In 1988 The Federal Highway Administration issued FHWA Technical Advisory T5140.20 entitled "Scour at Bridges". It required the States to evaluate the scour risk at all bridges over water. Accompanying the Advisory was the publication "Interim Procedures for Evaluating Scour at Bridges." The "Interim Procedures" delineated the scour problem at highway encroachments and crossings as 1) stream instability and channel movement, 2) long term degradation or aggradation, 3) live bed or clear-water contraction scour and 4) local scour at piers and abutments. The "Interim Procedures" provided guidance and equations for evaluating scour. This was the first time a manual was written that gave comprehensive methods and recommended equations for the hydraulic analysis to determine scour depths for design of foundations of new bridges or evaluation of existing bridges and to protect the river environment. Subsequently, the "Interim Procedures" were updated and issued as Hydraulic Engineering Circulars 18. The Fourth Edition of HEC-18 is summarized in this paper.
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