Innovations in Erosion Protection

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Innovations in Erosion Protection
Norton Fitzwarren Dam
Murray Seaton
Date 23 January 2008
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Dam Description

• Earth flood alleviation dam 450 m long, 5m high
• Floods smaller than 100 year return period with
  20 allowance for climate change pass through
  culvert
• Some flow from floods large than 100 year plus
  20 event overtops crest
• The whole crest and downstream shoulder acts as
  spillway
• Category A dam designed to pass the Probable
  Maximum Flood (PMF) of 272 m3/s

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Section through Downstream Shoulder
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Spillway Erosion Protection Armorflex

• Pre-cast concrete blocks. Polyester cables
  passing through the blocks enable them to be
  supplied as mats
• Installed on a geotextile on the crest, top of
  downstream shoulder and along the downstream toe
  of the embankment
• When topsoiled and grassed forms a reinforced
  grass waterway. Erosion protection of blocks
  enhanced by root anchorage and soil wedging
  between blocks
• Can withstand flow velocities up to 8 m/s
• Design guidance CIRIA Report 116 Design of
  reinforced grass waterways
• Approx material cost 12/m2

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Spillway Erosion Protection Armorflex
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Spillway Erosion Protection Armorflex
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Spillway Erosion Protection Stepped Blocks

• Pre-cast concrete blocks.
• Installed on a underlayer comprising granular
  material and geotextiles.
• The granular material provides an even foundation
  for the blocks and, in combination with drainage
  vents in the blocks and toe, helps prevent the
  build up of pressure beneath the blocks.
• The geotextile protects the subsoil from erosion
  due to flow in the granular material and prevents
  soil particles from the subsoil being washed out
  by seepage.

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Spillway Erosion Protection Stepped Blocks

• Installed on the downstream shoulder where the
  hydraulic jump forms
• Design guidance CIRIA Special Publication 142
  Design of stepped block spillways
• Block sizes designed and blocks manufactured at
  pre-casting plant
• Approx material cost 70/m2. Installation
  costs are also higher than for the Armorflex
  system.

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Spillway Erosion Protection Stepped Blocks
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Spillway Erosion Protection Stepped Blocks
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Spillway Erosion Protection Stepped Blocks
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Stepped Blocks A Brief History

• Initially developed at the Moscow Institute of
  Civil Engineering in the 1960s and 1970s
• CIRIA study arose due to
• Interest in low-cost methods of upgrading the
  capacity of spillways on existing dams
• Limitations in the performance of plain, parallel
  sided concrete blocks and other low cost erosion
  protection systems
• Stepped block spillway built at North West
  Waters Brushes Clough Reservoir and checked
  against the guidance in the CIRIA report

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Stepped Blocks Advantages

• Hydrodynamically stable because
• The upstream edge of the block is shielded from
  the flow
• The flow pattern causes a low-pressure zone
  downstream of each step connected by drainage
  vents to the underlayer and limits development of
  seepage flow
• The top surface of the block experiences a
  downthrust
• The stepped profile dissipates energy, reducing
  velocity and consequently reducing the energy to
  be dissipated in the tailwater at the toe

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Spillway Erosion Protection Stepped Blocks
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Stepped Blocks Advantages

• They can withstand high unit flow intensities
• They can be designed to withstand the forces
  generated by hydraulic jumps
• Flexibility of system allows differential
  settlement

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Conclusion

• Stepped blocks are a useful erosion protection
  option for spillways on erodible slopes, both for
  new structures and existing structures requiring
  increased flow capacity
• The ability to design the stepped blocks to
  withstand the forces generated by hydraulic jumps
  makes them particularly useful