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<br />0481
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<br />.
<br />
<br />the watershliid. The upper one-tl];i,r,d of the area consists of rocky
<br />rolling h:blls, with medi\1lll to sp,arse vf!getation and small sloping,. " e.,!."
<br />meadow arElas near the divides. 'The remainder of the drainage area . "(',CO
<br />consists of a deep canyon, the north side of which is very steep and" f
<br />rocky w;l,;th znany sandstone cliffs. There are a f.ew short, steep,
<br />rocky ai-royos. The south side of the canyon is less precipitous but"
<br />is rocky, medium to sparsely vegetated, and the distance to the divide
<br />is further than on the north side, One. large arroyo about 1.8 miles ''.i...
<br />long enters the canyon just above the dam site. Several small s:l;eep'
<br />arroyos enter the canyon ,from the south s~de:. The predominant geo-,',;'"
<br />logic formations are of l:bhe Upper CretaceoUs period and consiatof the
<br />Vermejo formation inths' higher portions of the area and the ,'rrinidad
<br />sandstone in, the middle ,and lower portion. Pierre shale is, eltposeQ. in
<br />the main canyon in the reservoir area and slightly above.
<br />
<br />. .:'~
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<br />,-:
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<br />
<br />2-05. No sediment samples have been secured from Pinon Canon..
<br />Estimates of ,the sediment runoff were, made from reconnaissance sUt'yey.:p",
<br />within the drainage area considering tlle geology, slope, v1.sible 'fr.
<br />erosion, soil depths, vei>etation and other cilaracteristics:: and from ',I.
<br />comparison of this stream with the 8,verage Sediment contributlon rates ....
<br />for streams of similar areas.' Based on these: data, it. is: :e'stlma.ted
<br />that -the reservoir will receive about 45 acrei.'l'eet of sediinent (in-
<br />cluding', the bed loa9.) in 50 years. . This would':result in an average .
<br />reservoir depletion rate Of 0.9 acre-foot per year.! Since the streal)l"
<br />is epli:emeral ,and the cjrainage area is subject to SEWere' ttnmderstorms,- ;:
<br />it is believed that most of the sedilnent will be mov&d .into the reser-.,:,
<br />voir during-high flows. Thus the quantity of'iletti,metlt re'acl;lil1g the . '.
<br />reservoir fl:oom year .to,year will vai~ consid:erat\.ly. Cwd-th.weather.cQn,1'r.:: ' .
<br />ditions..i "'It is possiq;I,e that al~g'e'p'ercenf:a~ .ot::-tiliie'i,total. 5Cl'Cye,ar. .
<br />se"di.inent :Jne:y be movech into the re~!!ltvoir dud.'hgaJre:LativeJ.yshort .. ....
<br />period' of tlme ..;I;t' is belieVed" the trap"efficiency.of itlhe. re.se:rvoi.;',,",,:;,i.,
<br />will be high because pi the flashihature::of the heavy:'sediment: cl!I'I1T,""' ,;,)
<br />ing flows and the large percentage6f coarse mater,i-alb indicat,eq:.,:,:,
<br />Also, the portion of the sediment moving as bed load will be gre~,\> -,1':0'.',
<br />beoause of the steE1P slopes. Ther,eforEj!, the, trap efficiency has been
<br />considered as 100 percent. "
<br />
<br />III - DESIGN FLOODS
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<br />.;,. ,I' "
<br />3-01. Standard pi-ojec'\; floOd. - After studying all major storms
<br />of' past OCCurrence in the. general vicinity. of Trinidad, it was found
<br />tl1atthe2-6,.June 1921 storm whici\,cEintered near Pueblo, Cplor.~Q.o,.
<br />was, 'the 'l1Iost:suitable for,standf!Tq."w()ject'flood COl'lsider:at:j:on qn the.
<br />Pi:non".(lanon drainage basin. This I1torm -wa's, the !t:y"~e e>f,'..stcmn.1;hat,
<br />:'c&uId~ell be expected to o,ccur over "the"iir'iliriage'.'.'arell:'1iith the least
<br />amount:'of modification. The stprJJl cent,ei<:itE;'itt- puelblo, was transposed
<br />without rotation 8 miles" east, 75 mil136 slluth; and::centered over
<br />Pinon Canon. This transposition required an adjnistliient of orographic:
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<br />,....3-
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<br />Appendix I
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