culvert design workshop - local technical assistance program · design procedures • perform site...
TRANSCRIPT
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Culvert Design Workshop
Erik Carlson, P.E. MDOT Hydraulics Unit
February 2008
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Objectives
• Discuss required input data
• Discuss design criteria
• Discuss design procedures
• Answer questions
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Topics Not Covered
• Basic fluid mechanics
• Hydrologic design
• Theory behind equations
• Weir flow with culverts
• Recommend taking NHI’s Culvert Design
Workshop
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References
http://www.michigan.gov/stormwatermgt/0,1607,7-205--93193--,00.html
http://isddc.dot.gov/OLPFiles/FHWA/012545.pdf
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Required Input Data
• Hydrologic Information • Drainage Area
– If greater than or equal to 20 acres, use MDEQ SCS
procedures
– If less than 20 acres, use Rational Method
– If greater than 2 square miles, request discharge
information from MDEQ.
• Flood Discharge Information
– 10 year (10% chance)
– 50 year (2% chance)
– 100 year (1% chance)
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Required Input Data
• Design Requirements • Site Constraints
– Tailwater conditions
– Headwater constraints
– Existing culvert information
– Discharge constraints
Tailwater
Headwater
Flow
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Design Requirements
• MDOT culverts are
typically sized for a 50
year (2% chance)
flood
• Check for harmful
interference in a 100
year (1% chance)
flood
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Design Requirements
• Design headwater usually
controlled by site features
• Buildings
• Crops
• Adjacent drainage
structures (i.e. ditch
culverts)
• Road features
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Design Requirements
• The design
headwater should be
no greater than the
elevation where flow
diverts around culvert
(i.e. bypass flow)
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Design Requirements
• Design Headwater – General Procedures
• Target is 0.9D for circular and elliptical culverts;
nearly full for box culverts for a 50 year storm (2%
chance).
• Maximum allowable headwater is 1.5 below the
edge of shoulder for a 50 year storm (2% chance).
Use with caution for outlet velocities.
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Design Requirements
• Outlet Velocity
• Beware of scour at outlets
• In many cases, energy
dissipation required if over
6 ft/s (unless consistent
with downstream channel).
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Design Requirements
• Tailwater Determination
• Perform site visit to determine downstream
controls
• If downstream controls are present, a detailed
survey will be required of the channel • Tailwater determined using Manning’s Equation
• If no downstream controls are present, the
tailwater may be estimated using the average of
critical depth and barrel diameter.
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Design Requirements
• Based on MDOT design, what is the
design (target) headwater depth for the
following?
• Answer: 856.00 ft (+/-) for a 50 year storm
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Design Requirements
• Based on MDOT Design, what is the
maximum headwater depth for the following?
• Answer: 857.50 ft for a 100 year storm
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Design Procedures
• Perform site visit to determine controlling features.
• Tailwater
• Existing culvert information (inlet type, culvert shape, culvert material, etc.)
• Headwater constraints
• Visible problems with existing culverts
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Design Procedures
• Rules of Thumb
• Most culverts in Michigan are designed
for outlet control.
• Culverts steeper than 1% are generally
governed by inlet control.
• Culverts in inlet control may have issues
with cavitation and outlet velocities using
conventional design.
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Design Procedures
• Definitions
• Inlet Control – Occurs when the culvert
barrel is capable of conveying more flow
than the inlet will accept
• Outlet Control – Occurs when the culvert
barrel is not capable of conveying as
much flow as the opening will accept.
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Design Procedures
• Calculations
• Headwater values for both inlet and outlet
control are calculated and compared to
determine appropriate (and controlling)
headwater value.
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Design Procedures
• Inlet Control
• Factors influencing inlet control
include inlet area, inlet edge
configuration, and inlet shape.
• Cavitation may occur for submerged
inlets designed in inlet control.
• Most culverts in Michigan are not
designed in inlet control.
