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<br />area could be treated as a separate channel but this case <br />represents a situation for which flow continuity may not be <br />appropriate because some of the approach overbank flow will <br />probably end up in the main channel in the contracted <br />section. <br /> <br />For the set-back <br />with <br />Q2 <br /> <br />portion apply equation 2a given in Chapter 2 <br /> <br />= <br /> <br />Qob2 <br /> <br />W2 = W setbacK <br /> <br />where: <br /> <br />Qob2 <br /> <br />W setback <br /> <br />= overbank flow through the contracted section for <br />the left of right overbank area. <br /> <br />= distance the abutment is set back from the main <br />channel <br /> <br />Y2 = <br /> <br />Q~t2 <br /> <br />3 <br />,. <br /> <br />(5 ) <br /> <br />2 <br />120 Dso 2 <br /> <br />",2 <br />I'Y fie r;back <br /> <br />The flow in the overbank area set-back fron the main channel <br />can be determined using a water surface model like WSPRO <br />(23), A conservative assumption for the setback overbank area <br />would be that all of the overbank flow at the upstream <br />section stays on the overbank as it passes through the <br />contraction. Then, <br /> <br />Qobl = Qob2 <br /> <br />If the abutment is set back only a small distance from the bank <br />(less than 3 to 5 times the depth of flow through the bridge), <br />there is the possibility that the combination of contraction <br />scour and abutment scour may destroy the bank. Also, the two <br />scour mechanisms are not independent. Then consideration should <br />be given of using a guide bank or of ripraping the bank and bed <br />under the bridge in the overflow area using HEC. 11 (24) to <br />determine the riprap size. <br /> <br />Also, Laursen's abutment scour equations given in Appendix B will <br />estimate both contraction and local scour at abutments but will <br />not give contraction scour for the channel. <br /> <br />... <br /> <br />41 <br />