What This Solves
Calculates the most hydraulically efficient channel cross-section dimensions that maximize flow capacity for a given cross-sectional area.
Best Used When
- You want to minimize excavation while maximizing channel capacity
- You are designing a new channel and want the most efficient geometry as a starting point
- You need to compare the efficiency of rectangular, trapezoidal, and triangular sections
Do NOT Use When
- You already know the channel geometry and need to calculate flow capacity — Use Manning's Channel Calculator
- You need to evaluate channel lining adequacy for erosion protection — Use Channel Lining Calculator
Key Assumptions
- The optimal section is derived from minimizing wetted perimeter for a given area
- Side slopes are stable and can be constructed at the optimal angle
- Roughness is uniform across the entire wetted perimeter
- The channel is prismatic (constant cross-section) along its length
Input Quality Notes
The theoretical best section may not be practical due to soil stability, construction constraints, or right-of-way limits. Use it as a design starting point, then adjust for site conditions.
Optimal Section Proportions
| Shape | Optimal Proportions | R/y Ratio | Efficiency* |
|---|---|---|---|
| Semicircle | Theoretical optimum | 0.500 | 100% |
| Rectangular | b = 2y (width = 2 x depth) | 0.5 | ~95% |
| Trapezoidal | z = 0.577, b = 1.15y (half-hexagon) | 0.5 | ~97% |
| Triangular | z = 1.0 (45-degree sides) | 0.3536 | ~83% |
* Efficiency compared to semicircular section for same flow area
Ready to Calculate
Select a channel shape and design approach to find optimal dimensions.
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Last verified: February 2026