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I I I I I I I I I :1 I I I I I I I I I 17 .:~ Channel Storage Channel storage upstream from the impoundment structure is approximately 25 acre-feet at elevation 3925.5 ft. Water depths at several locations upstream are as follows: Locati on Depth (ft.) North Division Avenue (Node 4) 10 North 7th Avenue B ri dge 7 Franklin St. Bridge (Node 3) 4 It is emphasized that the impoundment structure should be operated in a manner that minimizes seepage problems. Because the Pioneer Ditch is also a drain, storage times after every storm event should be as short as possible. Accoun,tlilg for channel storage, net storage volume in the impoundment structure should be 160 and 190 acre-feet for existing conditions and future development, respectively. Design Criteria For Near~Term Improvements Design parameters for near-term improvements are summarized below. All bottlenecks upstream from the proposed impoundment structure should be removed. Upgraded crossings should be designed to convey the peak flows summarized in Table 4. Design of 10-year peak flows at all crossings should be computed from Figure 3 using a peak flow adjustment factor of 1.5 for developed areas upstream from each crossing (see example calculations for Node 2). The storage capacity of the proposed impoundment structure at Node 4 should be approximately 190 acre-feet. Maximum water surface elevation of the storage pool from a 10-year, 6 hour runoff event should be 3,925.5 ft. m.s.1. The maximum rate of outflow should not exceed 620 c.f.s. The structure should be proportioned s,o that desi gn outf1.ow rate and maximum storage occur at the same stage (3,925.5 ft.). Maximum water surface elevation in the Pioneer Ditch immediately downstream from the spillway should not exceed 3,922.5 ft. m.s.1. All near-term improvements Shou1 d be designed so that they sustain minimal damage from a regional 100-year flood event.