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Verifying Roughness Coefficients
for
Stream Channels in Arizona
USGS Professional Paper 1584, Verification
of roughness coefficients for selected natural and constructed stream
channels in Arizona, by Jeff Phillips and Todd Ingersoll,
is just one of a series of publications written over the years to
help designers and engineers select roughness coefficients for stream
channels. The best known publication of this genre, long considered
the standard reference for estimating roughness coefficients by
the “visual comparison” approach, is Roughness Characteristics
of Natural Channels by Harry H. Barnes, Jr. (USGS Water
Supply Paper 1849). The 1967 publication contains color photographs
and descriptive data for over 50 stream channels in the United States.
Other similar investigations include n-values determined
for 21 high-gradient streams in Colorado (Jarrett, 1985), 15 floodplains
in the southeastern United States (Acement and Schneider, 1989),
78 rivers and canals in New Zealand (Hicks and Mason, 1991) 67 gravel-bed
streams in Canada (Bray, 1979) and 21 perennial channels in New
York State (Coon, 1995). Few n-verification measurements
have however been made in drylands such as the arid and semiarid
regions of the southwestern United States.
USGS Professional Paper 1584 presents
verified Manning’s roughness coefficient values for 37 discharge
measurements at 14 selected streams in Arizona. Selected sites
include unstable alluvial sites, high-gradient boulder-strewn channels,
and manmade flood control channels.
The verification-measurement data are
used to develop empirical relations between channel and hydraulic
components and Manning’s n. The relations include an equation
for gravel-bed streams that relates Manning’s n to relative
roughness and an equation to determine the effects of vegetation
on total roughness.
The equation
developed for base values of n for gravel-bed channels in Arizona
have substantially lower n values for a given R and d50 compared to similar equation from other
parts (Figure 1). The larger values for other channels may have
resulted from the flow-retarding effects associated with channel
irregularities, poorly sorted coarser bed material, or more bank
vegetation compared to Arizona streams.
Figure 1. Relation of Manning’s n and relative roughness
for gravel-bed stream channels in Arizona and throughout the world.
All trend lines are plotted for values of d50
equal to 0.30 foot, and values of R (hydraulic radius) range from
0.6 to about 8 feet.
Phillips, Jeff V., and Todd L.
Ingersoll, 1998. Verification of roughness coefficients
for selected natural and constructed stream channels in Arizona.
U.S. Geological Survey Prof. Paper 1584, 77 pages.
Copies are available from libraries
or they may be purchased from the USGS Information Services, Denver,
CO (303) 202-4700 for $14.
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