Water Resources Applications Software

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Summary of CGAP

       cgap - Channel Geometry Analysis Program

       CGAP permits the analysis, interpretation, and quantification of the
       physical properties of an open-channel reach as defined by a
       sequence of cross sections.  The primary function of the program is
       to compute the area, width, wetted perimeter, and hydraulic radius
       of cross sections at successive increments of water-surface
       elevation (stage) from data that consist of coordinate pairs of
       cross-channel distances and land-surface or channel-bottom
       elevations.  Longitudinal rates-of-change of cross-sectional
       properties are also computed.  In addition, the mean properties
       (including discharge as computed using the Manning's equation) of a
       channel reach and the cross-sectional area and channel widths as
       functions of stage for subdivisions of a cross section can be
       computed.  CGAP was developed to compute, display, and otherwise
       format cross-sectional data for use with simulation models.  CGAP
       produces files of data in the required input format for BRANCH and
       HSPF (the latter after use of the program genftbl--generate F-
       Table--provided with CGAP).  CGAP has 17 output options that produce
       the following forms of output:  tabular lists, sequential files, and
       (or) graphical plots in digital or line-printer format.

       Version 3.5 1995/10/06 - Enhancement to include output of wetted
          perimeter for mean group option 6 (BRANCH model geometry table).

       Version 3.3 1995/07/18 - Code clean-up and bug fixes, print
          Manning's value and wetted perimeter for subdivision tables.

       Version 2.7 1994/02/15 - Added automatic update of revision number
          and date, completed port to DOS-based computers, made minor bug
          fixes and code cleanup.

       Version June 01, 1993 - Added option 17, which produces mean groups
          of cross-sectional properties including discharge as a function
          of stage as computed using Manning's equation.  Option 16
          modified to allow computation of discharge for subdivisions of
          cross sections based on horizontally varying roughness.

       Version 92/02/01 - First Hydrologic Analysis Software Support
          Program release, code cleaned up and made UNIX compatible.

       Version 91/09/05 - First UNIX release.

       Version 90/12/05 - Improved interactive update of existing CGAP
          input records, allowing selective processing of a subset of the
          input cross sections.

       Input data consists of land-surface or channel-bottom elevations
       measured and referenced horizontally to a channel-bank location and
       referenced vertically to a common datum.  These data should be
       measured along a line normal to the flow direction at the thalweg of
       the channel.  Sufficient number of points must be obtained so that
       linear interpolation between points accurately describes the cross-
       sectional properties at all possible stages of flow.  A minimum of
       two data points below and a single channel must be present at the
       minimum elevation of interest in order for CGAP to perform
       computations.  Other input data include datum adjustments, segment
       lengths, skew angles, and program-control parameters, such as output
       options, stage range of computations, computation increment, and
       plot scaling factors.  Input is expected in the inch-pound system of

       The program provides 16 output options that format the input cross-
       sectional data or the computed cross-sectional properties as digital
       plots, line-printer plots, tabular lists, and (or) sequential files.
       These output formats include: plots of cross sections; plots of
       cross-sectional area and (or) channel width as functions of stage;
       tabular lists of computed, stage-dependent, cross-sectional
       properties (area, width, wetted perimeter, hydraulic radius, average
       depth, and symmetry); files of computed cross-sectional properties;
       plots of cross sections in isometric projection; plots of cross-
       sectional area at fixed stage as a function of longitudinal distance
       along an open-channel reach; files of computed cross-sectional area
       as a function of stage for subdivisions of a cross section; and
       files of computed discharge, area, top width, and conveyance as
       functions of stage for mean groups of one or more cross sections.
       Output is in the inch-pound system of units.  The graphical output
       is dependent on the capabilities of the graphics library linked with
       the program (either GKS or CalComp compatible).

       CGAP is written in Fortran 77 with the following extensions:  use of
       include files, variable names longer than 6 characters, and
       reference to compiler-dependent system date and time routines.  CGAP
       requires a utility library (libutl) that includes the software/user,
       software/computer, and software/graphics interaction routines.  CGAP
       graphics are coded using CalComp graphics calls.  The libutl
       software provides graphics libraries to convert CalComp graphic
       references to Graphical Kernel System (GKS) library references and
       Interactive Software Services's INTERACTER library references.
       Generally, the program is easily installed on most computer systems
       that have access to one of the mentioned graphics libraries.  The
       code has been used on UNIX-based computers and DOS-based 386 or
       greater computers having a math coprocessor and 4 mb of memory.

       Regan, R.S., and Schaffranek, R.W., 1985, A computer program for
          analyzing channel geometry:  U.S. Geological Survey Water-
          Resources Investigations Report 85-4335, 40 p.

       Implementation and Calibration of Unsteady Open-Channel Flow
       Transport Models (SW3091TC), offered annually at the USGS National
       Training Center.

       Operation and Distribution:
          U.S. Geological Survey
          Hydrologic Analysis Software Support Program
          437 National Center
          Reston, VA 20192

       Official versions of U.S. Geological Survey water-resources analysis
       software are available for electronic retrieval via the World Wide
       Web (WWW) at:


       and via anonymous File Transfer Protocol (FTP) from:

         (path: /pub/software).

       The WWW page and anonymous FTP directory from which the CGAP
       software can be retrieved are, respectively:


       branch(1) - One-dimensional Branch-network flow model
       libutl(1) - Utility libraries for simulation models

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