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<br />STEP 2, ANALYZE LONG TERN BED EU"JATION CHANGE. <br /> <br />Analysis of the U, S. Geological Survey stage discharge relation <br />at a gaging station fi,'e miles downstream of the bridge site <br />indicates that there is a.long term decrease in bed elevation. <br />This decrease is not gradual but averages about 0.02 ft per year. <br />It results from erosion of a bed rock control located downstream <br />of the gage. Because this is a sand bed stream this shift will be <br />reflected in a long tel~ bed elevation decrease at the bridge <br />site of 2 ft in 100 ye~rs, The decrease dose not appear to <br />affect the stream hydrc.ulics but the main channel is getting <br />deeper with respect to the banks. <br /> <br />Will add 2 ft of long t:erm bed elevation change to the <br />contraction and local scour but will not take into consideration <br />that with a deeper main channel that there is the possibility <br />that the overbank flow will decrease in time. Also, will not <br />consider that the hydrclulics will change. This is a conservative <br />approach. <br /> <br />STEP 3. EVALUATE SCOUP ANi'.LYSIS ,'!ETHOD <br /> <br />Contraction scour will be limited to around 6 ft by sizing <br />the bridge opening and/or the use of relief bridge if <br />necessary. <br /> <br />. <br /> <br />Scour components will develop independencly so analysis <br />method given in Chapter 4 sLep 3 will be used. <br /> <br />The velocity in the pier and abutment scour equations will <br />be adjusted by coefficients times the mean velocity to <br />account for the increase or decrease in velocity resulting <br />from their location in the flow, <br /> <br />STEP 4. CONTRACTION SCOUR <br /> <br />Problem 1. <br /> <br />Contraction scour with abutments set into the channel (CASE <br />1a) . <br /> <br />y 0 6 W <br />2 = ( rne2) "7 ( -E) K, <br />Y1 Ornel WC2 <br /> <br />(2 ) <br /> <br />64 <br />