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Design Procedures
• Inlet Control
• Is the inlet control headwater affected
by switching from a manufactured end
section to just the pipe with the
grooved end projecting?
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Design Procedures
• Inlet Control
• HW/D values found in Nomographs in
HDS-5 or MDOT Drainage Manual
• HW added to upstream invert
• Beware of correct inlet configuration
HWinlet
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Design Procedures
• Inlet Control
• Find the inlet control HW value for the
following:
HWinlet
Prop. 36” diameter concrete culvert
56 feet long
Square edge with headwall
Q = 40 cfs
• Answer: 3.3 feet (Chart 1B)
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Design Procedures
• Outlet Control
• Based on the Energy Equation
• Factors influencing outlet control include
inlet area, inlet edge configuration, inlet
shape, barrel roughness, barrel area,
barrel shape, barrel length, barrel slope,
and tailwater elevation.
• Most culverts in Michigan are designed
for outlet control (conventional design).
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Design Procedure • Outlet Control
• Equations:
Ldu Hg
VTW
g
VHW
O
22
22
gjbofeL HHHHHHH
A
QV
)1.,5( EqHDS
)6.,5( EqHDS
)2.,5( EqHDS
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Design Procedure • Outlet Control
• Typically calculated as follows:
• Bend and grate losses neglected
• Elevations must be known
LHTWHWO
g
V
R
LnkH eL
2
291
2
33.1
2
)7.,5( EqHDS
)5.,5( EqHDS
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Design Procedure
• Outlet Control
• If elevations unknown, use the following
equation:
LSHTWHW L 0
TW
HL = He + Hv + Hf
D SL
HW=TW+HL-SoL
)7.5.,( EqMDOT
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Design Procedure
• Outlet Control
• Hydraulic
radius and
velocity must
be adjusted for
partial depth
• Use outlet
depth (TW) for
partial elements
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Design Procedures
• Outlet Control
• Find the headloss for the following:
Prop. 36” diameter concrete culvert
56 feet long
Square edge with headwall
Q = 40 cfs
TW= 3 ft.
HL = He + Hv + Hf
Slope = 0.002 ft/ft
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Design Procedures
• Outlet Control
07.7
4
3
4
22
D
A
75.04
3
4
4
2
D
D
D
P
AR
66.507.7
40
A
QV
5.0ek
012.0n
92.02.322
66.5
75.0
56012.0295.01
2
291
2
33.1
22
33.1
2
g
V
R
LnkH eL
Full flow at outlet:
sft.
ft.
ft/s
ft.
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Design Procedures
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Design Procedures
• Outlet Control
• Find the velocity for the following:
Prop. 36” diameter concrete culvert
56 feet long
Square edge with headwall
Q = 40 cfs
TW= 2.75 ft.
HL = He + Hv + Hf
Slope = 0.002 ft/ft
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Design Procedures
• Outlet Control
07.7
4
3
4
22
D
A
96.0
A
A
89.579.6
40
A
QV
sft. (full flow)
(Hydraulic Elements chart)
79.607.796.096.0 AA sft.
ft/s
92.03
75.2
D
d
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d/D = 0.92
A’/A = 0.96 R’/R = 1.17
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Design Procedure
• Miscellaneous Items
• Recommend using actual surveyed
elevations when sizing culverts to reduce
errors.
• Tailwater • Use surveyed cross-sections for all crossings with a
defined bed and bank in channel (which includes
“blue lines” on USGS maps).
• Don’t forget to check for any downstream
structures!
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Design Procedure
• Miscellaneous Items
• Beware of evidence of
problems in the field. • Scour/erosion/headcuts
• Debris lines at inlet
• High water marks
• Culvert damage
• Road cracking over
culvert
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Design Procedure
• Miscellaneous Items
• Bury the inverts
below the flowline
as described on
page 5-16 in the
MDOT Drainage
Manual
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Questions?