The constant reshaping of sandy shorelines is a dynamic natural process driven by the transfer of energy from fluids like water and air to sediment grains. This continuous movement of sand, known as coastal sediment transport, dictates coastal geomorphology, determining the shape and slope of beaches worldwide. The active zone of the beach is governed by a complex interplay of forces that move sand toward, away from, and parallel to the coastline.
The Immediate Force Wave Energy
The most direct and energetic driver of sand movement within the active beach zone is the force generated by breaking waves. As waves approach the shore and shoal, their orbital motion is compressed, causing them to steepen and eventually crash, transferring significant kinetic energy to the water and underlying sediment. This wave action is responsible for the cross-shore transport of sand, moving it perpendicular to the coastline, either onshore or offshore.
Once a wave breaks, the turbulent sheet of water rushing up the beach face is called the swash, and it carries sand grains upward. The swash moves sand toward the land, often as a suspended load. Following the swash, gravity pulls the water back down the slope of the beach in a flow known as the backwash.
The backwash, typically thinner and less turbulent than the swash, carries sand back toward the sea, primarily as a bedload. The balance between the swash and backwash dictates whether the beach experiences net deposition or net erosion. This balance is largely determined by the type of wave dominating the environment.
Waves are broadly categorized into two types based on their effect on the beach profile. Constructive waves are characterized by a stronger swash and a weaker backwash, leading to a net movement of sand onto the beach and causing beach accretion. These waves are generally lower in energy and frequency, occurring in calm weather conditions.
Conversely, destructive waves possess a weaker swash but a significantly stronger backwash, resulting in a net transport of sand offshore. These high-energy, high-frequency waves, often associated with storms, erode the beach, moving sediment to form offshore sandbars. The constant adjustment between these wave types means the beach is always seeking a new equilibrium profile.
Horizontal Transport Longshore Currents
While wave energy dictates the onshore-offshore movement of sand, longshore drift transports massive volumes of sediment parallel to the coastline. This movement occurs when waves approach the shore at an oblique angle, rather than arriving perfectly parallel to the beach. The angled approach generates the horizontal force required for longshore transport.
When an angled wave breaks, its swash carries sand up the beach face at the same oblique angle as the incoming wave. However, the backwash, governed by gravity, pulls the water and sediment straight back down the steepest slope, which is perpendicular to the shoreline. This asymmetrical motion causes each grain of sand to move in a sawtooth or zigzag pattern along the beach face.
This repeated angled movement creates a net displacement of sand along the coast, known as longshore drift. This process generates a parallel flow of water, the longshore current, which runs within the surf zone. This current provides the continuous medium for carrying suspended sand over long distances, acting as a “river of sand” flowing along the shoreline.
The rate of longshore transport is directly related to the energy of the waves and the angle at which they strike the coast. Although longshore drift is the dominant parallel mover, other current-driven features, such as rip currents, also influence the system. Rip currents are strong, narrow offshore flows that return water from the surf zone back to the sea, carrying sand perpendicularly away from the beach.
Above the Waterline Wind Movement
Beyond the reach of the waves, sand is moved by the atmosphere in a process called aeolian transport, which primarily affects the dry, upper beach and the backshore areas. Wind acts as the fluid medium, picking up and carrying dry sand grains inland to form complex dune systems. This mechanism is a significant source of sediment input for coastal dunes.
For wind to initiate movement, it must reach a certain minimum velocity, known as the fluid threshold velocity, required to dislodge the sand grains. Once movement begins, collisions between grains can lead to a cascade effect, allowing transport to continue even if the wind speed slightly drops below the initial threshold. The effectiveness of wind transport is often limited by factors like sand moisture content, as wet sand clumps together and is much heavier to move.
Even a small amount of moisture can significantly reduce the sand transport rate to almost zero, even under strong winds. Consequently, aeolian processes are most active on wide beaches where there is a large source of dry, exposed sand. The transported sand typically accumulates around obstacles, such as driftwood or vegetation, which act as nuclei for the formation and migration of coastal dunes.
The Mechanics of Sand Transport
Regardless of the driving force—wave, current, or wind—sand is physically transported in one of three primary modes: bedload, saltation, and suspension. These modes describe the specific way a sand grain moves relative to the surface it rests on.
Bedload transport involves the heavier sand grains that remain in close contact with the bed surface. These grains are moved by being rolled or slid along the bottom, a process sometimes called traction or surface creep. This mechanism is governed by the flow-induced drag forces acting on the particle.
Saltation is a distinct mechanism where grains move in a series of short hops or bounces. The fluid force lifts the particle briefly, which then follows a ballistic trajectory before gravity pulls it back down to the surface. Upon impact, the landing grain can strike and mobilize other stationary grains, maintaining the transport process.
Finally, suspension is the mode of transport for the finest and lightest sand grains, which are carried fully within the water or air column. These particles are lifted and supported by the turbulent eddies within the fluid, remaining aloft for extended periods. This allows them to be transported much farther and faster than grains moved by bedload or saltation.