Free Online Manning Formula Trapezoidal Channel Calculator

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Manning Formula Uniform Trapezoidal Channel Flow at Given Slope and Depth

Inputs

Bottom width, b		X
Side slope 1 (horiz./vert.)		X
Side slope 2 (horiz./vert.)		X
Manning roughness, n ? Strickler B/B (See notes)		X
Channel slope, S		X
Flow depth, y		X
Bend Angle ? (for riprap sizing)		X
Rock specific gravity (2.65)		X
Design rock size, D50 Isbash Maynord Searcy * (See notes)		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
Flow, Q	Flow, Q	X
Velocity head, h_v	Velocity head, h_v	X
Top width, T	Top width, T	X
Froude number, F	Froude number, F	X
Average shear stress (tractive force), tau	Average shear stress (tractive force), tau	X
n for design rock size per Strickler	n for design rock size per Strickler	X
n for design rock size per Blodgett	n for design rock size per Blodgett	X
n for design rock size per Bathurst	n for design rock size per Bathurst	X
Blodgett vs. Bathurst	Blodgett vs. Bathurst	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 radio button (BB recommended) and a design rock size radio button (Isbash recommended). Fine-tune depth and rock size safety factor to get your desired flow with an even rock size. Every time you change any input value, the following iteration cycle happens: 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.