Detention vs. Retention: Understanding the Difference and Choosing the Right Approach

Detention and retention are two fundamental approaches to stormwater management, yet they’re frequently confused. Understanding their differences is essential for selecting the right approach for your project.

The Fundamental Difference

Detention: Temporary Storage

Detention temporarily holds stormwater and releases it slowly over time. Water is detained but not permanently kept.

Key Characteristics:

• Water is stored temporarily during storms
• Outlets release water at controlled rates
• Basin empties between storm events
• Often called “dry ponds” when no permanent pool
• Reduces peak flow rates downstream

Retention: Permanent Storage or Removal

Retention permanently removes stormwater from the runoff stream—either by holding it permanently or eliminating it through infiltration/evaporation.

Key Characteristics:

• Water is permanently retained or eliminated
• May have permanent pool (wet pond)
• Or may infiltrate entirely (dry retention)
• Reduces both peak flow AND volume
• Removes water from downstream system

Visual Comparison

Detention Basin (Dry Pond)

Storm Event                After Storm
     ↓                          ↓
┌─────────────────┐    ┌─────────────────┐
│  ~~~~~~~~~~~~   │    │                 │
│  ~~~~~~~~~~~~   │    │                 │
│  ~~~~~~~~~~~~   │    │                 │
│      Water      │    │   Empty/Dry     │
│                 │    │                 │
└────────┬────────┘    └────────┬────────┘
         │                      │
         ↓                      ↓
    Controlled              No flow
     outflow             (basin drains)

Retention Basin (Wet Pond)

Storm Event                After Storm
     ↓                          ↓
┌─────────────────┐    ┌─────────────────┐
│  ~~~~~~~~~~~~   │    │  ~~~~~~~~~~~~   │
│  ~~~~~~~~~~~~   │    │  ~~~~~~~~~~~~   │
│  ~~~~~~~~~~~~   │    │  Permanent      │
│  Storm volume   │    │    Pool         │
│  Permanent Pool │    │                 │
└────────┬────────┘    └─────────────────┘
         │                      │
         ↓                      ↓
     Overflow              No outflow
    (if needed)         (pool level stable)

Infiltration Basin (Dry Retention)

Storm Event                After Storm
     ↓                          ↓
┌─────────────────┐    ┌─────────────────┐
│  ~~~~~~~~~~~~   │    │                 │
│  ~~~~~~~~~~~~   │    │                 │
│  Storm volume   │    │   Empty/Dry     │
│                 │    │                 │
├─────────────────┤    ├─────────────────┤
│    Infiltrates  │    │ Water absorbed  │
│     ↓  ↓  ↓     │    │   into ground   │
└─────────────────┘    └─────────────────┘
         │                      │
         ↓                      ↓
    No surface              Volume
     outflow               removed

Types of Each System

Detention Systems

Dry Detention Basin:

• Empties completely between storms
• Grassy bottom, usable as open space
• Outlet(s) control release rate
• Most common type

Extended Detention Basin:

• Holds water longer (24-48 hours)
• Provides water quality benefits
• Still drains completely
• Better settling of pollutants

Underground Detention:

• Pipes, chambers, or vaults
• Used where surface area is limited
• Higher cost, space-efficient
• Limited water quality benefit

Retention Systems

Wet Retention Pond:

• Permanent pool of water
• Storm volume stored above pool
• Excellent water quality treatment
• Aesthetic and habitat value

Infiltration Basin:

• All water infiltrates into ground
• Requires permeable soils
• No outlet structure needed
• Eliminates runoff volume

Infiltration Trench:

• Narrow, gravel-filled trench
• Water infiltrates through bottom
• Good for linear applications
• Moderate volumes only

Comparison Table

When to Use Each Approach

Choose Detention When:

1. Only peak flow control needed - No volume reduction requirement
2. Limited land available - Can be smaller than retention
3. Dual use desired - Playfields, parking (in extreme storms)
4. Poor soils - Clay or high groundwater
5. Cost is primary concern - Generally less expensive
6. Downstream flooding is the issue - Reduces peak flows

Choose Wet Retention When:

1. Water quality is critical - Lake or stream protection
2. Aesthetic value important - Amenity ponds
3. Wildlife habitat desired - Waterfowl, fish
4. Sufficient land available - Needs larger footprint
5. Nutrient removal needed - Phosphorus, nitrogen
6. Regulatory requirement - Some areas mandate

Choose Infiltration When:

1. Volume reduction required - Regulations require it
2. Suitable soils exist - Sandy or permeable
3. Groundwater recharge beneficial - Aquifer protection
4. Peak AND volume control needed - Comprehensive management
5. No receiving system available - Nowhere to discharge
6. Low pollutant risk - Clean runoff sources

