FLOOD06195

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Measurment of Peak Discharge at Culverts by Indirect Methods
• Date: 6/1/1968
• Prepared By: G.L. Bodhaine
• Floodplain - Doc Type: Educational/Technical/Reference Information

38

TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Types 1, 2, and 3 flow

For culverts, the ratio of channel contraction,
m, is defined as (I-AlAI) where A is the area
of flow at the terminal section and AI is the
area of the approach section. Because the value
of m is usually large for flood flows, the labora-
tory tests placed emphasis on the condition.
However, tests on flow through bridge openings
demonstrate that the discharge coefficient varies
almost linearly between values of m from 0 to
0.80, and that the coefficient reaches a mini-
mum value atm=0.80. All coefficients given
herein are for an m of 0.80. If the contraction
ratio is smaller than 0.80, the value of C may
be computed by inl<lrpolating between the value
of C listed for an m of 0.80 and a value of C
of 0.98 for an m of O. The following formula

m-O

0,1

0,95

0,90

v

0,80

0,75
0,75

may be used in place of interpolation: C(ad-
justed) =0.98- (0.98- O)mI0.80. This formula
is shown in graph form in figure 19. This adjust-
ment is made as the last step in the computation
of the discharge coefficient. Example 10 of the
sample computations shows this adjustment,

Flush setting in vertical headwall

Pipe culverts
The discharge coefficient for square-ended
pipes set flush in a vertical headwall is a func-
tion of the ratio of the headwater height to the
pipe diameter, (h ,-z) ID, The coefficient for flow
types 1, 2, and 3 can be determined from
figure 20,
If the entrance to the pipe is rounded or
beveled, compute the discharge coefficient by

0,80

0,85 0,90
C for m of 0.80

0,95

Figure 19.-Adjustm~nt to discharge coefficient for degree of channel contradion.

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