The term hydraulic power in a geographical sense refers to a form of coastal erosion. This occurs when waves from a body of water compress air into the pores of rock formations on a coastline.
Air is often trapped in small and large holes within the rock formations of a coastline. When waves crash into the coastline they force the air within these holes to become compressed. The air in the rock compresses until it eventually breaks free and smashes the rock apart. This is referred to as hydraulic action or hydraulic power.
Coastal erosion is predominantly caused by the waves of a body of water eroding the rock and sand of a coastline. Erosion is completed in three ways: hydraulic power, corrosion and attrition.
Erosion caused by hydraulic power often takes place at the base of a coastal headland, with a horizontal notch cut out of the rock. This results in the base of the rock disappearing until the top of the cliff is left as an overhang that eventually collapses into the ocean.