When a groin is placed perpendicular to up-current longshore currents, what is the expected long-term beach effect?

• A. Accumulation of sand up-current and downdrift starvation
• B. Erosion on all beaches
• C. No effect
• D. Accumulation down-current

Answer: (A)

Longshore transport moves sediment along the shore as currents run parallel to the beach. A groin built perpendicular to the shoreline interrupts that alongshore movement, acting as a barrier for the sediment flowing with the longshore current. As sand encounters the groin, it accumulates on the up-current side, widening that portion of the beach. But because the groin blocks the continual supply of sand to beaches downdrift, those beaches lose sediment over time and erode.

So, the long-term result is sand buildup up-current of the groin and sand/starvation downdrift, which matches accumulation on the up-current side and erosion or reduced sediment supply further along the coast.