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Preliminary Design of Dam and Spillways Recommended Design The embankment design, as shown in Figure 4-2, proposes the removal of the lower portion of the existing dam- and the construction of a new core built from the dam foundation to the new crest elevation. New shell zones will be placed immediately upstream of the core and over the downstream filters, completing the embankment. The existing drainage system will be abandoned and a new chimney drain would be constructed and connected to a blanket drain that would discharge seepage through the toe of the downstream embankment. For control of seepage, a new key trench would be excavated 5 feet into the bedrock and a cement-bentonite slurry diaphragm wall would be constructed in the bedrock to a depth approximately 20 feet beneath the dam and 100 feet in the abutments. The dam raise will be a zoned, rolled-fill embankment with a 25-foot wide crest, a 3:1 upstream slope, and a 2.5:1 downstream slope. The upstream slope will be covered with a combination of high quality riprap and bedding to protect against runoff and wave action, and native grasses. The downstream slope will be covered with native dryland grass to protect against rainfall and wind erosion. Since there is the potential for settlement of the old and new embankments as well as the foundation in association with the dam raise, the embankment will be constructed with a camber which varies from 2 feet at the centerline of the embankment, tapering to zero at the abutments. The riprap on the upstream slope of the raised embankment will extend from the riprap at the crest of the existing dam at an elevation of 6,375 feet to an elevation of 6,409 feet which is the crest of the new emergency spillway. The reservoir water level will reach this upper limit of riprap only approximately once in five hundred years. A wind setup and wave runup analysis was completed for the riprap protected embankment to determine the maximum height of the embankment potentially impacted by wave action. This was based on a 60 minute wind velocity of 53 miles per hour from the northwest across the reservoir toward the dam. The maximum wind setup plus wave runup will affect a vertical height of 1.5 feet (6,407.5 feet) above the normal full pool elevation of 6,406 feet. This analysis confirmed that the riprap will be high enough to also provide protection against wave. action. The geotechnical investigation included a review of the quantity and quality of on-site embankment construction materials based on the information obtained from the test pits at the six borrow areas located around the reservoir site. There appears to be adequate earthfill and random fill from the proposed borrow areas. Commercial borrow areas for the sand (Hard Rock Sand and Gravel Company) and the riprap (S&K Construction Company) were located and judged to be sufficient in size to provide the adequate quantities of quality material needed for the dam embankment construction. A seismic stability analysis was conducted on the raised dam for the conditions listed below. The results of this analysis showed that the minimum factor of safety requirements of the State of Colorado were satisfied for all conditions. 1) Upstream Slope, Steady State Seepage 2) Downstream Slope, Steady State Seepage 4-4