Scour at Contracted Bridges (2006) / Chapter Skim
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Pages 103-111
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From page 103... ...
92 CHAPTER 3: INTERPRETATIONS, APPRAISAL AND APPLICATION RECOMMENDED MODIFICATIONS TO SCOUR PREDICTION METHODOLOGY The analysis of field data collected at the 15 sites during the NCHRP 24-14 project, and 146 sites in South Carolina (Benedict, 2003) has provided further recognition of the complex nature of scour at contracted bridges and a basis to recommend modifications to the HEC-18 (Richardson and Davis, 2001)
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From page 104... ...
93 Adding Abutment and Contraction Scour Field observations of scour at many bridges indicate that conceptual separation of contraction and abutment scour as described in Hydraulic Engineering Circular-18 (HEC-18) (Richardson and Davis, 2001)
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From page 105... ...
94 Although the overall effects of flow contraction and the local flow curvature that occurs around abutments can be conveniently separated conceptually, the resulting scour pattern cannot be separated into contraction- and abutment-scour components. The cause of the specific scour patterns is believed to be highly sensitive to local field conditions.
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From page 106... ...
95 Two-dimensional models such as FESWMS-2DH can provide information on the bend effects on flow distribution; however, judgment in the adjustment of flow parameters may be required where two-dimensional models may not be cost effective. Considerations such as the relative amount of flow in the channel and floodplain, the bend radius of curvature, the position of the bend with respect to the bridge opening, and the flow velocity in the channel and floodplain are a few factors that influence the effect of bends on the flow distribution near a bridge.
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From page 107... ...
96 Location of Scour Holes Analysis of the field data also has revealed that the location of scour in a contracted bridge opening is highly variable and does not follow the patterns typically reported from laboratory experiments. The longitudinal location of contraction and abutment scour holes can be dependent upon site specific factors such as the configuration of scour protection, guide banks, bridge length, channel alignment, and bed material.
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From page 108... ...
97 scour study (Benedict, 2003) , may provide the information necessary to develop prediction equations that will more accurately represent field conditions and reduce the costs of overdesigning bridge foundations.
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From page 109... ...
98 Comparison of abutment-scour predictions with observed scour depths showed that typically the abutment-scour equations over-predict the depth of scour, often substantially. Analysis of the cause of the inaccuracies of the predictions showed that the primary problem lies in the abutment-scour equations rather than in the model used to estimate the hydraulic parameters.
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From page 110... ...
99 applicability of these methods. These methods should be applied with caution and substantiated with additional field observations.
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From page 111... ...
100 dimensional model appears to be a better alternative to simply using the multi-dimensional model to determine the hydraulic parameters for the scour equations; therefore, the need for a multi-dimensional model is highly site specific but where applicable can provide valuable information on the expected flow and scour patterns. ERODIBILITY AND GEOTECHNICAL PROPERTIES OF MATERIALS The drastic overprediction of scour using HEC-18 methods on floodplains where no scour was observed at a large percentage of sites in the study of Benedict (2003)
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