Coastal Erosion and Transportation by Longshore Drift

Coastal erosion is the wearing away of the shoreline by waves through hydraulic action, abrasion, attrition and solution, and longshore drift is the zigzag movement of sediment along the coast, caused by waves that hit the beach at an angle. In brief: waves approach the beach at an angle set by the prevailing wind, so the swash carries sediment up the beach at that angle, while the backwash returns straight down the slope under gravity. Repeated many times, this moves sand and pebbles along the coast, and structures called groynes are built to slow it.

Erosion and transport determine where cliffs retreat and where beaches grow. This guide explains the four erosion processes, how sediment is carried, how longshore drift works, compares the processes in a table, works through calculations of retreat and drift, outlines the history of coastal management and corrects common mistakes.

How do waves erode the coast?

There are four main processes of erosion.

• Hydraulic action: waves crash against the cliff and force water and air into cracks. The compressed air and the pressure exerted when the water retreats widen the cracks and loosen rock.
• Abrasion (corrasion): waves pick up rocks, pebbles and sand and hurl them against the cliff, which scours and undercuts it, like sandpaper.
• Attrition: the rocks and pebbles carried by the water collide with each other and wear down. They become smaller, smoother and rounder.
• Solution (corrosion): slightly acidic seawater dissolves some rocks, mainly limestone and chalk.

Subaerial processes, such as weathering and mass movement on the cliff face, also contribute, because rain and frost weaken the rock and let it slump or fall, and the waves remove the debris.

What controls the rate of erosion?

The rate depends on wave energy (fetch, storms, destructive waves), the geology (resistant rocks such as granite erode slowly, soft clay and boulder clay erode quickly), the structure of the rock (joints and faults create weaknesses), the beach in front of the cliff, which absorbs wave energy, and human activity, such as defenses that reduce erosion in one place but may increase it elsewhere. Rapidly retreating coasts include the soft glacial till cliffs of Holderness in eastern England, where the cliff is generally reported to retreat by about 2 m per year on average.

How is sediment transported along the coast?

The same four types of transport that occur in rivers apply: traction (large pebbles rolled on the sea bed), saltation (small stones and sand bouncing), suspension (fine sediment carried in the water) and solution (dissolved minerals). The dominant movement along the shore is longshore drift.

How does longshore drift work?

Longshore drift happens in a series of steps:

1. The prevailing wind drives waves toward the coast at an oblique angle.
2. The swash carries sediment up the beach in the direction of the waves, at an angle.
3. The backwash flows straight back down the steepest slope, pulled by gravity, and carries the sediment with it.
4. The sediment is therefore shifted a short distance along the beach with each wave, in a zigzag path.
5. Over days and years, the material moves along the coast in one general direction, which is the direction of the dominant wave approach.

On the south coast of England, where waves from the southwest dominate, the drift is mostly from west to east. Where the drift is blocked or the supply of sediment ends, deposits accumulate or beaches are starved. Groynes, wooden or rock barriers built at right angles to the beach, trap sediment on the updrift side and widen the beach, but the downdrift side loses material and may erode faster. Sediment moving along the coast is divided into sediment cells, which are used for management.

Coastal processes compared

Worked example: retreat and drift

1. Cliff retreat: a cliff retreats by 2 m a year. In 50 years it loses 2 × 50 = 100 m.
2. Time until a house is lost: a house stands 30 m from the cliff edge. At 2 m per year, the edge reaches it after 30 ÷ 2 = 15 years.
3. Volume of rock lost: a 5 km (5,000 m) stretch of cliff 20 m high retreats by 2 m. The volume removed is 5,000 × 20 × 2 = 200,000 m³ per year. With a density of 2.0 tonnes per cubic meter for soft clay, this is about 400,000 tonnes.
4. Drift rate (hypothetical figures): a groyne traps 12,000 m³ of sand on its updrift side in a year, so the drift rate is about 12,000 m³ per year. If 4 groynes of similar size are built along the beach and each traps this amount, 4 × 12,000 = 48,000 m³ per year is held back, which is sand the downdrift beaches no longer receive.
5. Zigzag path: if each wave moves a pebble 1 m along the beach and 6 waves arrive per minute, the pebble travels 6 m per minute, or 360 m per hour, as long as it is within the swash zone. In practice, movement is much slower because pebbles are only moved by larger waves.

How have people responded to coastal change?

Communities have tried to protect land from the sea for centuries, from sea walls to timber groynes. In the 20th century, engineers built large concrete defenses, and later it became clear that blocking drift in one place can cause erosion farther along the coast. After severe storm surges, for example in 1953 on the North Sea coasts of the Netherlands and eastern England, defenses were upgraded. In the 1990s, England and Wales began to divide the coast into sediment cells and prepared Shoreline Management Plans, which choose between holding the line, managed realignment and other options for each stretch of coast.

Which mistakes are common?

Mistake 1: Thinking that backwash moves at an angle

The backwash flows straight down the slope. This is what creates the zigzag.

Mistake 2: Confusing erosion with transport

Erosion wears material away. Transport moves it. Both happen at the same time.

Mistake 3: Assuming that all coasts erode at the same rate

Soft rock, such as clay and sand, retreats much faster than hard rock, such as granite.

Mistake 4: Believing that a groyne stops erosion everywhere

It traps sediment on one side and may starve the beach on the other, shifting the problem.