Retention Basin Calculator

Calculator

Example data table

Formula used

• Rainfall depth: d = i × t
• Runoff volume: V = C × d × A × (1 − loss%)
• Peak inflow (rational approximation): Q_in = C × i × A
• Outflow volume: V_out = Q_allow × t
• Required storage: V_s = max(0, V − V_out)
• Design storage: V_design = V_s × (1 + safety%)
• Effective depth: h = depth − freeboard
• Frustum volume check: V = h/3 × (A₁ + A₂ + √(A₁A₂))

How to use this calculator

1. Enter the drainage area and select its unit.
2. Choose a runoff coefficient that matches land cover.
3. Set a design rainfall intensity and storm duration.
4. Enter the allowable outflow limit from your criteria.
5. Provide basin depth, freeboard, and side slopes.
6. Select rectangular or circular shape and aspect ratio.
7. Use loss and safety factors to reflect uncertainty.
8. Click Calculate, then export results if needed.

Professional notes for retention basin sizing

1) Why retention basins matter

Retention basins store stormwater to reduce downstream flooding, erosion, and surcharge on buried networks. Many sites target a controlled release rate that matches predevelopment flow, protects culverts, and reduces peak stage in channels. This calculator gives a fast planning estimate for storage and footprint.

2) Start with defensible site data

Collect drainage area from survey or GIS, then confirm flow paths during a field walk. A 2.5 ha catchment with 60 mm/hr intensity for 30 minutes produces 45 mm rainfall depth. With C = 0.55, the gross runoff volume is about 619 m³ before loss and safety adjustments.

3) Runoff coefficient selection

Runoff coefficient C summarizes imperviousness and surface response. Low-density residential often ranges 0.30–0.55, commercial pavements commonly 0.70–0.95, and landscaped areas may be 0.10–0.35. Use a weighted C when subareas differ, and document assumptions for review.

4) Rainfall intensity and duration choices

Choose intensity from local IDF curves for the required return period. Duration strongly affects volume because depth equals intensity times time. If agency criteria use a critical duration, run several durations (for example 15, 30, 60 minutes) and compare storage demand.

5) Allowable outflow and drawdown

The allowable outflow reflects policy, downstream capacity, and risk tolerance. A common operational goal is complete drawdown within 24–48 hours to recover storage and limit standing water. This calculator estimates drawdown using storage divided by allowable flow, which is a conservative first check.

6) Storage, loss factor, and safety margin

Loss factor reduces runoff volume to represent interception, surface depression storage, and infiltration. On permeable soils, a 5–15% reduction may be reasonable for screening, while tight clays may justify 0–5%. Safety margin (often 10–25%) helps cover clogging, grading tolerances, and forecast uncertainty.

7) Geometry: depth, freeboard, and slopes

Effective depth equals total depth minus freeboard. Many designs use 0.30 m freeboard (or 1 ft) and side slopes near 3H:1V for maintainability and safety. The calculator sizes either a rectangular or circular basin and checks volume using a frustum approximation.

8) Practical checks before final design

Confirm available footprint, utility conflicts, inlet/outlet elevations, and safe access for maintenance. Verify that the outlet structure can reliably limit discharge and that emergency spillways are included where required. Final design typically uses routing with stage-storage-discharge curves and local standards.

Use these results for early planning, budgeting, and option comparison only.

FAQs

1) What is the difference between detention and retention?

Detention temporarily stores runoff and releases it later. Retention typically includes permanent storage or infiltration features. Many projects use the term “retention basin” loosely; confirm the intended function and design criteria for your jurisdiction.

2) Which runoff coefficient should I use?

Use a value that matches land cover and imperviousness, ideally weighted by subarea. If unsure, run a low and high scenario (for example 0.45 and 0.75) to understand sensitivity before finalizing assumptions.

3) How do I choose the storm duration?

Start with the duration required by your approving agency or the critical duration from local guidance. For screening, test multiple durations (15–60 minutes) and size for the largest storage requirement.

4) Why does the calculator include freeboard?

Freeboard reserves height above the design water surface to reduce overtopping risk and allow for wind, wave, and operational uncertainty. Storage is computed using effective depth, so increasing freeboard reduces available storage and increases footprint.

5) What if allowable outflow is zero?

If outflow is set to zero, the model assumes no discharge during the storm, so required storage approaches the total runoff volume. This can be a conservative check for blocked outlets, but it may overestimate typical operational conditions.

6) Are infiltration and soil permeability fully modeled?

No. The loss factor provides a simple reduction for screening. For infiltration basins, you should model measured infiltration rates, seasonal groundwater, clogging factors, and compliance drawdown windows using a dedicated infiltration design method.

7) How accurate are the basin dimensions?

Dimensions are planning estimates based on simplified event volume sizing and a frustum geometry. Final layouts should include grading constraints, berm widths, side-slope benches if needed, and detailed hydraulic outlet sizing to meet standards.