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I I I I I I I I I I I I I I I I I I I SECTIONSIX Investigation and Rehabilitation Measures 6.1 INVESTIGATION 6.1.1 General The existing outlet works has deteriorated significantly since construction of the dam. Leakage of water from the conduit has resulted in embankment material loss, or piping, from around the conduit. This was observed during excavation of the outlet works area in December 1998. Piping was also observed in the area downstream of the outlet works in November 1998. Based on the age and condition of the existing outlet works and the foundation material encountered, the outlet works should not be rehabilitated and should be demolished and replaced. A new outlet works should be constructed and the embankment rebuilt. The investigations performed are summarized below, followed by recommended repair and rehabilitation measures. 6.1.2 Site Reconnaissance A site reconnaissance was performed on the Black Hills Reservoir Dam on November 17,1998 by our Messrs. Arnold and Reichler. The files in the Colorado State Engineer's Office on Black Hills Reservoir Dam were reviewed and a meeting was held with the Colorado State Engineer's Office staffin Pueblo, Colorado. As an additional component of the site reconnaissance, surveying of site was performed in January and October of 1999, as discussed in Section 3.5. The reservoir had been drained at the time of our site reconnaissance. There was a small base flow in two streams entering the intake structure. The crest of the dam was generaU y covered with weeds, and there were signs of rodent activity on the crest and downstream slope. The crest was also somewhat irregular in areas due to gravel, which may have been placed to minimize rodent activity. Along the central portion of the dam, over the outlet works, the crest was narrow (on the order of2 to 6 feet). The downstream slope was irregular in areas due to erosion. Vegetation at the downstream toe of the dam indicates that seepage has been occurring beneath and through the dam. Seepage and erosion were particularly significant around the outlet structures. The concrete in the intake and outlet structures was significantly deteriorated. An erosion channel (up to 3 feet in diameter) had formed in the streambank near the left side of the outlet structure. There were also numerous signs of significant seepage and piping beneath the dam/outlet works structure including an active seepage "conduit," about 4 inches in diameter, located about 15 feet downstream ofthe outlet structure. The seepage conduit was flowing during our reconnaissance even though the reservoir had been essentially drained. Based on our site reconnaissance, review of inspection reports and records in the State Engineer's Office, the dam and outlet works is in poor condition. Photographs of dam and outlet works are shown in Attachment A. 6.2 RECOMMENDED REPAIR AND REHABILITATION MEASURES The rehabilitation measures discussed below include the deteriorated conditions encountered at the existing outlet works that led to the zero storage restriction on the facility in October 1998. URS Greiner Woodward Clyde 6800Q44333.00IR2.doc 11311OO{3:18 PM)IPROJECTS 6-1