Design of Open Channels and Culverts

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Design of Open Channels and Culverts
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Transverse Slopes

• Removes water from pavement surfaces in shortest
  amount of time possible

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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
• Minimum 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

• Trapezoidal generally preferred considering
  hydraulics, maintenance, and safety
• V-shaped less desirable from safety point of
  view and maintenance

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Flow Velocity

• Depends on lining type
• Typically 1 to 5 slopes used
• Should be high enough to prevent deposit of
  transported material (sedimentation)
• For most linings, problem if S lt 1
• 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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Side Ditch/Open Channel Design-Basics

• Find expected Q at point of interest (see
  previous lecture)
• Select a cross section for the slope, and any
  erosion control needed
• Mannings formula used 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,f)
• 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 Hydraulic Design Charts
• 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 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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Check lining for max depth of flow
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Rounded
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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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Median drain
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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 (with respect to
  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 and roadway cross section at
  culvert crossing location (with approximation of
  stream elevation) best is natural stream
  location, alignment, and slope (may be expensive
  though)
• Establish inlet/outlet elevations, length, and
  slope of culvert

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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

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Headwater Depth

• Constriction due to culvert creates increase in
  depth of water just upstream
• Allowable level of headwater upstream usually
  controls culvert size and inlet geometry
• Allowable headwater depth depends on topography
  and land use in immediate vicinity

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Types of culvert flow

• Type of flow depends on total energy available
  between inlet and outlet
• 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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Ans.
Example Design ElevHW 230.5 Stream bed at
inlet 224.0 Drop 6.5 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 - OK
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Types of culvert flow

• 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 Stream elev at inlet 240 200
culvert Outlet invert 240-0.02x200
220.0 Given tail water depth 6.5 Check
critical depth 4.3 from fig. 17.23 (next
page) Depth to hydraulic grade line (dcD)/2
4.7 lt 6.5, use 6.5 Head drop 3.3 (from
chart) 220.06.53.3 229.8lt230.5 OK
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