Full Equations Utilities (FEQUTL) Model for the Approximation of Hydraulic Characteristics of Open Channels and Control Structures During Unsteady Flow

# 5.3 CHANRAT Command

Update available for optimization of interpolated table values

Purpose: A 2-D table for flows through a short, prismatic channel that simulates the overbank flow bypassing a culvert or bridge opening is computed with the CHANRAT command. The rating for the flow through the channel resulting from the differences in water-surface elevation across it is given in the table. Additional discussion of this command is presented in section 3.3.

Notes: The results for each upstream head and partial free drop are printed out with the CHANRAT command. Also printed is a column of values headed by QDRERR, which is the same as the integration error tolerance so long as no special problems result during computation. If the value of QDRERR becomes large, this indicates that the stopping depth tolerances are too small or that there is some error in the program.
LINE 1
Variable: TABLE
Format: 7X, I5
Example: TABLE #= 9900
Explanation:
 TABLE gives table number for the table to be computed in FEQUTL.
LINE 2
Variables: TABTYP, ERRKND, INTHOW, EPSINT, NDDABS, NDDREL
Format: A4,1X,I5,5A5
Example: TYPE = 13
Explanation:
 TABTYP gives the type of 2-D table to produce. Currently, tables of type 6 and of type 13 can be computed in the CHANRAT command. ERRKND specifies the error measure to apply in the adaptive integration. An absolute error is selected if ERRKND = 0, and a relative error is selected if ERRKND = 1. An absolute error criterion is the default option. INTHOW is the method utilized for integration. Simpson adaptive integration is selected if INTHOW = 1. Simpson adaptive integration is the only option available. Others may be added in the future. EPSINT is the tolerance value for the adaptive integration and has a default value of 0.1. This default value means that the estimated integration error must be less than 0.1 ft before computations are completed in the adaptive routine for numerical integration. NDDABS is the absolute deviation from normal depth used to control the integration near the singularity in the governing equation at normal depth. The default value for NDDABS is 0.005 ft. NDDREL is the relative deviation from normal depth used to control the integration near the singularity in the governing equation at normal depth. The default value for NDDREL is 0.005.

Variables other than type are given default values if omitted. These default values should be applied and changed only when necessary. These defaults should be changed only with caution.
LINE 3
Variable: LABEL
Format: A5,1X,A50
Example: LABEL = Overbank flow at Golf Course Bridge on Dinky Creek.
Explanation:
 LABEL gives a user-defined label to identify the resulting table.
LINE 4
Variable: XSTAB
Format: A6,1X,I5
Example: XSTAB #= 100
Explanation:
 XSTAB gives the table number of the cross-section table defining the shape of the channel. This table must contain critical flow. Therefore, it must be type 22 or type 25.
LINE 5
Variable: BOTSLP
Format: A6,1X,F10.0
Example: BOTSLP = 0.005
Explanation:
 BOTSLP gives the bottom slope of the channel with decline downstream taken as positive. The bottom slope may be negative or zero. The flow must be subcritical. If a flow is supercritical in CHANRAT, an error message will be issued and the rating table will not be computed.
LINE 6
Variable: LENGTH, ELEV
Format: A6,1X,F10.0, 1X, 8X,F10.0
Example: LENGTH = 30.0 MIDELEV = 256.00
Explanation:
 LENGTH gives the length of the channel, which must be > 0. ELEV gives the elevation of the midpoint. The elevation of the midpoint is used to compute the elevation for head to be stored in the table for later checking with the value given for elevation for head in FEQ when this table is used. The elevation for head is computed in FEQUTL as the maximum of the two end-point elevations computed from the given slope and one-half the length. In the current example, if the bottom slope was 0.005, the elevation for head would be 256.075.
LINE 7
Format: A80
Explanation:
 HEAD gives a descriptive label for subsequent input.
LINE 8
Variable: NFRAC
Format: 6X,I5
Example: NFRAC = 21
Explanation:
 NFRAC gives the number of partial free-drop fractions to use in computing the tail-water heads. A complete discussion of partial free-drop fractions for computation of 2-D function tables is given in section 11.2 of the documentation report for the Full Equations model (Franz and Melching, 1997).
LINE 9
Variable: POWER
Format: 6X,F10.0
Example: POWER = 2.0
Explanation:
 POWER gives the power used to distribute the partial free drops from 0 to 1. The proportion of free drop is given by for i = 1, ..., NFRAC.
LINE 10 (Repeated as needed to give the ascending sequence of upstream heads.)
Variable: HUVEC(I)
Format: F10.0
Explanation:
 HUVEC gives the upstream heads to use in computing the 2-D table. The input of the list is terminated when a negative head is encountered. Head is measured from the end of the channel with the higher elevation. A nonzero head must result in a nonzero flow. Thus, if the bottom slope is positive, the datum for head is the upstream end of the channel; and if negative, the datum for head is the downstream end of the channel.