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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />i I <br />I <br />'I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />-21- <br /> <br />IV.5 Alternative Modification Schemes <br /> <br />Based on the results of the evaluations described in the preceding sec- <br />tions, as well as discussions with the SEO, the following four alternative <br />modification schemes capable of providing the acceptable flood routing protec- <br />tion were identified: <br /> <br />Scheme No.1 - Spillway enlargement to safely pass the PMF. <br /> <br />Scheme No.2 - Soil-cement overtopping to safely pass the PMF. <br /> <br />Scheme No.3 - Spillway enlargement to safely pass 50 percent of the PMF <br />in conjunction with an early warning system to evacuate <br />downstream residents. <br /> <br />Scheme No.4 - Overtopping protection using a cable-tied modular block <br />mattress system to safely pass the PMF. <br /> <br />Feasibility level hydraulic routings and designs indicated the following <br />requirements for each of the schemes: <br /> <br />Scheme <br /> <br />Design Di scharge (1) Discharge(l) Embankment <br />Discharge Elevation Width Crest <br />(cfs) (cfs) (ft) Elevation (ft) <br />17,000 9725 120 9738 to 9739 <br />16,500 9733 to 9736 300 9739 to 9742 <br /> <br />1 - PMF Spi 11 way <br /> <br />2 - PMF Soil-Cement <br />Overtopping Section <br /> <br />3 - 50% PMF Spillway <br />wi Early Warning <br />System <br /> <br />8,500 9730 <br /> <br />120 <br /> <br />9738 to 9739 <br /> <br />4 - PMF Modular Block <br />Overtopping Section <br /> <br />16,500 9733 to 9736 <br /> <br />300 <br /> <br />9739 to 9742 <br /> <br />Note: <br /> <br />(1) Assumes concrete Ogee overflow section. Concrete broad-crested weir and <br />riprap-lined overflow sections were considered but crest length require- <br />ments were significantly in excess of room available in the left abutment <br />area. <br /> <br />As indicated, a crest raise of 2 to 3 feet above the existing crest would <br />be required for schemes 1 and 3. The total discharge of 17,000 cfs for scheme <br />1 includes consideration of additional attenuation of the inflow due to <br />increased reservoir storage created by the crest raise. The actual final <br />crest elevation would be established during final design, based on detailed <br />flood routings, backwater evaluations, and consideration of freeboard require- <br />ments. For purposes of these feasibility designs, the design discharge for <br />the 50-percent PMF spillway was assumed to be 50 percent of 17,000 cfs, or <br />