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<br />HYDRAULIC AN~LYJ~ <br />A hydraulic analysis was completed by Hydro-Triad Ltd. 'in April, 1979, <br />(Reference 1) to determine water surface py'ofiles for the lO.-year, 50-year, <br />lOa-year, and 500-year floods on Montrose Arroyo, Cedar Creek and Dry Cedar <br />Creek. These water surface el evations were computed uSing the HEC-II com- <br />puter model for backwater analysis. The cu-Ivert crossings on Montrose <br />Arroyo are insufficient to pass the laO-year flow; consequently, during <br />such an event a considerable amount of flow would be lost from the channel <br />to flow overland. This shallow flooding would flow away from Montrose <br />Arroyo west and northwest to the Uncompahgre River. Some of this flow <br />would pond along the east side of the D&RGW Railroad embanklJ1E!nt between <br />South 1st and North 2nd Streets. The previous hydraulic computations were <br />carried out based on the premise that immediately downstream of each cross- <br />ing all spilled flow is considered to be back in the channel. and any ex- <br />cess is again shown as spilling at the next inadequate crossing. This <br />procedure was used by Hydro-Triad Ltd. to prevent the analysis from becom- <br />ing superseeded each time an upstream culvert is improved, and to illustrate <br />a conservative situation. <br />In order to evaluate existing damages, it is necessary to have the <br />floodplain and flood profiles defined for the existing conditions. Therefore, <br />Gingery Associates conducted hydraulic analyses to determine flood elevations <br />and delineate flooding limits that reflect the actual situation that exists <br />in Montrose. The analyses were performed in the follm~ing manner: Proceed- <br />ing in a downstream direction, at the f-irst inadequate crossing (South 12th <br />Sty'eet), calculations were made based on cu"lvert rating curves and existing <br />topography to determine the amount of fl ow lost from the channel. DO~lI1stream <br />of the crossing the discharge in the channel was reduced by this amount. This <br /> <br />- 1 g - <br />