Basic Hydraulics: Culverts – I
Concepts• A culvert conveys surface water through a roadway
embankment or away from the highway right-of-way (ROW) or into a channel along the ROW
• In addition to the hydraulic function, the culvert must also support construction and highway traffic and earth loads; therefore, culvert design involves both hydraulic and structural design
• Culverts are considered minor structures, but they are of great importance to adequate drainage and the integrity of the facility.
Definitions
• Culvert = relatively short length of conduit used to transport water through an embankment
• Components
• Outlet
• Barrel(s)
• Inlet
Shapes• Typically, several shapes provide hydraulically
adequate design alternatives:
Multiple barrel culverts
Materials• Commonly used culvert materials include concrete
(reinforced and non-reinforced), steel (smooth and corrugated), aluminum (smooth and corrugated), and plastic (smooth and corrugated)
• The selection of material for a culvert depends on:• structure strength, • hydraulic efficiency, • installation, local construction practices, • durability, • cost.
Pertinent dimensions
Circular culvert – diameter
Box culvert – rise & span
Terminology
HW = headwater
TW = tailwater
Headwater• Headwater is the depth of water on the entrance
or upstream side of the culvert as measured from the inlet invert
• The Tailwater is the depth of water on the exit or downstream side of the culvert, as measured from the downstream invert
Culvert hydraulics
• What we need to know:• For given discharge, what size culvert is required
to carry the flow without overtopping embankment?
• For given culvert size, will specified discharge overtop?
• For given culvert size, what is capacity without overtopping?
• To answer these, must compute headwater depth, using principles of hydraulics
What’s in control?
• “Control” of culvert flow may be:• At inlet (inlet control) • At outlet (outlet control).
• Analysis of a culvert requires us to:• Assume inlet control, calculate headwater
depth.• Assume outlet control, calculate headwater
depth for control at outlet.• Pick higher of two values as appropriate
headwater value.
Data needed for culvert analysis
Item Inlet control Outlet control
Inlet area X X
Inlet edge configuration X X
Barrel shape X X
Barrel roughness X
Barrel area X
Barrel length X
Barrel slope X X
Tailwater elevation X
Inlet control
• Occurs when flow capacity of entrance is less than flow capacity of barrel.
• Control section is located just inside culvert entrance.
• Water surface passes through critical depth.• Inlet capacity depends on entrance geometry.
Computing inletcontrol headwater
• Culvert headwater depth for inlet control depends on the inlet shape and efficiency
• Federal Highway Administration (FHWA) developed a series of nomographs for standard culvert shapes that compute headwater depth for inlet control
Using FHWA nomographs
• Identify the culvert type (concrete pipe, box, CMP, etc.).
• Find correct nomograph.• Start on the pipe diameter scale line• Draw a straight line through the discharge scale line• Read the value of HW/D from the appropriate HW/D
scale for the entrance type for your culvert.• Usually two or three “scales” on nomograph that
represent the type of inlet. • For example, on next page, scales for (1) square
edge with head wall; (2) groove end with headwall; (3) groove end projecting
Example of FHWA nomograph
Scale HW/D HW (ft)
(1) 2.5 8.8
(2) 2.1 7.4
(3) 2.2 7.7
Example: D = 42 in. Q = 120 cfs
Entrance treatment
Source: HEC-2 Users Manual
Entrance efficiency
Outlet control
• Occurs when flow capacity is limited by downstream conditions (high-tailwater) or by capacity of the barrel
Outlet control examples
Outlet control computations
• To analyze, use energy equation:
where Zup = upstream invert elevation; HW = depth at inlet; Vup = average velocity upstream; g = acceleration of gravity; Zdown = downstream invert elevation; TW = depth at outlet; Vdown = average velocity downstream; hL = total energy loss through culvert.
• Since Vup Vdown we can simplify the equation
Lupdown hTWZZHW )(
Energy loss equations
• Energy loss is
in which hL= total head loss; he= entrance loss; hf = friction loss; ho= outlet (exit) loss
• Friction loss estimated with Manning’s equation
in which L = culvert length (feet); Q = flow rate in the culvert (cfs); n = Manning's roughness coefficient; A = area of flow (square feet); R = hydraulic radius (feet)
ofeL hhhh
Entrance loss coefficients(For outlet control only)
Type of Structure and Design of Entrance Coefficient, ken
Concrete Pipe Projecting from Fill (no headwall):
Socket end of pipeSquare cut end of pipe
0.20.5
Concrete Pipe with Headwall or Headwall andWingwalls:
Socket end of pipe (grooved end)Square cut end of pipeRounded entrance, with rounding radius = 1/12 of diameter
0.20.50.2
Concrete Pipe:
Mitered to conform to fill slopeEnd section conformed to fill slopeBeveled edges, 33.7 or 45 degree bevelsSide slope tapered inlet
0.70.50.20.2
Corrugated Metal Pipe or Pipe-Arch:
Projected from fill (no headwall)Headwall or headwall and wingwalls square edgeMitered to conform to fill slopeEnd section conformed to fill slopeBeveled edges, 33.7 or 45 degree bevelsSide slope tapered inlet
0.90.50.70.50.20.2
Weir flow
• Flow over the roadway can be computed as weir flow.
• Check the the headwater elevation to see if weir flow occurs.• If headwater elevation is higher than the
roadway, use iterative procedure, balancing weir and culvert flow.
• Solution found when weir flow and culvert flow produce same headwater elevations.
Qtotal = Qweir + Qculvert = Qgiven
Flow analysis for culverts
Flow Rate (cfs)
Roadway Crest
Top of Culvert
Inlet Control
Outlet Control Culvert Plus RoadwayOvertopping
Headw
ate
r Ele
vati
on (
ft)