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<br />CHAPTER 2 <br /> <br />BASIC CONCEPTS AND DEFINITIONS OF SCOUR <br /> <br />A. GENERAL <br /> <br />Scour is the result of the erosive action of flowing water, <br />excavating and carrying away material from the bed and banks of <br />streams. Different materials scour at different rates. Loose <br />granular soils are rapidly eroded by flowing water, while <br />cohesive or cemented soils are more scour resistant. However, <br />ultimate scour in cohesive or cemented soils can be as deep as <br />scour in sandbed streams, Scour will reach its maximum depth in <br />sand and gravel bed materials in hours; cohesive bed materials in <br />days; glacial tills, sand stones and shales in months; limestones <br />in years and dense granites in centuries. Massive rock <br />formations with few discontinuities are highly resistant to scour <br />during the lifetime of a typical bridge. <br /> <br />Designers and inspectors need to carefully study site specific <br />subsurface information in evaluating scour potential at bridges, <br />giving particular attention to foundations on rock. <br /> <br />This entire document relates to scour in the riverine context. <br />That is, scour resulting from flow in one direction, downstream. <br />In coastal areas of the Nation, highway associated transverse <br />and/or longitudinal stream encroachments are subject to tidal <br />flow. The determination of scour in tidal situations has not <br />been studied sufficiently to permit its inclusion. The best <br />guidance for determination of tidal scour until research and <br />operational experience give direction is judicious use of the <br />material developed for the riverine situation in this document. <br /> <br />B. TOTAL SCOUR <br /> <br />Total scour at a highway crossing is comprised of three <br />components: <br /> <br />1. Aqqradation and Deqradation. These are long-term stream <br />bed elevation changes due to natural or man induced causes <br />within the reach of the river on which the bridge is <br />located. Aggradation involves the deposition of material <br />eroded from other sections of a stream reach, whereas <br />degradation involves the lowering or scouring of the bed of <br />a stream. <br /> <br />2, Contraction Scour. Contraction scour in a natural channel <br />involves the removal of material from the bed and banks <br /> <br />7 <br />