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<br />the local scour there. A guide bank at that abutment decreases the risk from <br />scour of that abutment from this returning overbank flow. Also, relief bridges <br />in the approaches, by decreasing the amount of flow returning to the natural <br />channel decrease the scour problem at the bridge cross section. <br /> <br />Other factors that can cause general scour are: (1) a natural stream <br />constriction: (2) long approaches over the flood plain to the bridge: (3) ice <br />formation or jams; (4) berm forming along the banks by sediment deposits: (5) <br />island or bar formations upstream or down stre.. of the bridge opening: (6) <br />debris: and (7) the growth of vegetation in the channel or flood plain. <br /> <br />To determine the magnitude of contraction scour from a variable backwater <br />requires a study of the stream system to (1) determine if there will be <br />variable backwater and: (2) if this condition exists to determine the ..gnitude <br />of contraction scour for this condition. The VSPRO computer model is of <br />particular value in determining if backwater effects exist and the magnitude of <br />the effects on the velocity and depth of flow. <br /> <br />General scour of the bridge opening may be concentrated in one area. If the <br />bridge is located on or close to a bend the scour will be concentrated on the <br />outer part of the bend. In fact there may be deposition on the inner portion <br />of the bend, further concentrating the flow, which increases the scour at the <br />outer part of the bend. Also, at bends the thalweg (the part of the stre.. <br />where the flow or velocity is largest) will shift toward the center of the <br />stream as the flow increases. This can increase scour and the non uniform <br />distribution of the scour in the bridge opening. Equations for calculating <br />contraction scour are presented in Chapter 4. <br /> <br />-- <br /> <br />E. LOCAL SCOUR <br /> <br />The basic mechanism causing local scour at a pier or abutment is the fOMDItion <br />of vortices at their base. The formation of these vortices results fr~ the <br />pileup of water on the upstream face and subsequent acceleration of the flow <br />around the nose of the pier or embankment. The action of the vortex is to <br />remove bed materials away from the base region. If the transport rate of <br />sediment away from the local region is greater than the transport rate into the <br />region, a scour hole develops. As the depth of scour is increased, the <br />strength of the vortex or vortices is reduced, the transport rate is reduced, <br />and an equilibrium is reestablished and scouring ceases. <br /> <br />With a pier, in addition to the vortex around the base, the horseshoe vortex, <br />there is a vertical vortex downstream of the pier, the wake vortex, figure 2.1. <br />Both vortices remove Mlterial from around the pier. However, immediately <br />downstream of a long pier there is often deposition of material. <br /> <br />6 <br />