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Estimate the detention storage volume needed to hold a developed site's peak runoff back to its pre-development rate, then size the orifice or weir outlet that controls the release. Sizing uses the Modified Rational Method with standard outlet hydraulics for preliminary detention basin design.
Preliminary answer: size storage first, then prove performance by routing
Use the Pond Storage tab for an order-of-magnitude detention volume and the other tabs to size one orifice or weir at a specified head. A worked stage-storage-discharge table below shows the separate routing data you will ultimately need; the interactive tabs do not generate a project table or multi-stage outlet assembly. Route complete inflow hydrographs, test every required return period, and check tailwater, freeboard, drawdown, emergency overflow, safety, and maintenance.
Still choosing a facility type? Compare the design objectives, routing workflow, and maintenance implications in the detention vs. retention guide.
Calculation Mode
Pond Storage: Calculate preliminary detention storage volume using the Modified Rational Method.
Detention Pond Sizing Overview
This calculator provides preliminary sizing for detention ponds and outlet structures. The Modified Rational tab maximizes the difference between constant-rate inflow and outflow blocks over storm durations at or above the time of concentration. The outlet tabs size one orifice or weir at a specified head; they do not build a multi-stage outlet or routing table.
Key equations:
- Critical duration: td* = max[tc, tc(Qi / 2Qo)2]
- Storage: Vs = Vi - Vo at td* (with the tc boundary enforced)
- Orifice: Q = Cd x A x sqrt(2gh)
- Weir: Q = Cw x L x H1.5
Typical Coefficients
| Structure Type | Coefficient | Notes |
|---|---|---|
| Sharp-crested orifice | 0.60 | Standard value |
| Short tube orifice | 0.80 | L/D = 2-3 |
| Rectangular weir (US) | 3.10 | FHWA Hydraulic Toolbox default |
| Cipolletti weir (US) | 3.37 | 1:4 side slopes |
| 90-degree V-notch | 2.50 | Q = C x H^2.5 |
Source: FHWA HEC-22 (2009), Chapter 7-8.
For educational purposes only. Not a substitute for professional engineering judgment.
Interactive model
Interactive detention pond storage and outlet model
Explore how pond geometry and water stage change storage, then see how the low-level orifice and overflow weir shape the stage-discharge relationship.
Open full model (opens in a new tab)How pond sizing works
Detention works by storing the extra runoff that development creates and letting it out slowly so the downstream peak is no higher than before. This tool covers the two core design steps: estimating the storage volume, and sizing the outlet structure that meters the flow.
1. Required storage — Modified Rational Method
The Modified Rational Method used here scales the Rational peak with duration using i = itc(tc/td)0.5 and assumes a constant target release. Because the supplied Rational peak assumes the full watershed is contributing, the duration search is limited to td ≥ tc. The required storage is the maximum difference between constant-rate inflow and outflow volumes over that domain.
td* = max(tc, tc[Qi/(2Qo)]2)
Vs = Qi2tc/(4Qo) when Qo ≤
Qi/2;
Vs = (Qi − Qo)tc when Qi/2
< Qo < Qi.
- Vs = required detention storage (acre-ft or m³)
- Qi = post-development peak inflow (cfs or m³/s)
- Qo = target release rate, defaults to the pre-development peak (cfs or m³/s)
- tc = time of concentration (min)
Two intermediate values support this. The routing coefficient is the ratio of outflow to inflow, r = Qo ÷ Qi. The critical storm duration — the duration that maximizes that storage envelope — is the larger of tc and the stationary-point duration shown above. The calculation does not cap long durations at 10tc; it warns when the result is far beyond the base duration because a site-specific IDF curve and full routing become especially important. The optional rainfall intensity at tc is display-only: the entered Qi already embodies rainfall intensity, C, and area, so changing that optional field does not change storage. When r ≥ 1, the outlet can pass the peak inflow and this simplified method reports no detention requirement.
