What structure is built to trap sand moving along the coast?

• A. Jetties
• B. Barrier Islands
• C. Breakwater
• D. Groins

Answer: (D)

Sand moves along the coast because waves hit the shore at an angle, driving sediment in a alongshore (longshore) flow. A groin is a structure built perpendicular to the shoreline to interrupt that alongshore movement. By blocking the continuous transport of sand, it causes sediment to accumulate on the updrift side, building a wider beach there and helping to reduce erosion locally. The other options have different primary purposes: jetties protect harbor entrances and influence sediment patterns but aren’t designed specifically to trap sand along a beach; barrier islands are natural deposits formed by longshore processes rather than man-made devices; breakwaters reduce wave energy to shelter harbors and can cause sediment to collect behind them, but their main function isn’t to intercept alongshore sand movement the way a groin does.