5.5: Wave-Driven Sediment Transport

Ocean waves, the physical expression of energy moving through the ocean, transfer some of their energy to sediments as the waves break at the mouth of a river. If given sufficient energy by the moving water, the sand will roll, hop, or become suspended. And if the waves arrive at an angle relative to the river mouth—as opposed to straight on—a current of water—the longshore current—will be generated along the shore. This current—driven by the energy of the waves—can move sediments down the beach.

Longshore Transport

Once the river-transported grains of sand reach the ocean, they keep moving down the beach with the assistance of the energy provided by the wave-generated longshore current. This current moves sand (and even you!) down the beach if it’s strong enough. Have you ever gone into the water on a day with high surf and found yourself way down the beach a few minutes later? You’ve experienced the longshore current.

Unlike ocean currents, the longshore current operates only within the surf zone. As a wave approaches at an angle, one part of the wave reaches the beach before the rest, and the water temporarily “piles up.” Just like water flowing downhill owing to the force of gravity, the water piled up from the wave flows “downstream,” that is, in the direction opposite the incoming wave. The result is the longshore current.

Grains of sand within the longshore current generally move parallel to the beach. But because of the back-and-forth nature of waves, the suspended materials often take a zig-zag path down the beach. This wave-generated movement of sand grains and other materials is called longshore transport.

Enormous volumes of sand may be transported along coastlines. Patsch and Griggs (2006) report that up to a million cubic yards of sand may move southward along the California coast annually. Considering that one cubic yard of sand weighs roughly 2,700 pounds—about the weight of a Kia Forte (Kia 2023)—that’s a million Kias worth of sediment traveling down our coast each year. That’s a lot of sand. Sedimentologists estimate that tens of thousands to possibly more than two million cubic yards of sand move southward along the US East Coast annually (e.g., van Gaalen et al. 2016).

The longshore transport of sediments by the longshore current has been eloquently referred to as a “river of sand” (Encyclopedia Britannica Films 1966). Just like a terrestrial river that moves sand from the mountains to the ocean, the river of sand (i.e., the longshore current) moves sand down the beach from the mouths of the rivers. This conceptual model of longshore transport proves useful for envisioning the transport of sediments along beaches, as we shall see.

Cross-Shore Transport

Waves also carry sediments back and forth across the beach, a process called cross-shore transport. When a wave strikes a beach, it transfers enough energy to the sand to cause the sand to become temporarily suspended. Once suspended, the sand flows with the motion of the water. As the wave slides up the beach, it carries sand with it. If the wave energy is high enough, the sand will remain in suspension and be carried back out toward the sea as the wave recedes from the shore. If the wave energy is low, the sand will be deposited at the point where the wave can no longer carry it. Simply put, high-energy waves move sand away from the beach and low-energy waves move sand onto the beach.

When big waves pound the shore, as they typically do in winter in Southern California, they remove sand from the beach face. The excavated sand moves offshore. In the deeper water, where the waves’ energy is reduced, the sand settles and forms sandbars. When gentle waves caress the shore, as they typically do in summer in Southern California, they move sand from sandbars onto the beach. A sandbar acts as a reservoir of sand, at least on a temporary basis.

It’s quite dramatic to see the changes that can occur on a beach. Beaches are ever changing, which is just another thing that makes them so darn interesting.

The Beach Profile

The movement of sand onto or away from the beach face by cross-shore transport alters the appearance of a beach, what is known as the beach profile. Like a profile of a person when viewed from the side, a beach profile refers to the changing slope of the beach from the backshore to the foreshore.

Measurements of beach profiles provide a simple and useful means for tracking seasonal and other kinds of changes in beaches. First described in 1961 by University of Southern California marine geologist Kenneth O. Emery (1914–1998), the method requires only two 2-meter-long measuring sticks and a tape measure. By holding the sticks in a line perpendicular to the beach and separating them by a known distance, the slope of the beach can be determined by observing the marks on the beach-side stick where the top of the oceanside stick lines up with the horizon. Known as the Emery method, this tool for determining beach profiles has found wide adoption among beach managers, beach scientists, and students. (See Emery 1961.) If you take an oceanography field class, you’re very likely to encounter this technique in your studies.

Contributions from Beach Bluff Erosion

Recent research on some California beaches suggests that a percentage of their sand comes directly from the erosion of coastal bluffs, a type of rounded cliff found on coastlines. For example, bluff erosion contributes 31 percent of the sediments found on Laguna Beach (Patsch and Griggs 2007). Oceanside receives 80 percent of its sediments from the erosion of coastal bluffs (Young et al. 2010). Of course, many of these bluffs were created by sedimentary processes in times past. But recognition of beach bluff erosion adds a new wrinkle to the explanation of where sand comes from on our beaches.

Submarine Canyons

The final step in our journey from the mountains to the sea takes us to the resting place for sediments in the ocean. In many places along the coastline, the river of sand suddenly stops, as if the sediments have disappeared. In fact, as it turns out, sediments are drained from beaches by the presence of a submarine canyon, a steep-sided underwater valley whose shallow end—its head—comes close to shore.

Sediments carried by the longshore current often accumulate where a landform—such as a headland—interrupts their flow. The sediments may eventually move around the landform, but if a submarine canyon is present, they will be deposited at the head of the canyon. Here gravity takes over. If the mass of deposited sediment grows too large, it may become unstable, at which point it will tumble down the canyon. Such underwater landslides are called turbidity currents and they are one of the forces that create submarine canyons.

Submarine canyons can be found along all coastlines of the world. The Southern California Continental Borderland—roughly the coastal waters from Point Conception to the US-Mexican border—boasts 11 submarine canyons and 2 sea valleys (a similar feature) that drain sediments to basins (Normack et al. 2009). This includes the Newport submarine canyon, a branch of which lies less than 500 feet (150 m) from the Newport Beach pier (Felix and Gorsline 1971). Submarine canyons serve as a major sink for beach sediments and represent an important pathway for delivering sediments to deep basins (e.g., Sweet and Blum 2016).