Calculate preliminary detention pond storage requirements using the Modified Rational Method. Size orifice and weir outlets for stormwater management facilities. Professional-grade hydrology tool for civil engineers.
Pond Storage: Calculate preliminary detention storage volume using the Modified Rational Method.
Orifice Sizing: Size a circular orifice outlet for a target discharge at a given head.
Weir Sizing: Size a weir outlet structure (rectangular, V-notch, or Cipolletti) for a target discharge.
This calculator provides preliminary sizing for detention ponds and outlet structures. The Modified Rational Method estimates required storage volume based on the difference between post-development and pre-development peak flows.
Key equations:
| Structure Type | Coefficient | Notes |
|---|---|---|
| Sharp-crested orifice | 0.60 | Standard value |
| Short tube orifice | 0.80 | L/D = 2-3 |
| Rectangular weir (US) | 3.33 | Sharp-crested |
| 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.
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