Swale Volume Calculator

Inputs

Example data table

Example volumes assume full depth storage and no freeboard or safety factor.

Formula used

• Volume: V = A × L (cross-sectional area times swale length)
• Rectangular area: A = b × d
• Triangular area: A = ½ × T × d (or A = z × d² when T = 2 z d)
• Trapezoid area: A = (b + T)/2 × d (or T = b + 2 z d)
• Wetted perimeter: rectangle P = b + 2d, trapezoid P = b + 2d √(1+z²)
• Hydraulic radius: R_h = A / P (useful for hydraulic checks)

Effective storage applies freeboard and safety factor: V_eff = V × (1 − freeboard) × safety.

How to use this calculator

1. Select your swale shape that matches the excavation profile.
2. Enter swale length and depth, then choose consistent units.
3. Provide geometry: bottom width plus slope, or top width mode.
4. Set freeboard to keep capacity above the stored water level.
5. Add a safety factor for sediment, vegetation, and tolerances.
6. Optional: enable storm check, then enter drainage area and rain depth.
7. Press Calculate to see results above, then export CSV or PDF.

Professional guide to swale storage sizing

1) Why storage volume matters

Swales act as shallow conveyance and temporary storage for runoff. Storage volume helps you confirm that the channel can detain a target storm depth long enough to reduce peak flow and improve water quality. In many site plans, storage is paired with controlled outlets or check dams.

2) Cross‑section choices in practice

Trapezoids are common because they balance stability and capacity. Rectangles appear in lined channels and tight corridors. V‑shapes can fit narrow rights‑of‑way but concentrate flow, so confirm erosion control, lining needs, and safety at pedestrian edges.

3) Typical geometry ranges

For residential and light commercial sites, depths often fall between 0.30–0.60 m with bottom widths around 0.30–1.20 m. Side slopes are frequently 3H:1V or flatter for mowable turf. Longitudinal slopes commonly range from 1–4% depending on terrain and lining.

4) How the calculator builds area

The tool computes cross‑sectional area A from your shape, then multiplies by length L to get volume V. Trapezoids use A = (b + T)/2 × d and can derive top width from a side slope z using T = b + 2zd. Rectangles use A = b × d, while triangles use A = ½Td or A = zd².

5) Freeboard and safety margins

Freeboard reserves a portion of depth above the design water surface. A common starting point is 10–20% of depth, increased where debris or wave action is expected. A safety factor (often 1.1–1.3) can cover sediment accumulation, vegetation growth, and construction tolerances over time.

6) Storm volume comparison

If you enable the storm check, required volume is calculated as drainage area × rain depth × runoff coefficient C. Typical C values are about 0.20 for lawns, 0.60 for roofs, and 0.90 for pavements. Many preliminary checks use 25–50 mm events, then adjust using local IDF guidance and allowable release rates.

7) Units and reporting for field teams

Designers often size in meters and verify in liters for smaller projects, while contractors may prefer cubic feet and gallons. The CSV output supports quick quantity takeoffs, and the PDF summary helps attach assumptions to submittals, inspection notes, and client handover packages.

8) Practical checks beyond volume

Volume alone does not guarantee performance. Confirm side slopes meet stability and maintenance targets, verify longitudinal slope and lining for shear resistance, and consider inlet spacing. Where infiltration is intended, coordinate with soil tests and drawdown criteria. Also review outlets, overflow paths, and sediment forebays to keep capacity functional.

FAQs

1) What is swale storage volume?

It is the temporary water volume a swale can hold along its length, based on its cross‑sectional area and length, before overtopping the design depth.

2) Should I choose trapezoid or triangle?

Trapezoids usually provide easier maintenance and more stable side slopes. Triangular sections fit narrow spaces but may increase flow velocity and erosion risk without lining.

3) What side slope values are common?

Mowable turf swales often use 3H:1V or flatter. Steeper slopes may require reinforcement, higher‑quality compaction, or hard lining depending on soil and velocities.

4) How do freeboard and safety factor change results?

Freeboard reduces usable depth to keep capacity above the water surface. The safety factor multiplies the final capacity to reflect uncertainty, sediment, vegetation, and construction variation.

5) What runoff coefficient should I use?

Use a value matched to surface type and local guidance. A quick starting point is 0.20 for lawns, 0.60 for roofs, and 0.90 for pavements, then refine by land‑use mix.

6) Does this replace hydraulic flow design?

No. It estimates storage capacity. For final design, also check flow capacity, velocity, shear stress, inlet/outlet controls, and the required drawdown time set by local standards.

7) Why is hydraulic radius shown?

Hydraulic radius supports later conveyance checks using common open‑channel equations. It is computed as area divided by wetted perimeter and helps compare shapes consistently.