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� � <br />This landslide was also studied by Chen and Associates, and mapping and analysis was <br />presented in their 1983 report. They also installed two inclinometers in the slide mass in 1982, <br />but I have not found any records of readings. <br />The presence of these landslides not only drives concern on the conditions in and around the <br />dam, but also in the reservoir area. A very large landslide (rock avalanche) occurred in the early <br />1990s on West Creek a few miles upstream of the reservoir. A fluctuating reservoir can promote <br />instability due to increased pore water pressures in the reservoir slopes and could precipitate <br />future landsIide movements that could cause a rapid evacuation of the reservoir basin due to <br />displacement of a large mass of rock abruptly into the reservoir. I recommend that detailed <br />studies of the nature of slides in the area should be performed prior to proceeding with <br />enlargement of the dam and reservoir. If possible new readings should be taken of the two <br />inclinometers to evaluate if movement has occurred in the last 24 years. <br />4. D EXISTING CONDITIONS AND HISTORY OF THE DAM <br />The current existing dam facility layout is shown on Figure 3. The outlet tunnel construction <br />began in 1908. The tunnel is an unlined horseshoe shaped 1 I-foot high by 15-foot wide rock <br />tunnel in the right abutment. This tunnel initially served as a diversion twuiel for the Rio Grande <br />River that allowed for dam construction in the dry. I entered the tunnel downstream of the gates <br />during our reconnaissance in October 2006. Rock conditions axe quite good and consist of <br />strong and massive welded tuff with no support except at the portal. <br />Dam plans were submitted and approved by the State in 1910. The project was completed in <br />1913. The original outlet works had five stide gates. In the first year of operation of the <br />reservoir, the outlet works suffered severe damage due to vibration and erosion, and a major <br />repair job was immediately done in 191 S with the gates refurbished and two of the gates plugged <br />with concrete so that the dam was Ieft with only three operating gates. Later gate repairs were <br />designed by Wheeler and Associates in 1983. There were problems with that repair, and <br />additional gate repairs were designed by HARZA Engineering which were constructed in 1987. <br />There has been continuing discussion with the SEO over the acceptable capacity of the outlet <br />works on a ma,in stem river such as the Rio Grande. The State has required that the outlet works <br />be able to discharge 2,500 cfs. Currently, up to about 1,804 cfs can be reliabIy discharged <br />through the outlet works before severe vibrations are still experienced due to a hydraulic jump <br />which occurs in the tunnel downstream of the gates. A solution to this was proposed by HARZA <br />Engineering, and that would be to drill a shaft just downstream of the gates to allow air to <br />escape. It is believed that this may significantly reduce the vibratibn and allow the outlet to <br />reliably discharge the full 2,500 cfs maarimum capacity. <br />The original dam was 100 feet high with a crest elevation of 9449. It was built as an earthen and <br />rockfill dam. The maximum section is shown on Figure 4. The upstream one-third was earthfill <br />with a flat 4:1 (horizontal to vertical) slope. The downstream slope was rockfill with a 1.5:1 <br />slope. <br />;._ � _ � -3- <br />�.` � <br />