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<br />' J11L1987 <br />lkpt. of Higtlway <br />State of Coloradr <br />r_OIST. 2 OFF1f , <br /> <br />~~ <br />REPORT <br />POTENTIAL SCOUR HAZARD <br />U. S. HIGHWAY 50 BRIDGE OVER ARIiANSA <br />~~ at <br />L~RMAR, COLORADO <br />INTRODUCTION <br />Gravel arining operations downstream of the U.r/~ Highway 5U <br />bridge over the Arkansas River at Lamar, C;olorad~~F~s.created <br />Foux• ponds (gravel pits). une of Lhe gravel~p1~>a is just <br />downstream of the b {'L 00 Ft. 1. 1'Iris pit about{ 60 i't <br />deep and has a ut•face at•e~ of about :15d,U0 cubic fee ,.' There <br />is a high dike aroun a-- pstream and stream si of the pit. <br />This dike was built in the spring of 1983. The upstream dike is <br />between the bridge and the pit and blocks all but about 700 ft of <br />the approxamate 55U Ct buttom opening oC the bridgr_. A HEi: Z <br />water surface profile stud,v determined that. it would take around <br />20,000 cfs to overtop the dike. <br />The gravel mining operator has proposed to increase Lhe size of <br />his operation by moving his sCr•eam side dire to the nu.rth, see <br />fig. 1. This would curve the lU0 t't. stream channel ar~~und the <br />left Inorthl abutment. <br />The purpose oi' this report is to evaluate the hazard to the <br />bridge of the present gravel pit and of the proposed channel <br />change and enlargement of the pit. <br />SITE CONDITIONS <br />The Arkansas River at this location, altlrungh only 16 miles <br />downstream from John Plartin Uam which was completed iri 1945, <br />still has a large t'lood Clow potential. 'Che 100 yr flood is <br />40,000 cfs and in 1965 a peak flow ui' i2,8U0 cfs occurred <br />lFEP1A,1982). Flow from above Juhn Plartin Uam did not r•or..tribute <br />to the 1965 peak. FEPIA, 1982, gives the lU yr and 50 yr flood <br />peaks as 11,200 and 28,OU'~fs respectively. <br />There are two bridges tparallelf at this site to carry the four <br />lanes of traffic. The bridge openings are approximately 550 ft. <br />wide at the stream bed. Both bridges are orr 8 piers. These piers <br />rest on footings which are on piles. The footings are at <br />elevation about 3596 and the piles are 58 ft. long. Bedrock is <br />at elevation 35-15 to 3550 Ct. <br />The bed material oi' the river between bedrock and the surface is <br />mostly sand for Lhe first 3 to ~ I'L thou ~fravelly sand with some <br />,~ <br />1 <br />