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<br />HY D[<9LO(~Y_ <br /> <br />Peak Dischar~ <br />Flood discharges for the major drainageways in this report were obtained <br />fmm a report compl eted in Apri 1, 1979 by Hydm- Tri ad, L td., (Reference 1). <br />As a part of that previous study, runoff hydrographs were developed using <br />;'fv~wJl <br />the Soil Conservation Servi ce procedure for -each sub..bas in;: The hydrographs <br /> <br />were then combined using Muskingum routing to yield discharges for Montrose <br /> <br />Arroyo, Cedar Creek and Dry Cedar Creek. <br /> <br />Ditch cmssings in the study basin, with the exception of the South <br />Cf..'" </~- <br />Canal, were-iiafldlcd b:f ossumi1'l!j-t-ltem to be flowing full at times of flood <br />such that tributary runoff would flow across unimpeded. The South Canal, <br /> <br />because of its large capacity and safe operating record was assumed to <br />collect runoff draining directly into -it and convey it out of the basin. <br /> <br />The impact of snowmelt runoff was eva 1 ua ted and determi ned to be <br /> <br />significant only for the higher frequency events. The dischilY'ges in Tabl e <br /> <br />5 include the effects of rainfall and snowmelt runoff. <br /> <br />Urban Hydrology <br /> <br />As part of the previous study, peak di,'charges were determined for <br /> <br /> <br />each of the local urban sub-basins. These discharges were calculated <br /> <br />using the rational formula and rainfall data kept by the National Weather <br />Service, (Reference 3) and the Uncompahgre Valley Water Users Association, <br /> <br />(Reference 4). <br /> <br />The peak discharges calculated fat' the local urban areas were routed <br />and combi ned by Gi ngery Associ ates to determi ne the effect of urban dra i n- <br />age on the study streams. The individual urban sub-basins were combined <br /> <br />by 1 aggi ng the hydro graphs of upper bas ins to account for travel time to <br /> <br />the lower areas, then adding the hydr09raphs. The results are summarized <br /> <br />in Table 6. It should be noted that the urban runoff is of short duration <br /> <br />and w-ill have subsided before the upstream peak flows reach the city. <br /> <br />- 17 - <br />