Depositional Coastal Landforms: Beaches, Spits, and Dunes

Depositional coastal landforms are features built from sediment that waves, currents and wind have dropped along the shore, including beaches, spits, bars, tombolos, salt marshes and sand dunes. In brief: deposition happens where energy falls, for example in sheltered bays, at river mouths or where the coast changes direction. A beach is an accumulation of sand or shingle between the low and high tide lines; a spit is a long ridge of sediment attached to the land at one end; and dunes are mounds of wind-blown sand held in place by plants such as marram grass.

These landforms protect coasts from waves, provide habitats and attract tourists, but they are also dynamic and can change within a single storm. This guide explains where and why sediment is deposited, describes each landform, compares them in a table, works through calculations of beach slope and dune sequence, outlines how they have been studied and corrects common mistakes.

When do waves deposit sediment?

Waves deposit sediment when their energy is low or when they carry more sediment than they can move. This occurs in sheltered bays, in calm weather with constructive waves, where the coastline changes direction, at the mouth of rivers where the flow slows down and where the supply of sediment from cliffs, rivers or the sea floor is large. Sediment is moved along the coast by longshore drift and deposited where it stops.

How do beaches form?

A beach is a gently sloping area of sand, shingle (pebbles) or both. It forms between the low-tide line and the highest point reached by storm waves. Sandy beaches have gentle gradients, because fine sediment is carried up and laid down by constructive waves, whereas shingle beaches are steeper, as water drains quickly through the large gaps between pebbles and the backwash is weak. A beach often has ridges called berms, which mark the limit of high tides or storms. The sediment on a beach is often sorted by size: larger pebbles lie higher up the beach and finer material lower down, and shape and size are used to study the wave conditions.

How do spits, bars and tombolos form?

A spit forms where the coast changes direction suddenly, such as at a river mouth or a bend. Longshore drift keeps carrying sediment along, and it is deposited in the sheltered, deeper water beyond the bend, so a ridge of sand or shingle grows out into the water. Strong winds and waves from another direction can curve the end, creating a recurved end or hook. Behind the spit, calm water allows the formation of a salt marsh. Spurn Head on the Humber estuary in England and Hurst Castle Spit in Hampshire are well-known examples, and Spurn Head has been breached by storm surges more than once.

A bar forms when a spit grows across a bay and closes it off, trapping a lagoon behind it. A tombolo is a bar that joins an island to the mainland or to another island, as at Chesil Beach, which links the Isle of Portland to the Dorset coast. A barrier island is a long, narrow sand island parallel to the coast, common along the Gulf and Atlantic coasts of the United States. A cuspate foreland, such as Dungeness in Kent, is a triangular projection of shingle ridges.

How do sand dunes form?

Dunes require a large supply of dry sand, onshore winds and an obstacle that slows the wind. Wind moves sand grains by saltation. When it meets an obstacle, such as seaweed or a clump of grass, the wind slows and drops the sand. Pioneer plants then trap more sand, and the dune grows. Dunes develop in a sequence, called a succession, as you move inland.

1. Embryo dunes: small mounds near the high tide line, colonized by salt-tolerant plants such as sea couch grass.
2. Fore dunes (yellow dunes): taller dunes with marram grass, whose long roots and rhizomes bind the sand. The sand is still light yellow.
3. Grey dunes: older dunes where decaying plants add humus, giving a grey color, and more plant species can grow.
4. Dune slack and heath or woodland: damp hollows between dunes and, finally, stable ground with shrubs and trees.

Dunes act as natural sea defenses and as stores of sand. When vegetation is destroyed by trampling, a blowout can develop and the dune can erode.

What are salt marshes?

Salt marshes form in sheltered places, such as behind spits or in estuaries, where fine silt and clay settle in calm water. Plants that tolerate salt, such as glasswort and cordgrass, trap the sediment, and the surface builds up above the tide level over time. Marshes absorb wave energy, provide homes for birds and fish and store carbon, and they are increasingly used in managed realignment schemes.

Depositional landforms compared

Worked example: slope, growth and succession

1. Beach slope: a beach profile drops 3 m over a horizontal distance of 60 m. The gradient is 3 ÷ 60 = 0.05, or about 2.9 degrees. A steep shingle beach that drops 3 m over 15 m has a gradient of 0.2, or about 11.3 degrees.
2. Sediment budget: a beach receives 25,000 m³ of sand per year from drift and loses 20,000 m³ to the sea. The net gain is 25,000 − 20,000 = 5,000 m³. Spread over a beach 1,000 m long and 50 m wide (50,000 m²), this raises the surface by 5,000 ÷ 50,000 = 0.1 m, or 10 cm, per year.
3. Spit growth: if a spit extends by 15 m a year, it grows 15 × 60 = 900 m in 60 years, as long as the sediment supply continues.
4. Dune sequence: if dunes advance seaward by about 1 m a year and the embryo dunes are at the shore, a dune ridge 100 m inland is roughly 100 years old (a simplified estimate, since rates vary).
5. Sand transport: a wind that moves 50 g of sand per meter of width per second would move 50 × 3,600 = 180,000 g = 180 kg per meter of width per hour (an illustrative value for strong wind, since transport depends on wind speed cubed).

How have these landforms been studied?

Geomorphologists have studied beaches and dunes since the 19th century. The British scientist Ralph Bagnold published his study The Physics of Blown Sand and Desert Dunes in 1941, explaining how wind moves sand and how dunes form. Later work on beach profiles and sediment cells has been used to manage coasts. Beach nourishment, in which sand is pumped onto beaches, started in the 20th century and is now widespread. Satellites and drones are used for monitoring changes in spits, dunes and barrier islands, which respond quickly to storms and sea-level rise.

Which mistakes are common?

Mistake 1: Assuming that beaches are permanent

Beaches change with seasons, storms and sediment supply, and some are shrinking because supply has been cut by dams or defenses.

Mistake 2: Confusing a spit with a bar

A spit is attached to land at one end only. A bar extends across a bay and traps a lagoon.

Mistake 3: Thinking that dunes are just piles of loose sand

Plants such as marram grass bind and build dunes. Without vegetation, dunes move or erode quickly.

Mistake 4: Treating a salt marsh as wasteland

Salt marshes reduce wave energy, support wildlife and store carbon, which makes them valuable for flood protection.