Rock Riffle Design Course

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Rock Riffle Design Course
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Goals of Streambank Restoration

• Work With Nature
• Reduce Sediment
• Protect Cropland
• Protect infra-structure
• Improve Water Quality
• Improve Aquatic Habitat
• Make it Affordable

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Lanes Balance
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200 years of Human Activity on the Landscape

• Cleared the Timber
• Plowed the Prairie
• Drained the Wetlands
• Straightened the Streams
• Leveed the Floodplains
• Built Cities with Large Areas of Concrete,
  Asphalt and Rooftops

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Results of Human Activity on Stream dynamics

• Increased Runoff
• Increased Stream Slope
• Reduced Floodplain Width

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Design Philosophy

• Work WITH the Natural Processes.
• Know Where Nature is Taking the Stream.
• Determine the Cause of The Problem
• Treat Only the Cause
• Let Nature Finish Healing The Banks
• Keep the Cost Down

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Channel Evolution Model (CEM)
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Stream Stabilization Techniques

• 4 Major Practices Used in Illinois
  
• Rock Riffle Grade Control Structures (stage II or
  III)
• Stone Toe Protection (stage III or IV)
• Bendway Weirs (stage IV)
• Stream Barbs (stage IV)

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Rock Riffle Grade Control Structures (stage II/
III streams)

• Stabilize the Bed
• Creates Riffle/Pool Sequence
• Dissipates Energy
• Aerates Water
• Permit Fish Passage
• Aesthetically Attractive

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Determine Stability

• Review I E Data
• CEM Stage?
• Entrenchment Ratio?

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McCray Stream Site

• Valley slope 0.0028
• Surveyed slope ave. 0.0024 (slightly lower)
• Bed in Riffle ---Clay (exposed in riffle bed)
• Width/Depth ratio 10.3 (just over 10)
• Entrenchment Ratio 1.28 (just under 1.4)
• 2 elements suggest Grade Control
• 2 elements outside suggested range (barely)

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Channel Geometry Considerations

• Sine Wave Flow
• Anticipated Scour Depth
• Radius Curvature/Bankfull Width Ratio (should be
  more than 1.8)
• Existing Riffle Location
• Existing Riffle Spacing

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Scour Depth

• Assume Scour Depth will equal Max. Bankfull Flow
  Depth
• Assumes bed material allows full Sine Wave Flow
  to develop over time

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Radius of Curvature

• Ave radius 2.3 times bankfull width (Leopold)
• Range generally from 1.6 to 4.5 times bankfull
  width (natural streams)
• 2.3 is the optimum for hydraulic efficiency
• 1.8 is the suggested minimum under guidelines

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Locating Riffles

• 5 to 7 Bankfull widths (28 ft)
• Expect Spacing--140 to 196 ft.
• Located at Cross-over Points

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Read The Channel

• Knickzones and Headcuts
• May be hundreds of feet long
• Little or no bedload in crossover pts.
• Will not be bedload material (sand wave)
• Increasing bank height downstream?
• Stable Points? (Bedrock, Culvert, etc)

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Gather Survey Data

• Bed Profile
• Water Surface Profile
• Typical X-Section _at_ Riffles (detailed)
• Low Bank Profile
• Stream Cross-over Points (Stations) or surveyed
  planform

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Plot profile

• Channel Grade (riffle to riffle)
• Riffle Spacing
• Compare low bank vs. channel grade
• Pool Depths
• Stable Points

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Riffles
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Existing Riffles?

• Stations 000, 425, 590, 825, 1020
• 1150, 1320, 1375, 1475 1550,1640, 2020
• 12 riffles in 2020 ft. --Ave. Spacing 168 ft.
• Max. Spacing --425 ft.
• Min. Spacing-- 55 ft.

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Low Bank
Grade Line
Profile
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Bank Heights
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Plot Cross-Section(s)

• Bankfull depths
• Riffle Crest Elev.

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Determine New Gradeline

• Slope (will be greater than existing slope)
• Stable Upstream(flood out headcut/knickzone)
• Stable Downstream(Culvert/Stable Grade, etc)
• Blend into existing channel bed (generally 1.0
  ft. height or less _at_ last riffle)

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Locate Riffles and Crest Elev.

• Geometrically Correct
• Uniform Gradeline
• 6 Bankfull Width spacing?
• Backwater to Toe of Upstream Riffle

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Planned Riffles
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Riffle Spacing

• Existing Riffles range from 55 ft. to 425 ft.
• (2 to 15 bankfull widths)
• Ave. 168 ft. (6 bankfull widths)
• Planned Riffles range from 210 ft. to 370 ft.
• (7.5 to 13 bankfull widths)
• Ave. 233 ft. (8.3 bankfull widths)
• Too Wide?

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Riffle Spacing (cont.)

• Concern is natural flow pattern and siltation
  problems if spacing is too wide.
• Experience has not shown spacing to be as
  critical as once thought (especially with silt
  and sand bed streams)
• Attempt to locate riffles near existing riffles
  at cross-over points and put backwater on tail of
  upstream riffle.

