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<br />Notes. <br />1 . QmcZ <br /> <br />It is the:otal flow going through the bridge <br />opening, Often it is the sum of the flow in the main <br />channel up3tream of the bridge and the overbank <br />flow. <br /> <br />2. Qmcl <br /> <br />is th~ flo~ in the main channel upstream of the <br />bridge. <br /> <br />3. The Manning n ratio can be significant for a condition of <br />dune bed in the main channel and a corresponding plain <br />bed, washed out dunes or antidunes in the contracted <br />channel (7), HOIlEVER LAURSENS EQUATION DOES NOT <br />CORRECTLY ACCOUllT FOR THE INCREASE IN TRANSPORT THAT WILL <br />OCCUR AS THE RE~;ULT OF THE BED PLANING OUT WHICH <br />DECREASES RESIS',ANCE TO FLOI'l AND INCREASES VELOCITY AND <br />THE TRANSPORT OF BED l".ATERIAL AT THE BRIDGE. THAT IS, <br />LAURSEN'S EQUAT::ON INDICATES A DECREASE IN SCOUR FOR THIS <br />CASE WHEREAS IN REALITY THERE IS AN INCREASE IN SCOUR <br />DEPTH. THEREFORE SET THE TIW n VALUES EQUAL, <br /> <br />4. The average wid.:h of the bridge opening (l'IeZ) ~'" normally <br />taken as the bo~tom width, with the width of the piers <br />subtracted. <br /> <br />5. Laursen's eguatLon will overestimate tha dep~h of sco~r <br />at the bridge if the bridge is located at the upstream <br />end of the contraction or if the contraction is the <br />result of the bridge abutments and piers, At this time, <br />however, it is ~he best eg~ation available, <br /> <br />CASE la. <br /> <br />Case 1a involves contraction of the channel and overbank <br />flow. In this case, <br /> <br />Qmcl < QmeZ <br /> <br />QmcZ <br /> <br />= total flow going through the bridge, It eguals <br />plus Qoverb."k (Qob) less ~ny fl,?W going over ~he <br />roadway, through a rellef brldge or otherwlse <br />bypassing the main bridge, <br />> Wez <br /> <br />Qmcl <br /> <br />We1 <br /> <br />Wc2 <br /> <br />= bottom wid~h of the channel at the bridge less the <br />width of p.lers. <br /> <br />n1 = nz <br /> <br />.. <br /> <br />38 <br />