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


5.18 QCLIMIT Command

Purpose: A reasonable estimate of the maximum flow in a closed conduit when the conduit is flowing full is computed in the QCLIMIT command for defining the free-flow limit for the EXPCON command (section 5.7). Critical flow is used in EXPCON to determine the location of a control, but when a closed conduit is flowing full, critical flow is undefined. Therefore, some reasonable flow must be set as the maximum possible so that EXPCON can be used with closed conduits. QCLIMIT should be utilized for each closed-conduit cross section appearing in the EXPCON command.

Variable: TABLE
Format: 7X, I5
Example: TABLE #= 291

TABLE is the table number of the cross-section table to be limited by critical flow. This table must already have been input to FEQUTL with the FTABIN command (section 5.13) or have been saved with SAVE22 or SAVE25 on a previous computation for a closed conduit (sections 5.16, 5.17, and 5.19). The table must be of type 22 or 25.

Variable: FACTOR
Format: A7, F10.0
Example: FACTOR = 1.25

FACTOR is a multiplying factor to change the limiting value from that given by the built-in extrapolation. The limiting value is computed in QCLIMIT assuming that the logarithm of critical flow and the logarithm of water-surface height are linearly related. The two topwidths in the closed conduit that are closest to the hypothetical slot at the top of the conduit are detected and utilized for extrapolation. Critical flow is undefined in a closed conduit. The critical flow produced with the top width of the hypothetical slot is so large that the computations in EXPCON will be nonsensical to applications. EXPCON must include a maximum flow that is reasonable. When the cross section is such that a physically meaningful free surface is present, then critical flow represents this value. In other cases, a value that is reasonable must be provided. The friction slope calculated in the computation of the maximum flow is printed so that the reasonableness of the maximum flow can be assessed. If the friction slope in the closed conduit at maximum flow is too small, then FACTOR should be used to increase the maximum flow so that the friction slope will be so large that no flow greater than the maximum is possible. The goal is to obtain a maximum flow that is larger than any that will be calculated but not a maximum flow that is several times larger than any calculated flow.

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