Boundary example: for Qi = 25 cfs, Qo = 20 cfs, and tc = 30 min, the unconstrained stationary point is 11.72 min. That is below the valid td ≥ tc domain, so 30 min governs. The constant-rate volume difference is (25 − 20) × 30 × 60 = 9,000 ft³, or 0.207 acre-ft.
2. Outlet sizing — orifice and weir equations
The outlet structure controls the release rate. The calculator sizes three common types:
- Orifice: Q = Cd × A × √(2gh). Solving for area gives A = Q ÷ (Cd × √(2gh)), and the circular diameter is D = √(4A ÷ π). Here g = 32.2 ft/s² (US) or 9.81 m/s² (SI), and h is the head above the orifice centerline.
- Rectangular / Cipolletti weir: Q = C × L × H1.5, so the required crest length is L = Q ÷ (C × H1.5), where H is the head over the crest.
- V-notch weir: Q = C × tan(θ/2) × H2.5, solved for the notch angle θ that passes the target discharge.
Methods and coefficients follow FHWA HEC-22 (Urban Drainage Design Manual), McCuen (2005), NRCS TR-55, the USBR Water Measurement Manual, and ASCE MOP 77.
Outlet discharge coefficients
Default discharge coefficients used by the outlet sizing tabs. US values are for use with feet and cfs; SI values are for use with meters and m³/s. Enter a custom coefficient if your structure differs from these standard sharp-crested cases.
| Outlet type | Equation | Coefficient (US) | Coefficient (SI) | Source |
|---|---|---|---|---|
| Sharp-crested orifice (Cd) | Q = Cd·A·√(2gh) | 0.60 | 0.60 | Mays (2011); HEC-22 |
| Rectangular weir | Q = C·L·H1.5 | 3.10 | 1.7115 | FHWA Hydraulic Toolbox |
| Cipolletti (trapezoidal) weir | Q = C·L·H1.5 | 3.367 | 1.859 | USBR Water Measurement Manual |
| 90° V-notch weir | Q = C·tan(θ/2)·H2.5 | ~2.5 | ~1.38 | Mays (2011) |
Minimum recommended orifice diameter to limit clogging: 4 in (100 mm), per ASCE MOP 77. Coefficients assume free (unsubmerged), fully ventilated flow with negligible approach velocity. The alternative 3.33 US / 1.84 SI rectangular-weir coefficient produces about 7% more discharge at the same crest length and head; use it only when the applicable standard or reviewing agency calls for it.
Worked stage-storage-discharge example
This illustrative table is generated at build time by the page's calculation library; it is a worked reference, not an output produced by the interactive tabs. The example basin has a 100 ft by 50 ft bottom, 3H:1V side and end slopes, a 9 in low-level orifice at elevation 0, and a 10 ft rectangular overflow weir at elevation 5. Storage follows the FHWA Hydraulic Toolbox average-end-area convention; the weir uses C = 3.10.
| Stage (ft) | Storage (acre-ft) | Discharge (cfs) |
|---|---|---|
| 0.0 | 0.000 | 0.00 |
| 1.0 | 0.126 | 2.13 |
| 2.0 | 0.274 | 3.01 |
| 3.0 | 0.448 | 3.68 |
| 4.0 | 0.651 | 4.25 |
| 5.0 | 0.884 | 4.76 |
| 6.0 | 1.150 | 36.21 |
Illustrative geometry only. A design table should use surveyed elevations, the actual outlet assembly, tailwater and submergence checks, and the increment required by the routing model.
Inputs and assumptions
What you need to enter
- Pre- and post-development peak flow (cfs or m³/s), typically from the Rational Method
- Time of concentration of the watershed (minutes)
- An optional rainfall intensity at the time of concentration for a display-only IDF check
- An optional target release rate if your ordinance is stricter than the pre-development peak
Method assumptions and limits
- Constant-rate Rational Method inflow block and constant outflow at each duration
- Duration domain td ≥ tc; no peak-flow extrapolation below tc
- Generic intensity ratio (tc/td)0.5; use local IDF data for final design
- Best for small to medium watersheds (under ~200 acres)
- Preliminary sizing only — verify with level-pool routing
- Does not size multi-stage outlets or account for tailwater
Frequently asked questions
What method does this detention pond calculator use?
