Retention Pond Size Estimator (Simplified)
Estimate retention pond storage volume and surface area to capture a target fraction of stormwater runoff. For educational and early planning use only, not a substitute for detailed engineering design.
Drainage Area
Design Storm Rainfall
Runoff Coefficient
Typical values: 0.1–0.3 for permeable/vegetated areas, 0.5–0.7 for mixed surfaces, 0.7–0.95 for highly impervious surfaces.
Retention Pond Design
Fraction of runoff to capture (e.g., 0.8 = 80%)
Estimate Retention Pond Size for Your Site
Enter your drainage area, rainfall depth, runoff coefficient, and pond design parameters to estimate the storage volume and surface area needed to capture a target fraction of stormwater runoff.
Tip: A typical target capture fraction is 80% (0.8), and common pond design depths range from 3 to 8 feet depending on site constraints and regulatory requirements.
Understanding Retention Pond Sizing
What is a Retention Pond?
A retention pond (also called a wet pond or stormwater basin) is a designed depression that collects and stores stormwater runoff from a drainage area. Unlike detention ponds that temporarily hold water and release it slowly, retention ponds are designed to permanently hold a pool of water, with stored runoff released over time through evaporation, infiltration, or controlled discharge.
How This Simplified Estimator Works
This tool uses a two-step approach:
1. Runoff Volume = Drainage Area × Rainfall Depth × Runoff Coefficient
2. Design Storage = Runoff Volume × Target Capture Fraction × (1 + Freeboard)
3. Surface Area = Design Storage ÷ Pond Design Depth
The pond dimensions are then approximated using a rectangular shape with a specified length-to-width ratio (default 2:1).
Key Input Parameters
- Drainage Area: The land area that contributes runoff to the pond location.
- Rainfall Depth: The design storm rainfall amount, typically based on local standards (e.g., 10-year, 25-year storm).
- Runoff Coefficient: Fraction of rainfall that becomes surface runoff (0.1–0.95 depending on surface type).
- Target Capture Fraction: What percentage of runoff you want to capture (e.g., 80% = 0.8).
- Pond Design Depth: The average or effective water depth of the pond (typically 3–8 feet).
Typical Pond Design Depths
| Pond Type | Typical Depth Range |
|---|---|
| Shallow Retention / Wetland | 2–4 ft (0.6–1.2 m) |
| Standard Retention Pond | 4–8 ft (1.2–2.4 m) |
| Deep Detention Basin | 6–12 ft (1.8–3.7 m) |
| Underground Storage | Varies by system |
Limitations of Simplified Models
This tool provides a rough approximation only. Real retention pond design must account for many factors not considered here:
- Inflow hydrographs and peak flow rates
- Outlet structure sizing and discharge rates
- Side slopes, embankment stability, and freeboard requirements
- Sediment accumulation and maintenance access
- Groundwater levels and liner requirements
- Emergency spillway design for extreme events
- Water quality treatment and detention time requirements
- Local regulations and permitting criteria
How to Use Results for High-Level Thinking Only
Use these estimates for:
- Early-stage feasibility discussions about site development
- Understanding approximate land requirements for stormwater management
- Comparing relative pond sizes for different scenarios
- Initial conversations with engineers about pond sizing
Do NOT use these results for: final engineering design, construction drawings, permit applications, dam safety analysis, or any regulatory compliance documentation.
Important: This calculator is for educational and early planning purposes only. It does not replace detailed hydrologic and hydraulic modeling, civil engineering design, or professional consultation. Always work with licensed engineers and local stormwater authorities for real projects.
Frequently Asked Questions
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