Detention and retention are the two fundamental approaches to stormwater quantity management. They sound similar but serve different purposes, have different design criteria, and require different maintenance. Confusing the two leads to systems that do not meet the permit requirements and sites that either flood or discharge too much runoff.

Detention: Store and Release

A detention system temporarily stores stormwater during a storm event and releases it at a controlled rate over a period of hours or days. The stored volume eventually leaves the site — it just leaves more slowly than it arrived. The purpose is to reduce the peak discharge rate to match or approximate the pre-development rate, preventing downstream flooding and erosion.

How it works: Stormwater flows into a basin, vault, or tank through an inlet. The outlet structure has a restricted opening (orifice, weir, or flow control device) that limits the discharge rate. As water flows in faster than it can flow out, the water level rises, storing the excess volume. After the storm passes and inflow decreases, the stored water drains out through the restricted outlet.

Sizing: Detention volume is determined by hydrologic analysis — typically a comparison of the pre-development and post-development hydrographs for the design storm (10-year, 25-year, or 100-year depending on the jurisdiction). The required storage volume is the area between the inflow hydrograph and the controlled outflow hydrograph. For a typical commercial site, detention volumes range from 0.5 to 2.0 inches of storage per acre of contributing impervious area.

Common types:

• Surface detention basins — open basins that flood during storms and drain between storms. Can be integrated into landscape areas, playfields, or parking lots (parking lot detention). Lowest cost per cubic foot of storage.
• Underground detention vaults — precast or cast-in-place concrete boxes below grade. Used on urban sites where surface area is not available. More expensive but invisible.
• Underground chamber systems — modular plastic arch chambers (StormTech, Cultec) or pipe arrays installed below parking lots or landscape areas. Cost-effective for large volumes.
• Oversized storm drain pipes — the storm drain pipe itself is upsized to provide inline storage. The outlet is restricted at the downstream end. Works for small projects with limited space.

Retention: Store and Keep

A retention system captures stormwater and prevents it from leaving the site. The water is permanently removed from the runoff stream through infiltration into the ground, evapotranspiration by plants, or harvesting for reuse (irrigation, toilet flushing). No controlled release to the storm drain system.

How it works: Stormwater flows into a facility (bioretention basin, infiltration trench, dry well, rain garden, or cistern) that either infiltrates the water into the subgrade, stores it for reuse, or holds it until it evapotranspires. The facility has no positive outlet to the storm drain system for the design storm. An overflow for larger storms is typically provided to prevent uncontrolled flooding.

Sizing: Retention volume is based on the "design capture volume" specified by the permit — typically the runoff from the 85th percentile storm (about 0.75 to 1.0 inches of rainfall in California) or the first flush volume. The facility must be able to absorb, infiltrate, or store this volume between storm events.

Common types:

• Bioretention basins — landscaped basins with engineered soil media. Water infiltrates through the soil and into the subgrade (if soil is permeable) or is collected by an underdrain (if soil is clay). Bioretention with an underdrain provides water quality treatment but not true retention.
• Infiltration trenches and dry wells — underground gravel-filled trenches or chambers that store water and allow it to infiltrate into permeable subgrade soils.
• Cisterns and rain tanks — above-grade or below-grade storage tanks that capture roof runoff for irrigation or non-potable indoor use. True retention because the water is consumed rather than discharged.
• Pervious pavement — with a full-infiltration design (no underdrain), pervious pavement retains the water it captures in the aggregate reservoir until it infiltrates.

Which One Does Your Permit Require?

This varies by jurisdiction and permit:

Design Considerations

For Detention

• The outlet structure design is critical. An improperly sized orifice can either over-detain (holding water too long, not draining between storms) or under-detain (releasing water too quickly, not reducing the peak).
• Maintenance access must be provided. Underground vaults need manholes large enough for equipment access. Surface basins need vehicle access for sediment removal.
• The emergency overflow must be designed to handle storms larger than the design storm without flooding adjacent properties or structures.

For Retention

• Soil infiltration rate is the governing parameter. If the native soil cannot absorb the design volume between storm events (typically 48 to 72 hours), the facility will not drain and will not be ready for the next storm.
• Seasonal high groundwater must be at least 3 feet below the bottom of the facility to prevent groundwater mounding and contamination.
• Retention facilities in contaminated soils or industrial sites may not be permitted because infiltrated stormwater could mobilize contaminants.

Cost Comparison

Detention is generally less expensive than retention because it does not depend on soil conditions and can use standardized manufactured products (vaults, chambers). Retention requires favorable site conditions (permeable soils, deep groundwater) and may require oversized facilities if soil infiltration rates are marginal.

However, retention can reduce or eliminate storm drain connection fees, reduce the size of downstream pipes, and provide landscape amenity value (bioretention basins). The total project cost comparison should include the downstream infrastructure savings, not just the BMP cost in isolation.