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<br />~ <br /> <br />- <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />I. INTRODUCTION <br /> <br />This Floodplain Information Report investigates the existence and severity <br />of flood hazards on East Plum Creek from U.S. Highway 85 upstream for <br />approximately 2.5 miles to Section 22 Road. The area is currently <br />undeveloped and includes the proposed Brookside Business Center development. <br />Figure 1 presents the location of East Plum Creek studied in this report. <br /> <br />The East Plum Creek basin (see Figure 2) has its headwaters 20 miles south <br />of Castle. Rock above Palmer Lake. The drainage area of the basin is <br />approximately 85 square miles at the U.S. Highway 85 bridge which is the <br />downstream study limit. The channel meanders through the floodplain and is <br />subject to change by flooding. The area within the 100-year floodplain is <br />mostly sandy wet alluvial land. The soils consist of sandy clay, or silty <br />or clayey sands over medium dense to very dense clayey sands. Erosion and <br />depOSition is common due to flooding. The vegetation consists of willows, <br />annua 1 grasses, sedges, and weeds.. <br /> <br />II. ENGINEERING METHODS <br /> <br />Hydrology: The flood discharges used to develop the floodplain information <br />presented in this report are from the draft Flood Insurance Study (FrS) for <br />the reach of East Plum Creek upstream of U.S. Highway 85 (Wilcox). That <br />study, prepared for the Federal Emergency Management Agency (FEMA) by J. F. <br />Sato and Associates, is in progress and is essentially an upstream extension <br />of previous Flood Insurance Studies for Douglas County (1980) and the Town <br />of Castle Rock (1978). The flood discharges used in the FrS by Sato are <br />consistent with the discharges in the previously pUblished FIS's. <br /> <br />The peak discharges are shown in Table 1 below: <br /> <br />TABLE 1 <br />PEAK DISCHARGES (cfs) <br /> <br />la-YEAR <br />5190 <br /> <br />IOO-YEAR <br />14120 <br /> <br />SOO-YEAR <br />23640 <br /> <br />50-YEAR <br />10600 <br />