Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Wister DamLocation: OklahomaSummary: Near failure of an embankment dam due to internal erosionThe descriptions of this case history are extracted from several articles written bySherard, including his 1986 article, and an article by Rutledge and Gould (1973).Arthur Casagrande (1950) also discussed this case history.Lake Wister is located in the San Bois Mountains on the Poteau River in far easternOklahoma. The Tulsa District Corps of Engineers designed and built the project.Construction began in April 1946, and the project was placed in full flood controloperation in December 1949. The embankment dam is a rolled, impervious earthfillwith rock-protected slopes. The embankment dam was constructed as ahomogeneous clay fill without a chimney filter. At the time the embankment damwas constructed, chimney filters were not a standard design element in major damsas they are now. The embankment dam is 5,700 feet long and rises to a maximumheight of 99 feet above the streambed. Later tests on soils from the embankmentdam conclusively demonstrated that the clays were highly dispersive. The bedrock inthe area is Pennsylvanian age shale known to commonly produce residual soils withdispersive properties.Heavy rains caused the reservoir to fill quickly beginning in January 1949. Whenwater had risen to a height of about 60 feet, muddy water was seen discharging onthe downstream slope of the embankment dam. The quantity of flow was initiallyestimated at 2,200 gal/min, and it increased to about 8,000-9,000 gal/min in the nextseveral days. The spillway radial gates were opened, and within 3 days, the reservoirhad dropped about 13 feet. This exposed tunnels on the upstream slope, throughwhich the water was entering the embankment dam. The tunnels were about 2 feetin diameter and extended along the upstream face of the embankment dam for adistance of about 300 feet, at about the same elevation on the slope. Dye wasinjected into a vortex on the upstream slope, and the test showed the water wasflowing along a nearly horizontal seam in the embankment dam for a distance ofabout 740 feet, with a head on the tunnel at the time flow began of only about13 feet (a gradient of a little over 50:1). The dye tests showed the time for flow totravel this distance was less than 13 minutes, a velocity of about 1 foot per second.Figure B-81 below shows an idealized sketch of the embankment cross section withthe flow path causing the erosion identified.After the reservoir level had dropped farther, the erosion tunnels exposed wereexcavated and plugged, and several remedial measures were implemented, includingB-117