Limitations/Extensions of Manning Equation

Applications of the Manning Equation

• in Texas the most common application is in channel and culvert design, e.g. for highways
• also common in forestry/rangeland/agriculture to estimate flood travel time in land-use management
• often the only tool avaliable for small-area engineering, e.g. to calculate required curb height (see sec. 10.4, Storm Drains/Pavement Drainage)

Regional Variation

Figure 11: Regional variation in $n$. A comparision of gravel-bottom streams in Central Arizona vs. U.S. average. $R$ is hydraulic radius,$d_{50}$ is median grain size. Some of the Arizona sites have been channelized. After AZ Geological Survey .

Helpful Guides

• the USGS publishes a pictoral guide to help in selecting $n$. An online version is available at USGS Verified Roughness Characteristics website
• the standard government guide (USGS and Federal Highway Administration) is Arcement, Jr. and Schneider (1990), also available online at the Federal Highway Administration
• regional calibrations are available for many states, e.g. Texas (actually this is a strictly empirical approach that indirectly incorporates $n$).