Retention (wet) ponds keep a permanent pool that treats runoff; detention (dry) ponds store runoff and drain. Size the pool, storage and outlet step by step.
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A retention pond (wet pond) holds a permanent pool that stays full between storms. Runoff from each storm pushes out pool water, and sediment and nutrients settle, so its main job is water-quality treatment. A detention pond (dry pond) stores runoff for a short time and releases it through a restricted outlet until it is empty, so its main job is peak-flow control.
Most retention ponds do both. The permanent pool is sized for the water-quality volume, and detention storage above it holds post-development peaks to the allowed release. An orifice or riser controls that storage, and an emergency spillway passes the largest storm.
Terms vary by state. Florida calls permanently wet ponds that release through an outlet wet detention systems, and uses retention for storage that does not discharge to surface water. Use the definitions in the manual you design to.
Size the pond from the bottom up. Each layer has its own design storm and its own control.
The Simple Method, as written in the Maryland manual, gives the runoff from the 90% storm:
WQv is in ac-ft, P is the rainfall depth in inches (Maryland uses 1.0 in east and 0.9 in west), A is in acres and I is percent impervious. A standard wet pond holds all of it below the outlet, with a forebay at each inlet.
The pool is already full when a storm starts, so flood storage must sit above it. On small sites the Virginia critical-storm-duration method (VDOT Eq. 11.4) gives a first estimate:
V is in ft³, Qi = C·i·A for a storm of duration Td (min), and qo is the allowable release. Try several durations and keep the largest volume, then confirm it by routing.
An orifice at the normal pool sets the pool level and limits the release:
h is the head on the orifice centroid, and Cd is about 0.6. Weirs or riser openings higher up handle larger storms. An emergency spillway passes the extreme flood, often with the principal outlet assumed blocked, and freeboard sits above that.
Workflow follows FHWA HEC-22 Fourth Edition, Chapter 10. Local criteria control the project-specific numeric requirements.
These values come from the Maryland manual and a county manual based on Georgia's. Your manual's numbers govern, and a dam-safety review may add more.
| Item | Typical requirement (source) |
|---|---|
| Contributing drainage area | 10 acres minimum, 25 acres or more preferred, unless groundwater keeps the pool wet (Maryland). Georgia-based manuals set 25 acres for wet ponds unless a water balance shows the pool will hold. MD 3.1.1; Knox Co. 4.3.1 |
| Permanent pool volume | All of the WQv for a standard wet pond. A wet extended-detention pond can hold part of it as extended detention above the pool. MD 3.1 (P-2, P-3) |
| Sediment forebay | At each major inlet; 0.1 in of runoff per impervious acre. Its volume counts toward the WQv. MD 3.1.2, 3.1.3 |
| Pool depth | 3–4 ft minimum and 8 ft maximum; deeper pools stratify and go anoxic. Knox Co. 4.3.1 |
| Flow path | Length-to-width 1.5:1 or longer; more than 3:1 is better. MD 3.1.4; Knox Co. 4.3.1 |
| Interior side slopes | 3:1 (h:v) or flatter, ending on a safety bench. MD 3.1.6 |
| Benches around pools 4 ft or deeper | Safety bench (6% maximum slope) plus aquatic bench (18 in maximum depth), 15 ft combined width. Waivable where slopes are 4:1 or flatter. MD 3.1.5, 3.1.6 |
| Low-flow orifice | 3 in minimum with an external trash rack. MD 3.1.6 |
| Freeboard | 1 ft above the 10-year high water (Maryland ponds exempt from MD-378 review) or 1 ft above the extreme-flood water surface (Georgia-based manuals). MD 3.1.1; Knox Co. 4.3.1 |
| Pond drain | A valved drain that can empty the pond within 24 hours. MD 3.1.6 |
MD = Maryland Stormwater Design Manual (2000, rev. 2009), Vol. I, section number. Knox Co. = Knox County, Tennessee Stormwater Management Manual, Vol. 2, §4.3.1, adapted from the Georgia manual. Full references are at the end of this page.
These criteria illustrate the method and follow the pattern of the Maryland manual. Your jurisdiction sets the events, depths and methods.
