A bar in physical geography is a depositional landform, defined as a long, narrow ridge of sand, gravel, or other sediment built up within a body of water. This submerged or partially exposed feature is created by the action of waves, currents, or tides in oceans, lakes, or rivers. The composition typically reflects the available material, often sand, but it can also be shingle, silt, or gravel. Bars are dynamic features that demonstrate how the energy of moving water shapes coastlines by depositing material where wave or current energy decreases.
Formation and Characteristics
The creation of a bar begins with the movement of sediment, a process heavily influenced by wave action and currents. When waves approach the shore, they stir up material from the seabed and transport it. This sediment is moved through a process called longshore drift, where waves approach the shore at an angle, carrying material up the beach, and the backwash pulls it straight back down, resulting in a zig-zag movement along the coastline.
Deposition occurs when the transporting energy of the water decreases, causing the sediment load to be dropped. This often happens in the nearshore zone where waves begin to break or where a coastline changes direction, slowing the current. The material accumulates to form a ridge that runs generally parallel to the coast. For instance, during strong storm events, high-energy waves erode sand from the beach and deposit it just offshore, building a temporary bar.
Bars are generally categorized based on their position relative to the water level. Offshore bars, also known as longshore bars, are submerged ridges that lie parallel to the coast. These features are highly mobile, shifting their position seaward during strong, destructive wave conditions, and then migrating back toward the shore during calmer periods of constructive waves. The material composing a bar is sorted by the water’s energy, with the size of the particles relating to the strength of the currents that moved them. A breakpoint bar forms precisely where incoming waves consistently break, impacting how waves transform before reaching the shore.
Different Coastal Bar Configurations
The term “bar” applies to several distinct coastal landforms, defined by their final shape and location. These configurations represent different stages of sediment accumulation, primarily driven by longshore drift depositing material in low-energy environments.
A spit is a bar attached to the land at one end that extends out into the open water, typically where the coastline abruptly changes direction. Longshore drift carries sediment past the corner and deposits it in the relatively sheltered water. The end of a spit may curve inland toward the shore due to wave refraction or secondary currents, forming a recurved end.
A baymouth bar forms when a spit grows across the entire entrance of a bay, connecting two headlands. This effectively seals off the bay from the open ocean, creating a calm, sheltered lagoon behind it. Complete sealing is rare, as freshwater inflow or tidal exchange often maintains a small inlet channel.
Barrier islands are a distinct type of bar configuration—long, narrow islands of sand that run parallel to the mainland coast, separated by a lagoon or marsh. These landforms are larger and more stable than typical offshore bars, evolving over centuries in areas with abundant sediment and gently sloping coasts. They act as a dynamic buffer for the mainland, notably along the U.S. Gulf and Atlantic coasts.
Significance in the Coastal Environment
Coastal bars play a significant role in the natural dynamics of the shoreline. They function as natural barriers, absorbing the energy of incoming waves and storm surges before they reach the mainland. This wave attenuation helps protect the coast from erosion and reduces the impact of storms on infrastructure and inland ecosystems.
The landward side of exposed bars, such as spits and barrier islands, becomes a sheltered, low-energy environment. This shelter facilitates the development of unique and productive ecological habitats, including lagoons, tidal flats, and salt marshes. These wetlands are biologically rich, supporting diverse wildlife, specialized plants, and numerous species of birds and fish.
Bars have important implications for both navigation and coastal engineering. Submerged offshore bars present a hazard to maritime traffic by creating shallow shoals where ships can run aground. In coastal defense, artificial bars or reefs are sometimes installed to decrease wave loads on the shore and reduce erosion by causing waves to break farther out at sea.