What is the difference between a detention pond and a retention pond?

A detention pond (dry pond) stores storm runoff temporarily and releases it through an outlet at a controlled rate, then drains empty; its main job is cutting peak flow. A retention pond (wet pond) keeps a permanent pool of water between storms and stores runoff above that pool; its main job is water-quality treatment by settling, with peak control added in the storage above the pool. In some regions, especially arid ones, 'retention' instead means a basin with no surface outlet that holds runoff until it infiltrates or evaporates.

Which is better, a detention pond or a retention pond?

It depends on the requirement. If the rule is peak-flow control only, a dry detention basin is usually cheaper and smaller in footprint. If the site must meet a pollutant-removal or nutrient requirement, a wet retention pond removes far more sediment and nutrients than a dry basin, but needs a reliable water supply to hold its pool, more area and more maintenance. Infiltration retention only works where soils drain and groundwater is deep.

How long does a detention pond take to drain?

A dry detention basin sized only for peak control empties within hours of the storm. An extended detention basin is designed to release its water-quality or channel-protection volume slowly, commonly over about 24 to 48 hours (Maryland's channel-protection standard is 24-hour extended detention of the 1-year, 24-hour storm). North Carolina's wet-pond criteria draw the temporary pool down over 2 to 5 days. The California Department of Public Health asks for structural stormwater BMPs to drain within 96 hours so mosquitoes cannot complete development.

Can a detention pond have standing water?

A dry detention pond should not hold water for more than a few days after a storm. Persistent standing water usually means a clogged outlet or low-flow orifice, a bottom set below the seasonal water table, or poor grading; it needs maintenance, not acceptance. A pond designed with a permanent pool is a wet pond (retention pond), which is a different practice with its own design rules.

How do you size a detention pond?

Start with the allowable release rate (usually the pre-development peak) and the post-development inflow. For small sites a preliminary volume comes from the Modified Rational Method, maximizing V = 60[C·i·A·td − Qa(td + tc)/2] over storm duration td. For larger sites use the TR-55 storage ratio Vs/Vr from qo/qi, then confirm the outlet and stage-storage by routing the full hydrograph (storage-indication method) for every design storm.

What is extended detention?

Extended detention is a dry or wet basin whose outlet includes a small orifice that releases a specified volume, typically the water-quality or channel-protection volume, over a long drawdown time such as 24 to 48 hours. The slow release lets sediment settle and reduces the erosive duration of high flows downstream, which a basic peak-shaving dry pond does not do.

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Detention vs Retention Ponds — Reference

By Michael Flynn, PE · water resources & dam safety engineer · Last updated Sep 29, 2026

What each kind of stormwater pond is for, how they differ, the drawdown and design rules that actually govern them, and two quick ways to get a first storage volume. Detention controls how fast runoff leaves; retention controls what stays behind — a permanent pool that treats water, or (in arid-region usage) water that never leaves except by infiltration and evaporation.

Quick answers

Detention pond: a dry basin that fills during a storm, releases through a sized outlet, and drains empty. Its job is peak-flow control.

Retention pond: a wet pond with a permanent pool that stores runoff above the pool. Its job is water-quality treatment by settling, often with peak control on top.

Regional catch: in arid-region and some local codes, “retention” means a basin with no surface outlet that holds runoff until it infiltrates — check your jurisdiction’s definition.

Extended detention: a detention basin with a small orifice that releases the water-quality or channel-protection volume slowly, commonly over 24–48 hours.

Which to use: peak control only → dry detention; pollutant or nutrient removal required → wet pond or extended detention; good soils and deep groundwater → infiltration basin.

Definitions

Practice	Also called	What it does
Dry detention basin	Detention pond, dry pond	Stores runoff temporarily; outlet releases it at or below an allowable peak; empty between storms.
Extended detention (ED) basin	Dry ED pond	Dry basin plus a small low-level orifice that drains a set volume slowly (hours to days) for settling and downstream channel protection.
Wet pond	Retention pond, wet detention pond, wet basin	Holds a permanent pool below the outlet invert; storm runoff is stored above the pool (temporary pool) and released through the outlet.
Infiltration / retention basin	Retention basin (arid-region usage), recharge basin	No surface outlet for the design storm; runoff leaves by infiltration and evaporation. Needs permeable soil and separation to groundwater.

Terminology trap. “Wet detention” and “retention” describe the same permanent-pool pond in much of the eastern US, while Arizona-style usage reserves “retention” for basins that hold the design volume with no outflow. Read the definition section of the governing manual before assuming either word.

