Vegetated Waterways

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Vegetated Waterways

• AST 324
• Carl E. Anderson

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Vegetated WaterwaysLearning Objectives

• Understand the concept of permissible velocity as
  used in waterway design
• Be able to use design tables, charts and
  equations to determine the required dimensions
  for a vegetated waterway

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Vegetated Waterways
Think of water flowing down a channel as a
soapbox racer going down a racetrack. The only
force causing motion is gravity. What factors
would cause the racer (water) to move down the
track (channel) faster?
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If you wanted to find a tire that would get
better traction, what characteristics would you
look for?
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Vegetated Waterways

• To control erosion we need to make sure the water
  moves at a slow velocity down the channel
• Leaves and Stems of Vegetation will increase the
  roughness and slow down the water
• Taller vegetation will slow down the water more

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Vegetated Waterways

• Water flows faster when it is deeper
• A waterway that is shallow will have lower
  velocity and less erosion
• A wider channel will have less depth and thus
  lower flow velocity

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Vegetated Waterways

• Bare soil has no protection from the erosive
  force of flowing water
• Plant leaves and stems will PAVE the channel and
  protect the soil from erosion.
• More dense vegetation will provide more
  protection.

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Vegetated Waterways

• The Permissible Velocity is a speed limit on the
  water. If the water exceeds this speed limit,
  there will likely be erosion.
• Vegetation Increases the Permissible Velocity for
  a Waterway
• More Dense Vegetation allows a Higher Permissible
  Velocity

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Vegetated Waterways

• The ACTUAL VELOCITY of flow is determined by the
  channel roughness, slope and shape (R).
• The roughness is determined by the height of the
  vegetation.
• The PERMISSIBLE VELOCITY is the SPEED LIMIT,
  determined by the density of the vegetation.

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Vegetation VR vs n Curves
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Vegetated Waterway Design

• Dense sod Mowed 6-10 inches tall
• Unmowed 11-24 inches tall
• Easily Eroded Soil on a 6 field slope
• Permissible Velocity 4 feet / sec
• Parabolic Cross-Section
• Design Capacity 75 CFS

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Vegetated Waterway Design

• C-Retardance when mowed
• From C-Retardance Chart R 0.51 ft.
• For Parabolic channel d 1.5R 0.77 ft.
• A Q / V 75 / 4 18.75 sq.ft.
• t 3A / 2d (3)(18.75)/(2)/(0.77) 36.53 ft.

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Vegetated Waterway Design

• B-Retardance when unmowed
• Estimate Freeboard of 0.25 feet
• D 0.77 0.25 1.02 ft.
• R 2/3 D (2/3)(1.02) 0.68 ft.
• From the B-Retardance Chart V 2.9 ft/sec lt 4.0

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Vegetated Waterway Design

• A 2/3 TD (2/3) (42.04) (2.9) 28.59 ft.
• Q AV (28.59)(2.9) 82.9 CFS