Rv = 0.05 + 0.009 × 50 = 0.50. WQv = 1.0 × 0.50 × 10 / 12 = 0.417 ac-ft (18,150 ft³). The forebay holds 0.1 in over the 5.0 impervious acres, which is 1,815 ft³. That volume is part of the pool, not extra to it.
Apply VDOT Eq. 11.4 with tc = 15 min and qo = 6.74 cfs at each Atlas 14 duration. A longer storm has a lower peak but more volume, so the storage peaks at an intermediate duration:
| Storm duration Td | 10-yr intensity | Inflow peak Qi | Storage V |
|---|---|---|---|
| 15 min | 4.60 in/hr | 25.3 cfs | 20,900 ft³ |
| 30 min | 3.37 in/hr | 18.5 cfs | 26,900 ft³ |
| 60 min | 2.18 in/hr | 12.0 cfs | 29,200 ft³ |
| 120 min | 1.34 in/hr | 7.37 cfs | 25,900 ft³ |
The 60-minute storm governs: 29,200 ft³ (0.67 ac-ft). On a smooth IDF curve through the 15- and 60-minute values, storage peaks at about 56 minutes with the same volume (VDOT Eq. 11.5). A single triangular hydrograph at tc (VDOT Eq. 11.3, base 2tc) gives only 16,700 ft³, 43% less. Preliminary methods can disagree this much, which is why you check several durations and then route.
Try a bottom of 85 × 20 ft at elevation 99.0, with 4:1 interior side slopes. Slopes that flat let Maryland waive the benches. Set the normal pool at 104.0, giving a 5-ft pool.
Volumes use the prismoidal formula for a rectangular basin. The average-end-area method agrees within 0.1% on 0.5-ft slices but runs high on coarse ones (about 6% for the pool taken as one 5-ft slice).
The orifice invert sits at the normal pool, so it sets the pool level. For 6.74 cfs at the stage that holds 29,200 ft³ (about 106.96), with Cd = 0.6 and the head measured to the centroid, the orifice equation gives D = 12.8 in. Round down to a 12-in orifice so the release stays under the limit. It passes 5.98 cfs with the water at the crest. You can check the orifice in the Pond Sizing calculator (2.42 ft of head on the centerline).
| Stage (ft) | Surface area (ft²) | Storage above pool (ft³) | Orifice outflow (cfs) |
|---|---|---|---|
| 104.0 | 7,500 | 0 | 0.00 |
| 104.5 | 8,260 | 3,940 | 0.87 |
| 105.0 | 9,040 | 8,260 | 2.67 |
| 105.5 | 9,860 | 12,990 | 3.78 |
| 106.0 | 10,720 | 18,130 | 4.63 |
| 106.5 | 11,600 | 23,710 | 5.35 |
| 107.0 | 12,520 | 29,740 | 5.98 |
Below the crown (105.0) the orifice runs part-full, so the table uses the wetted area and the head on its centroid.
Route each Modified Rational hydrograph through the table with level-pool (Modified Puls) routing. Each hydrograph rises over tc, holds Qi until Td, then falls over tc.
| Storm duration | Inflow peak | Routed peak outflow | Peak stage |
|---|---|---|---|
| 15 min | 25.3 cfs | 4.67 cfs | 106.02 |
| 30 min | 18.5 cfs | 5.50 cfs | 106.62 |
| 60 min | 12.0 cfs | 5.77 cfs | 106.83 |
| 120 min | 7.37 cfs | 5.43 cfs | 106.56 |
The 60-minute storm is still the worst case. The peak outflow is 5.77 cfs, under the 6.74 cfs limit. The peak stage is 106.83, 0.17 ft below the crest, using 27,600 ft³. Here the Step 3 estimate was a little conservative. The routing sets the final size, not the estimate. The Level-Pool Routing calculator runs the same check for one hydrograph at a time.
For the 100-year Rational peak, apply the frequency factor Cf = 1.25 that some agencies use (HEC-22 Sec. 3.2.2.1; FHWA does not endorse it): Q100 = (1.25 × 0.55) × 6.92 × 10 = 47.6 cfs. Assume the orifice is blocked. A 20-ft broad-crested spillway at 107.0, with C = 2.6 from the low end of the usual range, carries it at H = (47.6 / (2.6 × 20))2/3 = 0.94 ft. That puts the water surface at 107.94. A top of embankment at 109.0 leaves 1.06 ft of freeboard, which meets both 1-ft rules in the table above. Size other spillway shapes with the weir calculator.
