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


5.19 SEWER Command

Purpose: A cross-section function table for a single circular conduit is computed in the SEWER command. A cross-section table is computed in the SEWER command only for the barrel of a culvert or for a storm sewer. No culvert losses are computed. Computation of culvert losses is done in the CULVERT command (section 5.5) with the cross-section function table developed in SEWER and entered on Line 14 of the input to the CULVERT command as part of the description of the culvert.

Notes: In closed conduits, the conveyance will decrease with depth as the conduit approaches the full-flow condition. Two options are allowed in FEQUTL. The first option, selected by giving a positive table number, is to propagate the conveyance of the full conduit to smaller depths where the conveyance may actually be higher than the full-flow conveyance so that the maximum conveyance in the conduit is the full-flow conveyance. This means that the maximum conveyance in the conduit that usually results at depths close to the crown of the pipe is overridden. This is done because the point of maximum conveyance is often close enough to the crown of the conduit such that attainment of the maximum conveyance is unlikely. As the water surface approaches the crown of the conduit, the flow can easily oscillate between full flow and part-full flow as the air cavity is removed. This results because the normal undulations in the water surface have great effects in that the pipe is filled momentarily. Also, the normal variation in depth in the pipe will often contribute to the instability close to the crown. In the second option, selected by giving a negative table number, warning messages for decreasing conveyance are issued, but the decreasing conveyance is retained for further computations. The conveyance in the hypothetical slot is forced to be a constant with the value given by the full-flow conveyance in both options.

Variable: TAB, CIN
Format: 7X, I5, A50
Example: TABLE #= 9 NOOUT

TAB is the table number of the cross-section table computed in FEQUTL.

CIN are user-specified options for cross-section table computations. The user may specify the same options after the table number as in FEQX (section 5.8). If the table number is positive, the conveyance is not permitted to be larger than the full-flow conveyance at any water level. If the table number is negative, the conveyance is computed with no modifications.

Variable: D
Format: 9X, F10.0
Example: DIAMETER = 3.5

D is the diameter of the pipe in feet or meters.

Variable: NSIDES
Format: 7X, I5
Example: NSIDES = 30

NSIDES is the number of sides in the polygon used to approximate the circular conduit. NSIDES should be 10. The polygon is sized so that the area of the polygon and the area of the circular conduit are the same when the conduit is flowing full. A hypothetical slot is added to the top of the pipe and included in the cross-section table to maintain a free surface at all stage levels.

Variable: WSLOT
Format: 6X, F10.0
Example: WSLOT = 0.01

WSLOT is the width of the hypothetical slot to maintain a free surface in the pipe. The width should be small so that the area of flow is not greatly increased for the expected surcharge levels.

Variable: HSLOT
Format: 6X, F10.0
Example: HSLOT = 50.0

HSLOT is the height of the hypothetical slot above the conduit invert. HSLOT is overridden on output, so that any value greater than the top of the pipe is acceptable.

Variable: N
Format: 2X, F10.0
Example: N = 0.025

N is Manning's n for the conduit.

The following two lines are optional. The lines are used to specify a mud line in the conduit to represent the accumulation of sediment in the conduit. The mud line is horizontal in all cases with a single pipe.

Variables: ZMUD
Format: 5X, F10.0
Example: MUDL = 0.55

ZMUD is the thickness of the sediment measured from the invert of the pipe. The part of the conduit boundary that is below the sediment line is truncated in the capacity computations.

Variables: NMUD
Format: 5X, F10.0
Example: NMUD = 0.035

NMUD is the Manning's n for the sediment in the pipe. A perimeter-weighted composite Manning's n for the pipe is computed if a mud line is present.

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