Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsThe embankment dam was constructed of low plasticity soils susceptible tobackward erosion piping, and the failure could have occurred from this mechanism.The high seepage pressures caused by the conduit leak would have been sufficient toinitiate backward erosion piping, and the exit face for the seepage was not protectedby an adequate filter. Another possibility is that the pressurized leakage hydraulicallyfractured the earthfill surrounding the conduit and the failure occurred from themechanism of internal erosion. Probably, a combination of these mechanisms wasresponsible for the earthfill erosion. Regardless of the mechanism, the failuredemonstrated the dangers of a pressurized conduit that develops a defect allowingfull reservoir head to be imposed on soil that is not protected by filters.The embankment dam failure resulted in the loss of three lives and approximately$35 million in property damage in the town of Estes Park. The resulting dam breakflood also overtopped and failed the Cascade Lake Dam located 6.7 milesdownstream of Lawn Lake Dam. Cascade Lake Dam was a 12-foot high concretegravity dam with a reservoir with a storage volume of 12.1 acre-feet. The concretedam was overtopped by an estimated 4.2 feet. Extensive erosional damage alsooccurred along the Roaring and Fall Rivers downstream of Lawn Lake Dam. Theriver channels were widened by several tens of feet in some locations, and scourdepths varied from 5 to 50 feet. A large alluvial fan was also created at theconfluence of the Roaring and Fall Rivers. The alluvial fan covered an area ofapproximately 42 acres and had an estimated maximum thickness of 44 feet. Thefan also dammed the Fall River, creating a reservoir with a surface area of 17 acres.The largest boulder believed to have been moved by the floodwaters was estimatedto have a weight of 450 tons.Lesson learnedIf possible, construction of fully pressurized conduits beneath embankment damsshould be avoided. When pressurized conduits are constructed within embankmentdams, the joints in the conduit should be properly designed to assure watertightnessunder all loading conditions.ReferenceU.S. Geological Survey, Hydrology, Geomorphology, and Dam-Break Modeling of the July 15,1982, Lawn Lake Dam and Cascade Lake Dam Failures, Larimer County, Colorado,Open-File Report 84-612, 1982.B-56