Longshore drift
Sediment transport parallel to the coast by wave-driven currents.
Longshore drift is a geological process that transports sediments such as clay, silt, pebbles, sand, shingle, and shells along a coast parallel to the shoreline. It is driven by the longshore current, which is generated by oblique incoming wind squeezing water along the coast, and occurs within the surf zone. The process is also known as littoral drift and includes beach drift, where swash and backwash move beach sand in a sawtooth fashion downbeach.
Quick Facts
- Alternative name
- littoral drift
- Sediments transported
- clay
- silt
- pebbles
- sand
- shingle
- shells
- Example spit
- New Brighton spit
- Canterbury
- New Zealand
- Example barrier/spit system
- Kaitorete Spit
- Canterbury
- New Zealand
- Age of kaitorete spit
- last 8,000 years
Facts from the source article.
Development of longshore drift theories
Early observations of sediment displacement along coasts were made by fishermen, sailors, and locals, who noted that sand and gravel appeared to move down beaches without understanding the mechanics. Systematic investigation began in the mid-1800s, when scientists sought to explain sediment movement. Among the first to propose theories were French engineer Jean-Baptiste Fourier and Irish geologist Robert Mallet, who studied wave action and sediment transport, though the term "longshore drift" was not yet used. Their work focused on understanding waves and their role in resuspending and moving sand and pebbles, helping to explain coastal morphological features. The complete significance of these mechanisms was not fully realized at that time.
Overview
Numerous calculations consider factors that produce longshore drift, including formulas such as those by Bijker (1967, 1971), Engelund and Hansen (1967), Ackers and White (1973), Bailard and Inman (1981), Van Rijn (1984), and Watanabe (1992). Common factors in these formulas are suspended and bed load transport, waves (breaking and non-breaking), and shear exerted by waves or associated flow. Longshore drift plays a large role in shoreline evolution; changes in sediment supply, wind direction, or other coastal influences can alter drift dramatically, affecting beach system formation and profile. Such changes can result from geological alterations (e.g., erosion, backshore changes, emergence of headlands), changes in hydrodynamic forces (e.g., wave diffraction in headland and offshore bank environments), new tidal inlets and deltas, alterations of the sediment budget (e.g., switch from drift to swash alignment, exhaustion of sediment sources), and human intervention (e.g., cliff protection, groynes, detached breakwaters). The sediment budget accounts for sources and sinks such as rivers, lagoons, eroding land sources, artificial sources like nourishment, artificial sinks like mining, offshore transport, deposition on shore, and gullies. Inlet ebb-tidal shoals store sand transported by longshore transport and may transfer sand to other beach systems.
Natural features
Spits form when longshore drift travels past a point where the dominant drift direction and shoreline do not veer in the same direction, such as a river mouth or re-entrant. They are affected by the strength of wave-driven current, wave angle, and wave height. A spit has a proximal end, constantly attached to land, which may form a barrier between the sea and an estuary or lagoon, and a distal end, detached from land, which may take a complex hook-shape due to varying wave directions. An example is the New Brighton spit in Canterbury, New Zealand, created by longshore drift from the Waimakariri River; it is currently in equilibrium but undergoes alternate phases of deposition and erosion. Barrier systems are attached to land at both ends and are generally widest at the down-drift end, enclosing an estuary or lagoon, such as Lake Ellesmere / Te Waihora enclosed by the Kaitorete Spit, or hapua at river-coast interfaces like the mouth of the Rakaia River. The Kaitorete Spit in Canterbury, New Zealand, has existed below Banks Peninsula for 8,000 years, undergoing changes due to avulsion of the Waimakariri River, erosion, and phases of open marine conditions; around 500 years Before Present, longshore drift from its eastern end created the barrier, retained by ongoing transport.
Human influences
Groynes are shore protection structures placed at equal intervals along the coastline to stop coastal erosion, typically crossing the intertidal zone. They are used on shores with low net and high annual longshore drift to retain sediments lost in storm surges. Common groyne designs include zig-zag groynes, which dissipate destructive flows in wave-induced currents or breaking waves; T-head groynes, which reduce wave height through wave diffraction; and ‘Y’ head, a fish-tail groyne system. Artificial headlands are created to protect beaches or bays, involving accretion of sediments on the up-drift side and moderate erosion on the down-drift end, aiming for a stabilised system that allows material to accumulate in beaches further along the shore. Detached breakwaters are shore protection structures built to accumulate sandy material to accommodate drawdown in storm conditions; they have no connection to the shoreline, allowing currents and sediment to pass between the breakwater and shore, forming a region of reduced wave energy that encourages sand deposition on the lee side. They are generally used similarly to groynes to build up material between the coast and the structure.
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