Infiltration Basin

What Is an Infiltration Basin?

An infiltration basin is a stormwater management system designed to collect rainwater runoff from impervious surfaces such as roadways, rooftops, and parking lots and infiltrate it through the soil rather than allowing it to leave the site. Infiltration practices are a broad category of BMPs that includes most systems designed to return stormwater to the ground, and they can take many different forms depending on the site conditions and design requirements.

When rainwater falls onto impervious surfaces, it picks up pollutants including oils, sediments, heavy metals, and nutrients. Infiltration systems ensure those pollutants are filtered as the water moves through the soil, keeping groundwater clean and safe. Most systems include a pre-treatment component to trap larger sediments, a storage area (above or below ground) that holds water in a stone reservoir, and a sand layer at the bottom to provide final filtration as the water exits the system.

The specific design of an infiltration practice depends on the quality and type of soil at the site, the layout of the contributing impervious surfaces, topographic factors such as slope and elevation, and the volume of runoff that needs to be managed. Related stormwater systems that use infiltration principles include bioretention facilities, permeable pavement systems, and wet ponds, each of which is covered in its own section of this Learning Center.

How Do I Inspect My Infiltration Basin?

Infiltration systems should be inspected regularly to ensure they remain healthy and functioning efficiently. The inspection should begin with any pre-treatment components. Look for sediment buildup or debris that could clog the pre-treatment and prevent it from working properly.

After heavy rain, observe the drawdown rate of the system for two to three consecutive days and record the water levels each time you check. The water level should drop significantly as time passes. If standing water remains after the third day, there is likely a clogging issue within the system.

For above-ground systems, check for sediment buildup in the area where rainwater enters the system. Ensure that nearby vegetation is not creating an obstruction directly above the system. Leaf litter, fruits, and other vegetative materials can cause clogging. Check the health of any vegetation the system uses and confirm that overall cover density is 90% or higher. If the vegetation is unhealthy or sparse, replanting will be necessary. Look for weedy growth on the surface, which may indicate sediment deposition or clogging. Inspect internal and external side slopes for evidence of erosion or slumping.

For underground systems, verify that all valves, manholes, and access locks can be opened properly. Inspect the maintenance access and observation well for signs of debris or sediment buildup. Check pipes and other conveyance structures to ensure water is flowing properly and that sediment is not being carried into the system. Remove any sediment you find.

How Do I Maintain My Infiltration Basin?

Infiltration practices come in many varieties and require proper care to continue functioning as intended. Maintenance tasks vary by system type and season, but the general framework includes the following:

As needed: Replace pea gravel, topsoil, and top surface filter fabric when clogged. Mow vegetated filter strips as necessary and remove the clippings.

Quarterly: Ensure that the contributing drainage area, inlets, and facility surface are clear of debris and that the contributing drainage area is stabilized. Remove sediment and oil/grease from pre-treatment devices and overflow structures. Repair undercut and eroded areas at inflow and outflow structures.

Annually: Clean out accumulated sediment from the pre-treatment cell. Have a licensed professional perform a comprehensive inspection and document the findings.

Exact Stormwater performs thorough forensic investigations when infiltration systems are underperforming. Our team determines the root cause of problems and creates targeted remediation plans to restore system function.

How Does an Infiltration Basin Get Installed?

Installation begins with excavation of the land according to the design dimensions. Filter fabric is then lined across the sides of the trench. This fabric serves primarily to prevent outside materials from migrating into the stone reservoir, which could cause clogging over time. Natural soils are used to fill any gaps between the fabric and the sides of the excavation.

The system layers are installed from the bottom up, starting with a sand filtration layer. Before adding stone, the observation well is anchored to the bottom of the excavation. Stone is then added in one-foot increments to build the storage reservoir. Once the practice is filled with stone, either gravel or sod and native grasses are used to top it off and help it blend with the surrounding landscape.

Exact Stormwater provides professional installation services for all types of infiltration practices.

Frequently Asked Questions