A sand bar is a dynamic, underwater or partially exposed ridge of sand or other coarse sediment that forms parallel to the coastline. These landforms result from a constant geological process where ocean energy moves and deposits material from the seabed. Understanding sand bar formation requires recognizing the interplay between available sediment and the powerful forces of moving water. This construction process is driven by the ocean’s energy as it encounters shallower waters near the shore.
The Essential Ingredients and Energy Sources
The formation of any sand bar depends on two components: an adequate supply of sediment and sufficient energy to transport it. Coastal sediment, primarily sand, shingle, and gravel, originates from various sources. These sources include river runoff, the erosion of coastal cliffs, and material previously deposited on the continental shelf. Without a continuous input of this material, erosion would quickly wear away any developing structure.
The primary movers of this sediment are the hydrodynamic forces of the ocean, including waves, tides, and currents. Waves, especially those generated by storms, provide the initial energy to lift and suspend the sediment from the seafloor. Tides and persistent ocean currents then transport this suspended material over long distances. The combination of these forces dictates where the sediment is picked up and deposited to build a bar structure.
Formation of Offshore Bars (Submerged)
Offshore bars are submerged ridges running parallel to the coast, created primarily by the dynamics of breaking waves. As waves travel into shallow water, they begin to “feel” the seabed, causing them to slow down and steepen. The powerful zone where waves finally break, known as the surf zone, is where the main action of bar formation occurs.
When a wave plunges or spills, its energy scours the seabed, lifting sediment into the water column. The wave’s forward momentum pushes some material toward the shore, but a significant portion is pulled back seaward by the returning water, known as the undertow or backwash. This seaward-moving water deposits the sediment just beyond the breaking point, where the wave energy is dissipated. The repeated action of this scouring and redeposition builds a mound of sand that grows into a submerged bar crest.
The submerged bars are not continuous and often have breaks or channels running through them. These channels are typically scoured deeper by focused currents, such as rip currents, which flow rapidly seaward. The presence of a sand bar causes waves to break earlier, forcing water piled up near the shore to return to the ocean through these low points. These offshore ridges directly influence coastal water circulation and present a hazard to swimmers.
Formation of Coastal Bars and Spits (Emergent)
Coastal bars and spits are distinct from offshore bars because they are emergent features, meaning they rise above the water surface. Their formation is driven by lateral sediment movement, known as longshore drift. Longshore drift occurs when waves approach the shoreline at an oblique angle. The swash, or water rushing up the beach, moves sediment diagonally, while the backwash pulls it straight back down the slope.
This zigzag motion effectively transports quantities of sediment along the coastline. A spit begins to form when this longshore current reaches an abrupt change in the coastline, such as a headland or a bay mouth. At this point, the current spreads out and loses velocity, causing the carried sediment to be deposited into the open water. The resulting feature is a narrow tongue of sand or shingle extending outward from the shore.
If a spit continues to grow across the mouth of a bay, it can connect two headlands, forming a coastal bar, sometimes called a bay-mouth bar. This structure effectively separates the bay from the open ocean, creating a sheltered lagoon behind the new landform. If a spit connects the mainland to a nearby island, the feature is termed a tombolo, creating a sand bridge where none existed before. These emergent features are constantly reshaped by shifting wave directions and storm events, making them highly dynamic structures.