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Monument Lake Dam Feasibility Study 25 130VLE.'f I I I I I I I I I I I I I I I I I I I Chapter IV . Formulation of Alternatives with an end sill. Figure IV-3 shows that the chute walls are IS-foot high reinforced concrete, which are required to protect the sandstone on the left abutment and the embankment on the right abutment Spillway Alternative S2 - Concrete Chute As shown in Figure IV-4, Alternative S2 involves protecting the downstream discharge chute floor with I-foot of reinforced concrete. The Chute is sloped from the invert of the existing control section down to the natural streambed. The chut(: is uniform in width to develop a predicable flow into the stilling basin. Figure IV-5 shows that the energy of the water is dissipated in a stilling basin. The stilling basin is comprised of a 4-foot high-eM sill and 2-foot high chute blocks. Ten-foot high training walls are used to protect the left and right abutments from erosion. B. Embankment Alternatives Two alternatives were prepared to rehabilitate the Monument Lake Dam embankment. Both alternatives include a partial excavation of the crest and upstream and downstream portions ofthe embankment to remove all of the existing trees and roots in the embankment, installing an internal drainage system and toe drain in the embankment, and rebuilding the embankment back to its current configuration. Based on the stability analysis performed for the existing dam, the stability of the existing embankment is consider~d to be adequate, and therefore, it is not necessary to flatten either the upstream slope or downstream slope. Two methods for rehabilitating the dam 'Were not considered due to potential magnitude of construction costs, they included: I) upstream liner and 2) slurry wall through the dam axis (core trench). These methods of rehabilitating the dam were not considered due to the following: . The trees will have to be removed for any selected alternative, based on SEO guidelines. Most of the cost associated with rehabilitating the embankment is the removal of the trees. The investigated alternatives both use a standard downstream filter combination. A core trench or upstream liner would be considerably more expensive compared to the filter system. These alternatives would have been consideted if the embankment was not so over-grown with trees.