Design Principles

Detention Basin Design

Volume Calculation: Size to store the difference between inflow and outflow:

Outlet Design: Multiple outlets for different design storms:

• Low-flow orifice for small storms
• Mid-level opening for 10-year
• Emergency spillway for extreme events

Key Parameters:

• Inflow hydrograph (SCS or other method)
• Maximum allowable release rate
• Stage-storage relationship
• Stage-discharge for outlets

Calculate detention requirements →

Wet Retention Pond Design

Permanent Pool:

• Size for pollutant settling (typically 0.5-1.0 inch × watershed acres)
• Depth: 4-8 feet (average 4-6 feet)
• Configured to maximize flow path

Surcharge Volume:

• Storm volume stored above permanent pool
• May be controlled or overflow
• Provides peak attenuation

Key Parameters:

• Mean pool depth
• Length-to-width ratio (>3:1 preferred)
• Volume above pool
• Outlet configuration

Infiltration Design

Infiltration Rate: Critical for sizing:

Where:

• V = Volume infiltrated
• A = Bottom area
• f = Infiltration rate
• t = Time

Safety Factor: Use design rate of ½ tested rate (safety factor of 2)

Key Parameters:

• Soil permeability (conduct testing)
• Groundwater depth
• Separation from utilities/foundations
• Pretreatment for sediment

Water Quality Performance

Pollutant Removal Comparison

Key Takeaway: For water quality, wet retention > infiltration > dry detention

Enhanced Detention

Extend detention time for better quality:

Extended Detention (ED):

• 24-48 hour drawdown time
• Improves TSS removal to 60-80%
• Small additional volume

ED + Micropool:

• Small permanent pool at outlet
• Improves overall performance
• Reduces resuspension

Maintenance Requirements

Dry Detention Maintenance

Wet Retention Maintenance

Infiltration Maintenance

Common Design Mistakes

Detention Mistakes

1. Outlet sized incorrectly - Doesn’t achieve target release rate
2. No emergency spillway - Dam safety hazard
3. Outlet clogs - Undersized or unprotected orifice
4. Inadequate freeboard - Overtopping during large storms
5. Poor access - Can’t maintain or remove sediment

Retention Mistakes

1. Insufficient pool depth - Resuspension during storms
2. Short-circuiting - Flow path too direct
3. No forebay - Sediment fills main pool
4. Steep banks - Safety hazard, erosion
5. No aeration - Water quality problems

Infiltration Mistakes

1. No soil testing - Actual rates differ from assumed
2. Compaction during construction - Destroys infiltration
3. No pretreatment - Rapid clogging
4. Undersized - Doesn’t handle design volume
5. Too close to structures - Foundation damage

Hybrid Approaches

Many effective designs combine approaches:

Detention + Wetland

• Dry detention for peak control
• Wetland forebay for quality
• Best of both worlds

Retention + Infiltration

• Wet pool for treatment
• Permeable bottom for partial infiltration
• Volume and quality benefits

Treatment Train

• Bioretention → Detention → Wetland
• Each component contributes
• Comprehensive water quality

Selection Flowchart

1. Is volume reduction required?

   • Yes → Consider retention or infiltration
   • No → Detention may be sufficient

2. What are the soil conditions?

   • Permeable (>0.5 in/hr) → Infiltration viable
   • Impermeable → Wet retention or detention

3. Is water quality critical?

   • High priority → Wet retention preferred
   • Moderate → Extended detention or hybrid
   • Low priority → Standard detention OK

4. How much land is available?

   • Abundant → Wet retention
   • Limited → Detention or underground
   • Very limited → Underground or regional facility

5. What are the regulations?

   • Follow local requirements first

Summary

The best approach often combines multiple strategies, tailored to site conditions and regulatory requirements.

Related Calculators

• Detention Pond Calculator →
• SCS Curve Number Calculator →
• Dry Well Calculator →
• Infiltration Trench Calculator →

References

1. Environmental Protection Agency. (2004). Stormwater best management practice design guide, Volume 2: Vegetative biofilters. EPA.

2. American Society of Civil Engineers. (2017). Design and construction of urban stormwater management systems (ASCE Manual of Practice No. 77). ASCE Press.

3. Urban Drainage and Flood Control District. (2016). Urban storm drainage criteria manual, Volume 3: Best management practices. Denver, CO.

4. Center for Watershed Protection. (2007). National pollutant removal performance database (Version 3). CWP.

5. Minnesota Pollution Control Agency. (2020). Minnesota stormwater manual. MPCA.

6. Prince George’s County, MD. (2007). Bioretention manual. Department of Environmental Resources.