Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure B-56.—Sinkhole in the dam crest the night of December 11, 1996.As an immediate response to the threat of an embankment dam breach, countymaintenance staff filled the sinkhole with some 200 yards of angular cobbles andboulders. The State dam safety staff saw no viable alternative to the county’s schemeto address the immediate crisis. Finer grained soils would likely have been sluicedthrough the top of the collapsed box conduit. This could have worsened thesituation by plugging what limited outlet capacity remained after sediment had largelyblocked the conduit. Nonetheless, it was obvious that the rockfill was but an interimmeasure, and immediate follow-up action was necessary to lower the reservoir andpermanently resolve the public safety threat. Three days of pumping were necessaryto lower the reservoir to allow excavating a trapezoidally shaped breach of theembankment dam (figure 57). The floor of the breach was armored with a geotextilefabric and capped with much of the rock originally dumped into the void the nightof the failure. To improve fish passage, an attempt was made to include a number ofpools along the breach channel at the direction of the Washington State Departmentof Fish and Wildlife. The Washington State Water Quality staff assisted inblanketing the disturbed sections of the embankment dam with hay to minimizefurther sediments entering the water course.Damage downstream was limited to the streambed. Primarily, it occurred in theform of stream habitat degradation from sediment deposition. Many of the salmoneggs in this fish-producing stream were smothered under sediments for severalhundreds of yards downstream of the embankment dam. As bad as it was, theemergency action prevented a likely failure of the embankment dam. Thus, theB-70