As the wave front enters shallow water, the leading edge of the wave hits the shallow water sooner than the rest of the wave front and slows down, bending the wave as it moves ashore. The shoreward movement of the wave thus forms a current whose net flow is parallel to the shore in the surf zone. The speed of the longshore current increases with increasing wave height, decreasing wave period, increasing angle of wave front to beach, and increasing beach slope. Once established, the current moves at a speed of about one knot in the same direction as the advancing wave train. Longshore currents are more prevalent along lengthy straight coastlines.
Sandbars often form in areas where longshore currents frequently occur.
Longshore currents transport significant amounts of sand and sediment suspended by wave action in the surf zone along the shore. When the current enters deeper water, forward momentum diminishes and the sediment settles to the bottom. This can erode the beach in one area and build it in another. Unfortunately a considerable amount of sediment is dumped into shipping channels and harbors, which requires expensive dredging to remove. Rip currents are formed when longshore currents, moving parallel to the coastline, are deflected seaward by bottom irregularities, or meet another current deflecting the flow to seaward.