What Is the Retention Basin Drawdown Time Calculator?
The Retention Basin Drawdown Time Calculator determines the emptying time of stormwater retention and detention basins via bottom control orifices.
How Does the Retention Basin Drawdown Time Calculator Work?
Integrates the Torricelli orifice discharge equation over a variable falling water surface level.
Retention Basin Drawdown Time Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
t_{drain} = \frac{2 A_{pond}}{C_d A_o \sqrt{2 g}} \left( \sqrt{h_0} - \sqrt{h_f} \right)
Integrates Torricelli orifice discharge over a falling pond water elevation.
How to Use the Retention Basin Drawdown Time Calculator
- Input initial water depth and mean pond water surface area.
- Enter control orifice diameter and target drawdown percentage.
- Review total emptying hours and initial peak flow rate.
Step-by-Step Example Calculation
Subdivision Stormwater Detention Basin
Input Values:
permanentPoolDepthM:1.5
topSurfaceAreaM2:2500
orificeDiameterMm:150
drawdownTargetFraction:0.95
orificeDischargeCoefficient:0.62
Worked Steps: Drawdown takes 26.2 hours, perfectly matching the municipal 24-48 hr water quality requirement.
Understanding Your Result
Total Drawdown Time: Duration required to drain the water quality volume.
Peak Initial Flow: Maximum discharge when the pond is full.
Average Flow Rate: Mean discharge over the drainage cycle.
Factors That Affect the Result
- Orifice area scales with diameter squared (d²); doubling orifice size cuts drawdown time by a factor of 4.
When Should You Use This Calculator?
- Stormwater management compliance, EPA NPDES Phase II permits, and flood retention basin design.
Assumptions & Limitations
- Assumes zero continuous baseflow inflow during the post-storm drawdown period.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Standard falling-head orifice integration.