Stormwater Retention vs Detention: What’s the Difference and When to Use Each

Civil engineers often need to decide between retention and detention in stormwater plans. These terms are sometimes mixed up in project documents, which can lead to failed inspections, redesigns, and extra costs.

This guide explains the main differences between retention and detention systems, when to use each one, and how modern modular stormwater systems can provide both functions in the same space.

The Core Difference in 30 Seconds

Detention systems temporarily store stormwater and then gradually release it. Water enters, is held, and then drains out at a controlled rate.

Retention systems capture stormwater and retain it on-site, either by allowing infiltration into the ground or by storing it for future use.

Detention acts as a temporary barrier for water, while retention serves as its final destination.

Detention Systems: Delay and Release

Detention systems manage peak flow rates by temporarily storing runoff that enters faster than it can be discharged. The system fills, holds excess water, and then releases it through a controlled outlet.

How Detention Works

A typical underground detention system includes:

• Storage volume sized for the design storm (often 10-year or 100-year events)
• Inlet structures that direct runoff into the system
• Outlet control (orifice plate, weir, or flow restrictor) that limits discharge rate
• Overflow provisions for storms exceeding design capacity

The primary metric is detention time, or the duration water remains before release. Most codes require post-development peak discharge to match or be less than pre-development levels.

When Detention Makes Sense

Detention works well when:

• Local codes focus on peak flow control rather than volume reduction.
• Native soils have low permeability (clay, hardpan) that prevents infiltration.
• The site discharges to an existing storm sewer with capacity constraints.
• Groundwater contamination concerns rule out infiltration.

Large commercial sites often use detention systems to manage high peak flows that could otherwise overwhelm existing infrastructure. For detailed cost analysis of detention options, see our underground detention cost comparison guide.

Retention Systems: Capture and Keep

Retention systems remove stormwater from the drainage network entirely. Water either infiltrates into the ground or is stored for later use, preventing discharge to the storm drain.

How Retention Works

Retention systems take several forms:

Infiltration-based retention allows captured water to percolate into surrounding soils. This requires:

• Adequate soil permeability (sandy or gravelly soils work best)
• Sufficient setback from foundations, property lines, and the water table
• Pretreatment to remove sediment that would clog the infiltration surface

Storage-based retention holds water in tanks or cisterns for irrigation, toilet flushing, or industrial processes. This requires:

• Waterproof containment (lined systems, sealed tanks)
• Pumps and distribution infrastructure for reuse
• Treatment appropriate for the intended use

When Retention Makes Sense

Retention fits projects where:

• Regulations require volume reduction or zero discharge.
• Soil conditions support infiltration (percolation rates above 0.5 in/hr)
• Water reuse offers operational savings (large irrigation demands, process water needs)
• LID/green infrastructure credits reduce other site requirements

Many jurisdictions now require retaining the first inch of rainfall on site, as it typically contains the highest concentration of pollutants.

Side-by-Side Comparison

The Hybrid Approach: Detention with Partial Retention

Most projects benefit from combining detention and retention in a single system rather than relying on only one method.

Detention with infiltration captures the design storm, allows a portion to infiltrate, and slowly releases the excess. This satisfies both peak flow and volume reduction requirements.

The math works like this: if native soils can infiltrate 2 inches of water over a 72-hour drain-down period, and the design storm produces 4 inches of runoff, half the volume is retained, while the other half is detained and released.

This hybrid approach can make the whole system smaller. Instead of sizing detention and retention separately, you can count infiltration during detention toward volume reduction.

Underground Modular Systems: Flexibility Built In

Traditional detention systems use concrete vaults or excavated basins, while retention systems use infiltration trenches or ponds. Each method has specific limitations:

• Concrete vaults offer no infiltration—pure detention only.
• Open basins consume surface area and create safety/maintenance concerns.
• Infiltration trenches clog quickly without robust pretreatment.

Modern geocellular modules provide a versatile solution. These plastic structures with significant open space can be wrapped in various materials to meet specific requirements:

• Impermeable geomembrane for pure detention (waterproof containment)
• Permeable geotextile for infiltration-based retention
• Partial wrapping for hybrid detention/retention

The same module can be used in different ways depending on how it is installed. For example, a 10,000-gallon system might have an impermeable wrap on three sides and the bottom, and a permeable fabric on the fourth side next to soils that allow infiltration.

