Coastal Dynamics: The Geomorphology of Spits
An in-depth exploration of these dynamic coastal landforms, from their formation by longshore drift to their ecological and human significance.
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Defining a Spit
A Dynamic Coastal Feature
A spit, often referred to as a sandspit, represents a distinctive depositional landform found along coasts and lake shores. Fundamentally, it is a bar or beach composed of sediment that extends from a headland or a point where the coastline abruptly changes direction, projecting into a body of water. This dynamic feature is a direct result of the continuous processes of longshore drift and longshore currents.
The Role of Longshore Drift
The primary mechanism behind spit formation is longshore drift. This process occurs when waves approach a beach at an oblique angle, causing sediment (sand, gravel, pebbles) to be transported along the shoreline in a characteristic zigzag pattern. Concurrently, longshore currents, generated by the same wave action, further facilitate the movement of this sediment through the water parallel to the coast. It is the combined action of these forces that drives the accumulation of material to form a spit.[1][2][3]
Hydrology & Formation
Sediment Deposition Dynamics
Spits typically begin to form where the coastline experiences a significant re-entrance, such as at a headland or the entrance to a cove. At these points, the longshore current, which carries a substantial load of sediment, tends to spread out and dissipate. As the current loses energy, its capacity to transport the full sediment load diminishes, leading to the deposition of much of this material. Initially, this forms a submerged bar of sediment.
Building Above the Surface
For this submerged bar to evolve into an above-water spit, the complementary process of littoral drift is essential. Littoral drift continues to transport sediment in the direction of wave breaking, gradually building the bar upwards until it emerges above the water's surface. Without this continuous supply and transport, the submerged bar would simply be leveled off underwater rather than developing into a prominent landform.[1]
Critical Angle and Growth Limits
Spits are particularly prone to forming when longshore drift encounters a section of headland where the change in coastal direction exceeds approximately 30 degrees. The spit will continue to extend into the sea or lake until the opposing water pressure, often from a river outflow or strong tidal currents, becomes too significant to allow further sand deposition. Once formed, vegetation may colonize the spit, contributing to its stabilization and often enhancing its fertility.
Recurved and Complex Spits
Wave refraction can play a crucial role in shaping the distal (seaward) end of a spit. As waves bend around the end of the landform, they can carry sediment to form a distinctive hook or recurved shape. In situations where wave refraction occurs from multiple directions, the result can be a more intricate, complex spit. However, waves approaching the spit from directions other than obliquely along its length can impede its growth, cause it to shorten, or even lead to its complete destruction.[4]
Evolution & Related Forms
Sediment Sources and Vulnerability
The sediments that constitute spits originate from various sources, including material carried by rivers and eroded from coastal bluffs. Changes in these source areas can have profound effects on the stability and morphology of spits and other coastal landforms. For instance, upstream activities such as logging and farming can increase the sediment load in rivers, potentially harming intertidal environments around spits by smothering delicate habitats. Conversely, the construction of roads or bulkheads along bluffs can drastically reduce the volume of eroded sediment, leading to an insufficient supply of material to maintain the spit's integrity.
From Spit to Island or Bar
The long-term evolution of a spit depends heavily on the continuity of its sediment supply and the prevailing marine conditions. If the supply of sediment is interrupted, the sand at the proximal (landward) end of the spit may be gradually transported towards its distal end, eventually leading to the formation of an isolated island. Alternatively, if the sediment supply remains uninterrupted and the spit is not breached by the sea (or a river, in the case of an estuary), it can continue to grow until both ends connect to the mainland, forming a coastal bar. This process often creates a sheltered lagoon behind the newly formed bar.
The Tombolo Connection
A specialized form of spit evolution occurs when an offshore island is situated near a point where the coast changes direction. If a spit continues to grow and extends sufficiently to connect this island to the mainland, the resulting landform is known as a tombolo. This demonstrates the dynamic interplay between sediment transport, coastal morphology, and the presence of existing landmasses.
Global Examples
The World's Longest Spits
The title of the world's longest spit is often debated, with several contenders reaching impressive lengths. Both the Arabat Spit in the Sea of Azov and the Younghusband Peninsula in South Australia are approximately 110 kilometers (68 miles) long.[5][6][7] In North Carolina, the Bodie Island may also qualify as one of the longest, measuring approximately 70 miles (110 km) to its terminus at the Oregon Inlet, a length achieved after the natural closure of the Roanoke inlet in 1811.[8]
Freshwater and Notable Formations
Not all significant spits are found in marine environments. Long Point, Ontario, stands as the longest spit in a freshwater body, extending approximately 32 kilometers (20 miles) into Lake Erie. In New Zealand, Farewell Spit, also 32 kilometers long, is located in the north-west of the South Island. Its formation is attributed to strong prevailing winds and currents that transport sand eroded from the Southern Alps, depositing it into Golden Bay.
European and Adriatic Wonders
Europe hosts several prominent spits. Spurn Point at the Humber in the UK is approximately 4.8 kilometers (3.0 miles) long, while Chesil Beach connects the Isle of Portland to the mainland. The Curonian Spit, a remarkable 98-kilometer (61-mile) landform, is divided between Lithuania and Russia's Kaliningrad Oblast, separating the Curonian Lagoon from the Baltic Sea. Similarly, the Vistula Spit in Poland separates the Vistula Lagoon from Gdaลsk Bay. Further south, Zlatni Rat, a distinctive pebble beach on the Croatian island of Braฤ, dynamically bends slightly west or east in response to Adriatic currents and weather conditions.
Human Interaction
Historical Settlement Patterns
Throughout prehistory, certain spit formations have been strategically chosen by human populations as sites for habitation. These locations were often favored due to their advantageous proximity to marine resources, facilitating the exploitation of fish, shellfish, and other coastal provisions. A notable example is the prehistorical settlement of the Chumash Native American people on Morro Bay, which leveraged the unique geographical advantages offered by the spit formation for sustenance and community development.[9]
Managing Coastal Change
The dynamic nature of spits means they are constantly evolving, influenced by natural processes and, increasingly, by human activities. Understanding the sediment budget and the forces of longshore drift is critical for coastal management. Human interventions, such as dredging, construction of coastal defenses, or alterations to river systems, can significantly impact the delicate balance of sediment supply and transport, potentially leading to erosion or accretion that affects both natural ecosystems and human infrastructure on or near these landforms.
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References
References
- Duane, D.B. and James, W.R., 1980, "Littoral transport in the surf zone elucidated by an Eulerian sediment tracer experiment:" Journal of Sedimentary Petrology. vol. 50, p. 929-942
- Evans, O.F. 1942, "The origin of spits, bars and related structures:" Journal of Geology, v. 50, p. 846-863
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Important Notice
This page was generated by an Artificial Intelligence and is intended for informational and educational purposes only. The content is based on a snapshot of publicly available data from Wikipedia and may not be entirely accurate, complete, or up-to-date.
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