NAME
ncalc - Manning's n value calculation program
ABSTRACT
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).
METHOD
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.
HISTORY
Version 2.6 1995/03/30 - code clean-up and minor bug fixes
1993 version - first version for use on PC's and the Data General
AViiON
1985 version - program converted to use on PC's
1975 version - original version
DATA REQUIREMENTS
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.
OUTPUT OPTIONS
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.
SYSTEM REQUIREMENTS
NCALC is written in Fortran 77 with the following extensions: use
of integer*2 declarations, use of include files, and reference to
compiler-dependent system date and time routines. Generally, the
program is easily installed on most computer systems. The code has
been used on UNIX-based computers and DOS-based 386 or greater
computers having a math coprocessor and 1 mb of memory.
DOCUMENTATION
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.
REFERENCES
Barnes, H.H., Jr., 1967, Roughness characteristics of natural
channels: U.S. Geological Survey Water-Supply Paper 1849, 213 p.
Jarrett, R.D., 1985, Determination of roughness coefficients for
streams in Colorado: U.S. Geological Survey Water-Resources
Investigations Report 85-4004, 54 p.
TRAINING
Basic Hydraulic Principles (SW1004TC), offered semiannually at the
USGS National Training Center.
Surface-Water Hydraulic Analysis (SW2009TC), offered annually at the
USGS National Training Center.
CONTACTS
Operation and Distribution:
U.S. Geological Survey
Hydrologic Analysis Software Support Program
437 National Center
Reston, VA 20192
h2osoft@usgs.gov
Official versions of U.S. Geological Survey water-resources analysis
software are available for electronic retrieval via the World Wide
Web (WWW) at:
http://water.usgs.gov/software/
and via anonymous File Transfer Protocol (FTP) from:
water.usgs.gov (path: /pub/software).
The WWW page and anonymous FTP directory from which the NCALC
software can be retrieved are, respectively:
http://water.usgs.gov/software/ncalc.html
--and--
/pub/software/surface_water/ncalc
If you would like to obtain the price of and (or) order paper copies
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U.S. Geological Survey
Branch of Information Services
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Denver CO 80225-0286
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