Channel Design

1
Channel Design

• Stable Drainage Conveyance Channel Design using
  Flexible Linings
• Reference FHWA HEC 15

2
Relevance

• Roadside channel is included in highway
  right-of-way
• Channel gradient typically parallels highway
  grade
• Mild gradients can lead to severe hydraulic
  conditions (e.g., erosion, flooding)

3
Equilibrium

• Design a channel that performs within acceptable
  limits of stability
• Channel boundaries may be
• rigid (static equilibrium)
• channel lining resists erosive forces of flow,
  but subject to failure if a portion is damaged
  (frost, slumping)
• flexible (dynamic equilibrium)
• sediment supply rate equals sediment transport
  rate
• Flexible linings can tolerate some change in
  shape w/o loss of integrity
• bank instability and lateral migration cannot be
  tolerated in highway drainage channels

4
Flexible Lining

• Generally less expensive
• Permit infiltration
• Natural looking
• Simulate natural flow
• Habitat opportunities
• Pollutant removal

5
Flexible Linings

• Subject to erpsion
• Channel geometry limited within right of way

6
Analysis Methods Static Equilibrium

• permissible velocity
• 1920s research by Bureau of Reclamation
  (empirical)
• permissible tractive force
• focus on shear stresses at interface between
  lining and fluid
• more realistic model

7
Tractive Force

• Hydrodynamic force of water flowing in a channel
• Basis of design tractive forces should be less
  than the critical shear stress of lining

t average shear stress on channel g unit
weight of water (9810 N/m3 R hydraulic radius S
average bed slope
8
Permissible Shear and Velocity

• By substitution in Mannings equation, we get a
  relationship between permissible velocity and
  permissible shear stress

9
Shear Stress Determination

• Maximum shear stress in a straight channel
  occurs on the channel bed and is less than or
  equal to the shear stress at maximum depth of
  flow (d)
• Flow on a bend creates higher stresses on channel
  sides than in straight section.
• Values for K can be found in charts

10
Shear Stress Distribution - Bottom
11
Shear Stress Distribution - Bend
12
Flexible Lining Design

• Procedure
• compute normal depth
• determine shear stress at maximum depth
• permissible shear stress is force required to
  initiate movement of lining
• keep shear stress less than permissible shear
  stress

13
Design Procedure

• Select a flexible lining and determine
  permissible shear stress
• estimate flow depth range for lining, channel
  geometry
• determine Mannings n for lining and flow depth

• Calculate flow depth
• Compare calculated depth with estimated depth to
  a closure of lt0.03 m.
• Repeat
• Calculate shear stress at normal depth
• Determine needs for bends and side slopes

14
Determining Permissible Shear Stress

• Research and field studies yield tabulated
  results for different linings (see Table 2)
• For rock riprap and large diameter stone (with
  D50 in meters), permissible shear stress is

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Permissible Shear Stress for Non-Cohesive Soils
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Determining Maximum Shear Stress

• Use equation
• For channel bends, use Chart 10 to determine
  value of Kb
• The length of protection downstream is function
  of roughness of lining and depth of flow (see
  Chart 11)

18
Kb Factor for Bends (Chart 10)
19
Lining Design Continued

• Compute Normal Depth
• Mannings Equation
• See table 3

20
Mannings Roughness Coefficient (n)
21
Side Slope Stability

• Channel side slopes greater than 1V3H with
  gravel or riprap may be unstable
• Depends upon angle of repose for rock

22
Allowable Velocity
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Design Considerations - Riprap Lining

• Riprap gradation and thickness
• smooth size distribution curve
• smaller stones fill interstices between large
  stones - interlock
• D100/D50 and D50/D20 between 1.5 and 3
• Use angular rock round rock is OK on slopes
  lt1V 3H (length is 3X thickness or breadth)
• Thickness of layer 1.5 X thickness of largest
  rock diameter

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Rock Riprap Filter Design

• Use of granular filter material under rock riprap
  (overlying a soil base).