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<br />b. CASE 2, CONTRACTION SCOUR, NO OVERBANK FLOW. Live-bed <br />scour) <br /> <br />Case 2 is a special ca~e where there is not overbank flow and the <br />main channel narrows either naturally or due to the bridge piers <br />or the abutment and emtankment occupying part of the rrain <br />channel, Assuming that the main channel is transporting bed <br />material (live-bed) thEn equation 1 applies and redUCES to <br /> <br />Y2 <br />Y, <br /> <br />;v~. .Ie.. <br />= (--"".) <br />hlc2 <br /> <br />(6 ) <br /> <br />Although the computaticns are the same for cases 2a, 2b, and 2c <br />the latter two cases rEpresent situations where contraction scour <br />is not bridge related, Never the less this contraction scour is <br />flood related and need~ to be considered int the design or <br />evaluation of a foundation. In the case of 2b Laursen's long <br />contraction scour giver in equation 1 is conservative. <br /> <br />c. CASE 3. CONTRACTION SCOUR, RELIEF BRI::lGE ~.;'IT:1 :10 BED <br />MATERIAL TRANSPORT. (CLEAR-WATER SCOUR) <br /> <br />Case 3 applies to a relief <br />no bed material transpcrt. <br />in Chapter 2, <br /> <br />bridge on a floodplain where there is <br />Use Laursen's 1963 equation (9) given <br /> <br />with some algebraic maripulation it is as follows: <br /> <br />Ys = 0,13 [ <br />Y, <br /> <br />Q <br /> <br />6 <br />] 7" <br /> <br />- 1 <br /> <br />(7 ) <br /> <br />1 7 <br />D:! Y16 W2 <br /> <br />Ys = <br />Yl = <br />Q = <br />Dm = <br /> <br />Depth of scour, <br /> <br />Depth of flow on the flood plain upstream of the <br />relief bridge. <br />Discharge through the relief bridge. <br /> <br />Effective mean diameter of the bed material <br />(1.25 D~) in the bridge opening. <br /> <br />~ <br /> <br />42 <br />