When the wave retreats the air rushes out of the gap, and causes cliff material to break away is known as hydraulic action. Attrition when waves cause rocks and pebbles to bump into each other and break up.
Hydraulic action is when the force of fast-flowing water hits the bed and air into cracks in the bedrock, whereas attrition is a process in which rocks are carried in the flow of a river and repeatedly knock into each other, which causes the rocks to break.
Attrition is the collision between sediment particles that breaks into smaller pebbles. Hydraulic action is the force of water against the banks, which expand and fracture the rock over time.
In coastal areas wave hydraulic action is often the most important form of erosion.
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