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<br />PREFACE <br /> <br />This FHWA publication, Hydraulic Engineering Circular No, 18 <br />(HEC-18), "Scour at Bridges," provides procedures for the design, <br />evaluation and inspection of bridges for scour. It is a reVlSlon <br />of the publication, "Interim Procedures for Evaluating Scour at <br />Bridges," which was issued in September 1988 as part the FHWA <br />Technical Advisory 5140.20, "Scour at Bridges."(5) It contains <br />revisions as the result of the use o.f the Technical Advisory by <br />the highway community. <br /> <br />The principal changes are 1) the inclusion of Niell's equation <br />for beginning of motion for coarse bed material in Chapter 2; 2) <br />only one scour analysis method is given in Step 3 of Chapter 4 <br />with the second method presented in the appendix; 3) the removal <br />of all but one abutment scour equations to the appendices; 4) the <br />recommendation to use guide banks (spur dikes) and/or riprap to <br />protect abutments from scour, thereby minimizing the need to <br />compute abutment scour; 5) the addition of procedures to <br />calculate local pier scour when footings or pile caps are <br />exposed, when multiple columns are at an angle to the flow and <br />when pile groups are exposed; 6) the addition of a discussion of <br />local pier scour when pressure flow occurs; i.e., the bridge deck <br />is submerged; 7) the inclusion of an equation to calculate the <br />width of the pier scour hole; 8) the elimination of the equation <br />to calculate the worse case (deepest) local pier scour from <br />Chapter 5; 9) a slight modification in the equation to determine <br />riprap size given in Chapter 7 to include recent research; and <br />10) extensive editorial changes. Also, some changes were made-in <br />the appendices, Principally the inclusion of North carolina"s <br />scour evaluation procedure in place of Minnesota's. <br /> <br />v <br />