The Pond Storage tab uses a Modified Rational storage envelope with a constant target outflow and a power-law intensity-duration assumption. It is a preliminary method for small to medium watersheds (generally under about 200 acres), not a routed design hydrograph. Final designs should be confirmed with reservoir (level-pool) routing for every required storm.
How is the required detention storage volume calculated?
The calculator maximizes a constant-rate Modified Rational inflow block over storm durations at or above the time of concentration, tc. When Qo is at most half of Qi, the interior maximum gives Vs = Qi² · tc / (4 · Qo). When Qi/2 < Qo < Qi, the unconstrained maximum would fall below tc, so the valid boundary governs and Vs = (Qi − Qo) · tc. When Qo is at least Qi, this screening method reports no detention requirement. The result is converted to acre-feet (US) or cubic meters (SI).
What is the target release rate, and what should I use?
The target release rate (Qo) is the maximum discharge the pond outlet is allowed to pass. Most stormwater regulations require the post-development peak to be held to the pre-development peak for one or more design storms, so the calculator defaults Qo to the pre-development peak flow if you leave the override blank. Always check your local ordinance — some jurisdictions require matching multiple return periods (for example the 2-, 10-, and 100-year storms).
What orifice and weir coefficients does the calculator use?
The outlet tabs use Cd = 0.60 for a sharp-edged orifice, C = 3.10 (US) / 1.7115 (SI) for a rectangular weir, C = 3.367 (US) / 1.859 (SI) for a Cipolletti weir, and about 2.5 (US) / 1.38 (SI) for a 90-degree V-notch. The rectangular value intentionally matches the FHWA Hydraulic Toolbox default; 3.33 / 1.84 is a common alternate sharp-crested coefficient, so use a custom value when the reviewing agency specifies it.
Why does the calculator recommend a minimum orifice size?
Small orifices clog easily with leaves, sediment, and trash. Following ASCE Manual of Practice 77, the calculator flags any computed orifice smaller than 4 inches (100 mm) in diameter. If your hydraulic calculation calls for a smaller opening, use at least the minimum size with a flow-control plate, or add a trash rack or debris guard upstream.
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What This Solves
Estimates preliminary detention storage from supplied peak flows and sizes one orifice or weir outlet at a specified head. A separate worked table illustrates stage-storage-discharge data, but the interactive tool does not generate a project routing table or pond dimensions.
Best Used When
- You need a preliminary storage estimate from known pre- and post-development peaks, time of concentration, and allowable release
- You want to size an individual orifice or weir for a target discharge at a known head
- You are screening detention feasibility before developing surveyed stage-storage data and routing hydrographs
Do NOT Use When
- You need an underground detention system instead of a surface pond — Use Underground Detention Calculator
- You need to route a full inflow hydrograph through the pond to get the outflow hydrograph — Use Level Pool Routing Calculator
- You want to compare several screening-level storage methods — Use Storage Volume Calculator
Key Assumptions
- The supplied Rational peak applies at tc and scales only for tested durations td >= tc
- Inflow and outflow are constant-rate blocks at each tested duration
- The generic peak-flow duration ratio is (tc/td)^0.5
- Each outlet sizing result uses a free-flow orifice or weir equation with a standard or user-entered coefficient
- Tailwater elevation does not significantly affect outlet discharge
Input Quality Notes
Peak flows must already incorporate the selected design storm. The optional rainfall intensity is display-only and does not change storage. Develop surveyed stage-storage data and the complete multi-stage outlet curve, then verify every required storm with hydraulic routing.
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Last verified: August 2026