Cowan Roughness Calculator
Estimate Manning's roughness coefficient (n).
Manning's n Value
0.000Cowan's Method for Roughness
The Cowan Method estimates Manning's roughness coefficient (n) by separating channel and flow characteristics into factors. It can support an engineering assessment, but it does not prescribe an authority-approved value or replace field review.
Formula: n = (n0 + n1 + n2 + n3 + n4) × m5
- n0: Base material of the channel
- n1: Degree of surface irregularity
- n2: Variations in shape and size of the channel cross section
- n3: Relative effect of obstructions
- n4: Effect of vegetation
- m5: Correction factor for meandering of the channel
Separating the factors makes the estimate easier to review. Select values from observed channel conditions and project-appropriate references, then compare model results with observations when suitable calibration data are available.
Go to CalculatorUsing the estimate in a flood model
The Cowan result is an estimate that can inform the initial choice of Manning n in a flood model. Assess terrain and cross-section properties, vegetation, obstructions, water level and flow conditions together. Comparison with available observations and roughness sensitivity tests helps justify the choice. The calculator result alone does not replace a project report or an authority opinion.
Method reference: USACE — Manning n selection and the Cowan method
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