Swale Flow Calculator

Flow Rate Q (m³/s)

0.50

Channel Slope S (%)

2.0

Convert to fraction for calculations (2% => 0.02).

Manning’s n

0.015

Grass-lined ~0.03, Concrete ~0.013

Bottom Width b (m)

3.0

Example: 0.1–10 m

Side Slope z (H:V)

2.0

e.g., 2 => 2:1 horizontal:vertical

Approx. Depth for Q: -- m

Velocity (at that depth): -- m/s

Hydraulic Radius (R): -- m

Flow Area (A): -- m²

Description & Methodology

This tool calculates the swale flow characteristics for a trapezoidal channel using Manning’s Equation:

v = (1 / n) * R^(2/3) * S^(1/2)

where v is velocity (m/s), n is Manning’s roughness coefficient, R is hydraulic radius (m), and S is the channel slope (dimensionless).

The flow area (A) and wetted perimeter (P) of a trapezoidal channel are:

A = y * b + z * y² and P = b + 2y√(1+z²)

The hydraulic radius is R = A / P. By evaluating velocity (and therefore flow Q = A·v) for incremental depths, we can plot a non-linear velocity vs. depth curve.

Adjust the sliders for flow rate (Q), channel slope (S), Manning's n, bottom width (b), and side slope (z) to see real-time updates of the velocity–depth relationship.