BIORETENTION/RAIN GARDENS

What Are Bioretention Facilities and Rain Gardens?

A bioretention facility, sometimes called a rain garden, is a nature-inspired stormwater management system designed to capture and treat stormwater runoff. These systems function as engineered mini-ecosystems that use specialized soil, organic matter, and selected vegetation to filter pollutants from stormwater before it enters the groundwater or downstream waterways.

A typical bioretention facility consists of a shallow depression filled with a carefully designed soil media mix. When it rains, stormwater runoff flows into the facility, either through surface grading or through a network of pipes. The depression acts as a capture zone, holding the water temporarily while it percolates through the soil media. Physical and biological treatment processes remove pollutants as the water filters down through the layers.

Plants play a critical role in the bioretention process. Their root systems stabilize the soil media and create pathways for water infiltration. As plants grow, they absorb nutrients from the water, further improving water quality. The vegetation also provides habitat value and visual appeal, making bioretention facilities an attractive component of a site's landscape.

In heavy rainfall, bioretention facilities are designed with an overflow mechanism to release excess water in a controlled manner, preventing flooding and erosion while still allowing for natural infiltration during the initial stages of the storm.

Bioretention systems are related to other infiltration-based stormwater practices, including infiltration basins and permeable pavement systems, all of which manage stormwater by returning it to the ground rather than piping it off site.

How Do I Inspect My Bioretention Facility?

Bioretention facilities require regular inspections to remain effective. Early identification of problems is critical to preventing small issues from becoming costly repairs. Here is what to look for during an inspection.

Walk around the bioretention area and look for obvious signs of trouble: standing water that has not drained within 48 hours, erosion within the facility or on its banks, and dead or damaged vegetation. Check for mulch washouts, as well as sand or sediment buildup, especially around inflow points. These are potential signs of clogging across the surface of the system.

Evaluate the health of the vegetation. Look for signs of stress such as wilting, discoloration, or dieback. Dead plants should be replaced promptly. Weeds and invasive species should be removed. Vegetated cover of 75% to 90% throughout the facility is necessary to maintain proper function.

Inspect the outlet structure to verify there is no accumulated debris blocking water flow. Check the overflow mechanism to make sure it is clear and functioning properly so that excess water is released appropriately during heavy rainfall.

How Do I Maintain My Bioretention Facility?

Bioretention facilities are nature-based systems that require ongoing care to function as designed. Common maintenance issues include erosion and scouring within the facility, standing water that fails to drain, dead or stressed vegetation, and clogged inlets or outlets.

If your system is not draining properly or appears to be underperforming, Exact Stormwater can perform a forensic investigation to determine the root cause. Common causes include clogged soil media, failed underdrain systems, and sediment accumulation in the pre-treatment area. Our team will diagnose the problem and create a targeted plan to restore function.

For bioretention facilities with bank erosion or slope instability, HydroTurf turf reinforcement mats provide a durable solution for slope stabilization that supports vegetation growth while preventing further erosion.

How Does a Bioretention Facility Get Installed?

Before a bioretention system can be installed, the contributing drainage area (the area where runoff rainwater will enter the system) must be stabilized with vegetation. Then the excavation for the bioretention area is made according to the design depth and dimensions.

Geotextile fabric is lined across the sides of the excavation. The stone layer is installed first, from the bottom, with the amount based on the design specifications. During this process, the underdrain pipe is installed as well. The engineered soil media, consisting of a mix of soil, sand, and organic material, is what makes up most of the underground portion of the system. This media is installed in 12-inch increments until the desired elevation is reached, with the process spread over several days to allow proper settling.

The above-ground area is prepared for vegetation by creating planting holes. Surface cover, usually mulch, river stone, or turf, is applied to top off the main filtering area. Finally, the specified vegetation is installed. Plants should be actively watered during dry periods for two months after installation to establish root systems.

Frequently Asked Questions