Why can rapid drawdown of a reservoir cause an embankment dam to fail?

Rapid drawdown of a reservoir can cause an embankment dam to fail because of the behavior of pore water pressure within the dam's structure.

When the water level in the reservoir drops quickly, the pore water pressure inside the upstream face of the dam may not be able to adjust or dissipate at the same rate as the receding free water surface. This lag creates severe seepage gradients within the soil structure. These gradients can destabilize the upstream face, potentially leading to a slide or structural failure.1

To mitigate this risk, engineers often place a zone of freely draining, coarser-graded fill on the upstream face of the dam, which allows the pore water pressure to equalize more effectively during rapid changes in water levels.1