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<br />I <br /> <br />HYDROLOGIC ANO HYDRAULIC DETERMINATIONS <br /> <br />I <br />II <br />I <br /> <br />I <br /> <br />I <br /> <br />Hood Chara~ter1stics <br /> <br />I <br /> <br />str~a~ measurements at three separate locations. The locations ~nd values <br />obtained are shown in thc Technical Addendulll. For purposes of defining II <br />values for the remaining channel, a distinction was made betwoen the steep, <br />narrow river val1~y above station 500+00 and the wider valley and gentler <br />river slopes below this point. Above this station an n of .072 was used <br />while downstream of this point .051 was taken to be the value of n. <br /> <br />I <br /> <br />Flood flo~s on the Uncompahgre River within the study reach result from <br />the rapid melting of the mountain snowpack during the period from ~ay to <br />early July. Snowmelt 1IIay occasionally be augmented by rain. The runoff <br />from snowmelt typically sustains periods of high flows and marked daily <br />fluctuation. Although Su~er cloudbursts are a flood threat to occupied <br />areas at the mouth of tributaries of the Uncompahgre River, examination <br />ofatreamflowrecordsrevealthatthecloudburstsarenotafloodthreat <br />to the main channel. <br /> <br />I <br /> <br />Some cooments are in order on unusual topographical situations occurring <br />within the study reach. The topographic maps of the valley indicated that <br />the river may have had other channels besides the one presently occupied. <br />In particular, thnt portion of the linkedhydrauHe chnnnds called "The <br />;.'estAnn," appears at One ti"'e to h,we been the major river channel. At <br />some locations, this channel is several feet below the present main channe~ <br />and scpnrnted from it by an artificinl dike. Since flooding on the river u <br />caused primarily by snowmelt, floods may have flows at or near the peak last- <br />ing for several days. Due to this circumstance, t~o distinct stages of a <br />100-year flood were identified along the reach which included the Wcst Arm: <br />stage 1) early initial flooding which is contained by the c~isting river <br />channel; and, stage 2) eventual erosion of the dike which produces signifi- <br />cant flows into the West Arm. As the dike erodes, sheet flows occur 1uto <br />the West Arm as shown on the flooded areas maps. This sheet flooding will <br />rarely exceed one foot in depth. <br />A second unusual feature of sections of the river valley within the study <br />reach is large areas lying below the 100-year flood profile, but not having <br />distinct surface hydraulic connection to the main flood channel. These <br />areas play no part in the cOnveyance of the flood, but may be inundated due <br />to groundwater flow or flow through small sutface channels not easily de- <br />fined. Areas of this nature have been identified separately as "areas be- <br />10'" flood <>lev.~tjnn". <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Hydrolo~ic Analysis <br /> <br />I <br /> <br />I <br /> <br />A hydrologic analysis was cattied out to establish the peak discharge- <br />frequency relationships for floods of 10-, 50-, 100-, and 500-year recur- <br />tence intervals for the streams in the study area. <br /> <br />I <br /> <br />I <br /> <br />For the Uncompahgre River, Dallas Creek and Pleasant Valley Creek flood- <br />flow frequency data wcre based on statistical analyses of strcam flow re- <br />cords covering a 54-year period at gaging stations operated by the U. S. <br />GeologIcal Survey (Reference 3). These analyses followed the standard Log- <br />Pearson Type III Method as outlined by the Water Resources Council (Refer- <br />ence 5). Discharges ate shown in Table 3. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Diseharges foc Portland and Cascade Creek were determined by the SCS Method <br />described in the SCS National Engineering Handbook (Reference 6) and SCS <br />Publication "Proc~_dures for Determining Peak Flows in Colorado" (Reference <br />8). Dlsch..rg,;.sitndflooupLoftlesarenotshown in tne reporr forPorUand <br />and Cas<'ilue Creek because of topographic condition e_~plained in the section <br />"Study Area Description." Dischnrges are discussed in the Technical Adden- <br />dum. Dts<,_hilrges for the Approximate Study Ar"as on pleasant Valley Creek <br />and Dallas Creek w"re estim~ted by the methods described in technical Manual <br />No.1, Colorado Water ConsHvation Board a"dU. S. Geologkal Survey (Refer- <br />ence No.1), and are not shown in this report. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Athi<dfeature"l1usualtomostrtvervalleysisthesmal1steepttibutaries <br />which interest the main river valley at right angles. These occur along <br />most of the study reach. Flooding from these tributaries usually occurs as <br />a result of sUICI,,,er rainstorms. This flooding will be localized at the Itlo"th <br />of the tributary, b'Jt 'MY be of extre",e severity ~".. tn warp,. "...d/nT dphri~ <br />dsmage. An investigation of such flooding on Portland and Cascade Creeks <br />was carried out using data from the hydrology report, topographic maps, <br />field observations and interviews. the results of this portion of the re- <br />port are ~hown on rhe flooded aTeas maps, Plate 19. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />~iraulic Anal~ <br />Based upon the hydrologic report, a study was made to estimate water surface <br />profiles for floods of selected reC\lrrence intervals. These "ere the 10-, <br />SO-, 100-, and 500-year floods. Data supplied by aerial topographic mapping <br />and field surveying were used to construct " river model of the study reach. <br />The basis for this model was the Il~C II, Water Surf<lce Modeling Program of <br />the U. S. Army Corps of Engineers (R~ferFnce 10). Location of cross sec- <br />tionsused in the construction of the model are shown on thc flooded ar ea <br />maps, Plates three through eighteen. Water surface profiles generated by <br />the model for the selected floods have b~en plotted on the flood profiles, <br />Plates 19 through 32, and the areale>:tent of floodbgby thc lO().-yenrevent <br />is sho'JO on the flooded areas maps, Plates 3 through 18. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Table J shows the water surface elevarions at selected refe~ence points for <br />the 10-, 50-, 100-, and SOO-year floods, along with the discharges in CFS <br />used in the hydraulic analysis. ~ater surface elevations shown are fot clear <br />water conditions. Debris accumulations from tributary streams, especially <br />in the upper portion of the study reach near portland and Cascade Creeks, <br />and near bridges where the channel narrows, mny cause blockage and raise the <br />surface elevations abov" those shown. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />WOlter .~urfa"e "lev.~t!onn '''c not _~h<>Wn for Portland and Cascade Creeks for <br />reasons ""pJained before in the section "Study Area Description". <br /> <br />Roughn.,ss coefficients, ~nnnine's <br /> <br />"" <br />" <br /> <br />values, wpre field det<:,rmined hy <br /> <br />I <br /> <br />I <br /> <br />Waler surfaee elevations ure flot tabulated <br />on Dallas Creek snd Pleasant Valley Creek. <br /> <br />for theappro>:iftlate study r",,-ches <br /> <br />I <br /> <br />-24_ <br /> <br />I <br /> <br />-25- <br />