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Evaluating scour at bridges.
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    This document is the third of HEC 18, i.e., presents the state of knowledge and practice for the design, evaluation, and inspection of bridges for scour. It contains updated material not included in the second edition dated April 1993. This document is a revision to HEC 18 dated April 1993 which, in turn, was an update of HEC 18 dated Feburary 1991 and of the publication, "Interim Procedures for Evaluating Scour at Bridges," issued in September 1988 as part of the FHWA Technical Advisory T 5140.20, "Scour at Bridges." T 5140.20 has since been superseded by T 5410.23, "Evaluating Scour at Bridges" dated October 28,1991. This document contains revisions obtained from further scour-related developments and use of the 1993 edition of HEC 18 by the highway community. The principal change from the 1993 edition of HEC 18 is the use of metric (SI) units of measurement. Additional changes are: a discussion of backwater effects on contraction of scour and the use of the water surface or the energy grade line as the reference line for measuring contraction scour depths, addition of a coefficient to the pier scour equation to account for the armoring effect of large particle sizes in the bed material, and addition of an equation to compute the coefficient applied to the pier scour equation when there is an angle of attack. Figures have been added to clarify computation of pier scour for exposed footings, pile caps placed in the flow, multiple columns skewed to the flow, and a scour resulting from pressure flow. This document includes a method to compute scour depths for pressure flow when a bridge deck is submerged. A method to compute H for constricted tidal waterways is given along with the procedure used by Maryland SHD to evaluate scour for bridges over tidal waterways. A discussion of computer models to determine the value of the hydraulic variables for scour analysis of bridges over tidal waterways has been added and information on scour detection equipment has been updated based on recent research. Finally, minor errors in the text and figures have been corrected.
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