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<br />~ <br />III <br />( <br />fC <br />~ <br /> <br />determining \Jater surface profiles on the respective streams during project design <br />fIcus) it was assurued that a flO\J of 20,000 c. f. s. in the Purgatoire River 'tvould <br />coi:1ciJe 1Vith the project design fIm., of 140,000 c.f.s. in the Arkansas River. <br />A flow of 8,000 c.f.s. in the Arkansas River was assumed to coincide with the <br />project design flml of 102,000 c.f.s. in the Purgatoire River. <br /> <br />7-05. Levee grades ahd free board. The flood profile derived for a dis- <br />charge corresponding to a stage t\W feet above the project design flood profile <br />at a critical section at the dmmstream end of the project was used to define <br />locations where extra freeboard allowance would be needed. A discharge about <br />equal to the standard project flood of 200,800 c.f.s. was determined to be the <br />capacity of the critical section vlith fImV' two feet above the design flood <br />elevation; therefore, the standard project flood profile, as tabulated in table <br />1, was considered appropriate for use as a guide to indicate locations where <br />greater than minimum freeboard is needed. In order to prevent initial over- <br />topping and to minimize possible failure of levees upstream from city develop- <br />ments, levee grades from the AT&SF railroad upstream to station 0+00 will be a <br />minimum of t\W feet above the standard project flood profile. This will result <br />in a maximum freeboard of 7.5 feet, a minimum of 4 feet, and provide for buildup <br />of head at the t\'lO bridges. Baclumter efeects above the bridges reflect a debris <br />factor of two feet on each side of the piers. Dounstream from the railroad to <br />the end of the project, the levee grad" would vary from t;70 feet above the <br />standard project flood profile at the r2'lroad bridge to one foot above the <br />stanuard project flood profile at tile ammstream end of the project. Levee <br />grades are shmvu on plates 2 through 17. <br /> <br />7-06. Hater surface profiles. ~vater surface profiles ,-1ere cor.lputed by the <br />standard step o.ethod utilizing ~'.iannin.;s equation. Profil~s "vere coru.puteu for <br />the design flood of 140,000 c. f. s. under existing and improved conditions. The <br />design I>ater surface profile is sho"n on plates 2 t;,rough 17. Profiles were <br />also determined for the 5- and 25-year design flows of 13,000 c.f.s. and ~6,50J <br />c.f.s., respectively, for interior drainage studies. Field investigations WC~~ <br />I:l"ade to determine roughness coefficients for the project reach. A channel "rl" <br />value of O.J20 Has selecteJ.. This value lvas confir11ed from stase-discharge studies <br />of flow measurenents at the Las Animas stream gaging station. These studies <br />indicated that a channel l1u" value of 0.025 would be representative for a cLis- <br />ci~rge of 5,000 c.f.s. but that 0.020 would be appropriate for discharges greater <br />than- 8,000 c.f.s. Overbaul" r~ugIlness coefficients 1-1ere deterrr.ined to vary fro7J <br />0,035 to O.el70. <br /> <br />7-07. Dack\-later computations \vere supplemented 1i:lith special studies at t:le <br />u. S. lIigln-;ray 50 uridge and tile AT&SF ~aih.]ay bridge. The Yarnell fOITi'.ula Has <br />used to compute the head 10s::3 throug:l tile opening for the higlnvay .Jridge. <br />V2 <br />I:" = 21:(;( + lOw - 0.6) (a + l5(4)---2 <br />~ 2g <br /> <br />L <br /> <br />In t:lis for:nula, rIJ is the difference in Hater depths upstream and GO'tJl1stream <br />of tne bridge obstruction, i~ is the emprical coefficient based upon the pier <br />snape, w is the ratio of t~1e velocity head to "'.~ater depth Clv/J.'J) of the dmlil- <br />strcar.~ chzl.Lluel section, and a is tILe. channel contraction ratio...J The selectc.J. <br /> <br />9 <br /> <br />