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

2

TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

the entrance; the culvert entrance; the culvert
barrel; the culvert outlet, the farthest down-
stream section of the barrel; and the tailwater
representing the getaway cbannel.
The change in the water-surface profile in the
approach channel reflects the effect of accelera-
tion due to contraction of the cross-sectional
area. Loss of energy near the entrance is related
to the sudden contraction and subsequent
expansion of the live stream within the barrel,
and entrance geometry has an important influ-
ence on this loss. Loss of energy due to barrel
friction is usually minor, except in long rough
barrels on flat slopes, The important features
that control the stage-discharge relationship at
the approach section can be the occurrence of
critical depth in the culvert, the elevation of
the tailwater, the entrance or barrel geometry,
or a combination of these.
The peak discharge through a culvert is
determined by application of the continuity
equation and the energy equation between the
approach section and a section within the cul-
vert barrel. The location of t,he downstream
section depends on the state of flow in the cul-

1
.J
~
<:)
~
'i

TYPE

I
CRITICAL DEPTH
AT INLET

hl~ 1 15
D <,
h4/h~ <1.0
So >$c
~-,

ICRITICA~ DEPTH
AT OUTLET

,-

~
"-
\)

~

~.!_-__Z_ '" 1.5
D
"4/ne "'1.0
50< Sc

3
TRANQUIL FLOW
THROUGHOUT

111- z '" 1 5
D '
h4/D~ 1.0
h.,-''\,. 1.0

EXAMPLE

Q:;::CAc 1/2g(hl-Z+G1VI~ - c(,-hf J
1-- Lw"-,J t 2g 12

- ,::~L. I
h ,-T- . -I,...c:"::'f --'-
. \)ii.:' "'fI2)" ~ci"1-..-_'__.-:~"'77--~ -",.-
.';E.' d ;'~'________'__t~ __~~_3r"t~
21.: Datum'" 56 w.:.:..
":"

Q = CAcJ!2"gfhl-; ~~'Y?--d:~~h-;")
t<-l'~ 29 ,,"
'-1----"1
"i-.';f." ..::....-....:.~~.--.,..."""="'.....,.,,.
2j tlT..-' '4 '-Mr.' to
i&){ $,,"'" -.- ',~; 'oj itf.3 lqi;,
Datum" 50'< C;,...~

vert barrel. For example, if eritical flow OeeUTl':i
ut Ule eulvert ent.rance, the headwat.er elevation
is not a fUllction of either the barrel {riet.ioll
loss 01' the tail water elevation, and the tel'miulll
section is located at the upstream end of the
culvert.
Information obtained in the field 'llI"vey
includes the peak elevation of the water slIrfuce
upstream and dm,,'nstream from, t.he culvert and
the geometry of the culvert and approach
channel. Reliahle high-water marks can rarely
be found in the culvert harrel; therefore, the
type of flow that occurred durinI' t.he peak flo\\'
ealJnot always be deterrnilled direel.Iy from field
data, and classification beeomes n. trial-and-error
))rocedure.

General classification of flow

For convenience in cornputatiou, culvert no".
has been classified into six types on the husis
of the location of the eontrol section and the
relative heights of the headwater and tailwater
elevations. The six types of flow are illustrated
in figure 2, and pertinent characteristics of each
type are given in table 1. From this information

TYPE
4
SUBMERGED
OUTLET

hl-l J 0
D > ,
h4/D "1.0

5
RAPID FLOW
AT INLET

hl ::--.: '" 1 5
D > '
h4/D ;;: 1.0

6
FUll FLOW
FREE OUTFALL
hi -z _
0-- ;: 1.5
h4 /0 ~ 1.0

EXAMPLE
Q~CAo\/2g(i,; -'n.)
Vl-:;'-?~~~~
RO./3

So
,

Q~CAo Y2g(nl-z)

Fi9ure 2.-C1assificolion of culver' flow.

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