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<br />1 <br />' of Interstate 70. <br />frontage road. <br />' HYDROLOGY <br />1 <br /> <br />1 <br />1 <br /> <br />1 <br /> <br />1 <br />1 <br />1 <br />1 <br /> <br /> <br />Three existing ranch accesses will be upgraded and tied into the existing <br />The hydrology for this report is based on the Army Corps of Engineer's 1987 Floodplain study and <br />the "Hydrology Report for Town of Rangely, City of Rifle and Garfield County Flood Insurance <br />Study," by Simons, Li & Associates, Inc., (Contract #En1W-C-0942, January 1983). According to <br />the ACOE's Floodplain Study, the following are the peak storm events and their corresponding <br />flow rates: <br />PEAK STORM EVENT DISCHARGE (cfs) <br />10-yeaz 28.600 <br />50-year 38,100 <br />100-year 42,200 <br />500-year 51,800 <br />Table 1. Storm Frequency vs. Flow Rate <br />In 1984, the Colorado River experienced a peak flow in the amount of approximately 3,459 cubic <br />feet per second (per the "Floodplain Study and Report of the Colorado River From Rifle to Marnm <br />Creek", Schmueser, Gordon, Meyer). Thus, this event was slightly less than a 50-year storm event. <br />The higher flow rates are typically observed from April through the end of June. This trend is a <br />result of high snowmelt runoff during this period and early summer cloud bursts. <br />High Country Engineering, Inc (HCE) performed a visual inspection of the Colorado River along <br />the study area in April 2001. The Colorado River has incised, steep channel banks on the north <br />with sparse dryland vegetation. The banks along the south side of the River are lower and are less <br />defined in certain reaches and have irrigated grasses and trees present. Mature vegetation <br />consisting of cottonwoods and willows are established along the overbank areas. Several gravel <br />pits aze located along the southeast portion of the study azea and along the northwest portion of the <br />site (prior to the intake for the City of Rifle's water supply). The thalweg is highly meandering. <br />Islands, ox bows, braided -multiple channels, and slack-water areas are present along the River <br />from the Town of Silt through Rifle. These features indicate an unstable, meandering stream <br />system. Cobbles and gravel are the channel linings, indicating minimal erosion potential in the <br />channel bed. However, the banks appear to consist of fine cobbles to clay, which may erode during <br />greater storm events. <br />