Detention Pond Volume Calculator

Inputs

This calculator estimates required detention storage from a simplified inflow–outflow balance and compares it with an assumed pond storage volume from geometry.

Formula used

How to use this calculator

1. Select a unit system and choose how peak inflow is defined.
2. Enter storm duration and the maximum allowed outflow rate.
3. If computing inflow, enter runoff coefficient, rainfall intensity, and drainage area.
4. Pick a hydrograph shape; set plateau fraction p for trapezoids.
5. Choose a geometry method and fill the required pond dimensions.
6. Click Calculate to see required storage, provided volume, and margin.
7. Download CSV or PDF for submittals, checks, and documentation.

Detention pond storage guidance

1) Purpose of detention storage

Detention ponds reduce downstream peak flows by temporarily storing runoff and releasing it at a controlled rate. For permitting, agencies commonly set an allowable discharge tied to pre‑development flow or a fixed outlet limit. The calculator compares required storage from inflow–outflow imbalance with the storage you can build.

2) Selecting a design storm

Design storms are typically defined by return period and duration. Many municipal manuals require 2‑, 10‑, 25‑, or 100‑year events, and may specify critical duration. Use local IDF curves to obtain rainfall intensity i for the chosen duration and recurrence. Always document the storm parameters in your report.

3) Runoff coefficient data ranges

The rational coefficient C reflects imperviousness and hydrologic response. Typical planning ranges are: rooftops and pavement 0.85–0.95, dense commercial 0.70–0.90, residential 0.35–0.60, parks and lawns 0.15–0.35, and open soil 0.05–0.20. Calibrate C using site grading and land cover.

4) Interpreting the hydrograph factor

Storage depends on the time distribution of inflow. A triangular hydrograph assumes a linear rise and fall, giving average inflow fraction k = 0.5. A trapezoid adds a flat peak; increasing plateau fraction p increases k and storage demand. Use trapezoids when the watershed has a sustained peak.

5) Outflow control and safety margin

Outflow Qout represents what your outlet works can reliably pass without surcharge. If Qin is close to Qout, required storage may be small, but real ponds need freeboard and allowance for debris, clogging, and tailwater. A practical margin is often 10–25% above the computed minimum, subject to agency rules.

6) Geometry-based volume with side slopes

For basins with sloped sides, the prismoidal formula provides a robust estimate between bottom and top areas. Rectangular and circular options here assume constant side slopes (e.g., 3H:1V). Common maintenance slopes are 3:1 to 4:1; steeper slopes may require stabilization and access controls. Verify the storage‑elevation curve for final design.

7) Construction and sediment considerations

Construction drawings should include emergency spillway, low‑flow channel or forebay, and sediment cleanout access. Sediment can consume storage over time; some programs allocate 5–10% of volume for long‑term sedimentation. If your site is phased, plan temporary controls that protect the permanent pond from excessive deposition.

8) Documenting results for review

Record input data, units, assumptions, and the selected geometry method. Include peak inflow, allowable outflow, storm duration, computed required volume, and the provided storage. Attach a simple narrative describing how intensities were obtained and how dimensions were measured. Clear documentation speeds approvals and reduces redesign cycles.

FAQs

1) What does this calculator estimate?

It estimates required detention storage from the inflow–outflow difference over a chosen storm duration, then compares that requirement with an estimated pond storage volume from your selected geometry.

2) Is the rational method valid for large watersheds?

Rational calculations are typically used for small catchments where uniform intensity is reasonable. For larger basins, use a full hydrograph model and route flows through storage to capture timing and attenuation properly.

3) How should I choose rainfall intensity?

Use local IDF curves for the target return period and a duration consistent with agency guidance or critical duration. Enter intensity in mm/hr or in/hr to match the selected unit system.

4) Why does hydrograph shape change required volume?

Storage is the time‑integral of excess flow. A longer sustained peak (trapezoid with higher plateau fraction) increases the average inflow during the event, so required storage increases.

5) What geometry option should I use?

Use rectangular or circular prismoids for basins with sloped sides and a defined bottom. Use the trapezoidal prismatic option for uniform sections along length, or enter a known storage volume from a grading model.

6) Does this replace final pond routing?

No. It is a screening and documentation tool. Final design should include stage–storage–discharge relationships, tailwater checks, emergency overflow sizing, and routing per your governing stormwater manual.

7) Why do my results look too small or too large?

Check units, storm duration, and whether allowable outflow exceeds peak inflow. Also verify runoff coefficient and intensity values. Use realistic side slopes and depth, and apply any required freeboard or sediment allowances.

Example data table

Illustrative example only; always use local design criteria and verified hydrology.

Tip: keep side slopes consistent with safety, maintenance, and geotechnical constraints.