Why is overtopping especially dangerous for earthfill dams?

Overtopping is especially dangerous for earthfill dams because they are typically not designed to withstand the uncontrolled flow of water over their downstream slopes. Unlike concrete structures, earthfill dams are composed of soil and rock fragments that lack structural strength and possess low resistance to the erosive force of flowing water.123

When water overtops an earthen embankment, it can rapidly erode the dam’s material and its foundation. This process can lead to a catastrophic washout and the eventual failure of the entire structure. Because these dams are highly susceptible to such deterioration, they require an ample spillway—usually constructed to one side of the dam—to safely divert surplus water and prevent overtopping.12