Swale Capacity 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.

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