Which structure is typically built in pairs to guard harbor entrances from wave action?

• A. Seawalls
• B. Breakwater
• C. Jetties
• D. Groins

Answer: (C)

Jetties are long, solid structures that extend from the shore into the water, and they are built in pairs on opposite sides of a harbor entrance. This paired setup interrupts wave energy and cross-shore currents from both directions, helping keep the entrance open by reducing erosion and shoaling inside the harbor, which maintains a stable, navigable channel. Seawalls run along the shoreline to protect landward areas rather than protect a harbor mouth, so they don’t specifically guard the entrance. Breakwaters may shield a harbor by dampening waves, but they aren’t typically placed as the standard pair that directly guards the entrance itself. Groins extend from the beach to trap sand and influence littoral drift, not to protect a harbor entrance. So, the structure built in pairs to guard harbor entrances from wave action is the jetty.