Longshore Currents

Very rarely do wave trains approach a shoreline aligned perfectly parallel to the trend of the shoreline, much more common is for wave trains to arrive at an angle to the trend of the shoreline. The net result of waves breaking at an angle to shoreline is to produce a weak current known as a longshore current (LSC). This current flows parallel to the shoreline. Some of you may perhaps recall swimming in the ocean when the waves were relatively large and where you entered the water was not where you got out of the water. The reason for this is that the longshore current moved you slowly down the beach. This same wave induced current, in conjunction with sand being suspended by breaking waves, is responsible for moving sand along the beach (Fig. 12). For this reason, beaches have sometimes been referred to as rivers of sand because there is constant sediment transport as a result of longshore currents. The movement of sand parallel to the trend of the shore results in the development of features you will learn about in the next module such as spits, it can also increase the length of barrier islands, and the buildup of sand next to coastal structures such as groins and jetties that you will learn about in Unit II.

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Figure 2.12: Photograph showing waves approaching a New Jersey beach (U.S.A) at an angle, which during this day contributed to a longshore transport direction toward the right of the image.
Credit: M. Kulp