What term refers to the phenomenon that can trap a small vessel against the face of a dam?

The term that refers to the phenomenon that can trap a small vessel against the face of a dam is known as a boil or back roller. This occurs when water flowing over a dam or a rapid encounters the structure and drops, creating a turbulent and swirling current. As the water levels change, it creates a recirculating current that can engulf smaller vessels, making it difficult for them to navigate away from the dam.

Understanding this concept is crucial for boat operators, especially those navigating near dams or similar structures. Awareness of boils is essential for safety, as they can pose significant risks, including capsizing or being held in place against the structure, leading to dangerous situations for operators and passengers alike.

The other terms in the question refer to different water phenomena: a whirlpool is a rotating body of water, a vortical current generally relates to movement in a spiral direction, and a tidal surge describes water levels rising rapidly due to tidal forces. These do not specifically indicate the trapping effect observed with boils near dams.