Final answer:
Longshore drift occurs when waves approach the shore at an angle, leading to a zigzag movement of sand along the beach by the longshore current. This process is key for understanding sediment transport and is essential for effective coastal management.
Step-by-step explanation:
Longshore drift of sand on a beach occurs when waves approach the shore at an angle. This movement of sand is caused by the action of the longshore current, which is the dominant current in the nearshore zone and typically runs parallel to the shore. When waves hit the shore at an oblique angle, they carry sand and sediment in the direction of the wave's travel. As the water recedes back to the ocean, it pulls sand with it at a right angle to the shoreline due to gravity, initiating a zigzag pattern of sediment movement along the beach known as longshore drift.
This process is distinct from sediment deposition by rivers or tidal movements. When a river enters the sea, it often slows down dramatically, dropping its silt content, potentially forming deltas or estuarine areas depending on the silt content and wave action present. However, this river deposition is not the same as longshore drift.
The formation of ripples and dunes on the beach can also give clues to the direction of the moving water or wind, due to the asymmetrical nature of these features. Cross-bedding structures in the sedimentary record provide evidence of past directional flows. An understanding of these features is crucial for coastal management, including the protection of coastal communities from erosion caused by wave action. Therefore, the answer to the student's question is: a) waves approach the shore at an angle.