Design of Open Channels and Culverts CE453 Lecture 25

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Design of Open Channels and CulvertsCE453
Lecture 25
Ref Chapter 17 of your text and HYDRAULIC DESIGN
OF HIGHWAY CULVERTS, Hydraulic Design Series
Number 5, Federal Highway Administration,
Publication No. FHWA-NHI-01-020, September 2001
available at http//www.cflhd.gov/design/hyd/hds5_
03r.pdf, accessed March 18, 2006
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Design of Open Channels
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Longitudinal Slopes

• Gradient longitudinal direction of highway to
  facilitate movement of water along roadway

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Drains

• Along ROW
• Collect surface water

A typical intercepting drain placed in the
impervious zone http//www.big-o.com/constr/hel-
cor.htm
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Drainage Channels (Ditches)

• Design
• Adequate capacity
• Minimize hazard to traffic
• Hydraulic efficiency
• Ease of maintenance
• Desirable design (for safety) flat slopes,
  broad bottom, and liberal rounding

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Ditch Shape
Source Fabriform1.com

• Trapezoidal generally preferred considering
  hydraulics, maintenance, and safety

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Ditch Shape
Source Fabriform1.com

• V-shaped less desirable from safety point of
  view and maintenance

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Terms

• Steady Flow rate of discharge does not vary
  with time (Mannings applies)
• Uniform channel properties are constant along
  length of channel
• Slope
• Roughness
• Cross-section
• Water surface is parallel to slope of channel
• Non-uniform properties vary

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Terms

• Unsteady flow rate of discharge varies with time
  
• Critical depth
• a hydraulic control in design
• depth of water where flow changes from tranquil
  to rapid/shooting
• Critical velocity velocity corresponding to
  critical depth
• Critical slope slope corresponding to critical
  depth

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Flow Velocity

• Depends on lining type
• Should be high enough to prevent deposit of
  transported material (sedimentation)
• For most linings, problem if S lt 1 (generally
  velocity should be gt 2 fps when full)
• Should be low enough to prevent erosion (scour)
• For most types of linings, problem if S gt 5

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Use spillway or chute if ?elev is large
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Rip Rap for drainage over high slope
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Riprap (TN Design Manual)
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Side Ditch/Open Channel Design-Basics

• Estimate Q at point of interest
• Select ditch cross section
• Erosion control?
• Mannings formula for design
• Assume steady flow in a uniform channel

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Mannings Formula

• V R2/3S1/2 (metric) V 1.486
  R2/3S1/2
• n n
• where
• V mean velocity (m/sec or ft/sec)
• R hydraulic radius (m, ft) area of the cross
  section of flow (m2, ft2) divided by wetted
  perimeter (m,ft)
• S slope of channel
• n Mannings roughness coefficient

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Side Ditch/Open Channel Design-Basics

• Q VA
• Q discharge (ft3/sec, m3/sec)
• A area of flow cross section (ft2, m2)
• FHWA has developed charts
• to solve Mannings equation
• for different cross sections

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Open Channel Example

• Runoff 340 ft3/sec (Q)
• Slope 1
• Mannings n 0.015
• Determine necessary cross-section to handle
  estimated runoff
• Use rectangular channel 6-feet wide

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Open Channel Example

• Q 1.486 R2/3S1/2
• n
• Hydraulic radius, R a/P
• a area, P wetted perimeter

P
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Open Channel Example

• Flow depth d
• Area 6 feet x d
• Wetted perimeter 6 2d

Flow depth (d)
6 feet
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Example (continued)

• Q 1.486 a R2/3S1/2
• n
• 340 ft3/sec 1.486 (6d) (6d) 2/3 (0.01)1/2
• (6 2d)
• 0.015
• d ? 4 feet
• Channel area needs to be at least 4 x 6

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Example (continued)

• Find flow velocities.
• V 1.486 R2/3S1/2
• n
• with R a/P 6 ft x 4 ft 1.714
• 2(4ft) 6ft
• so, V 1.486(1.714)2/3 (0.01)1/2 14.2 ft/sec
• 0.015
• If you already know Q, simpler just to do
• VQ/A 340/24 14.2)

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Example (continued)

• Find critical velocities.
• From chart along critical curve, vc ? 13 ft/sec
• Critical slope 0.007
• Find critical depth yc (q2/g)1/3
• g 32.2 ft/sec2
• q flow per foot of width
• 340 ft3/sec /6 feet 56.67ft2/sec
• yc (56.672/32.2)1/3 4.64 feet gt depth of 4

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Source FHWA Hydraulic Design Charts
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A cut slope with ditch
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A fill slope
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Inlet or drain marker
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Ditch treatment near a bridge US 30 should pier
be protected?
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A fill slope
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Hidden Drain
Wheres the water going to end up?
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Median drain
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Design of Culverts
Source Michigan Design Manual
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Culvert Design - Basics

• Top of culvert not used as pavement surface
  (unlike bridge), usually less than 20 foot span
• gt 20 feet use a bridge
• Three locations
• Bottom of depression (no watercourse)
• Natural stream intersection with roadway
  (majority)
• Locations where side ditch surface drainage must
  cross roadway

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Hydrologic and Economic Considerations

• Alignment and grade of culvert (wrt roadway) are
  important
• Similar to open channel
• Design flow rate based on storm with acceptable
  return period (frequency)

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Culvert Design Steps

• Obtain site data
• Roadway cross section at culvert location
• (best is at existing channel)
• Establish inlet/outlet elevations, length, and
  slope of culvert

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Sometimes you want a dam why?
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Culvert Design Steps

• Determine allowable headwater depth (and probable
  tailwater depth)
• during design flood
• control on design size f(topography and nearby
  land use)
• Select type and size of culvert
• Examine need for energy dissipaters
• Emergency overflow?

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Headwater Depth

• Constriction due to culvert creates increase in
  depth of water just upstream
• Allowable/desirable level of headwater upstream
  usually controls culvert size and inlet geometry
• Allowable headwater depth depends on topography
  and land use in immediate vicinity, as well as
  need to protect roadway subgrade

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Inlet control

• Flow is controlled by headwater depth and inlet
  geometry
• Usually occurs when slope of culvert is steep and
  outlet is not submerged
• Supercritical, high v, low d
• Most typical
• Following methods ignore velocity head

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Example Design ElevHW 230.5 (max) Stream bed
at inlet 224.0 Drop 6.5 Peak Flow
250cfs 5x5 box HW/D 1.41 HW 1.41x5
7.1 Need 7.1, have 6.5 Drop box 0.6 below
stream _at_223.4 - OK
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Outlet control

• When flow is governed by combination of headwater
  depth, entrance geometry, tailwater elevation,
  and slope, roughness, and length of culvert
• Subcritical flow
• Frequently occur on flat slopes
• Concept is to find the required HW depth to
  sustain Q flow
• Tail water depth often not known (need a model),
  so may not be able to estimate for outlet control
  conditions

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Example Design ElevHW 230.5 Flow 250cfs 5x5
box (D5) Stream at invert 224 200
culvert Outlet invert 224-0.02x200 220.0
(note 223.4-.017x200) Given tail water depth
6.5 Check critical depth, dc 4.3 from fig.
17.23 Depth to hydraulic grade line (dcD)/2
4.7 lt 6.5, use 6.5
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Example (cont.) Design ElevHW 230.5 Flow
250cfs 5x5 box Outlet invert 220.0 Depth to
hydraulic grade line 6.5 Head drop 3.3
(from chart) 220.06.53.3 229.8lt230.5 OK
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