A coast is the interface where terrestrial and marine environments meet and interact. Geographically, a coast is understood as a transitional zone that extends both inland and offshore from the water’s edge. This area is sculpted by the complex interplay of atmospheric, oceanic, and geological forces acting over varying timescales. The coastal zone is a region of continuous energy exchange, where the land is constantly being reshaped by the ocean.
Defining the Coastal Zone
The coastal zone is a broad area subdivided into specific parts based on water level and the influence of wave action. The littoral zone, the area directly affected by marine processes, contains three primary segments transitioning from the sea toward the land. The nearshore zone lies seaward of the high-tide line, extending to the point where waves begin to break and friction with the seabed starts to distort wave shape.
The foreshore, also known as the intertidal zone, is the segment of the shore periodically exposed and covered by the rise and fall of the tides. This area stretches from the low-tide mark to the high-tide mark, enduring daily fluctuations in water coverage. The strandline is a specific feature within this zone, marking the highest reach of the tide or a storm surge where debris is deposited.
The backshore zone is the landward part of the coast that remains dry under normal conditions, situated above the mean high-tide level. This area is only affected by wave action during extreme events, such as severe storms or unusually high spring tides. Further seaward, the offshore zone extends into deeper water where waves no longer interact significantly with the seabed. The width of the entire coastal zone is highly variable, ranging from a few meters to several kilometers, depending on local topography.
Classification of Coastlines
Geographers classify coastlines using systems that reflect their dominant formative process or geological history. One primary method separates coasts based on the balance between erosion and sediment supply. Erosional coasts are characterized by material removal and feature rugged, high-relief landscapes such as steep sea cliffs and rocky platforms. These coasts develop where wave energy is high and the geology is resistant, often resulting in features like sea stacks and arches.
In contrast, depositional coasts are defined by a net accumulation of sediment, leading to the formation of low-relief features in areas of lower wave energy or abundant sediment supply. Examples include expansive sandy beaches, barrier islands, and marshy coastal wetlands. These classifications characterize the dominant physical state and expected landforms of a particular shoreline.
A second major classification system is based on relative sea-level change, influenced by global sea level (eustatic) and local tectonic movements. Emergent coasts are those where the land has risen relative to the sea, or sea level has dropped. This process often exposes former seabed features, resulting in elevated marine terraces and wave-cut platforms above the current high-tide mark.
Conversely, submergent coasts occur where the land is sinking or sea level is rising, causing the sea to flood pre-existing terrestrial features. A classic example is a fjord, a U-shaped glacial valley drowned by the rising sea. Estuaries, which are river valleys flooded by the sea, also represent a common feature of submergent coastlines.
Processes Shaping Coastal Landforms
The specific shape and character of any coast are determined by the mechanisms that transfer energy from the ocean to the land. Wave action is the most significant force; wave energy concentrates as it moves into shallow water and interacts with the seabed. This concentration causes the wave to slow down, increase in height, and eventually break, releasing its stored energy onto the shore.
Erosion is accomplished through several mechanical processes. Hydraulic action involves the force of water and compressed air within rock crevices breaking up the material. Abrasion occurs when waves use suspended sediment, such as sand and pebbles, to grind and wear away rock surfaces.
The process of wave refraction, where waves bend as they approach an irregular shoreline, focuses wave energy on headlands and diffuses it in bays. This action tends to straighten the overall configuration of the coast over time.
Tidal effects also contribute significantly to coastal morphology, particularly in areas with a large tidal range. Tidal currents transport vast quantities of sediment in and out of bays and estuaries, shaping the local depositional environment. The vertical rise and fall of the tide distributes the zone of wave action across the foreshore, ensuring erosion and deposition are not confined to a single narrow band.
Sediment transport, notably longshore drift, governs the movement of sand and gravel parallel to the shoreline. When waves approach the beach at an angle, the swash carries sediment obliquely up the beach face, but the backwash pulls it straight back down. This zigzag movement results in a net transport of material along the coast, which dictates the sediment budget and the stability of beaches and depositional features like spits and barrier islands.