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111-4 presently occurring in the channel (I ined channel and closed conduit) will serve to increase the downstream peaks and increase structural costs. To determine the effects of malnstem detEmtlon, a hydrograph was determined excluding Bas Ins A, B, C and D. This ~lOuld represent total storage of Basins C and D (Basins A and B are already stored in Standley Lake and Great Western Reservoir). By providing 1000 acre-feet of stor- age for total ponding of Basins C and D, the discharge at County Line Road is reduced from 14,000 cfs to 13,!i00 cfs" in other words, while having a profound effect on the sizing of flood plain structures and channels immediately downstream of U.S" Highway 36, the flood peak Is near I y regenerated by the end of the study area. The cos t of P I"OV i d- ing 1000 acre-feet of malnstem storage is estimated at $1.5 million, and we do not believe that this expense is justified (see Section VII I) nor Is it likely that such a dam 15 pol it Ical Iy feas ible. Related to malnstem detention storage Is the matter of on-site storm runoff detention. By using reasonable on-site detention over the entire basin, the resulting flood peak at the County Line Road might be reduced from the 14,000 cfs figure to lIS low lIS perhaps 10,000 cfs. In combination with mainstem detention storage on tributary channels, the flood peak could be reduced to as low "5 9,5CO cfs (the IDa-year runoff under existing conditions). It Is Important to note that the E,ffects of on-s ite, and tributary channel malnstem detention would significantly affect the Big Dry Creek tributaries. The on-site detention storage would particularly reduce the costs of storm sewers and other "initial storm runoff" f ac i I I ties. As there are presently no ordinances in force for on-site deten- tion, this alternate was not considered viablE' for this study; however, due to the phased implementation of the recommended alternate, adjust- ments can be made If on-s ite ponding ordinanCE'S are brought into force. 6. Tunnel. This alternate consists of constructing a tunnel for either all or a portion of the flood waters for a given design flood. The irrigation, and perhaps a portion of the flood flows, would be carried In the existing channel. While this alternate has many of the same, disadvantages as Alter- nate No.4, a tunnel deserves some add i t lonal thought. Used in combi- nation with the "uti 1 idor" concepts, this alte,rnate could serve to reduce surface flow peaks, provide additional storage, and also pro- vide a corridor through which other vital services may be delivered to rapidly urbanizing areas. The possibil ity also exists for con- sidering pumped storage power developmEmt In connection with Standley Lake serving as the upper storage reservoir.