About Detention Basin Calculator
The detention basin calculator sizes stormwater detention storage with the Modified Rational Method. For each candidate storm duration it reads the rainfall intensity from an Intensity-Duration-Frequency (IDF) curve, computes the Rational-method inflow peak Qi = C·i·A, and works out the storage needed to hold back the difference between the inflow and the allowable release over the storm.
Because rainfall intensity falls as duration increases while the storm volume grows, the required storage reaches a maximum at a particular critical duration. The tool searches the duration range you specify, reports the critical duration, the peak inflow, and the required detention volume, and plots the full storage-versus-duration curve so you can see the governing storm.
How It Works
- Enter the catchment area (ha), the composite runoff coefficient C, and the allowable outflow (pre-development or permitted release, m³/s).
- Enter the IDF coefficients a and b for i = a/(td+b), and the duration range and step to search.
- For each duration the tool computes intensity, inflow Qi = C·i·A and storage S = (Qi − Qa)·td.
- It returns the critical duration that maximises S, the peak inflow, and the required detention volume, with the storage curve plotted.
Worked Example
A 2 ha catchment (A = 20,000 m²) with composite runoff coefficient C = 0.7 must limit its release to the pre-development peak Qa = 0.10 m³/s. The IDF curve is i = 4000/(td + 20) mm/hr. For each storm duration the inflow is Qi = C·i·A/3.6e6 and the storage is S = (Qi − Qa)·td·60. Searching durations from 5 to 180 minutes in 5-minute steps, the storage peaks at a critical duration of 35 minutes, where i = 72.7 mm/hr and Qi = 0.283 m³/s, giving a required detention volume of about 384 m³. A finer 1-minute search refines the critical duration to 36 minutes with essentially the same 384 m³.
Formulas
- IDF rainfall intensity
i(td) = a / (td + b)- Rational Method inflow peak
Qi = C * i * A / 3.6e6- Required storage for a duration
S(td) = (Qi(td) - Qa) * td * 60- Critical duration
td* = argmax over td of S(td); required volume = S(td*)
Standards & References
- Modified Rational Method (Poertner, 1974)
- ASCE/WEF MOP 77 — Design and Construction of Urban Stormwater Management Systems
- Rational Method (Kuichling, 1889)
Frequently Asked Questions
What is the Modified Rational Method?
It extends the Rational Method peak-flow formula to estimate detention storage by treating the runoff hydrograph as a rectangle of height Qi and width equal to the storm duration. The storage needed is the volume between this inflow and the allowable outflow, evaluated for many durations to find the worst case.
Why search across storm durations?
Short storms are intense but brief, while long storms are mild but voluminous. The required storage is largest for an intermediate critical duration, so the method tests a range of durations and reports the one that maximises storage — that storm governs the basin size.
What is the allowable outflow?
It is the maximum release rate the basin may discharge, usually set to the pre-development peak flow or a value fixed by the drainage authority. The detention basin stores the excess so that the post-development peak leaving the site does not exceed this limit.
What units does the calculator use?
Catchment area is entered in hectares and converted to square metres internally, intensity is in mm/hr from the IDF curve, flows are in m³/s, durations in minutes, and the resulting detention volume is reported in cubic metres.