Open Channel Hydraulics Formulas (Trapezoidal Cross-Section)
Trapezoidal channels are the most widely used shape for engineered irrigation canals, storm drainage ditches, and culvert cross-sections due to stable soil bank slopes ($z:1$, where $z$ is horizontal distance per 1 vertical foot or meter).
1. Cross-Sectional Flow Area
A = (b + z × y) × y
2. Top Water Surface Width
T = b + 2 × z × y
3. Wetted Perimeter
The total perimeter of the channel bed and banks in contact with fluid:
P = b + 2 × y × √(1 + z²)
4. Hydraulic Radius
R = A / P