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Ms. Minerva Lee <br />October 16,2013 <br />Page 2 <br />3. Creation of a moderate sized hole on the left abutment dam/rock contact about 20 feet <br />below the crest <br />4. Minor damage to gunite facing of gravity dam <br />5. Increased loss of mortar between masonry blocks on downstream face <br />Figures 2 and 3 compare 2004 and October 2013 photographs of the dam's downstream face <br />near the crest. As shown on the figures, four vertical courses of masonry were lost. A fifth <br />course was damaged and chipped locally. <br />As can be seen on Figures 2 and 3, the failed section corresponds closely with an apparent seep <br />line seen in the 2004 photographs. Remnants of the upstream gunite face (sprayed concrete with <br />fine aggregate) can be seen on Figure 2. It appears the one to two-inch thick gunite facing, that <br />is believed was installed in the 1960s, only extends about five feet below the original crest. <br />During the flooding, the courses of masonry blocks below the gunite probably experienced uplift <br />forces, which contributed to failure. <br />The damage to the arch dam's left abutment buttress is shown on the upper photograph of Figure <br />4. This area may be where failure initiated in the upper dam masonry courses. Most of the arch <br />dam is confined within the narrow slot canyon in very good quality Colorado Red Granite. A <br />coherent arch in compression is formed throughout most of the dam structure, making it a strong <br />and robust structure. The exception to this is the top few feet of the dam, which because of the <br />topography, relies on this artificial buttress thrust block to engage full arching action in the upper <br />dam courses. <br />Review of flood videos revealed very turbulent flow in the area of the flow restriction caused by <br />the buttress. Masonry blocks in the buttress and adjoining arch dam were probably the first <br />blocks to fail and resulted in loss of arching action for remaining blocks in the upper dam <br />masonry courses. This loss of arching action, combined with hydrostatic uplift, flow velocity <br />and depth, cavitation, and debris impact all contributed to the failure. <br />A moderate sized hole at the left abutment rock/dam contact is shown on Figure 4 (lower <br />photograph). This area contains smooth and shiny masonry blocks as it is a major impact zone <br />for cascading water. The rock mass is locally closely jointed in this area, and water has plucked <br />out both abutment rock and dam masonry. The hole needs to be inspected up closer, but appears <br />to be two to three feet deep. The photographs also show that most of the mortar between <br />masonry blocks has been removed on the downstream face over time by the water flow and <br />freeze-thaw action. Our examination of the crest indicates that mortar loss of about six inches in <br />depth may be common. Since the typical outer face masonry blocks are 18 inches thick, about <br />one-third of the block to block contact has been lost over time. Closer examination of the <br />downstream face is needed to verify this conclusion. Additionally, it appears that there may be <br />similar openings in the rock/dam contact at lower portions of the right abutment. Further <br />examinations of this area, as well as the plunge pool at the toe of the dam are warranted.