Hydraulic action

Hydraulic action is erosion that occurs when the motion of water against a rock surface produces mechanical weathering. Most generally, it is the ability of moving water (flowing or waves) to dislodge and transport rock particles. Within this rubric are a number of specific erosional processes, including abrasion, attrition, corrasion, saltation, and scouring. Hydraulic action is distinguished from other types of water facilitated erosion, such as static erosion where water leaches salts and floats off organic material from unconsolidated sediments, and from chemical erosion more often called chemical weathering. It is a mechanical process, in which the moving water current flows against the banks and bed of a river, thereby removing rock particles.

A primary example of hydraulic action is a wave striking a cliff face which compresses the air in cracks of the rocks. This exerts pressure on the surrounding rock which can progressively crack, break, splinter and detach rock particles. This is followed by the decompression of the air as the wave retreats which can occur suddenly with explosive force which additionally weakens the rock. Cracks are gradually widened so each wave compresses more air, increasing the explosive force of its release. Thus, the effect intensifies in a 'positive feedback' system. Over time, as the cracks may grow they sometimes form a sea cave. The broken pieces that fall off produce two additional types of erosion, abrasion (sandpapering) and attrition. In corrasion, the newly formed chunks are thrown against the rock face. Attrition is a similar effect caused by eroded particles after they fall to the sea bed where they are subjected to further wave action. In coastal areas wave hydraulic action is often the most important form of erosion.