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<br />more precise analysis is warranted, a sediment transport <br />model like BRI STARRS could be used. <br /> <br />Calculation of contraction scour is presented in Chapter 4. <br /> <br />E. LOCAL SCOUR <br /> <br />The basic mechanism causing local scour at a pier or abutment is <br />the formation of vortices at their base. The formation of these <br />vortices results from the pileup of water on the upstream surface <br />and subsequent acceleration of the flow around the nose of the <br />pier or embankment. The action of the vortex removes bed <br />materials from the base region, with the transport rate of <br />sediment away from the base region greater than the transport <br />rate into the region, a scour hole develops. As the depth of <br />scour increases, the strength of the vortices is reduced, thereby <br />reducing the transport rate from the base region, and eventually <br />equilibrium is reestablished and scouring ceases. <br /> <br />In addition to a horseshoe vortex around the base of a pier, <br />there is a vertical vortex downstream of the pier called the wake <br />vortex, Figure 2,1, Both vortices remove material from the pier <br />base region. However, the intensity of these wake vortices <br />diminishes rapidly as the distance downstream of the pier <br />increases. Therefore, immediately downstrea~ of a long pier there <br />is often deposition of material, <br /> <br />'.'...' ,. <br /> <br />~ <br /> <br />,.... <br /> <br /> <br />Woke <br />~vortex <br /> <br /> <br />:~g <br /> <br />Horseshoe Vor tex <br /> <br />Figure 2.1 Schematic Representation of Scour at a Cylindrical <br />Pier. <br /> <br />15 <br />