Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011) / Chapter Skim
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60 CHAPTER 6. CONTRACTION SCOUR FORMULAS 6.1 DEFINITION OF CONTRACTION SCOUR Contraction scour is caused by flow acceleration due to narrowing of the channel cross section whether by natural reduction in the width of the main channel for a bankline abutment, or by redistribution of floodplain flow in the contracted section as a result of flow blockage by the bridge embankment for a setback abutment.
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From page 63... ...
61 Figure 6-1. Definition sketch for idealized long contraction scour (Q1 = main channel flowrate for live-bed scour; Q2 = total flowrate in channel at contracted section; dsc = contraction scour depth.
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62 bed contraction scour occurs for flow in the main channel only, the value of M is unity but B1/B2 becomes the primary independent variable gauging the degree of flow contraction as determined by main-channel geometry alone. Because clear-water contraction scour tends to occur only on the floodplain, the effective B1/B2 can be replaced by the ratio of discharges per unit width, qf2/qf1, for a streamtube that passes through the contracted floodplain.
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63 contraction with a bankline abutment, it can be seen that dimensionless scour depth depends only on (Qt/Qc)
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From page 66... ...
64 6.5 FIELD DATA ON CONTRACTION SCOUR As with abutment scour, there is a paucity of reliable field data for comparison with the contraction scour formulas in Table B-1. Two major problems with such comparisons is that: (1)
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From page 67... ...
65 In a follow-up study of live-bed contraction scour in the South Carolina Piedmont and the Coastal Plain, Benedict and Caldwell (2009) estimated the elevation of buried scour surfaces using ground-penetrating radar.
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