What This Solves
Calculates the required detention storage volume using multiple simplified methods (Modified Rational, Baker, FAA) without full hydrograph routing.
Best Used When
- You need a preliminary estimate of detention storage volume for site planning
- You want to compare storage estimates from several standard screening methods
- You are at the conceptual design stage and full routing analysis is not yet warranted
Do NOT Use When
- You need full hydrograph routing to verify pond and outlet sizing — Use Level Pool Routing Calculator
- You need to size the pond dimensions and outlet structure, not just the volume — Use Pond Sizing Calculator
Key Assumptions
- Simplified volume methods assume idealized inflow and outflow hydrograph shapes
- The allowable release rate is known and constant
- Each method has its own inherent assumptions about hydrograph shape and timing
- Results are preliminary and should be verified with full routing for final design
Input Quality Notes
Allowable release rate is typically set by local regulations (e.g., pre-development peak flow for a specified return period). Verify this value with the reviewing authority before sizing.
Estimate the detention storage volume needed to hold back stormwater and limit peak discharge to an allowable rate. Enter pre- and post-development peak flows for the Modified Rational Method, or use the Baker, FAA and Simplified quick methods for screening-level estimates — in US customary or metric units.
Calculation Method
Modified Rational Method: Calculate storage from peak flows using triangular hydrograph approximation. Best for preliminary sizing.
Storage Volume Overview
Detention storage volume is the amount of stormwater that must be temporarily held in a detention basin to reduce peak discharge rates to allowable levels.
The Modified Rational Method calculates storage as:
Where:
- Vs = Required storage volume
- Qi = Peak inflow (post-development)
- Qo = Allowable outflow (pre-development)
- tc = Time of concentration
- td = Critical storm duration
Method Comparison
| Method | Best For | Accuracy |
|---|---|---|
| Modified Rational | Preliminary sizing | Good for small watersheds |
| Baker Method | Quick estimates | Approximate |
| FAA Method | Airport drainage | Conservative |
| Simplified | Screening analysis | Order of magnitude |
For final design, use level pool routing with detailed stage-storage-discharge relationships.
For educational purposes only. Not a substitute for professional engineering judgment.
How storage volume is calculated
Detention works by storing the difference between the inflow hydrograph and the controlled outflow during the peak of a storm. The methods below estimate that stored volume.
Modified Rational Method (MRM)
The MRM extends the Rational Method by assuming a triangular inflow hydrograph and a constant outflow held at the allowable release rate. The required storage is:
The critical storm duration that maximizes storage is estimated as td = tc × Qi/Qo (you can also enter it directly). The factor 3600 converts cfs·hr to ft³ (or cms·hr to m³).
Quick approximation methods
The quick methods first compute peak inflow and outflow with the Rational equation Q = C·i·A (US customary, cfs) or Q = C·i·A / 360 (metric, m³/s), then apply:
- Baker: Vs = 0.87 × Qi × tc × (1 − Qo/Qi) × 3600
- FAA: Vs = tc × (Qi − Qo) × 60 (tc in minutes)
- Simplified: Vs = 0.5 × (Qi − Qo) × 2tc × 3600
Variable definitions:
- Vs = required storage volume (ft³ or m³)
- Qi = peak inflow, post-development (cfs or m³/s)
- Qo = allowable outflow, usually the pre-development peak (cfs or m³/s)
- tc = time of concentration (hr; quick methods take minutes)
- td = critical storm duration (hr)
- C = runoff coefficient (pre- and post-development)
- i = design rainfall intensity (in/hr or mm/hr)
- A = contributing drainage area (acres or ha)
When the allowable outflow Qo equals or exceeds the peak inflow Qi, no detention storage is required and the result is zero. For US customary results the volume is also reported in acre-feet (ft³ ÷ 43,560).
