Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Medford Quarry Wash Water Lake DamLocation: MarylandSummary: Failure of an embankment dam due to internal erosion along theconduitMedford Quarry Wash Water Lake Dam is a 26-foot high, significant hazardembankment dam. Downstream hazards include roadways, railroad tracks, and aresidence. The embankment dam is essentially an offstream basin, and nearly allinflow is pumped into the basin from a nearby wash plant.The embankment dam was constructed in 1988 as a “temporary sediment basin.”Although an engineer prepared plans for the structure, no engineering supervisionwas provided during construction. Local materials (decomposed shale and erodiblesilts) were used to construct the embankment dam. A CMP spillway withconventional antiseep collars was constructed in a trench excavated into thefoundation and backfilled with low plasticity silts and decomposed rock fragmentsfrom the excavation (figure B-54).The structure was placed into use the following year, and it failed upon first filling.When the pool level was only about 2 to 3 feet deep, flow along the outside of thepipe resulted in loss of the adjacent soils by internal erosion (figure B-55).Figure B-54.—The embankment dam was constructed with antiseep collars.B-67