Structural Advantages

High-quality modular systems can handle heavy loads. AASHTO H-20 rated modules can support fire truck traffic when installed under parking lots, unlike infiltration trenches and plastic chambers.

The high void ratio (typically 95%+) means more storage in less excavation. A system storing 10,000 gallons of water might occupy only 1,400 cubic feet of space, compared to 2,000+ cubic feet for gravel-based alternatives.

Design Considerations by System Type

Sizing Detention Systems

Detention sizing starts with hydrology:

1. Calculate pre-development runoff for the design storm.
2. Calculate post-development runoff for the same storm.
3. Determine the required peak reduction.
4. Route the hydrograph through the proposed outlet to find the needed storage.

Most jurisdictions specify the design storm event (such as a 10-year, 25-year, or 100-year event) and the permitted release rate. Some also require modeling multiple storm events to ensure system performance under various conditions.

Sizing Retention Systems

Retention sizing depends on the regulatory driver:

For infiltration credits, calculate the volume that will infiltrate during the drain-down period:

Infiltration Volume = Infiltration Rate × Bottom Area × Drain-down Time

If infiltration cannot handle all of the design volume, the rest needs to be managed with detention or overflow options.

For water reuse, size storage to capture the design storm while considering demand patterns. A system collecting roof runoff for irrigation should have sufficient capacity to meet needs during dry periods and prevent overflow during wet periods.

Site Assessment Checklist

Before committing to detention, retention, or a hybrid approach:

• ☐ Review the local stormwater ordinance for peak flow vs. volume requirements.
• ☐ Conduct soil borings to determine infiltration potential.
• ☐ Check setback requirements from structures, property lines, and wells.
• ☐ Verify seasonal high groundwater elevation.
• ☐ Assess contamination risk (is infiltration appropriate here?)
• ☐ Calculate water reuse demand if retention for reuse is considered.
• ☐ Evaluate maintenance access and long-term obligations.

Common Mistakes to Avoid

Do not assume all underground storage qualifies as detention. Some jurisdictions use “detention” to refer to any underground storage. Always review the specific requirements. If volume reduction or infiltration is required, retention is necessary, not just storage.

Do not forget about changes in groundwater during the year. An infiltration system that works in summer might fail if winter groundwater rises into the storage area. Always design for the highest expected groundwater level.

Undersizing pretreatment. Retention systems that rely on infiltration require clean water. Sediment buildup in the infiltration zone is a primary cause of failure. Upstream treatment, such as sediment forebays, filter strips, or hydrodynamic separators, significantly extends system life.

Do not forget about maintenance access. Underground systems need to be inspected and cleaned from time to time. Plan access points before you finish the system layout.

Installation Best Practices

Whether constructing detention, retention, or hybrid systems, installation quality is critical to long-term performance.

Foundation preparation matters more than most contractors realize. Differential settlement cracks the geomembrane liners and misaligns outlet structures. Compact subgrade to 95% proctor density minimum.

Geotextile and geomembrane installation requires trained crews. Seams are the weak point—field-welded seams should be tested before backfill covers them.

Backfill placement in lifts with proper compaction prevents voids that can later cause surface settlement.

For the complete installation sequence, see our submittal-to-closeout installation guide.

Making the Right Choice

Selecting between retention and detention typically depends on three main factors:

1. What does the code require? Some jurisdictions mandate retention for a minimum storm depth, while others focus solely on peak flow control through detention. Confirm requirements before beginning design.
2. What are the site conditions? Sandy soils with low groundwater support infiltration-based retention. Clay soils or high groundwater typically require detention with controlled release.
3. What provides the best value? Retention for reuse can offset costs through reduced water bills, while simple detention with rapid release may minimize system size and expense.

The best projects look at all three factors. Even sites with average soils can use a hybrid system that lets some water soak in and holds the rest, meeting both volume and peak flow needs.

Next Steps

If you are designing a stormwater system and need to determine whether a retention, detention, or hybrid approach is appropriate for your site:

1. Review your local stormwater ordinance for specific retention and detention requirements.
2. Conduct a geotechnical investigation to assess infiltration potential.
3. Model the hydrology for your design storm(s).
4. Compare system alternatives, including surface and underground options.
5. Request product specifications for modular systems that match your loading and storage requirements.

For technical specifications on modular detention and retention systems, contact our engineering team or download product data sheets.

The information in this article is intended for general guidance. Specific design requirements vary by jurisdiction. Always consult local stormwater regulations and work with a licensed professional engineer for project-specific applications.