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I I I :1 I I I I I I I I I I I I I I I 13 Assumptions The purpose of dentention storage at Node 4 is to reduce peak discharges in the reach between Nodes 4 and 5 to levels which will not produce local backwater flooding. In designing the impoundment, it was assumed that the structure would be located so that existing and proposed future outfalls from Service Areas 6,7. and 8 (Figure 5) discharge into the Ditch downstream from the spillway. (The reader is referred to Leaf and ReI. 1983 for a complete discussion of proposed improvements in the Master Drainage Plan). A summary of inflows from these Service Areas under future development is shown in Table 6. The 640 c.f.s. is less than the existing Ditch capacity summarized in Table 4. The water surface elevation in the vicinity of N. Sydney Avenue at a Ditch flow of 640 c.f.s. would be 3,922.7 feet m.s.l. It was also, assumed that the peak flows summarized in Table 6 will occur prior to the occurrence of spills from dentention storage due to shorter times of concentration. Thus peak outflows from the impoundment structure would be into an essentially empty channel downstream. Inflow Hydrographs Table 7 summarizes ten-year. six hour inflow hydrographs for existing conditions and assumed future development. It is assumed that the storage structure is proportioned so that the maximum outflow rate and maximum storage occur at the same stage. As seen in Table 7. storage capacity for an outflow of 620 c.f.s. is 185 and 215 acre-feet for existing conditions and future development respectively. These are conservative estimates since they do not take into cons iderati on the vol ume removed from storage by the outflow devi ces. The spillway crest elevation should be set so that the time period during which the reservoir fills to the spillway crest is adequate to allow storm runoff peaks entering the channel downstream to flow to the river. The maximum water surface elevation of the storage pool is 3,925.5 m.s.1. which is 10 feet above the flow line of the Pioneer Ditch.