All water channels, from natural stream beds to lined artificial channels, exhibit some resistance to water flow, and that resistance is referred to as roughness. Hydraulic roughness is not necessarily synonymous with physical roughness. All hydraulic conveyance formulas quantify roughness subjectively with a coefficient. In Manning’s Equation, the roughness coefficients, or n-values, for Texas streams and channels range from 0.200 to 0.012; values outside of this range are probably not realistic.

Determination of a proper n-value is the most difficult and critical of the engineering judgments required when using the Manning’s Equation.

You can find suggested values for Manning’s roughness coefficient (“n” values) in design charts such as the one shown in the file named nvalues.doc ( ). Any convenient, published design guide can be referenced for these values. Usually, reference to more than one guide can be productive in that more opinions are collected. You can find a productive and systematic approach for this task in the FHWA publication

TS-84-204

, Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains

.

However inexact and subjective the n-value determination may be, the n-values in a cross section are definite and unchangeable for a particular discharge and flow depth. Therefore, once you have carefully chosen the n-values, do not adjust them just to provide another answer. If there is uncertainty about particular n-value choices, consult a more experienced designer.

In some instances, such as a trapezoidal section under a bridge, the n-value may vary drastically within a section, but you should not subdivide the section. If the n-value varies as such, use a weighted n-value (n

w

). This procedure is defined by Equation 6‑15 as follows:

Equation 6-15.

where: