Coastal Transportation and Depositional Landforms

From moving sediment to building landforms

Coastal landscapes are not shaped only by material being removed. Waves and currents also move sediment and deposit it elsewhere, building beaches, spits and bars. Sediment includes loose sand, shingle and finer particles. It can reach the coast from eroding cliffs, rivers or neighbouring stretches of shoreline.

Transportation means moving this material; deposition means laying it down. These processes are connected: the sediment building a beach may have travelled along the coast before being deposited. When water loses energy, it becomes less able to carry sediment, so material is dropped. Sheltered water and increased friction with the seabed can encourage this loss of energy.

How longshore drift moves sediment along the coast

Longshore drift is the movement of sediment along a coastline. It happens when waves approach a beach at an angle, often because of the prevailing wind—the wind direction that occurs most frequently.

After a wave breaks, the swash carries water and sediment up the beach at an angle. Gravity then pulls the backwash straight down the beach slope, approximately at right angles to the shoreline. The next wave moves the sediment diagonally up the beach again.

Imagine following one pebble: it travels diagonally uphill, straight downhill, then diagonally uphill again. These repeated movements create a zigzag path with an overall movement along the coast. The direction depends on the angle from which the waves approach.

Longshore drift supplies sediment to places where depositional landforms can grow. It is a transportation process; deposition occurs when the transported material is dropped.

Constructive waves and beach building

A beach is an accumulation of deposited sand or shingle along the shoreline. Constructive waves tend to build beaches because their swash is stronger than their backwash. The swash carries sediment up the beach, while the weaker backwash removes less material. Repeated waves therefore leave a net gain of sediment.

Constructive waves are relatively low, with long wavelengths. Beach building is often associated with calmer conditions, including calmer periods during UK summers. A beach is a changing store of sediment rather than a permanent, unchanging feature: material can later be removed or moved alongshore.

Sandy beaches commonly develop in sheltered places such as bays and are usually wide and gently sloping. Shingle beaches contain pebbles and are typically narrower and steeper. Many beaches contain both sand and shingle, so these are broad patterns rather than two completely separate categories.

How a spit extends beyond the shoreline

A spit is a narrow ridge of sand or shingle attached to the coast at one end and extending out into the sea or across part of an estuary.

Its formation begins with longshore drift transporting sediment along the coast. Where the coastline changes direction, such as at the entrance to a bay or a river mouth, sediment can be carried beyond the existing shoreline. In shallow, sheltered water, energy decreases and sediment is deposited.

As more sediment arrives, the deposit builds up towards sea level and extends outwards. Continued sediment supply allows the spit to lengthen. At a river mouth, river flow can prevent sediment from accumulating right across the estuary, leaving the far end unattached.

Changes in wind and wave direction, or opposing currents, can curve the end into a hook. Not every spit has a hooked end. The water behind a spit is sheltered from stronger waves, allowing fine sediment to settle and mudflats or salt marshes to develop.

Spurn Point, at the Humber Estuary in north-east England, is a UK example of a spit. It illustrates how sediment transported along a coast can build a landform extending across part of an estuary.

When deposition forms a bar

A bar develops through similar transportation and deposition processes to a spit. If sediment continues to accumulate across a bay until the ridge reaches land on the other side, it joins the two sides of the bay. Water cut off behind the bar forms a lagoon.

The key distinction is the connection to land: a spit has one attached end, whereas a bar joins land at both ends. In both cases, longshore drift provides sediment and deposition builds the ridge. The shape of the coastline and the movement of water help determine which landform develops.