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<br />FIGURE 4.9 Abutment set at Edge of Main Channel <br />~ = 2.75 (ta) [( 1 Y. + 1)7/6 - 1J <br />q",cYo Yo 4.1 Yo <br /> <br />( 10) <br /> <br />I <br />I <br />~"" <br />,. <br />f' <br />'1 <br />I <br />I <br />I <br />I <br />I <br />I <br />I I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />CASE 4 ABUlMENT SCOUR AT RELIEF BRIDGE <br /> <br />Scour depth for a relief bridge on the overbank flow area having no bed <br />material transport is calculated using equation B where Y1 is average flow <br />depth on the flood plain. Use ap for a in the equation. Draw stre.. lines or <br />field observations to delineate where the separation point is for the flow <br />going to the ..in channel and to the relief bridge. (See Figure 4.B.) <br /> <br />CASE 5 ABUl1IIENT SET AT EDGE OF CIWIIIEl <br /> <br />The case of scour around an abutment set right at the edge of the ..in channel <br />as sketched in Figure 4.9 can be calculated with equation 10 proposed by <br />Laursen (1980) when to < tc on the flood plain. <br /> <br /> <br /> ", > TC . <br />I , Main , I <br />, , , <br />hT~ Channel , <br />L-rt To < Tc <br /> " Overbank <br />" I' " 1\ <br />, I I \ , , I \ <br />" I \ I , I , <br /> , \ I , , , <br /> <br /> <br />qmc = the unit discharge in the main channel, Q/W <br />Q = Discharge in main channel <br />W = width of the main channel <br />Qo = overbank flow discharge <br />Yo = overbank flow depth <br /> <br />Note 1. If there is no overbank flow for this case then there is no <br />appreciable scour. <br /> <br />Note 2. Values of calculated scour depth for the different conditions are <br />given in Figure 4.10. <br /> <br />36 <br />