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


5.4 CRITQ Command

Purpose: The critical flow in a constricted section as a function of the depth of water in an approach section is computed in this command. The resulting flow table contains the flow as defined by critical flow at the constriction, but the height of the water in the approach section is used as the argument. This command is most often used with the GRITTER command for the generalized Ritter (1892) solution to an instantaneous dam failure. LINE 1
Variables: TABLE, SAVEIT
Format: 7X, I5, 1X, A4
Example: TABLE #= 9900 SAVE

TABLE gives the table number for the table to be computed in FEQUTL.

SAVEIT indicates that the resulting table is saved internally within FEQUTL by entering SAVE so that the table can be referenced for use in later commands. If the SAVE option is omitted, the table is not saved and it cannot be referenced in later commands. For example, a critical-flow table must be given for the GRITTER command (section 5.14). Application of the SAVE option retains that table so that it is available when the GRITTER command is applied.

Variables: NAME, APPTAB
Format: A8, I5
Example: APPTAB #= 25

NAME must be APPTAB #=. APPTAB gives the table number for the cross-section table describing the shape of the channel conveying water to the critical section. This is called the approach cross section or just the approach section. The bottom elevation of the approach cross section cannot be higher than the bottom elevation of the critical section. The elevations of the bottom of each cross section are taken from the elevation entry in the cross-section table.

Variables: NAME, CONTAB
Format: A8, I5
Example: CONTAB #= 29

NAME must be CONTAB #=.

CONTAB gives the table number for the cross-section table defining the cross section at which critical flow is assumed to be present. The bottom of the constricted cross section must be at or above the elevation of the approach section. The elevations are taken from the elevation entry in the heading of the cross-section table. The constricted cross section must not be larger than the approach cross section. However, it can be the same shape and size.

If the same table number is used for both the approach and the constricted section, an error message that the cross section is not high enough will be issued. This results because an entry in the critical-flow table is computed in FEQUTL for each entry in the constricted section. The water-surface elevation required in the approach section to produce critical flow in the constricted section is always higher than the water-surface elevation in the constricted section. Therefore, the computations for the depth in the approach section must overtop the cross-section table when computing the larger depth values in the critical section. Extending the cross-section table does not solve the problem of tabulated cross section overtopping because this also causes the computations for critical flow to go to larger depths. The solution is to copy the cross-section table in the input to the FTABIN command (section 5.13) and change the table number and then extend this copied table to a value large enough to avoid the problem of tabulated cross-section overtopping.

Variables: NAME, CD
Format: A8, I5
Example: DISCOEF = 0.95

NAME must be DISCOEF =.

CD gives the discharge coefficient for the opening. The discharge coefficient applies to the flow in the constricted section.

Variable: LABEL
Format: A50
Example: Flow at partial failure of Upper Baker Dam.

LABEL gives up to a 50-character label that will appear to the left of the column headings in the constricted-flow table produced in CRITQ.

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