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Chapter IV - Formulation of Alternatives There are three basic structural alternatives (with several variations each) and one nonstructural alternative to providing the required additional spillway capacity at Left Hand Valley Reservoir. The first structural alternative is to widen the existing spillway sufficiently to pass the design flood. The second structural alternative is to construct an additional spillway which in concert with the existing spillway would provide a total capacity sufficient to pass the design flood. The third structural alternative is to lower the top of the north dam and make it overtoppable. The nonstructural alternative is essentially a "do nothing" alternative leaving the existing spillway in place and not providing any additional spillway capacity. Each plan is described in detail in the following sections. A. Plan A - No Action A non-structural alternative to providing additional spillway capacity is to do nothing. If ordered improvements to the spillway capacity are not completed the SEO would impose a storage restriction on the reservoir. Because of the magnitude of the design storm the storage restriction could effectively require the complete breach of the dam and loss of the storage reservoir. B. Plan B - Widening the Existing Spillway The existing spillway is located in a cut through the south abutment of the north dam forming Left Hand Valley Reservoir. It is trapezoidal in shape with a bottom width of approximately 30-40 feet and side slopes of 2: I horizontal to vertical. The south abutment of the north dam, which is also the north abutment of the south dam is a bedrock knob approximately 450 feet across from one dam to the other. The knob has an elevation at the top of 5,376 feet, or 18 feet above the tops of the dams and 25 feet above the spillway crest. Figure IV-I shows the two dams and the knob between them. 13 I I I I I I I I I I I I I I I I I I I