FLOOD08565

• County: Statewide
• Basin: Statewide
• Title: HEC-2 Water Surface Profiles
• Date: 8/1/1979
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

would plot at critical depth (below the actual water surface profile)
for both mild reaches of the stream. The final plotted profile should
incorporate computed results from both computations and an analysis of
the hydraulic jump. HEC-2 does not contain the capability to determine
the position of the hydraulic jump or energy losses associated with the
jump.

3. STARTING ELEVATION

The water surface elevation for the beginning cross section may be
specified in one of three ways: (l) as critical depth, (2) as a known
elevation, (3) by the slope area method. By setting the variable STRT
on the Jl card equal to minus one, critical depth will be computed and
used as the starting water surface elevation. This method is appropriate
at locations where critical or near critical conditions are known to
exist for the range of discharges being computed, e.g., a waterfall, weir
or a section of rapids. When a rating curve is available, the appropriate
starting elevation can be specified by variable WSEL on the Jl card.

For beginning by the slope area method, STRT is set equal to the
estimated slope of the energy grade line (must be a positive value) and
WSEL is used as the initial estimate of the water surface elevation. The
flows computed for the fixed slope are compared with the starting flow
and the depth is adjusted until the computed flow is within one percent
of the starting flow. The water surface elevation thus determined is
used as the starting water surface elevation for subsequent water surface
profile computations.

4. DISCHARGE

Discharge may be specified and altered in several ways. Variable Q
on the first job card (Jl card) to specify the starting discharge for
single profile runs. When it is desired to change the discharge for a
single profile run, the variable QNEW on the X2 card can be used to
permanently change the discharge at any cross section.

An alternate procedure utilizes the QT cards (discharge table) may
be used to specify from one to nineteen discharge values for single or
multiple profile runs. QT cards may be used to specify starting discharges
and to permanently change discharges at any cross section in a data set.
Variable INQ on the Jl card directs the program to the field of the QT
card that contains the discharge for that profile. When a value of FQ is
entered. all discharges on the X2 cards and discharges in the specified
INQ of the QT cards are multiplied by the value.

5. ENERGY LOSS COEFFICIENTS

Several types of loss coefficients are utilized by the program to
evaluate head losses: (1) Manning's 'n' values for friction loss, (2) con-
traction and expansion coefficients to evaluate transition (shock) losses,
and (3) bridge loss coefficients to evaluate losses related to weir
shape, pier configuration, and pressure flow.

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