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

48

9

TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

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2,0 ~ /'
1.75 "i"j'Q0
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17 percen ~
1.50
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1.25
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1.00 ~;T-" f---- -.-- __n_ --~- .--- ---- --,- _n __. _0._
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/' 6- by 4-ft box culvertl having rounded entrance
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50

100 150 200 250 300
DISCHARGE. IN CUBIC FEET PER SECOND

400

350

Figure 28.-Rating cUlVe showing transition from type 2 to type 6 flow in a box culvert.

to be 1.25 and 1.75, A straight line is drawn
between discharges computed at these ratios,
The break in a rating curve at a headwater-
diameter ratio of 1.5 for a circular culvert is
not nearly so severe as for a rectangular culvert.
This is due to the gradual contraction and re-
d1lction in area per unit of culvert diameter
as the top of the pipe is approached, Figure
29 is an example of a rating curve for a pipe
culvert showing the relatively small difference
in discharge between types 1 and 5 flow condi-
tion at. a headwater-diameter ratio of 1.5,
In this example the area of questionable flow
t,ype apparently lies between headwater-diam-
('.t(lf I'utio::-o of 1.2 und 1..). Therefore, a ~tmi~ht
line d1'l1wIl hC'tween these two puint~ will
Jlrtlddt~ It sllti~ructory trnnsition.
The trnnsitioll between t',vpes 2 und 0 flow
t hl'o\l~dl circular culvert.s {'un be made in the
~l1nlC nUlnner as for box culverts. The sprend
in disehllrge ut u. headwater-diameter nltio of
1..5 is ypry smull. Therefore Ii strnig-ht line
rlmwn bet.\'feen heudwuler-dia1l1eter rntios of
1.25 nnd 1.75 is nIl lI.Yerl\.g'e line. Figufe :30
shows It tnmsit,ion from t.yp€" 2 to type (i flow
in It circllhu culvert.
The nbove are some of t.he more common
j.ransitions. There u.re Inl1ny speciul ruses. such

. as between 5 and 6, or borderline between
4 and 5, or between 4 and 6, that must b.
treated on individual bases,
It is recommended that tbe foregoing pro-
cedures also be applied to discharges computed
at miscellaneous sites, The discharge should be
interpolated between the limiting values of the
transition whenever the headwater-diameter
ratio falls in that range.

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Type 4 flow

A rating curve for type 4 flow is shown in
figure 31. The development of a rating for either
box or pipe culverts is a simple procedure. In
any given culvert the discharge coefficient is
constant, and the pipe is flowing full, The only
two variables in the discharge equation are the
fall (h,-h,) and the discharges. Therefore a
constant can be computed to represent the
remaining factors in the equation. The rating
curve is determined simply by multiplying the
constant by various values of the square root
of the fall,

Type 3 flow

A rating curve for type 3 flow is not readily
developed, because the discharge is a function

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