Detention &Retention Systems

Detention systems temporarily store stormwater and release it slowly over time through a controlled outlet—the water eventually leaves the site. Retention systems store water permanently or until it infiltrates into the ground—no direct surface discharge. Detention manages peak flow rates; retention manages total volume. Many systems can function as either depending on design: adding an outlet creates detention, removing it and relying on infiltration creates retention.

Underground detention offers several advantages: maximizes usable land by placing storage beneath parking lots, buildings, or landscaping; eliminates safety concerns and liability of open water; reduces mosquito breeding habitat; avoids aesthetics issues of dry or poorly maintained ponds; provides consistent performance regardless of weather; and often reduces long-term maintenance costs. The higher upfront cost is offset by land value preservation, especially in commercial and urban areas.

Modular chambers are large-diameter arch-shaped structures installed in rows within an excavated area. Water enters through inlet pipes, fills the void space within and between chambers, and exits through outlet pipes (detention) or infiltrates through the stone base (retention). The open-bottom design provides 95%+ void space compared to 30-40% for pipe systems, allowing smaller excavations for equivalent storage volume.

Required storage volume depends on local regulations, drainage area, impervious coverage, pre/post development runoff, soil conditions, and outlet restrictions. Common methods include: storing the difference between pre- and post-development runoff for design storms (2-year, 10-year, 100-year); capturing first-flush water quality volume (typically 1" over impervious area); or meeting specific local detention ordinances. A civil engineer calculates requirements based on site-specific hydrology.

Yes, most chamber systems are designed for H-20 or HS-20 traffic loads when properly installed with appropriate cover depth (typically 12-24" minimum depending on product). Chambers can be installed under parking lots, driveways, roads, and fire lanes. Follow manufacturer specifications for minimum cover, backfill requirements, and compaction to ensure structural performance under traffic loads.

Maintenance includes: regular inspection of inlet/outlet structures; cleaning inlet catch basins and sediment forebays that capture debris before entering chambers; periodic inspection of chamber interiors via access points; removing accumulated sediment when it reaches specified levels (typically every 5-10 years); and maintaining vegetation over the system. Many systems are designed for jet-vac cleaning access. Proper upstream pretreatment significantly reduces internal maintenance needs.

An infiltration gallery is a detention/retention system designed specifically for groundwater recharge. Stormwater enters chambers with open bottoms resting on permeable native soil or engineered stone beds. Water infiltrates into the ground rather than discharging to surface waters. Galleries require suitable soil percolation rates (typically >0.5 in/hr) and adequate separation from groundwater and bedrock. They provide water quality treatment through soil filtration.

Detention systems improve water quality through several mechanisms: sedimentation as flow velocity slows in chambers; oil and grease separation in calmer water; pollutant uptake by microorganisms in biofilms; infiltration filtering for retention systems; and upstream pretreatment in catch basins and sediment forebays. Some systems incorporate specific water quality units, isolator rows, or filter cartridges for enhanced treatment.

Key factors include: available footprint and depth for installation; soil type and percolation rate (critical for infiltration systems); groundwater level (minimum separation required); proximity to foundations, utilities, and property lines; traffic loads above the system; inlet and outlet locations and elevations; and local regulatory requirements. A geotechnical investigation typically precedes design to characterize subsurface conditions.

Yes, detention chambers can store rainwater for later use in irrigation, toilet flushing, cooling, or industrial processes. The system design adds: a waterproof liner to prevent infiltration loss; submersible pumps for water retrieval; treatment as required for intended use; overflow connection to stormwater system; and water level monitoring. This dual-purpose approach provides stormwater management while creating a supplemental water supply.