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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />FIELD INVESTIGATIONS <br /> <br />Existina Facilities <br /> <br />The Jackson Lake Reservoir dam was constructed around 1901 and 1902 for <br />storage of irrigation water. The dam is approximately 2.0 miles in length, has upstream <br />side slopes of approximately 1.6 horizontal to 1.0 vertical, has downstream side slopes <br />of approximately 2.0 horizontal to 1.0 vertical, and is approximately 20 feet in <br />structural height. The dam embankment is believed to be a homogeneous earth fill <br />consisting of sand and silty sand of somewhat low density. The attached Figure 4 <br />. presents a plan view of the dam, and a typical cross-section is shown in Figure 5. <br /> <br />The dam embankment was originally constructed at a 1: 1 upstream slope, <br />possibly with a stone or riprap upstream slope .protection. In 1907 the dam slope was <br />flattened to about 1.5:1 and a 5-inch concrete facing; reinforced with J4-inch twisted <br />steel bars, was constructed on a portion of the dam. In 1912, additional concrete slabs <br />were constructed. Repairs and/or additions to the concrete slabs were made in 1914, <br />Through the years, there have been repairs, modifications, and additions to the concrete <br />facing. Records of overlaying limited sections of bad slab with new concrete slabs <br />began in about 1960, .," <br /> <br />The concrete slabs include a vertical parapet wall section at the top of the dam, <br />extending above the road on the crest from a few inches to approximately four feet. A <br />portion of the parapet wall has a wave breaker at the top consisting of a horizontal <br />concrete shelf ranging from seventeen inches to twenty four inches wide, placed <br />perpendicular to the vertical portion of the wall. This shelf has fallen off in some areas <br />and is generally in very poor condition along the much of the dam length. <br /> <br />The reservoir spillway consists of an uncontrolled earth section on the eastern <br />end of the reservoir, approximately 100 feet in width (according to Smith). The <br />reservoir drainage area is' 'approximately 20 square miles, including the lake surface, <br />Zorich-Erker Engineering, Inc. conducted a hydrologic and spillway adequacy <br /> <br />17 <br />