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Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011) / Chapter Skim
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6 CONTRACTION SCOUR FORMULAS
Pages 60-65

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From page 60...
... Although contraction scour will vary across the cross section in the field due to nonrectangular geometry and a nonuniform velocity distribution, it is often visualized and applied as a uniform decrease in bed elevation across the bridge opening. Floodplain contraction scour is usually treated separately from main channel contraction scour in compound channels.
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From page 62...
... He applied the Duboys sediment transport formula (Vanoni 1975) , which is generally considered a bed-load transport formula in which bed shear stress is the independent variable, for estimation of the sediment transport rate in the approach-flow and contracted sections.
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From page 63...
... emphasized the influence of armoring on live-bed scour depth by arguing that the ratio of the sediment sizes in the approach flow section and contracted section influence the contraction scour depth for large values of B1/B2 and σg1. He applied dimensional analysis to a series of laboratory experiments on live-bed and clear-water contraction scour in a long contraction (Lc/B1 ≥ 1.0)
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From page 64...
... the formulas are based on a much simpler set of flow conditions in the laboratory than found in the field; and (2) existing field data are primarily based either on measurements of contraction scour long after the flood event for which the hydraulic parameters may not be known, or on "flood chasing" techniques in which the time of scour measurement may not coincide with the occurrence of maximum temporal scour depth.
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From page 65...
... They proposed eliminating Q2/Q1 from the Laursen live-bed scour equation by assuming that all flow remains in the main channel in order to justify an envelope curve for the contraction scour depth, which depends only on the geometric contraction ratio. By comparing the maximum depth of scour with soil boring data, they concluded that the Piedmont data for scour depth were limited by a scour-resistant subsurface layer that consisted primarily of bedrock, but in a smaller number of cases it was composed of gravel or clay.
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Key Terms

  • scour depth
  • approach flow
  • main channel
  • scour formula
  • scour formulas

  • field data
  • cross section
  • contracted section
  • dimensional analysis
  • geometric contraction

  • contraction ratio
  • pier scour
  • abutment scour
  • sediment size
  • bed elevation

  • scour contraction
  • field contraction
  • vertical contraction
  • flow section
  • contracted flow

  • shear velocity
  • average bed
  • carolina piedmont
  • bridge section
  • critical value

  • envelope curve
  • bed contraction
  • evaluating contraction
  • floodplain contraction
  • dimensionless contraction


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