Side-by-Side Comparison

Feature	Dry detention	Extended detention	Wet (retention) pond	Infiltration basin
Permanent pool	No	No (small micropool optional)	Yes	No
Surface outlet	Yes, sized for peaks	Yes, low orifice + peak weir/riser	Yes, above pool	Emergency overflow only
Primary purpose	Peak attenuation	Peak + channel protection + some settling	Water quality + peak control	Volume reduction, recharge
Drawdown	Hours	Designed, commonly 24–48 h	Temporary pool only (e.g. 2–5 d in NC)	Set by soil infiltration rate
Pollutant removal	Low	Moderate	High (sediment, attached nutrients)	High for what infiltrates
Footprint	Smallest	Small–moderate	Largest (pool + shelf + freeboard)	Moderate; needs soil + water-table fit
Water supply needed	No	No	Yes — drainage area or groundwater must sustain the pool	No
Main maintenance	Mowing, outlet trash rack	Clogging of small orifice	Forebay sediment removal, vegetation, algae	Surface clogging; restore infiltration
Mosquito / safety	Low if it drains	Watch drawdown time	Permanent water: safety bench, fencing per local rule	Standing water = failure sign

Typical Design Criteria (named sources)

Item	Value	Source
Wet pond temporary-pool drawdown	2–5 days	NCDEQ Stormwater Design Manual, Part C-3 Wet Pond, MDC 7
Wet pond average main-pool depth	3–8 ft	NCDEQ Stormwater Design Manual, Part C-3 Wet Pond, MDC 2 (rev. 11-23-2020)
Wet pond forebay volume	15–20% of main pool	NCDEQ Stormwater Design Manual, Part C-3 Wet Pond, MDC 5
Channel-protection volume (Cp_v)	1-yr, 24-h storm held 24 h (12 h in Use III/IV waters)	Maryland Stormwater Design Manual (2000), Ch. 2 Unified Sizing Criteria
Drain time to prevent mosquito breeding	≤ 96 h	California Department of Public Health, Best Management Practices for Mosquito Control in California

These are representative, not universal — freeboard, side slopes, safety benches, embankment rules and release-rate criteria vary by jurisdiction. An impounding embankment can also fall under state dam-safety law once it reaches the statutory height and storage thresholds.

Preliminary Storage Volume

1. Modified Rational Method (small sites, critical-duration form)

Q_p(t_d) = C · i(t_d) · A
V_s = 60 · [ Q_p · t_d − Q_a · (t_d + t_c) / 2 ] (ft³; Q in cfs, t in min)

The post-development inflow is a trapezoid of duration t_d ≥ t_c; the outflow is assumed to rise linearly to the allowable release Q_a. Evaluate V_s for a range of storm durations from the local IDF curve and keep the maximum — the critical duration is usually longer than t_c. Use for small drainage areas where the Rational Method itself is accepted.

2. TR-55 storage ratio (Chapter 6, Figure 6-1)

V_s / V_r = C₀ + C₁(q_o/q_i) + C₂(q_o/q_i)² + C₃(q_o/q_i)³
V_r = Q · A (runoff depth × drainage area)

Rainfall distribution	C₀	C₁	C₂	C₃
Type I, IA	0.660	−1.76	1.96	−0.730
Type II, III	0.682	−1.43	1.64	−0.804

q_o/q_i	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
V_s/V_r, Type I/IA	0.503	0.381	0.289	0.223	0.179	0.152	0.138	0.133
V_s/V_r, Type II/III	0.555	0.455	0.379	0.321	0.277	0.241	0.209	0.176

q_o = allowable peak outflow, q_i = post-development peak inflow, Q = runoff depth from the curve-number method. TR-55 limits the relation to roughly 0.1 ≤ q_o/q_i ≤ 0.8 and a single-stage outlet; it is a planning estimate that can be off by 25% or more.

Worked example (TR-55)

20 ac drains to a proposed pond, Type II storm. Post-development runoff depth Q = 2.4 in, peak inflow q_i = 60 cfs, allowable release (pre-development peak) q_o = 25 cfs.
q_o/q_i = 25/60 = 0.417 → V_s/V_r = 0.682 − 1.43(0.417) + 1.64(0.417)² − 0.804(0.417)³ = 0.313
V_r = 2.4 in × 20 ac / 12 = 4.0 ac-ft → V_s = 0.313 × 4.0 = 1.25 ac-ft (≈ 54,500 ft³).
Next: lay out a stage-storage curve holding that volume, size the outlet, and route every design storm to confirm the peak.

Always route. Both methods give a first volume only. Final design needs a stage–storage–discharge relationship and hydrograph routing (storage-indication / modified Puls) for each design storm, including a check that the emergency spillway passes the largest required event with freeboard.

Sources: USDA NRCS, Urban Hydrology for Small Watersheds (TR-55, 1986), Chapter 6; NCDEQ Stormwater Design Manual, Part C-3 (Wet Pond); Maryland Department of the Environment, 2000 Maryland Stormwater Design Manual, Volumes I & II; ASCE/WEF, Design and Construction of Urban Stormwater Management Systems (MOP 23). Values are representative — the governing local manual controls.

Route a hydrograph through your pond: Reservoir routing calculator → · Size the outlet: Orifice · Weir · Want the whole set? Print all the cards →

Related cheat sheets and tools

Get the peak flows first: Rational Method with the runoff coefficient card for small sites, or NRCS curve-number runoff with the curve number card and the SCS storm distributions for larger ones. Size the outlet with the orifice and sharp-crested weir calculators, route the storm with reservoir routing, and check a wet pond’s residence time with hydraulic detention time. For full BMP design with a sealed report, see HydroComplete.

Detention vs Retention Ponds — Reference

Definitions

Side-by-Side Comparison

Typical Design Criteria (named sources)

Preliminary Storage Volume

1. Modified Rational Method (small sites, critical-duration form)

2. TR-55 storage ratio (Chapter 6, Figure 6-1)

Worked example (TR-55)

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