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<br />"ee <br />~ <br /> <br />Ofe <br /> <br />~ <br />I" <br />t; <br /> <br />5.0 FLOOD PLAIN ANALYSIS <br /> <br />Analyses of the flooding resulting from the 100-year recurrence interval <br />peak runoff in Ralston Creek were completed for the existing and the improved <br />channel conditions. The channel, flood plain and hydraulic conditions were <br />modeled using the U.S. Army Corps of Engineering HEC-2 Water Surface Profile <br />program. Use of th i s computer program enab 1 ed severa 1 a 1 ternat i ves to be <br />analyzed. The hydraulic conditions for the existing situation and the sel- <br />ected flood protection alternative are discussed below. <br /> <br />5.1 Existing Conditions <br /> <br />Near the Water Treatment Plant Site, the Ralston Creek Channel is <br /> <br />approximately 30 feet wide and three feet deep. Overbank widths range from <br /> <br />about 100 feet on the left (east) bank to l40-feet on the right (west) bank. <br /> <br />Further downstream at the proposed emergency pond location, the channel is <br />approximately 30 feet wide and four feet deep. There is no left bank overflow <br />area; the right bank overflow area is about 150 feet wide. <br /> <br />The channel and overbanks are shown in plan view on Figure 3 in the map <br />pocket. Photographs on Figure 6a. show the channel flood plain. <br /> <br />Downstream of the emergency pond site, the fill for the access road to <br />the mine office blocks the right bank flood plain. All overbank flows are <br />forced to the main channel. <br /> <br />A key element in the hydraulic analysis of the flood plain is the <br />selection of appropriate friction values or On" values. Ralston Creek in the <br /> <br />5-1 <br />