Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Pasture Canyon DamLocation: ArizonaSummary: Closed circuit television inspection of an outlet works conduitPasture Canyon Dam is located on the Hopi Indian Reservation in Arizona. PastureCanyon Dam is a homogenous embankment dam with a height of 17 feet. Theembankment dam crest is at elevation 4890.0 feet, 20 feet wide, and 632 feet inlength. The embankment dam was apparently founded on pervious, sandy alluvium.No information was available as to its construction. The embankment dam wascompleted in 1920s or 1930s and modified in 1975. The 1975 modification includeda 3-foot crest raise.Appurtenant structures at the site include an uncontrolled earthen spillway and anoutlet works. The outlet works is located within the embankment damapproximately 200 feet from the left abutment. The outlet works consists of aconcrete intake structure, approximately 55 feet of 12-inch by 12-inch masonryconduit connected to 35 feet of 14-inch diameter CMP connected to 20 feet of14-inch diameter concrete pipe. The intake structure contains a hand-operated slidegate. The discharge capacity of the outlet works has been estimated to be 9 ft 3 /s.The Bureau of Reclamation’s Technical Service Center performed a CCTVinspection of the outlet works conduit at Pasture Canyon Dam in April 2004. Theconduit was accessed for inspection, via an existing manhole located at thedownstream end of the outlet works. The camera-crawler was inserted into the14-inch diameter concrete pipe and was advanced upstream approximately 18 feet,where the concrete pipe ended and CMP began. Water clarity was somewhat poorand limited viewing throughout the conduit. The camera-crawler was advancedupstream within the CMP for approximately 35 feet, where the CMP ended and amasonry conduit began. The camera-crawler was advanced upstream within themasonry conduit for approximately 30 more feet, where numerous piles of sand wereobserved near the sidewalls of the masonry conduit. Figure B-59 shows a typical pileof sand near the sidewall of the conduit. In addition, just a few feet into the masonryconduit existed an open defect (crack) in the crown of the conduit. Figure B-60shows the defect at the crown of the conduit. This defect allowed sand materials toenter the conduit. Based on observations made during the CCTV inspection, it wasconcluded that Pasture Canyon Dam was in the process of failing by internal erosionor backward erosion piping, and immediate action was required. Visual monitoringof the embankment dam and monitoring of the area around the outlet works wasperformed every 4 hours. The Bureau of Indian Affairs (BIA) imposed reservoir andgate operating restrictions at Pasture Canyon Dam.B-77