Lecture 23
Culvert Hydraulic Behavior
I. Open-Channel Definitions
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The following open-channel definitions apply in most cases:
Canal
Flume
Chute
Culvert
Prismatic
Open channel of mild slope (subcritical flow) and relatively long;
could be lined or unlined
A channel built above the natural ground surface, usually of mild
slope and rectangular or circular cross section, crossing a
depression or running along the contour of a hillside
Like a flume, but having a steep slope (supercritical flow; Fr2 > 1.0)
and usually with some type of energy dissipation structure on the
downstream side (outlet)
One or more circular or rectangular pipes/conduits in parallel,
crossing under a road, canal, or other structure, either flowing full
(pressurized) or part full (open-channel flow); often used as a
cross-drainage structure
This means a constant cross-sectional shape with distance,
constant and uniform bed slope, and straight channel alignment,
applied to any of the above
II. Culvert Hydraulics & Flow Regimes
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A culvert can serve as a combination open-channel and closed conduit
structure, depending upon the type of flow condition in the culvert
Most of the research involving the hydraulics of culverts has been concerned
with the use of such structures under highways
Some of the research has focused on inlet control (free orifice flow) and
submerged outlet control (submerged orifice flow)
For culverts placed in an irrigation conveyance channel (i.e. not as a crossdrainage structure), free surface (open-channel) flow usually occurs in the
culvert
Culverts as part of an irrigation canal often pass under vehicular bridges
Typically, downstream hydraulic conditions will likely control the depth of flow
and the discharge in the culvert
Culvert hydraulics can be much more complex than might appear at first
glance
III. Flow Regimes
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The classification of the hydraulic behavior of culverts can take several forms
Three primary groupings can be used to describe the hydraulics of culverts
These groups are based on the three parts of the culvert that exert primary
control on culvert performance and capacity:
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(1) the inlet;
(2) the barrel; and
(3) the outlet
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Usually, one of these three primary controls determines the performance and
capacity of the culvert
An example of this is a projecting, square-edged inlet with the barrel on a
steep slope (Fr2 > 1.0) and flowing partly full: if the inlet is not submerged, the
upstream water level (headwater) is determined by the inlet characteristics
alone
In other cases, two or even all three primary controls can simultaneously
affect the performance and discharge capacity
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For example, if the inlet and outlet are submerged and the barrel is full, then a
designer can determine the headwater elevation by adding the outlet losses,
the barrel friction losses, and the inlet losses to the tailwater (downstream)
elevation (assuming the same specific energy in both the upstream &
downstream open channels)
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The classification is further subdivided under each main group, as shown in
the table below (Blaisdell 1966)
The classification indicates the number of factors the designer must consider
when determining the performance of a culvert and computing its capacity
under different regimes
Only those items that exert a control on the hydraulic performance of a culvert
are listed in the table
Many alternatives are possible for each control
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For example, each type of inlet will have a different effect on the culvert
performance
Many of the items listed in the table are inter-related, which further
complicates an already difficult problem
For instance, the depth of the flow just inside the culvert entrance depends on
the inlet geometry
If this depth is less than the normal depth of flow, a water surface profile must
be computed beginning with the contracted depth of flow to determine the
flow depth at the culvert outlet
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INLET CONTROL
A. Unsubmerged (Free Surface)
1. Weir (Modular Flow)
2. Surface profile (Non-Modular Flow)
B. Submerged (Inlet Crown Under Water)
1. Orifice (Free Orifice Flow)
2. Vortex (Non-Aerated Jet)
3. Full (Submerged Orifice Flow)
BARREL CONTROL
C. Length
1. Short
2. Long
D. Slope
1. Mild
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ii.
