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I I I I I I I I I I I I I I I I I I , Structurally, both spillways are feasible and will provide about the same level of safety. The spillway on the right abutment of the dam, however, will be expected to experience more erosion In the downstream channel due to the steeper natural grades In this area than In the area of the RCC section. The RCC spillway location also has an advantage In that the purchase of Rlght-Of- Way may not be required as Is the case with the spillway on the right abutment. For both spillways proposed the control section will be approximately 360 feet wide and the downstream channel will be grass lined with rlprap protection as required to ensure the safety of the dam. Erosion of the spillway channel down- stream of the dam will not be controlled If not required to ensure the stability of the dam or spillway control section. Both a 100% PMP flood and a 76% PMP flood were developed and routed through the dam spillway to determine the spillway size and freeboard for the two floods. The 100% PMP flood was routed through a 350 foot wide spillway which Indicated a depth of flow of 6.2 feet. For a spillway control elevation of gage height 36 feet, the maximum water level was 42.6 feet which Is USGS Elevation 5164.6 feet. Figures C-l and C-2 In Appendix C show the Inflow and outflow hydrographs, and the spillway stage curve for the 100% PMP flood. The 76% PMP flood was routed through a 300 foot spillway which Indicated a depth of flow of 5.3 feet or gage helgltt 41.3 feet. Figures C-3 and C-4 show the Inflow and outflow hydrographs, and the spillway stage curve for the 75% PMP flood. For the proposed elevation of the dam crest at Gage Height 43 feet the freeboard Is 1.7 feet and would meet the requirements of the State Engineer to maintain a minimum one foot of freeboard. If designed for the 100% PMP the one foot of freeboard Is not required. For the remainder of this study the 350 foot spillway will be used to determine the project requirements. In the final design analysis, the spillway Is expected to be between 300 to 350 feet In length. Until the final crest elevations are set and the final details are determined and discussed with the State Engineer, we believe It Is prudent to base the project costs on the 100% PMP flood. Service Spillway. A service spillway was also considered to handle the nuisance flows rather than utilizing the emergency spillway for nuisance flows. This was considered due to the erosion potential In the spillway channel and outlet channels for both spillway 10caWms and due to the flows possibly getting Into the borrow ditch along Highway #.1. As noted earlier, Highway #1 will be Inundated when ever there are major spills over the emergency spillway and the service spillway would be a means of minimizing the frequency of Inundation. The service spillway considered was a four foot square concrete overflow weir with a screened Inlet set at Gage Height 36 feet. The spillway will empty Into a concrete tower that will be Incorporated Into the outlet conduit and will be uncontrolled. The arrangement will be similar to a wetwell often used to house the control gate except no gates will be utilized. PAG! II Chapler V - No. 6 Vel8lblllty