MachineCalcs

Swale Capacity Calculator

Estimate drainage swale runoff, trapezoidal open-channel capacity, velocity, required flow depth, freeboard depth and limiting drainage area.

Hydraulics 11 inputs 9 results
Used in Piping Trades 28

Calculator

Inputs

View results
Area draining to the swale section being checked.
ft²
Use the local design rainfall intensity for the storm duration and return period being screened.
in/h
Weighted runoff coefficient for the watershed surface mix.
Extra factor applied to runoff for sediment, vegetation, maintenance, uncertainty and local criteria.
%
Flat bottom width of the swale section.
in
Water depth available for the design-flow capacity check, before adding freeboard.
in
Extra depth above design flow reserved for freeboard, sediment, vegetation and maintenance tolerance.
in
Horizontal run per one vertical rise on each side of the trapezoid.
Longitudinal channel slope as fall divided by run times 100.
%
Open-channel roughness coefficient for the swale lining and maintenance condition.
User-entered permissible velocity screen for erosion or lining limits.
ft/s

Results

Default result
Edit inputs
Design runoff flow(Qd)
174.5gpm
Pass

Raw runoff before allowance: 550.6 L/min.

Also computed

Swale capacity(Qc)Pass225.3gpm

Trapezoidal open-channel Manning capacity at the entered flow depth.

Capacity utilization(Uc)Pass77.47%

Flow velocity(v)Pass0.753ft/s

Velocity at the calculated Manning capacity, not only the design runoff.

Velocity utilization(Uv)Pass18.82%

Velocity-limited capacity: 4531 L/min.

Required flow depth(yreq)Caution3.528in

Depth required to carry design runoff with the entered bottom width, side slope, channel slope and roughness.

Bank depth with freeboard(yb)8in

Entered design flow depth plus freeboard allowance.

Swale preview: capacity 77 percent used, velocity 19 percent of targetDrainage swale previewTrapezoidal open-channel capacity from slope, roughness and flow depthdesign runoff661 L/minswale capacity853 L/minCapacity77%VelocityScreen only. Local stormwater criteria, erosion limits, vegetation, maintenance, check dams, tailwater and outlet conditions control final design.
Method notes 4 notes
  • Runoff screen uses Q = C*i*A with user-entered rainfall intensity, runoff coefficient and safety/clog factor.
  • Swale capacity uses a trapezoidal open-channel Manning approximation from bottom width, flow depth, side slope, longitudinal slope and roughness.
  • Velocity is checked against the user-entered target maximum. Soil, grass cover, lining, check dams and erosion-control criteria can set a lower permissible velocity.
  • Final swale design still needs local stormwater criteria, time of concentration, ponding and bypass checks, erosion/sediment control, maintenance access, vegetation condition, tailwater/outlet checks and civil review.

Swale capacity starts with runoff, Q = C*i*A, then checks whether a trapezoidal open channel can carry it. This calculator uses bottom width, side slope, flow depth, channel slope and Manning n to estimate capacity, checks velocity against an entered target, back-solves required flow depth and reports freeboard and limiting drainage area.

Continue workflow

All Hydraulics
Workflow placement Piping Trades Stage 2: Lay out the install
Next 1 Size catch basin Use catch-basin sizing when surface runoff needs inlet collection before or after the swale. Next 2 Size French drain Compare open-channel swale conveyance with subsurface French-drain storage for the same drainage area. Next 3 Size gutters Start with roof runoff, gutter capacity and downspout count when roof discharge feeds the swale. Chart Use reference data Pipe fitting takeout chart

How to use this calculator

  1. Enter drainage area. Use the area that drains to this swale section, not the entire site unless all runoff reaches this section.
  2. Set rainfall and runoff. Use local design rainfall intensity, a weighted runoff coefficient and any safety or clog allowance.
  3. Enter cross section. Add bottom width, design flow depth, freeboard allowance and side slope.
  4. Enter channel condition. Set longitudinal slope, Manning roughness and the target maximum velocity for the lining or vegetation.
  5. Check the bottleneck. Compare capacity utilization, velocity utilization, required flow depth and limiting drainage area.

How it works

Runoff starts with the Rational Method screen: Q = C i A where C is runoff coefficient, i is rainfall intensity and A is drainage area. The entered safety or clog allowance is applied after that raw runoff calculation.

The swale is treated as a trapezoidal open channel: A = y (b + z y) and P = b + 2 y sqrt(1 + z^2). Manning capacity then follows Q = (1/n) A Rh^(2/3) S^(1/2).

For nearby site-drainage checks, use the catch basin sizing calculator, French drain sizing calculator and pipe slope calculator.

Worked example

Verified against the live calculator

With the defaults, a 20,000 ft^2 drainage area at 2 in/h rainfall and C = 0.35 produces about 175 gpm of design runoff after the 20% allowance. A 1 ft bottom, 4 in design flow depth, 3:1 side slopes, 0.5% channel slope and n = 0.05 carry about 225 gpm, so the default section is close enough to make freeboard, erosion and maintenance checks meaningful.

Frequently asked questions

How do you calculate drainage swale capacity?

This calculator uses a trapezoidal open-channel Manning equation from bottom width, side slope, flow depth, channel slope and Manning roughness.

Does this choose the right rainfall intensity or design storm?

No. Enter the rainfall intensity and return-period criteria required by the project, local authority or drainage report.

Why is there a velocity check?

A swale can have enough flow area but still run too fast for the soil, grass, lining or erosion-control criteria. The target velocity is user-entered for that reason.

Is this a final stormwater design?

No. Final swale design still needs time of concentration, ponding and bypass checks, erosion control, maintenance condition, outlet/tailwater review and local stormwater approval.

Method & assumptions

  • Rainfall intensity and runoff coefficient are user-entered project criteria.
  • The swale section is a simple trapezoid with symmetric side slopes.
  • Manning roughness is user-entered and depends on grass height, lining, sediment and maintenance condition.
  • Velocity is compared with a user-entered target; the calculator does not choose a permissible erosion velocity.
  • Final design still needs time of concentration, ponding depth, bypass, outlet/tailwater, erosion control, vegetation, maintenance and local stormwater review.
Embed this calculator on your site free

Paste this where you want the calculator to appear. It stays in sync — same formulas, metric & imperial, light/dark — and a small credit link helps people find more tools.

Open widget

Live preview