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<br />D. STEP 4. CONTRACTION SCOUR <br /> <br />1. General <br /> <br />Contraction scour can be caused by different bridge site <br />conditions. There are four (4) conditions (cases) which are: <br /> <br />Case 1. Involves overbank flow on a flood plain being <br />forced back to the main channel by the approaches <br />to the bridge. <br /> <br />a. The river channel width becomes narrower either <br />due to the bridge abutments projecting into the <br />channel or the bridge being located at a narrowing <br />reach of the river (Ive, > WeZ) . <br /> <br />b. Does not involve any contraction of the main <br />channel but the overbank flow area is completely <br />obstructed by the embankment (We' = WeZ) . <br /> <br />c. Abutments set back from the stream channel <br />(W, < WeZ + Wsetbock). <br /> <br />Case 2. Flow is confined to the main channel: i.e., there <br />is no overbank flow. The normal river channel <br />width becomes narrower due to the bridge itself or <br />the bridge site being locaced at a narrower reach <br />of the river. <br /> <br />Case 3. A relief bridge in the overbank area with little <br />or no bed material transport in the overbank area: <br />i.e., clear-water scour. <br /> <br />Case 4. A relief bridge over a secondary stream in the <br />overbank area. (Similar to Case 1). <br /> <br />We' = bottom width of the main channel <br />Wcz = bottom width of the contracted section <br /> <br />These 4 cases are illustrated in Figure 4.1. <br />equations for solving each case are presented in the <br />sections. <br /> <br />The <br />following <br /> <br />... <br /> <br />35 <br />