FLOOD09199

• County: Statewide
• Basin: Statewide
• Title: HEC-2 Water Surface Profiles - Users Manual
• Date: 11/1/1976
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

The loss coefficient for friction, kf, should be computed using Manning's

t' h k ,9.1 n2L (English) or 19.6 n2L (Metric).
equa 10n were f = R4/3 R4/3

Multiple Culverts:

Q =

l,gH . AT IUK l' ,where AT = Total Area
equ v

Kequi v =

AT'

~ Al~
lL J Ki J

III. Coefficient of Discharge, "c" (Variable COFQ)

Under free flow conditions (discharge independent of tailwater), the
coefficient of discharge, "C", ranges from 2.5 to 3.1 (1.39 - 1.72
Metric) depending primarily upon the gross head of the crest ("C"
increases with head) and resistance to flow caused by obstructions
such as bridge railings, curbs, and other barriers. For road ap-
proaches with a trapezoidal shaped cross section, a coefficient of
3.0 would be reasonable. For flow over bridge decks, a value of 2.5
could be used.

When submerged flow (discharge affected by tailwater) occurs, the
coefficient "C" should be reduced. This is done automatically by
tbe computer program using Waterways Experiment Station Design
Chart 111-4 for an ogee shaped weir.

IV. Expansion and Contraction Coefficients

These coefficients are used to compute losses caused by changes in
the river cross sections. For long gradual transitions, the coef-
ficients are small. For short abrupt transitions, they are large.
The transition loss is computed as the coefficient times the dif-
ference in velocity head between cross sections.

Coeffi c i ent
Expansion Contraction
0.0 0.0
0.3 0.1
0.8 0.6

No transition
Gradual transitions
Abrupt transitions

r:xhibit 2
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