NCALC - Verification of Manning's Roughness Coefficient in Channels
VERSION/DATE: Version: 2.6 1995/03/30
OPERATING SYSTEM: DOS
DOCUMENTATION (39 Pages):
Jarrett, R.D., and Petsch, H.E., Jr., 1985, Computer program NCALC user's manual--verification of Manning's roughness coefficient in channels: U.S. Geological Survey Water-Resources Investigations Report 85-4317, 27 p.
Computations involving flow in open channels commonly require an evaluation of the roughness characteristics of the channel. The Manning equation has been used extensively as an indirect method for computing discharge or depths of flow in natural channels. The U.S. Geological Survey engages in a continuing effort to improve the understanding of flow resistance, usually in terms of Manning's roughness coefficient, n, in channels in the United States. Procedures for computing values of Manning's roughness coefficient from known discharge, water-surface profiles, and channel cross-sectional properties are presented and have been programmed for automatic computation. General theory, procedures for onsite investigations and surveys, a description of the use of the computer program, an example problem, and additional channel-roughness-verification research needs are provided (Jarrett and Petsch, 1985).
The Manning equation is used as the basis for computing the channel reach properties and for calculating n values in this program. In this application, calculations also are assumed to be valid for nonuniform reaches usually found in natural channels if the energy gradient is modified to reflect only the losses due to boundary friction (Barnes, 1967). The program has the capability to compute an n value for single or multiple cross sections.
Onsite surveys provide data for discharge, water-surface profiles, and channel cross-sectional properties. The program has the capability to compute roughness coefficients for 99 subreaches, using 99 cross sections. Each cross section can be defined by 3 to 999 ground points. There is no provision in the program for subdividing cross sections. An option is available to include an alternate procedure to compute roughness coefficients for an individual cross section, based on its localized water-surface slope, and is for use in relatively uniform channel reaches. This option is included for comparison and evaluation with basic computational procedures.
The computational results are output in four parts. The first part is a listing of the input data and the computed cross-sectional properties of each cross section. The second part, based on multisection analysis, is a listing of the roughness coefficient for each successive subreach as well as the roughness coefficient for the entire reach. A roughness coefficient for the entire reach also is computed by weighing the subreach coefficients against their respective friction heads. Generally, the accepted n value will be from the results for the total reach with all cross sections (the friction-head weighted n value). Consistency of separate subreaches is an indication of the reliability of the weighted n value for the entire reach. The third part, which is optional, is the alternative method that lists the roughness coefficients computed for each cross section. The fourth part is an option that lists the same items as the second part, except that the user may preselect various combinations of cross sections.