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

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MEASUREMENT OF PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODR :~,

the following general classification of types of
flow can be made:
1. If h./D is equal to or less than 1.0 and
(h,-z)/D is less than 1.5, only types 1,2,
and 3 flow are possible,
2. If h./D is greater than 1.0, only type 4 flow
is possible.
3. If h./D is equal to or less than 1.0 and
(h,-z)/D is equal to or greater than 1.5,
only types 5 and 6 flow are possible,
Further identification of the type of flow
requires a trial-and-error procedure which is
described in a subsequent section of this
chapter,

Discharge Equations

Discharge equations have been developed for
each type of flow by application of the con-
tinuity and energy equations between the
approach section and the terminal section. The
discharge may be computed directly from these
equations after the type of flow has been iden-
tified. Discharge equations for critical depth at
a section are used to identify flow types 1 and 2;
thus, these equations are also included in the
following sections,

Critical depth

Flow at critical depth may occur at either
the upstream or the downstream end of a
culvert, depending on the headwater elevation,
the slope of the culvert, and the tailwater
elevation. To obtain flow at critical depth, the
headwater elevation above the upstream invert
must be less than 1.5 times the diameter or
height of the culvert. Type 1 flow will occur if

the tailwat,er elevation is lower than t,he wnter-
surface elevation at criticnl depth, nnd if the
bed slope of the culvert is grenter thnu the
critical slope, Type 2 flow will occur if the I".d
slope is less than t,he miticnl slope,
Cntical depth, d" is the dept,h lit the poiut of
minimum specific energy for a gi\'en discharge
and cross section, The relation between specific
energy and depth is illustrated in figure :J,
The specific energy, Ho> is the height of the
energy grade line above the lowest point in the
cross section. Thus,

V'
Ho=d+-'
2g

where

Ho= specific energy,
d=maximum depth in the section,
V=mean velocity in the section, and
!I=acceleration of gravity,

It. can be shown that at the point of minimum
specific energy and critical depth, d"

Q' A'
-=-1
!I T

and

V' A
-~d",=T'
!I

where

Q=discharge,
A = area of cross section below the water
surface,
T=width of the section at the water sur-
face,
d,=maximum depth of water in the critical-
flow section, and
dm=mean depth in section=A/T,

Table 1.-Charaderistics of flow types
[D =mu1mum vertical height of ba.rrelllond diameter of circular culverts)

Flow Barrel flow I Location of Kind or control
ty", terminal seetion
.
1 Partly fuIL___ Inlet.. __ __ __. Critical depth_____u___
, 2 _____do________ Outlet__ _ __.. _____do________________
I
I 3 _.__.do........ _____do_______ Backwater_____ _ _ __ __ __
4 FuU__.__..... _____dCL______ _.___do_._..______.__._
5 Partly fuU.____ Inlet_________ Entrance geometry _ _ _ __
I 6 FuIL........_ Outlet.__.... Entrance and barrel
geometry.

---~--I
III . hj i
he "Ii

I Culm' ,10'" I ',;;' !

_ Steep...... <1.51
. Mild______. <1..5
. ._.__dO___n <1..\
'1 Any__.cm ~1.0 i
______do_____ 51.5,
!_.__.dO""'1 >1. r. I

i I

<1.0 :;;1.0
<1.0 :;;1.0
>1.0 :;;1.0
>1.0
:;;1.0
:;;1.0