thumb|Diagram demonstrating longshore drift:

Longshore drift from longshore current is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand, shingle, shells) along a coast parallel to the shoreline, which is dependent on the angle of incoming wave direction. Oblique incoming wind squeezes water along the coast, generating a water current that moves parallel to the coast. Longshore drift is simply the sediment moved by the longshore current. This current and sediment movement occurs within the surf zone. The process is also known as littoral drift.

Beach sand is also moved on such oblique wind days, due to the swash and backwash of water on the beach. Breaking surf sends water up the coast (swash) at an oblique angle and gravity then drains the water straight downslope (backwash) perpendicular to the shoreline. Thus beach sand can move downbeach in a sawtooth fashion many tens of meters (yards) per day. This process is called "beach drift", but some workers regard it as simply part of "longshore drift" because of the overall movement of sand parallel to the coast.

Longshore drift affects numerous sediment sizes as it works in slightly different ways depending on the sediment (e.g. the difference in long-shore drift of sediments from a sandy beach to that of sediments from a shingle beach). Sand is largely affected by the oscillatory force of breaking waves, the motion of sediment due to the impact of breaking waves and bed shear from long-shore current. Because shingle beaches are much steeper than sandy ones, plunging breakers are more likely to form, causing the majority of longshore transport to occur in the swash zone, due to a lack of an extended surf zone.

These formulas provide a different view of the processes that generate longshore drift. The most common factors taken into consideration in these formulas are:

  • Suspended and bed load transport
  • Waves, e.g., breaking and non-breaking
  • The shear exerted by waves or the flow associated with waves. This sediment can come from any source with examples of sources and sinks consisting of:
  • Rivers
  • Lagoons
  • Eroding land sources
  • Artificial sources e.g. nourishment
  • Artificial sinks e.g. mining/extraction
  • Offshore transport
  • Deposition of sediment on shore
  • Gullies through the land

This sediment then enters the coastal system and is transported by longshore drift. A good example of the sediment budget and longshore drift working together in the coastal system is inlet ebb-tidal shoals, which store sand that has been transported by long-shore transport. As well as storing sand these systems may also transfer or by pass sand into other beach systems, therefore inlet ebb-tidal (shoal) systems provide good sources and sinks for the sediment budget. As well as dominant drift direction, spits are affected by the strength of wave-driven current, wave angle and the height of incoming waves.

Spits are landforms that have two important features, with the first feature being the region at the up-drift end or proximal end (Hart et al., 2008). The proximal end is constantly attached to land (unless breached) and may form a slight “barrier” between the sea and an estuary or lagoon (called peresyp in the Russian tradition of geomorphology). The second important spit feature is the down-drift end or distal end, which is detached from land and in some cases, may take a complex hook-shape or curve, due to the influence of varying wave directions.

As an example, the New Brighton spit in Canterbury, New Zealand, was created by longshore drift of sediment from the Waimakariri River to the north. These barrier systems may enclose an estuary or lagoon system, like that of Lake Ellesmere / Te Waihora enclosed by the Kaitorete Spit or hapua which form at river-coast interface such as at the mouth of the Rakaia River.

The Kaitorete Spit in Canterbury, New Zealand, is a barrier/spit system (which generally falls under the definition of barrier, as both ends of the landform are attached to land, but has been named a spit) that has existed below Banks Peninsula for the last 8,000 years. This system has undergone numerous changes and fluctuations due to avulsion of the Waimakariri River (which now flows to the north of Banks Peninsula), erosion and phases of open marine conditions. The system underwent further changes  500 years Before Present, when longshore drift from the eastern end of the “spit” system created the barrier, which has been retained due to ongoing longshore transport.

Tidal inlets

left|thumb|[[Arcachon Bay in Southwest France.]]

The majority of tidal inlets on longshore drift shores accumulate sediment in flood and ebb shoals. Ebb-deltas may become stunted on highly exposed shores and in smaller spaces, whereas flood deltas are likely to increase in size when space is available in a bay or lagoon system.

The structuring of tidal inlets is also important for longshore drift: if an inlet is unstructured, sediment may by-pass the inlet and form bars at the down-drift part of the coast.

Over extensive periods, drifting sediment can 'leak' into deeper water, where the wind and waves driving longshore drift are weaker. This allows extensive sediment deposits to be built up off-shore, which is gradually transferred back to the coast as the sea level falls in long-term glacial cycles.