A 5-ft permanent pool (21,700 ft³) with 3 ft of detention above it (29,700 ft³). A 12-in orifice at 104.0 controls the outflow, a 20-ft emergency spillway sits at 107.0, and the embankment top is at 109.0.
Not covered here: channel-protection extended detention (Maryland's 1-year, 24-hour storm), other required events, and a water balance. At 10 acres the site meets Maryland's minimum but not the 25 acres Georgia-based manuals expect. Also not covered: embankment, geotechnical and dam-safety design, outlet protection, the pond drain and maintenance access.
Final design means routing every required storm through a stage-storage-discharge relationship with a multi-stage outlet. These programs do that:
For preliminary numbers before you build a model, use the free calculators on this site:
Every tool needs the same inputs: approved inflow hydrographs, a stage-storage table, outlet geometry and tailwater. The worked example is small enough to check by hand or in a spreadsheet.
A retention (wet) pond keeps a permanent pool between storms and treats runoff by settling. A detention (dry) pond stores runoff for a short time and releases it through a restricted outlet until it is empty. Most retention ponds also have detention storage above the pool, so one basin does both jobs.
The permanent pool holds the water-quality volume, WQv = P × Rv × A / 12. The detention storage above it comes from routing each design storm through the outlet. In the worked example, a 10-acre site at 50% impervious needs a pool of at least 0.42 ac-ft and gets 0.68 ac-ft of detention above it. Its water surface at the spillway crest covers about 2.9% of the site, before the embankment and access road.
Georgia-based manuals ask for a permanent pool at least 3–4 ft deep and no deeper than about 8 ft. Maryland requires safety and aquatic benches around pools 4 ft or deeper, but lets them be waived where the side slopes are 4:1 or flatter. Detention storage and freeboard sit on top of the pool. The worked example uses a 5-ft pool, 3 ft of detention and 2 ft more to the top of the embankment.
Only for small sites, and only where your manual allows it. Roanoke, Virginia, for example, allows the Modified Rational Method only for drainage areas under 20 acres with a time of concentration under 20 minutes. Maryland requires TR-55 and TR-20, or an approved equivalent, for its peak-control criteria. When you do use it, check a range of storm durations, not just the one equal to tc.
The free agency programs are USACE HEC-HMS, EPA SWMM and NRCS WinTR-20. Commercial packages such as HydroCAD and Bentley PondPack do the same level-pool routing with built-in stage-storage and outlet editors. For a first estimate before modeling, use the calculators on this page.
These tools are most useful for solving "retention & detention pond design" problems.
Estimate preliminary detention pond storage with the Modified Rational Method and size orifice or weir outlets from watershed and allowable-release criteria; use level-pool routing for final design.
Enter an inflow hydrograph plus stage-storage and stage-discharge data to route a level pool with the Modified Puls method and return the outflow hydrograph, peak attenuation, maximum stage, and storage.
Estimate preliminary detention storage with either a Modified Rational peak-flow approximation or one selected quick method (Baker, FAA, or simplified) using that method's inputs.
Enter runoff coefficient, rainfall intensity, drainage area, and return period to calculate Rational Method peak discharge (Q = CiA), adjusted C, and a transparent calculation breakdown.
Enter watershed length, slope, cover, and method-specific parameters to calculate time of concentration with Kirpich, FAA, NRCS Lag, Kerby-Hathaway, or TR-55 segmental methods.
Calculate discharge or solve for head for rectangular, Cipolletti, four V-notch, broad-crested, and irregular weirs using the FHWA Hydraulic Toolbox coefficients and irregular-profile workflow.
Design underground detention systems for stormwater management. Calculates storage volume, outlet sizing, and drawdown time per ASCE MOP 77 and FHWA HEC-22 methodology.
Calculate cistern sizing for rainwater harvesting. Determines collection potential, storage requirements, and annual water balance using ASABE S597.1 and ARCSA guidelines.
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