Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011) / Chapter Skim
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EXECUTIVE SUMMARY
EXECUTIVE SUMMARY
Pages 1-8
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Furthermore, comparisons of abutment scour depth estimations from existing formulas with field data and with engineering experience produce mixed results, partly because of the misperception that abutment scour formulas based on simplified laboratory experiments apply to all types of abutment scour, even to the most complicated field situations, and partly due to the difficulty of estimating the flow and sediment parameters required in existing scour formulas. Even with the foregoing complexities, some progress has been made in understanding abutment and contraction scour in the past twenty years or so, but future advances require identifying the most useful concepts and then winnowing and unifying some of these concepts into an AASHTO ~ Association of American State Highway and Transportation Officials 1 1
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Collected insights were integrated into an expert system of organizing, collating, and evaluating current knowledge to create a solid base from which future research needs could be effectively identified and outlined in order to advance the methodologies needed for engineers to design safer bridges. As part of the overall research approach, the following criteria were established to evaluate existing scour-depth prediction formulas in order to identify those that may provide promise and direction or even a framework for future research: 1.
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Whereas much of the laboratory research of recent years has focused on solid abutments that extend into the soil foundation, such as with sheet piles or other fairly rigid foundations, more attention should be focused in the future on erodible embankments. Recognition of the difference between erodible and solid abutments provides a factor for classifying existing scour prediction formulas and introduces the importance of geotechnical failure caused by hydraulic scour.
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Class II crossings are the most difficult in terms of scour prediction because of the interaction between the main channel and floodplain flows and the resultant redistribution of the flow in the contracted bridge section depending on how much of the floodplain flow is blocked by the embankment. A fourth scour condition might be added to the Type II crossing: Scour Condition AB for an abutment with a small setback distance in which both the floodplain and the bank of the main channel are erodible and the scour hole on the floodplain extends into the main channel.
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The reference depth would be obtained from well-established contraction scour formulas that depend on the assumption of equilibrium sediment transport in the live-bed case and the occurrence of critical conditions in the equilibrium scour hole in the clear-water case. The amplification factor would be developed as a function of degree of flow contraction caused by the constricted bridge opening as well as the local turbulence generated by flow obstruction and separation as described previously.
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RECOMMENDATIONS 1. Contraction scour should be viewed as a reference scour depth calculation while abutment scour should be taken as some multiple of contraction scour rather than additive to it.
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The leading methods for estimating scour depth better reflect parameter influences; f. Improved insights exist regarding abutment scour in clay; g.
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outlines the well-understood relationships between scour depth and significant parameters, summarized in Table 5-1. Notable examples of recent information include similitude in hydraulic modelling of flow distribution through a contracted bridge waterway, and the importance of flood-plain and embankment soil strengths.
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