earth science

Longshore drift

Longshore drift is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Longshore drift rather than just read about it. In short: 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.

Key takeaways

Longshore drift belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Longshore drift to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Longshore drift from memory before moving on to harder problems.

Reference excerpt

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.

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 an Irish 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 (1967, 1971) The Engelund and Hansen formula (1967) The Ackers and White formula (1973) The Bailard and Inman formula (1981) The Van Rijn formula (1984) The Watanabe formula (1992) 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:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Longshore drift

Start with the simplest possible case. Write down what Longshore drift claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Longshore drift before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Longshore drift ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Longshore drift

In research: Longshore drift appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Longshore drift in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Longshore drift is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coastal erosion, Coastal geography, Oceanographical terminology, so understanding it makes those chapters shorter.
In everyday life: Look for Longshore drift outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Longshore drift in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Longshore drift means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Longshore drift out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Longshore drift in simple terms?

Why does Longshore drift matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Longshore drift?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Longshore drift.