the bioretention area and extend its lifespan.

They are very flexible with regard to placement, although adequate land is necessary for installation. And, of course, land availability can directly impact the feasibility of applying them, especially in urban environments. Land use patterns also play a critical role, as different land use types in the surrounding area can influence the design, performance, and regulatory requirements of bioretention systems.

Future research directions for bioretention systems

As urban areas continue to grow and environmental challenges evolve, future research on bioretention systems will play a key role in advancing stormwater management practices:

• Can we use AI to optimize bioretention media composition and soil amendment strategies to maximize pollutant removal and infiltration rates?
• Researchers are also exploring new plant selection criteria that account for changing climate conditions, urban heat island effects, and the threat of invasive species.
• Another important direction is the adaptation of bioretention systems as “stormwater parks” for use in very densely developed urban areas with limited land and high impervious surface coverage.
• There is growing interest in evaluating the effectiveness of bioretention systems in removing emerging pollutants like microplastics and pharmaceuticals from stormwater.

Long-term studies on system performance and maintenance requirements will further enhance the sustainability and effectiveness of bioretention as a stormwater management practice, ensuring these systems continue to protect water quality and urban environments for years to come.

Go deeper into sustainable drainage

In addition to this article on bioretention systems, we have articles on swales, infiltration trenches, cellular storage, and porous pavement – plus plenty of other resources:

• Read the SuDS manual: We have a comprehensive Guide to Representing SuDS in InfoDrainage in accordance with the SuDS Manual Ciria 753.
• Features and functionality: Our documentation shows you how to input specific constraints to accurately model and size your bioretention area.
• Learn the basics: We have an excellent Fundamentals of Drainage Design video for budding civil engineers, urban planners, or just people who are interested in understanding how drainage systems work.
• Try it out: Don’t have a copy of InfoDrainage? We offer a 30-day free trial with no credit card required.
• Get it for free? Are you a student or educator? If so, we have some very good news for you.