Detention Pond Sizing Calculator

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.

Calculation Mode

Pond Storage: Calculate preliminary detention storage volume using the Modified Rational Method.

Input Parameters

Peak Flow Data

Enter pre- and post-development peak flows

cfs

Peak flow before development

cfs

Peak flow after development

Storm Parameters

Enter watershed timing; rainfall intensity is optional context

min

Watershed time of concentration

in/hr

Display-only IDF reference. Storage uses the supplied peak flow, so changing this value does not change the result.

Target Release (Optional)

Override default pre-development release rate

cfs

Leave blank to use pre-development peak

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

Typical discharge coefficients and notes for pond outlet structures
Structure TypeCoefficientNotes
Sharp-crested orifice0.60Standard value
Short tube orifice0.80L/D = 2-3
Rectangular weir (US)3.10FHWA Hydraulic Toolbox default
Cipolletti weir (US)3.371:4 side slopes
90-degree V-notch2.50Q = C x H^2.5

Source: FHWA HEC-22 (2009), Chapter 7-8.

For educational purposes only. Not a substitute for professional engineering judgment.

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