Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsProject name: Balman Reservoir DamLocation: ColoradoSummary: Reservoir evacuation by pumping and controlled breaching techniquesBalman Reservoir Dam is an earthfill embankment dam located in San IsabelNational Forest in south central Colorado near Cotopaxi and is owned by the U.S.Forest Service. The embankment dam has a maximum height of 31 feet with a crestlength of 75 feet and impounds approximately 51 acre-feet of water. Theembankment dam was constructed at approximately elevation 9,400 feet and islocated in a remote area. The embankment dam has a small earthcut, 12-foot widespillway and no gated outlet works. The embankment dam was built in 1965.On November 4, 1996, a large sinkhole was discovered on the upstream slope andcrest of the embankment dam. The sinkhole was measured to be approximately 8 to10 feet in diameter and approximately 6 to 8 feet deep. A small whirlpool wasobserved in the reservoir near the sinkhole, indicating the presence of continual flowinto the cavity and apparent sediment transport into the sinkhole. Cracks weredeveloping in the embankment dam crest above the sinkhole (figure B-16) andsloughing of the embankment materials into the sinkhole was observed. Extensivewater flow was occurring all along the downstream toe of the embankment dam andalong the right abutment groin up to approximately mid-height of the dam. Waterexiting the slope was also occurring on the downstream face above an 18-inchdiameter drainpipe for the chimney drain.After reviewing the original construction plans for the embankment dam and takinginto account its past poor operational performance and the worsening condition ofthe sinkhole and dam, it was apparent that the dam was experiencing an internalerosion failure, which could eventually result in a sudden and catastrophic breach ofthe dam and a release of the reservoir. The downstream hazard consists of a churchcamp, a hiking trail, a campground, and a county road. Based on this, it was decidedthat the reservoir needed to be lowered to a safe storage level and to a level where nomore seepage was exiting on the downstream face of the dam and along thedownstream toe. Since the embankment dam lacked an outlet works, it was decidedto try lowering the reservoir by an alternative means.The first attempt to lower the reservoir water surface to a safe level was made bydiverting the reservoir inflow away from the reservoir. The diversion structureabove Rainbow Lake, located upstream of Balman Reservoir, was adjusted onNovember 8, 1996 to direct all flows from the drainage basin into Rainbow Lake andaway from Balman Reservoir in the hope that this would help lower the waterB-18