Longshore drift

Longshore drift from longshore current is a geological process that consists of the transportation of sediments 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 at an oblique angle and gravity then drains the water straight downslope perpendicular to the shoreline. Thus beach sand can move downbeach in a sawtooth fashion many tens of meters 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. 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.

Development of longshore drift theories

The concept of longshore drift or transportation of sediment parallel to the shore by wave action has evolved considerably with time. Early observations related to sediment displacement can be traced back to coastal communities, but the formal scientific understanding of this started crystallising in the 19th and early 20th centuries. While such early perceptions were imprecise, this evolution has encouraged a gradually more sophisticated understanding of the processes occurring at coastlines. Understanding of the coastline processes has continued to evolve through a succession of developments that began many years ago.

Early observations

Erosion of coasts and sediment transport was known in ancient times, mostly in those parts of the world where dramatic changes of shores take place. However, these early observations were largely anecdotal. Fishermen, sailors and locals would note that sand and gravel seemingly "moved" down the beaches; they didn't fully understand the mechanics, however. Because of the general scientific knowledge, this was an interesting but somewhat misunderstood phenomenon.

19th century: first scientific studies

The systematic investigation into the coast processes, including those responsible for longshore drift, began in the mid-1800s when scientists tried to explain the processes of sediment movement along coasts. Among the first of such theories were those proposed by a French engineer, Jean-Baptiste Fourier, and a British geologist, Robert Mallet. They studied wave action and sediment transport; however, at that time, the term "longshore drift" was not yet coined. Instead, the principal focus was to understand the processes of waves and their impact on the resuspension and movement of sand and pebbles. The subject was of primary importance because it helped to explain the morphological features of any coast. However, while much is covered, the complete significance of such mechanisms was yet to be fully realised.

20th century: longshore drift defined

In the early years of the 20th century, longshore drift became much more refined in its explanation through oceanographers and coastal engineers. They realised that the angle of wave approach to the coast is of paramount importance to sediment transport. This then led to the development in the concept of "longshore currents," which in turn transport sediment along the coast. These currents then became recognised as the main agent of longshore drift. An important concept which emerged during this generation was that of the "drift-aligned" beach. It explained how beaches get to form as a result of prevailing wind and wave directions and that on one side of the beach deposition takes place, while on the other side, erosion does. While the mechanics were becoming more apparent, the interrelationship of the forces in play still proved quite problematic for those trying to manage coasts.

Overview

Longshore drift formulas

Numerous calculations take into consideration the factors that produce longshore drift.
These formulations are:

Bijker formula
The Engelund and Hansen formula
The Ackers and White formula
The Bailard and Inman formula
The Van Rijn formula
The Watanabe formula

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.
Features of shoreline change

Longshore drift plays a large role in the evolution of a shoreline, as if there is a slight change of sediment supply, wind direction, or any other coastal influence longshore drift can change dramatically, affecting the formation and evolution of a beach system or profile. These changes do not occur due to one factor within the coastal system, in fact there are numerous alterations that can occur within the coastal system that may affect the distribution and impact of longshore drift.
Some of these are:

Geological changes, e.g. erosion, backshore changes and emergence of headlands.
Change in hydrodynamic forces, e.g. change in wave diffraction in headland and offshore bank environments.
Change to hydrodynamic influences, e.g. the influence of new tidal inlets and deltas on drift.
Alterations of the sediment budget, e.g. switch of shorelines from drift to swash alignment, exhaustion of sediment sources.
The intervention of humans, e.g. cliff protection, groynes, detached breakwaters.
The sediment budget

The sediment budget takes into consideration sediment sources and sinks within a system. 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 systems provide good sources and sinks for the sediment budget.
Sediment deposition throughout a shoreline profile conforms to the null point hypothesis; where gravitational and hydraulic forces determine the settling velocity of grains in a seaward fining sediment distribution. Long shore occurs in a 90 to 80 degree backwash so it would be presented as a right angle with the wave line.

Natural features

This section consists of features of longshore drift that occur on a coast where long-shore drift occurs uninterrupted by man-made structures.

Spits

s are formed when longshore drift travels past a point where the dominant drift direction and shoreline do not veer in the same direction. 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. The proximal end is constantly attached to land and may form a slight “barrier” between the sea and an estuary or lagoon. 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. This spit system is currently in equilibrium but undergoes alternate phases of deposition and erosion.

Barriers

Barrier systems are attached to the land at both the proximal and distal ends and are generally widest at the down-drift end. 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 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, 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

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. Tidal inlets can act as sinks and sources for large amounts of material, which therefore impacts on adjacent parts of the coastline.
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. This may also depend on the inlet size, delta morphology, sediment rate and by-passing mechanism. Channel location variance and amount may also influence the impact of longshore drift on a tidal inlet.
Arcachon lagoon in southwest France is an example of a tidal inlet system, which provides large sources and sinks for longshore drift sediments. The impact of longshore drift sediments on this inlet system is highly influenced by the variation in the number of lagoon entrances and the location of these entrances. Any change in these factors can cause severe down-drift erosion or down-drift accretion of large swash bars.