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<br />RO(IQsAcross the Stream <br />Within the study reach on 80ar Creek there are five <br />bridge crossings. TheUnTonAvenlJe brIM'1p. near tho ups"treamend <br />waS considered to be washad out by a larga Iloo<l. Two other brIdges, <br />at Canon RO(Id and at Park Avenue, woulM be greatly overtopoed bV, <br />and woul<l not seriously Imoede, largelloodllows. At the State <br />Highway a bridge, the I ntermed Tate Reolonal Flood would flow overbank <br />around the left b<lnkabutment; the Star'ldard Project flood would over- <br />top the brldgaas well. TheSodaLakesRoadbrldqe Is Inadequate <br />to pass either large flood and would have the effect of raislnq water <br />levels on the upstream side of the bridge durl"9 these floods. The <br />affocts of bridges on flood flows are shown on the profIle and In <br />thetnblesofFloodPlalnReferenceData. <br />Moun"tYernonCreek Is crossed by three bridges wIthin <br />"the study reach. The waterway under Highway 8 bridge near the mouth <br />has lesscapacltythanthestre"mandconstrlctslloodflows. During <br />past floods the brIdge waterway has been plugged comple"tely wIth <br />bouldars and debris. Asocond brldgo, a very small, light structure. <br />cros~1 n9 01- bank level. WQ<J Id be I nunda"t"l<l by lar'le f lnod fin..." "nd <br />would not slg~1 f Icantly retard them. The thIrd brIdge Is <I or Iva"te <br />pedestrl~n bridge 1,500 feet upstream that would probably be washed <br />a..."y during a large flnod. <br />The elevations of the crossT~g structures relatlva "to <br />flood levels and to the Channel are shown on the profllosat the <br />end of thIs report. <br /> <br />I <br />, <br />, <br /> <br />Obstructlons to flood Flows <br />The bridges are the only obs"tructlons to II00d flows <br />wlthl n the c:hilnoe I. On eoar Creek t~e Stat" HTghway 11 a~d the Soda <br />Lakes ROdd bridges with theIr approach embnnkmonts raise the upstream <br />flood stagesseverlll feet higher than If the brldgos were not In <br />OlilCll, However. "the backwater e!fect <Jxter'lds onlv a short distance <br />upstream because of the steep strllam slopes. ~he other three bridges <br /> <br />, <br /> <br />on OellrCreek'and the brIdges on '1ount Yernon Creek llro subolerg&d <br />i1nd hllve little backwater effoctonfloodsof the size studied III <br />t~ls report. au i Id I ngs and trees a I so obstruct + lows on the II00d <br />pldin. Th60bs"tructlon by any single building Isusu311y insignifi- <br />cant, bu"ttheccrnbinedelfectof a group of buildings does retard <br />flood flowS and cause a great"r depth of flooding upstream from the <br />buildings. <br /> <br />, <br />