Why can large storm waves move boulders while small waves mainly move sand?

The ability of waves to move sediment is directly related to their size and the energy they exert on the ocean bottom as they approach shallow water.1

When waves travel into shallow areas, they cause sediment to become temporarily suspended, making it available for transport by coastal currents. The size of the wave determines the depth at which this interaction occurs and the size of the particles that can be moved:1

• Small waves: Even waves only a few tens of centimetres high possess enough energy to pick up and transport [sand].1
• Large storm waves: These waves have significantly more force and can move much larger materials, including cobbles and boulders.1

Essentially, larger waves generate the necessary energy to entrain and transport heavier rock debris, whereas smaller waves are limited to moving finer materials like sand. While small waves typically transport sediment toward the coast to be deposited on the beach, larger storm waves are often responsible for removing sediment from the coast and conveying it into deeper water.1