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<br />HYDROLOGIC ANO HYDRAULIC DETERMINATIONS
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<br />Hood Chara~ter1stics
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<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.
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<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.
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<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".
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<br />Hydrolo~ic Analysis
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<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.
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<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.
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<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.
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<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.
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<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.
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<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.
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<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".
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<br />Roughn.,ss coefficients, ~nnnine's
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<br />values, wpre field det<:,rmined hy
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<br />Waler surfaee elevations ure flot tabulated
<br />on Dallas Creek snd Pleasant Valley Creek.
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<br />for theappro>:iftlate study r",,-ches
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