When evaluating the impact of an engineered structure, such as a jetty or groin, on a beach, one would expect that a groin placed perpendicular to the longshore currents of the up-current would cause what long-term effect?

• A. Erosion of the up-current beach
• B. No significant effect
• C. Accumulation of the beach sands up-current of the groin and starvation of the downdrift beaches
• D. Accumulation downdrift of the groin and starvation of the up-current beaches

Answer: (C)

Longshore transport carries sand along the coast as waves approach at an angle. A groin placed perpendicular to that flow acts as a barrier to this alongshore movement. Sand moving toward the groin tends to pile up on the up-current side, creating an accretion there. Because the groin interrupts the alongshore supply, less sediment reaches beaches down the shoreline, so those downdrift beaches become starved and erode over time. This combination—accumulation up-current of the groin and erosion/starvation downdrift—is the expected long-term effect.