Retention Pond Sizing Calculator

About the Retention Pond Sizing Calculator

Retention ponds (also called detention basins or stormwater ponds) store runoff from storm events to prevent downstream flooding, reduce erosion, and improve water quality. Sizing depends on the volume of runoff generated by the design storm, which is a function of rainfall depth, drainage area, and the runoff coefficient.

For agricultural settings, retention ponds may also serve as irrigation reservoirs, capturing spring runoff for summer use. In regulated areas, they must meet minimum storage requirements set by local stormwater ordinances.

This calculator estimates the required storage volume from basic watershed parameters and converts to acre-feet, cubic feet, and gallons for comparison with pond construction plans. Use this page to estimate storage needs before comparing them with planned pond dimensions or irrigation reuse goals.

When This Page Helps

Under-sized ponds overtop and fail during large storms. Over-sized ponds waste land and construction cost. This page helps tie the design storm to a target storage volume before excavation or permitting decisions are made.

How to Use the Inputs

1. Enter the design storm rainfall depth in inches.
2. Enter the drainage area in acres.
3. Enter the runoff coefficient (C) for your land cover.
4. Read the required storage volume in acre-feet and cubic feet.
5. Compare with your pond design volume (use the Pond Storage Volume calculator).

Example Calculation

Tips & Best Practices

• Design storm: often 10-year or 25-year 24-hour event for agricultural ponds.
• Runoff coefficients: cropland 0.30–0.60, pasture 0.10–0.40, impervious 0.90–0.95.
• Include a freeboard of 1–2 ft above the design water level.
• A principal spillway (pipe) handles normal storms; an emergency spillway handles extreme events.
• Pond volume should exceed calculated runoff by 10–20% for sediment storage.
• Multi-purpose ponds (retention + irrigation) provide dual economic benefit.

Emergency Spillway Design

Every retention pond needs an emergency spillway to safely pass extreme storms without dam failure. The spillway is a broad, grassed channel at the dam crest. NRCS design standards (TR-60) specify spillway width and depth based on dam height and inflow. Never block or reduce the emergency spillway.

Sediment Storage

Retention ponds trap sediment from the watershed. Over time, sediment fills the lower pool, reducing storage capacity. Design includes a sediment storage pool (typically 10–20% of total volume) below the normal water level. Periodic dredging restores capacity.

Water Quality Benefits

Retention ponds reduce downstream nutrient and sediment loads by settling particles and allowing biological uptake. They can remove 60–80% of suspended sediment, 30–50% of total phosphorus, and 20–40% of total nitrogen. A well-vegetated buffer around the pond further improves water quality.

Frequently Asked Questions

• What is the design storm?

  The design storm is the rainfall event the pond is sized to handle. For agricultural retention, a 10-year or 25-year 24-hour storm is typical. NOAA Atlas 14 provides rainfall depths by frequency and location.

• What is the difference between retention and detention?

  A retention pond holds water permanently (like a lake). A detention basin stores water temporarily and drains within 24–72 hours through an outlet. Both manage stormwater, but detention actively releases water.

• How do I find the runoff coefficient?

  C depends on soil type, slope, and land cover. NRCS engineering handbooks and state drainage guides provide tables. For mixed-use watersheds, calculate a weighted average C.

• What about the SCS Curve Number method?

  The CN method (TR-55/TR-20) is more detailed and accounts for initial abstraction, soil moisture, and antecedent conditions. It's preferred for larger watersheds. The Rational Method used here is simpler and suitable for rough sizing.

• Does the pond need a permit?

  Usually yes, especially if it impounds water above a certain height (6–15 ft depending on state) or volume (15–50 ac-ft). Dam safety regulations, Army Corps 404 permits, and local stormwater permits may all apply.

• Can I use the pond for irrigation?

  Yes. A dual-purpose retention/irrigation pond captures spring runoff for use during the growing season. Size the pond to hold both the stormwater detention volume and the irrigation supply. Install a pump and draw-down structure.