The Situation
You are designing the stormwater management for a 5-acre residential subdivision. The pre-development site was mostly wooded with good soil (Hydrologic Soil Group B). The post-development site will have 20 single-family homes with driveways, roads, and lawns. The local jurisdiction requires that the post-development peak discharge for the 10-year storm does not exceed the pre-development peak.
You need to size a detention pond with an outlet structure to attenuate the increased peak flow.
Which Calculator to Use
The Pond Sizing Calculator determines the required storage volume and outlet dimensions to reduce the post-development peak flow to the pre-development rate.
Walking Through the Inputs
Pre-Development Peak Flow (Qpre)
Using the SCS Curve Number method for pre-development conditions:
- Area: 5 acres
- Curve Number: CN = 55 (woods in good condition, HSG B)
- Design storm: 10-year, 24-hour rainfall = 5.0 inches (typical Mid-Atlantic)
- Time of concentration: tc = 30 minutes (undeveloped watershed)
The pre-development peak flow is approximately 4.5 cfs.
Post-Development Peak Flow (Qpost)
For the developed condition:
- Area: 5 acres (same)
- Curve Number: CN = 70 (residential 1/4-acre lots, HSG B — weighted for roads, roofs, and lawns)
- Same design storm: 5.0 inches
- Time of concentration: tc = 20 minutes (faster due to impervious surfaces and grading)
The post-development undetained peak flow is approximately 12.5 cfs.
Target Release Rate (Qo = 4.5 cfs)
The outlet must limit the discharge to the pre-development peak of 4.5 cfs. This is the maximum flow the pond is allowed to release.
Time of Concentration (tc = 20 minutes)
This is the post-development time of concentration. The pond needs to store the difference between the inflow volume and the released volume during the critical storm duration.
Rainfall Intensity at tc (i = 5.5 in/hr)
The 10-year rainfall intensity for a 20-minute duration at this location is approximately 5.5 in/hr.
The Results
The calculator determines:
- Required storage volume: Approximately 18,000-22,000 cubic feet (0.41-0.50 acre-feet)
- Pond surface area: Approximately 4,000-6,000 square feet at the maximum water surface elevation (depending on the depth and side slopes)
- Outlet orifice diameter: Approximately 8-10 inches to restrict flow to 4.5 cfs at the maximum water depth
What This Means for Your Project
Pond Dimensions (Typical)
For a pond averaging 4 feet deep with 3:1 side slopes:
- Bottom area: approximately 3,000 sq ft (roughly 50 ft x 60 ft)
- Top area: approximately 5,400 sq ft (roughly 66 ft x 82 ft)
- Total footprint including maintenance access: approximately 8,000 sq ft
This is about 4% of the 5-acre site — typical for a subdivision detention pond.
Outlet Structure
The outlet typically consists of:
- Low-flow orifice: 8-10 inch circular opening for the 10-year storm control
- Emergency spillway: A broad-crested weir or vegetated swale at the top of the pond to safely pass the 100-year storm without overtopping the embankment
- Trash rack: A grate over the orifice to prevent clogging
- Anti-seep collar: Around the outlet pipe to prevent water from piping along the outside of the pipe through the embankment
Regulatory Requirements
- Submit detention calculations with the subdivision plat
- The pond must include a permanent pool for water quality (if required locally)
- A maintenance agreement must be recorded on the plat
- Annual inspection is typically required
Try This Scenario
Try this scenario in the Pond Sizing CalculatorNext Steps
- Use the SCS Curve Number Calculator to verify the pre- and post-development runoff volumes
- Design the outlet orifice with more precision using hydraulic analysis
- Verify 100-year storm overflow capacity with an emergency spillway check
- Consult local stormwater ordinances for water quality volume requirements