4.2  Longshore Transport

Modified from "Physical Geology" by Steven Earle.

Longshore Current and Longshore Drift

Most waves reach the shore at a small angle, and as each one arrives, it pushes water along the shore, creating what is known as a longshore current within the surf zone (the areas where waves are breaking) (Figure 13.2.1).  Longshore currents can move up to 4 km/hr, strong enough to carry people with them, as everyone knows who has been swimming in the ocean only to look up and see that they have been carried far down the beach from their towel!

Another important effect of waves reaching the shore at an angle is that when they wash up onto the beach, they do so at an angle, but when that same wave water washes back down the beach, it moves straight down the slope of the beach (Figure 13.2.2).  The upward-moving water, known as the swash, pushes sand particles along the beach, while the downward-moving water, the backwash, brings them straight back. With every wave that washes up and then down the beach, particles of sand are moved along the beach in a zigzag pattern.

The combined effects of sediment transport within the surf zone by the longshore current and sediment movement along the beach by swash and backwash is known as longshore transport, or littoral drift. Longshore transport moves a tremendous amount of sediment (sand) along coasts (both oceans and large lakes) around the world, and it is responsible for creating a variety of depositional features. 

Rip Currents

A rip current  (often incorrectly called a “rip tide”; they are not really related to tides) is another type of current that develops in the nearshore area, and has the effect of returning water that has been pushed up to the shore by incoming waves or accumulated through longshore currents, particularly converging longshore currents.  Rip currents often occur where there is a channel between sandbars that makes it easier for the retreating water to escape.  As shown in Figure 13.2.3, rip currents flow straight out from the shore, and because the water is directed through a narrow space, the current can be very strong.  The currents lose strength quickly just outside of the surf zone, but they can be dangerous to swimmers who get caught in them and are pulled away from shore.  Swimmers caught in a rip current should not try to swim directly back to shore, as it is difficult to fight the current and the swimmer can quickly tire.  Instead, swim parallel to the beach for a short distance until you are outside of the rip current, and then you can easily swim to shore.

Rip currents are visible in Figure 13.2.4, a beach at Tunquen in Chile near Valparaiso.  As is evident from the photo, the rips correspond with embayments in the beach profile.  Three of them are indicated with arrows, but it appears that there may be several others farther along the beach.