Free Online Manning Formula Trapezoidal Channel Calculator

Manning Formula Uniform Trapezoidal Channel Flow at Given Slope and Depth

Solve for depth, y, given Q — Using b, z₁, z₂, S, and n from the calculator form, finds the depth for a given Q.
Flow, Q:

Inputs

Bottom width, b		X
Side slope 1, z₁ (horiz./vert.)		X
Side slope 2, z₂ (horiz./vert.)		X
Manning roughness, n? Strickler B/B P&I		X
Channel slope, S		X
Flow depth, y		X
Bend Angle, β?		X
Rock specific gravity, sg ?		X
Design rock size, D₅₀ ? Isbash Maynord Searcy *		X

Results

Flow area, A	Flow area, A	X
Wetted perimeter, P_w	Wetted perimeter, P_w	X
Hydraulic radius, R_h	Hydraulic radius, R_h	X
Velocity, v	Velocity, v	X
Velocity check	Velocity check	X
Flow, Q	Flow, Q	X
Velocity head, h_v	Velocity head, h_v	X
Top width, T	Top width, T	X
Froude number, Fr	Froude number, Fr	X
Average shear stress, τ	Average shear stress, τ	X
n for design rock size (Strickler method)	n for design rock size (Strickler method)	X
n for design rock size (Blodgett method)	n for design rock size (Blodgett method)	X
n for design rock size (Bathurst method)	n for design rock size (Bathurst method)	X
Blodgett vs. Bathurst	Blodgett vs. Bathurst	X
n for design rock size (Phillips & Ingersoll method)	n for design rock size (Phillips & Ingersoll method)	X
P&I range check	P&I range check	X
Required bottom angular rock size, D₅₀ (Isbash & MC) ?	Required bottom angular rock size, D₅₀ (Isbash & MC) ?	X
Required side slope 1 angular rock size, D₅₀ (Isbash & MC) ?	Required side slope 1 angular rock size, D₅₀ (Isbash & MC) ?	X
Required side slope 2 angular rock size, D₅₀ (Isbash & MC) ?	Required side slope 2 angular rock size, D₅₀ (Isbash & MC) ?	X
Required angular rock size, D₅₀ (Maynord, Ruff, and Abt 1989)	Required angular rock size, D₅₀ (Maynord, Ruff, and Abt 1989)	X
Required angular rock size, D₅₀ (Searcy 1967)	Required angular rock size, D₅₀ (Searcy 1967)	X

Notes

Automated rock size and roughness design iteration

Choose a roughness option (Blodgett–Bathurst recommended) and a design rock size option (Isbash recommended). Adjust depth and rock size safety factor to reach your target flow with a uniform rock size. Each time you change an input, the calculator repeats these steps: 1. Roughness is calculated from design rock size. 2. The requested roughness calculation is copied to input roughness. 3. Channel flow and required rock size are calculated. 4. Design rock size is adjusted. 5. Repeat until error in the design rock size is very small.

Basic calculator (no iteration)

Enter your desired roughness value. Ignore the design rock size input area.

Velocity check

High velocity implies that there was a large elevation drop that created such high specific energy. That energy can be lost quickly at expansions, bends, or obstructions. Verify that this is reasonable for the site.