Storage method reference
Equations and coefficients as implemented in this calculator. All four are preliminary sizing methods — confirm final volumes with routed stage-storage-discharge analysis.
| Method | Storage equation | Best for | Character |
|---|---|---|---|
| Modified Rational | 0.5 (Qi−Qo)(tc+td) · 3600 | Preliminary basin sizing, small watersheds | Triangular hydrograph |
| Baker | 0.87 Qi tc (1−Qo/Qi) · 3600 | Quick estimates | Approximate (a = 0.87, b = 1.0) |
| FAA | tc (Qi−Qo) · 60 | Airport / general drainage | Conservative |
| Simplified | 0.5 (Qi−Qo) · 2tc · 3600 | Screening analysis | Order of magnitude |
Unit conversions used: 1 cfs·hr = 3600 ft³; 1 cms·hr = 3600 m³; 1 acre-ft = 43,560 ft³. Quick methods compute Qi and Qo from the Rational equation Q = CiA (US) or CiA/360 (metric).
Worked example
A site with a post-development peak of 25 cfs, an allowable (pre-development) release of 10 cfs, and a time of concentration of 30 minutes (0.5 hr), by the Modified Rational Method:
- Critical duration td = tc × Qi/Qo = 0.5 × 25/10 = 1.25 hr
- Storm duration = tc + td = 0.5 + 1.25 = 1.75 hr
- Required storage Vs = 0.5 × (25 − 10) × 1.75 × 3600 = 47,250 ft³
- In acre-feet: 47,250 ÷ 43,560 ≈ 1.08 acre-ft
Matches the calculator's verification case. By contrast, a Baker-method estimate for a 10-acre site (Cpost = 0.7, Cpre = 0.3, i = 4 in/hr, tc = 20 min) gives Qi = 28 cfs, Qo = 12 cfs and Vs ≈ 16,700 ft³.
Assumptions & limitations
Assumptions
- Inflow hydrograph approximated as triangular (or trapezoidal)
- Outflow can be held at the allowable release rate
- Critical storm duration produces the maximum storage requirement
- No significant losses within the detention facility
Limitations
- Simplified methods may underestimate complex hydrographs
- Outlet hydraulic controls are not modeled directly
- Assumes a single-peak inflow; no back-to-back storms
- Verify with detailed routing for final design
Methods and assumptions per HEC-22 (Urban Drainage Design Manual), TR-55 (Urban Hydrology for Small Watersheds, Ch. 6) and ASCE/WEF stormwater guidance.
Frequently asked questions
What is detention storage volume?
Detention storage volume is the amount of stormwater that must be temporarily held in a basin, chamber or tank so that the peak discharge leaving a site after development does not exceed the allowable (typically pre-development) release rate. The water is stored during the peak of the storm and released slowly through a controlled outlet.
Which method should I use — Modified Rational, Baker, FAA or Simplified?
Use the Modified Rational Method for preliminary detention sizing of small watersheds when you know pre- and post-development peak flows and the time of concentration. The Baker, FAA and Simplified methods are quick, screening-level approximations driven by the Rational equation (Q = CiA): Baker tends to be moderate, FAA is conservative, and Simplified is order-of-magnitude. For final design, all four should be confirmed with full level-pool (stage-storage-discharge) routing.
How does the Modified Rational Method estimate storage?
It assumes a triangular inflow hydrograph and a constant outflow held at the allowable release rate. Required storage is Vs = 0.5 × (Qi − Qo) × (tc + td) × 3600, where Qi is the post-development peak inflow, Qo is the allowable outflow, tc is the time of concentration and td is the critical storm duration that maximizes storage (estimated here as td = tc × Qi/Qo). The factor of 3600 converts cfs-hours (or cms-hours) to cubic feet (or cubic metres).
How do I convert the result to acre-feet?
In US customary units the calculator returns volume in cubic feet. Divide by 43,560 ft² per acre to get acre-feet — so 47,250 ft³ ÷ 43,560 ≈ 1.08 acre-ft. Acre-feet is the unit most municipal stormwater manuals and detention permits use for required storage.
Is this calculator suitable for final detention basin design?
No. These methods are intended for preliminary sizing, scenario comparison and planning. They simplify the inflow hydrograph and do not directly model the outlet hydraulics, multi-stage outlets or back-to-back storms. Final design should use a routed stage-storage-discharge analysis and meet your local stormwater manual and reviewing agency requirements.
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Last verified: February 2026