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I I I I I I I I I I I I I I I I I I I I i I Chapter V: PLAN OF DEVELOPMENT A,1. PHYSICAL PLAN Construction of Mountain Home Reservoir was initiated in 1908 and completed in 1910, The primary purpose of the reservoir was to store .agricultural irrigation water. The dam was constructed by the Trinchera Canal Company which later became the Trinchera Irrigation Company. The company has water rights for storage of 19,150 acre-feet but the reservoir has never held more than 13,500 acre-feet since 1959 and has a capa~Qf_ajlJ)ro.xim_~~Jy-_18.,5()_~~~re-feet at the llJ>ill~~~_~E.~~!._ ------ The dam has not been significantly modified since its original construction. Minor regrading and filling of the dam crest has taken place over the years. The dam in general is about the same as constructed and has not experienced any significant probiems since it was constructed. 1. Existim! Facilities. Mountain Home Reservoir is impounded by one dam that runs in approximately a North to South direction. The dam is approximately 450 feet long with a structural height at the outlet of approximately 153 feet and a hydraulic height of 126 feet. The dam embankment slope at the upstream face is approximately 3H (horizontal) to 1 V (vertical) with the downstream slope varies from approximately 1.5H to IV to 2.5H to IV. The crest width is approximately 20 foot with a 36 foot wide berm approximately 8 feet below the crest with an access road running across the berm, The crest of the dam is slightly depressed in some areas but is relatively uniform in elevation. The dam is composed of predominately silty sands and gravels in the upstream portion of the dam with a downstream rock-fill section. Figure V-I is a sketch of the dam cross section and the configuration as determined by Geowest in investigations conducted in 1988, The upstream slope protection on the dam is in good condition and consists of hand laid riprap approximately 18-inches in thickness, The upstream face of the dam is in good condition and uniform in shape and slope. The downstream face consists of dumped rock with boulders up to 6-feet in diameter, The downstream face is rough in shape and slope but is stable and in good condition, The emergency spillway for the reservoir is located in a natural swale to the southeast of the dam, The spillway inlet is approximately 1400 feet southeast of the dam with the spillway control section located approximately 800 feet from the dam. The control section is approximately 80 feet wide consisting of a rock- filled embankment. The spillway control section is at Elevation 8202 feet and the crest of the dam is at Elevation 8210 feet for a freeboard of 8 feet, The spillway channel is has a total length of approximately 3500 feet and discharges into Trinchera Creek approximately 1500 feet downstream of the dam toe. The spillway channel is unlined and flows through a natural low created by sliding and movement in the area of the overlying basalt rock. PAGE V-I Chapter V - Mountain Home Reservoir