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<br />CASE lc. <br /> <br />Case lc is very corrplex. The depth of contraction scour <br />depends on many factors such as: how far back from the bank <br />line the abutment is set, the condition of the bank (is it <br />easily eroded, are there trees on the bank, is it a high <br />bank, etc), is the stream narrower or wider at the bridge <br />than the upstream section, the magnitude of the overbank flow <br />that is returned tc the bridge opening, the distribution of <br />the flow in the bridge section, etc. <br /> <br />Case lc is a general situation that can be analyzeQ using <br />both contraction scour equations given in Chapter 2. The <br />contraction scour in the main channel portion is an <br />application of Equation 1, The only difference ir. this <br />portion of the cross section at the bridge and case la is <br />that the magnitude of Q~2 is not intuitively obvious, <br /> <br />Equation 1 for the main channel portion becomes <br /> <br />Yz <br />Y1 <br /> <br />5 <br />= ( Orne2) -=; <br /> <br />Qmcl <br /> <br />;v. /. <br />.~).. <br />",!C2 <br /> <br />(, . <br />.-:; <br /> <br />Q~l <br /> <br />= flow in upstream main channel, <br /> <br />Q~2 <br /> <br />= flow in the main channel portion of the bridge <br />cross section, <br />= bottom width of the upstream main channel. <br /> <br />We1 <br /> <br />We2 <br /> <br />= bottom width of the channel at the bridge less the <br />width of piers. <br /> <br />A water surface motel like WSPRO (23) can be used to <br />determine the distribution of flow between the main channel <br />and the set-back overbank areas in the contracted bridge <br />opening. <br /> <br />The set-back overb~nk area for case lc can be analyzed by <br />using the clear wat.er scour equations 2a or 2b described in <br />Chapter 2. Again, the problem will be in determining the <br />diSCharge that will be in tre overbank area. Each OVerbank <br /> <br />40 <br />