2. Steep
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Barrel slope less than critical slope
a. Part full, normal depth greater than critical depth
b. Full, not applicable
Barrel slope less than friction slope
a. Part full, depth increases along barrel
b. Full, barrel under pressure
Barrel slope steeper than critical slope
a. Part full, normal depth less than critical depth
b. Full, not applicable
Barrel slope steeper than friction slope
a. Part full, depth decreases along barrel (increases if
the inlet causes the depth inside the inlet to be less
than normal depth)
b. Full, barrel under suction
E. Discharge
1. Partially Full (Free-Surface Open-Channel Flow)
2. Slug and Mixture (Unsteady Flow)
3. Full (Closed Conduit Flow)
OUTLET CONTROL
F. Part Full (Free Surface Open Channel Flow)
1. Critical Depth (Free Flow)
2. Tailwater (Submerged Flow)
G. Full (Closed Conduit Flow)
1. Free (Free Orifice Flow)
2. Submerged (Submerged Orifice Flow)
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IV. Hydraulically Short & Long
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If the computed outlet depth exceeds the barrel height, the culvert is hydraulically
long, the barrel will fill, and the control will be the inlet, the barrel, and the outlet
If the computed depth at the outlet is less than the barrel height, the barrel is only
part full and the culvert is considered hydraulically short, will not fill, and the
control will remain at the inlet
Whether a culvert is hydraulically long or short depends on things such as the
barrel slope and the culvert material
For example, changing from corrugated pipe to concrete pipe can change the
hydraulic length of a culvert from long to short
A similar effect could result from a change in the inlet geometry
Flow in culverts is also controlled by the hydraulic capacity of one section of the
installation
The discharge is either controlled at the culvert entrance or at the outlet, and is
designated inlet control and outlet control, respectively
In general, inlet control will exist as long as the ability of the culvert pipe to carry
the flow exceeds the ability of water to enter the culvert through the inlet
Outlet control will exist when the ability of the pipe barrel to carry water away
from the entrance is less than the flow that actually enters the inlet
The location of the control section will shift as the relative capacities of the
entrance and barrel sections change with increasing or decreasing discharge
This means that it cannot be assumed that a given culvert will always operate
under the same hydraulic regime
V. Three Hydraulic Classifications
Inlet Control
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Inlet control means that the discharge capacity of a culvert is controlled at the
culvert entrance by the depth of headwater and the entrance geometry, including
the barrel shape and cross-sectional area
With inlet control, the roughness and length of the culvert barrel, as well as outlet
conditions (including depth of tailwater), are not factors in determining culvert
capacity
An increase in barrel slope reduces the headwater (inlet) depth, and any
correction for slope can be neglected for conventional or commonly used culverts
operating under inlet control
Barrel Control
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Under barrel control, the discharge in the culvert is controlled by the combined
hydraulic effects of the entrance (inlet), barrel length & slope, and roughness of
the pipe barrel
The characteristics of the flow do not always identify the type of flow
It is possible, particularly at low flows, for length, slope, and roughness to control
the discharge without causing the pipe to flow full
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But, this is not common at design discharges for most culverts
The usual condition for this type of flow at the design discharge is one in which
the pipe cross section flows full for a major portion (but not all) of the length of
the culvert
The discharge in this case is controlled by the combined effect of all hydraulic
factors
Outlet Control
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Culverts flowing with outlet control can have the barrel full of water or partly full
for either all or part of the barrel length
If the entire cross section of the barrel is filled with water for the total length of the
barrel, the culvert is said to be flowing full
VI. Culvert Flow Regimes
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The following is a slightly different classification of culvert flow
The flow through culverts can be divided into six categories (French 1985;
Bodhaine 1976), depending on the upstream and downstream free-surface water
elevations, and the elevations of the culvert inlet and outlet
The following categories are defined based on the design (maximum) discharge
capacity of a culvert
Type I Flow Inlet control. Critical depth occurs at or near the inlet:
(a) The slope of the culvert barrel is greater than the critical slope
(b) The downstream water surface elevation is lower than the elevation of the
water surface where critical flow occurs at the inlet
(c) The upstream water depth is less than approximately 1.5 times the barrel
height (or diameter)
Type II Flow Outlet control. Critical depth occurs at or near the outlet:
(a) The slope of the culvert barrel is less than critical slope
(b) The downstream water surface elevation is lower than the elevation of the
water surface where critical flow occurs at the outlet
(c) The upstream water depth is less than approximately 1.5 times the barrel
height (or diameter)
Type III Flow
Barrel control. Subcritical barrel flow, a gradually-varied flow
profile:
(a) The downstream water surface elevation is less than the height (or
diameter) of the barrel, but is more than the critical depth at the outlet
(b) The upstream water depth is less than approximately 1.5 times the barrel
height (or diameter)
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Type IV Flow Barrel control. Both the upstream and downstream ends of the
culvert are submerged, and the barrel is completely full of water.
The culvert behaves essentially like an orifice, but with additional
head loss due to the barrel.
Type V Flow
Inlet control. The barrel flows partially full and supercritical flow
occurs in the barrel downstream of the inlet:
(a) The slope of the culvert barrel is greater than the critical slope
(b) The upstream water depth is greater than approximately 1.5 times the
barrel height (or diameter)
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Type VI Flow Barrel control. The culvert is completely full of water:
(a) The upstream water depth is greater than approximately 1.5 times the
barrel height (or diameter)
(b) The outlet is unsubmerged (downstream depth less than the barrel height
or diameter)
VII. Additional Culvert Flow Regimes
References & Bibliography
Lindeburg, M.R. 1999. Civil engineering reference manual. 7th Ed. Professional Publications, Inc.,
Belmont, CA.
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