Estimate swale flow capacity and velocity using Manning's equation for a trapezoidal section.
Design flow uses the Rational Method: Q = 0.278 x C x i x A.
Typical Manning n values: short grass 0.03-0.04, dense grass 0.04-0.06.
Planning estimate only. Final drainage design should be checked against local standards, climate allowances, and site-specific geotechnical conditions.
This calculator combines the Rational Method and Manning's equation to estimate whether a trapezoidal swale can carry stormwater from your catchment. Enter catchment area, runoff coefficient, rainfall intensity, and swale geometry to compare design flow with channel capacity.
Drainage area: The impermeable or contributing area directing water into the swale.
Runoff coefficient: Fraction of rainfall that becomes runoff. Hard surfaces are higher; landscaped areas are lower.
Rainfall intensity: Storm intensity for your chosen return period and duration, usually sourced from local design data.
Manning's equation estimates open-channel velocity from hydraulic radius, slope, and roughness. Higher slopes and smoother channels increase velocity; rougher vegetation slows flow. Capacity is then calculated as flow area multiplied by velocity.
This tool reports both capacity and velocity checks so you can balance conveyance against erosion control.
A pass means calculated capacity is greater than or equal to design flow. Velocity guidance in this tool uses 1.0 m/s as a simple screening threshold for many vegetated swales.
If velocity is high, consider a flatter slope, wider section, denser vegetation, check dams, or additional attenuation upstream.
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