What Is the Dry Extended Detention Basin Drawdown Orifice Calculator?
The Dry Extended Detention Basin Drawdown Orifice Calculator determines the exact bottom orifice diameter required to control stormwater release rates.
How Does the Dry Extended Detention Basin Drawdown Orifice Calculator Work?
It computes the average volumetric flow rate and solves the Torricelli hydraulic equation across the mean driving head.
Dry Extended Detention Basin Drawdown Orifice Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Q_{avg} = \frac{\text{WQ}_v}{t_d}, \quad A_o = \frac{Q_{avg}}{C_d \sqrt{2 g h_{avg}}}, \quad d_o = \sqrt{\frac{4 A_o}{\pi}}
Torricelli orifice hydraulics integrated over the extended detention drawdown timeline.
How to Use the Dry Extended Detention Basin Drawdown Orifice Calculator
- Enter total water quality volume in m³.
- Specify average driving water head above orifice centerline.
- Input regulatory drawdown duration (e.g. 24, 36, or 48 hours).
Step-by-Step Example Calculation
2,500 m³ Residential Basin 36-Hour Drawdown
Input Values:
waterQualityVolumeM3:2500
averagePondingHeadM:1.2
targetDrawdownHours:36
orificeDischargeCoeffCd:0.6
Worked Steps: Requires a 84.8 mm diameter circular orifice plate discharging an average of 19.3 L/s.
Understanding Your Result
Orifice Diameter: Required circular opening in millimeters and inches.
Average Discharge: Sustained release rate in L/s.
Orifice Area: Total flow area across the control plate.
Factors That Affect the Result
- Deeper ponding head increases exit velocity, requiring smaller orifice openings.
- Longer target drain times demand smaller restrictive orifices.
When Should You Use This Calculator?
- Stormwater management pond design, regional flood attenuation basins, and commercial site drainage outlets.
Assumptions & Limitations
- Assumes steady drawdown across average head without submerged downstream tailwater backpressure.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Adheres to US EPA and standard municipal stormwater drainage guidelines.