Underground Detention Facilities

What is an underground detention system?

An underground detention system is a stormwater management structure installed beneath the ground surface to temporarily store runoff from rain events and release it at a controlled rate. These systems are designed to reduce the volume and velocity of stormwater that would otherwise flood streets, overwhelm drainage infrastructure, or erode downstream channels.

Many urban development projects, including shopping centers, business parks, and multi-family residential communities, create large amounts of impervious surface that generate significant runoff. Underground detention systems allow developers to manage that runoff without sacrificing valuable space above ground for a surface pond. The land above an underground detention system can be used for parking, buildings, landscaped areas, or other productive purposes.

When it rains, stormwater is directed into the underground system through inlet structures and stored temporarily. Once the rain intensity decreases, the stored water is released gradually, usually over a 48-hour period, through an outlet control structure that restricts the flow rate. This controlled release prevents flooding downstream and minimizes erosion.

There are several types of underground detention systems in common use, including corrugated metal and plastic pipes, precast concrete vaults, plastic arched chambers (such as StormTech), and modular plastic box systems (such as R-Tank and RainTank). While these systems differ significantly in appearance and construction, they all serve the same fundamental purpose: storing stormwater runoff underground and releasing it at a controlled rate.

How Do I Inspect My underground detention system?

Underground detention systems are out of sight by design, which makes regular inspections by a licensed professional critical to identifying problems before they lead to clogged drains, flooding, or regulatory fines. Most systems should be inspected quarterly through their first year and at least annually thereafter.

A thorough inspection begins with opening all access points, which may include manholes, hatches, or cleanout ports. If physical entry into the system is required, confined space training and safety protocols must be followed to mitigate the potential dangers of accessing enclosed underground areas. All connection points where pipes enter the system should be inspected, along with the outlet structure.

There are several signs visible from above ground that may indicate a problem with your underground detention system. Persistent pooling of water around the drainage area could suggest an issue with the inlet or outlet. Visible soil erosion or settlement around the system may point to structural problems beneath the surface. Unpleasant odors in the vicinity may indicate stagnant water or pollutant buildup. Debris, trash, or litter accumulating around inlet and outlet points can obstruct water flow and signal a need for cleaning. Changes in the surrounding landscape, such as new construction or grading changes, may also warrant an inspection to assess impacts on the system.

Inspection records should be kept on file and reviewed periodically to identify maintenance patterns. If it has been more than a year since the last inspection, or if you are observing any of the warning signs listed above, it is time to bring in a stormwater professional.

How Do I Maintain My Underground Detention System?

Underground detention systems require periodic maintenance to continue functioning as designed. The most common maintenance needs include removing accumulated sediment, clearing debris from inlets and outlets, and verifying that the outlet control structure is operating properly.

Exact Stormwater has the equipment and expertise to maintain all types of underground detention systems, including R-Tank, RainTank, StormTech, Cultec, concrete vaults, and pipe systems from any manufacturer. Our team performs thorough forensic investigations when systems are underperforming to determine the root cause and develop a targeted remediation plan.

If you have not had an inspection performed, our team of licensed professionals can do one for you. If you have already had an inspection and problems were documented, we can help resolve those issues and create a long-term maintenance plan to keep your system operating efficiently.

How Does an Underground Detention System Get Installed?

Installation of an underground detention system begins with excavation of the designated area according to the engineering design specifications. The dimensions, depth, and slope of the excavation must match the approved plans precisely to ensure the system performs as intended.

Once the excavation is complete, filter fabric is placed along the sides and bottom of the trench to prevent surrounding soil from migrating into the detention system and causing clogging. Inlet structures are installed to direct stormwater runoff into the system, and an outlet control structure is installed to restrict the outflow rate and provide overflow capacity.

The underground detention units are then assembled according to the design, with careful attention to proper alignment and connections between units. The area surrounding the system is backfilled with suitable materials, usually clean stone, with each layer compacted to provide stability and support. Some systems include an outer fabric envelope to contain the stone backfill and prevent soil migration.

Finally, the detention system is connected to the existing stormwater management infrastructure to ensure proper flow both into and out of the system. Exact Stormwater provides professional installation services for all types of underground detention systems.

Frequently Asked Questions