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Check for increased Flooding(when necessary)

• Bankfull Specific Energy Line
• Critical Flow Depth
• Critical Specific Energy Line
• Allowable Riffle Height Bankfull Specific
  Energy - Critical Specific Energy

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Max. Riffle Ht. CalculationSpecific Energy Line

• Energy Line Bkf Depth Velocity squared
  divided by 2 times gravity
• Energy Line 2.7 (5.1sq./ 2 x 32.2)
• Energy Line 2.7 (26.01/64.4)
• Energy Line 2.7 .40 3.1 ft.

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Max. Riffle Ht. CalculationCritical Flow Depth

• Critical Depth (Q sq. divided by gravity times
  width sq.) raised to the 1/3 power
• Critical Depth (350 sq./32.2 x 28 sq.) raised
  to 1/3 power
• Critical Depth (122,500/32.2 x 784) 1/3
• Critical Depth (122,500/28142) 1/3
• Critical Depth (4.35) 1/3power 1.63 ft.

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Max. Riffle Ht. CalculationCritical Energy Line

• Critical Energy Line 1.5 times Critical Flow
  Depth
• Critical Energy Line 1.5 x 1.63
• Critical energy Line 2.45 ft.

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Max. Riffle Ht. Calculation

• Max. Riffle Ht. Allowable without raising water
  surface profile or creating backwater effect
  Energy Line - Critical Energy Line
• Max. Riffle Ht 3.1 - 2.45
• Max. Riffle Ht. 0.65 ft.

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Max. Riffle Ht. Calculation Summary

• Specific Energy Line 3.1 ft.
• Critical Flow Depth 1.63 ft.
• Critical Specific Energy Line 2.45
• Max. Allowable Riffle Ht. W/O effecting water
  surface elev. Specific Energy Line - Critical
  Specific Energy Line or
• 3.1 ft. - 2.45 ft. 0.65 ft. max. ht.

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Critical Velocity Estimate

• Velocity at Critical Flow(Vc) (gravity x
  critical flow depth) raised to the 0.5 power
• Vc 32.2 x 1.63 (raised to 0.5 power)
• Vc 52.5 (raised to 0.5 power)
• Vc 7.24 ft/sec.

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What if I Raise Flood Surface Profile?

• Will I increase Out of Bank Flooding?
• Will I effect instream pipe outlets, tile, etc.
• Is the increase in flood surface profile
  acceptable to all effected parties?

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Compute Quantities and Cost

• Determine Stone Size from CriticalVelocity
  Calculations (double size for safety)
• Quantity Chart
• Average County Cost per Ton of Stone

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Cost of RipRap

• Statewide Average is about 25 per ton installed
• Check with local quarries/truckers for delivered
  price of A-5 or A-6 stone---Add 3 to 5 per ton
  for installation
• quarry run or shot-rock CAN be
  acceptable--Check with designer for O.K. before
  using and plan on being at site during delivery.

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ROCK RIFFLE

• CONSTRUCTION

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Construction Layout

• Riffles must be located at stream crossover
  points where flow goes from one side to the
  other. (riffles)
• Riffles should be located slightly upstream of
  existing riffle so backslope can blend into
  existing riffle--also places maximum water depth
  closer to apex of bend and dissipates energy in
  pool.

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Sequencing

• Build from Upstream to Downstream
• Riffles will not seal and pond water until
  voids are filled with sediment.
• Can speed up pooling by placing a few inches of
  fine silt/clay on front face of riffle.

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Bed Key

• 4 ft. minimum width
• 3 ft. minimum depth/or to bedrock
• Extends across channel bottom and 3-4 ft. into
  outer bank.
• Doesnt need to be larger material
• Do not over excavate! Easy to lose stone in bed
  key

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Bank Key

• 4 ft. minimum width
• 5 ft. minimum depth
• Construct with a 1.5 on 1 slope into bank so that
  as vertical banksstabilize the key is not
  exposed
• No need to fill key trench out to original
  bank.
• Top Elev. approx.1 ft. below top bank

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Riffle Crest

• Build Crest to elevation before starting on
  backslope
• Use largest material in gradation
• Place crest stones individually with thumb if
  possible
• Build Crest with slight V shape using control
  elevation as low point in v.

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Backslope

• Build with 201 backslope
• Place next largest stone available from gradation
  immediately downstream of Crest stones.
• Flare the backslope up the channel sides slightly
  (1.5 ft?)
• Leave backslope rough to aerate flow and
  provide resting places for fish passage.

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Blending into Exit Channel

• H-4 dimension is depth to place material below
  existing channel bed at exit.
• Most streambeds are soft enough that material
  can be pressed into channel bed without
  excavating

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Alignment

• Align Crest so that desired exit is at 90 degrees
  to Crest.
• If channel has a sharp bend near tail of riffle
  use Stone Toe Protection downstream to protect
  against increased velocity near riffle exit.
  (Riffle backslope has a 5 grade)

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Follow-up and Maintenance

• Check each structure after first major storm
  (near bankfull flow)
• Some movement and of smaller stone and sorting of
  material is expected.
• If any crest stone moves it should be repaired
  immediately