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<br />HYDROLOGIC AND HYDRAULIC DETERMINATIONS <br /> <br />Hollow reservoir ~nd Severance. <br />ch~ eff~~t ~f John Law Reservoir upon flQodf1ow~ w~s determined <br /> <br />Flood Characteristics <br /> <br />hy routing flood events through the reservoir. <br /> <br />Information on this <br /> <br />Floodflows on The Slough result from heavy rains during the <br /> <br />dam and re8ervoir lOllS also extracted from The United States Inv"ntory <br /> <br />period from May through August. <br />threat in this area. <br /> <br />Summer cloudbursts are a primary <br /> <br />of Dams. <br /> <br />Hydrologic Analysis <br /> <br />Hydraulic Analysis <br />Water surface elevations for the l~-, 50-, 100-, and 500-year <br />floods were computed using the I). S. Army Corps of Engineers step - <br /> <br />The hydrologic analysis for The Slough consisted of determining <br />the magnitude of the HI-, 50-, 100-, and 51l1l..year floods. Since <br /> <br />stream gauging data for the slough <br /> <br />w'" <br /> <br />00' <br /> <br />available, <br /> <br />no <br /> <br />backwater computer program HEC..2. The version used was the HEC-2 <br />release dated November 1976, updated August 1977, error correction <br /> <br />Environmental Protection Agency's Storm Water Management Monel (SWMM) <br /> <br />-e1, -~ 2, modi fication -5 ~ ,..51, ..52 ,-5 3. <br /> <br />No high water marks were <br /> <br />was used to develop a rainfall - runoff model of the basin. <br /> <br />Rainfall <br /> <br />located from whiCh to accurately calibrate the hydraulic model on The <br /> <br />input for the hydrologic model was derived from the National Oceanic <br /> <br />Slough. <br />Manning's "n" values ranged from 0.035 In the channel to between <br />fl. 07~ and 0.090 on the overbank. These "n" values were determined <br /> <br />and Atmospheric Administration's Atlas 2 for Colorado, published in <br />1973. This data was adjusted for expected"probabillty and is shown in <br /> <br />Table <br /> <br />1. <br /> <br />Val ues fa r the 5fl~"ye/lr flood were extrapolated. <br /> <br />based upon field inspections by the corps and are summarized in the <br /> <br />Infiltration losses of l"inch per hour were used based upon results of <br /> <br />Technical Addendum. <br /> <br />the ."\is.::ouri River 3.1sin Comprehen"ive .ramework :';tudy. <br /> <br />A detention <br /> <br />Starting water surface elevations were based upon flood rQuting <br /> <br />storage value fl.3 inch for pervious areas was used based upon studies <br /> <br />studies through John Law Reservoir. <br /> <br />These studies showed <br /> <br />'"' <br /> <br />conducted in the Ca"he 1/1 POLldr" River Basin. <br /> <br />Storage-dlsch"rge relationships were developed for the Black <br /> <br />following relationship ~etween flood frequency and depth of flow above <br />John Law Dam spillway, 50<J-year flood - over tops the dam~ 100..year <br />flood - 3.4 feet; 50..year flood - 3.1 feet; ann the 10-year flood <br /> <br />11011010' and Loop Lake reservoir sy>;terns. these relationships were <br /> <br />l;.:l::;od "pun "tv.","" ""oJ di!<charge oapaclty data for these dams and <br />("servoirs contained in the United States Inventory of Dams. These <br /> <br />1.6 feet. <br /> <br />relationships Showed that BlaCk Hollow reservoir has a significant <br /> <br />Road crossings In this study consisted of conugate<l metal OliO'es. <br />The efrect of culvert blockiHle was determined but ".<11; found to have a <br /> <br />efrect upon runoff from Rtorms centered over the entire Slough basin. <br />Because of this phenomenon, the final hydrologic analysis assumed a <br /> <br />1\<l'llitjilJl" "rr"ct exc"lJt fo( flooll[;o[ slLort rU<:U(rCnce intervals. <br /> <br />A <br /> <br />det~ il ed dese r i pti()n (Jf the hyd r aul ic anal ys Is is included in thf" <br /> <br />storm centered over the 16 sqllare mile drainage area between Black <br /> <br />Te,;hl1ical Addendum. <br /> <br />.. <br />u <br /> <br />12 <br />