Depositional Landforms

When waves lose their energy, they deposit the sediment they are carrying. Over time, the mass of sediment that settles creates depositional landforms. These landforms include beaches, spits, bars and sand dunes, each having its unique formations and features.

Beaches

Beaches are made up of eroded materials transported and deposited by the sea. Their formation is influenced by wave energy and the surrounding coastal geography.

  • Sandy beaches often occur in shallow areas like bays, where waves have less energy. They are made up of fine sand particles.
  • Pebble beaches form where cliffs are being eroded and where higher energy waves prevail. They consist of larger pebbles and stones.

Sandy beaches tend to have a gentle slope, while shingle or pebble beaches can be steeper. The smallest material is typically found nearest the water, where the waves break and erode the rocks through attrition.

Spits

Spits are elongated stretches of sand or shingle that project into the sea from the land. They form due to changes in the coastal shape or the presence of river mouths.

1. Sediment is carried along the coastline by a process called longshore drift.

2. When there is a change in the shape of the coastline, deposition occurs, resulting in the formation of a long, narrow ridge known as a spit.

3. If there is a change in wind direction, a hooked end can develop on the spit.

Spits act as natural barriers, preventing waves from passing through and creating sheltered areas behind them. In these sheltered spots, silt is deposited, forming salt marshes or mud flats.

Sometimes, a spit can grow across a bay and connect two headlands, forming a landform called a bar. Bars can create lagoons (essentially a shallow lake) behind them, which gradually fill with sediment over time.

Sand Dunes

Sand dunes are found along many coastal areas and vary in their form, structure and appearance.

They are formed by the following process:

1. There is first an abundant supply of sand. This sand can originate from various sources, including eroded rocks, rivers or offshore sediments carried onto the shore by waves.

2. Prevailing onshore winds carry sand particles landward.

3. As the wind encounters obstacles, such as vegetation or man-made structures, it slows down, losing its carrying capacity. The sand particles are then deposited, resulting in the accumulation of sand in certain areas.

4. The initial dune formation stage involves sand accumulation in small mounds known as embryo dunes. These dunes are usually low in height and lack a well-defined shape.

5. Over time, sand accumulates, and the dunes grow in height and size. The shape of the dunes is influenced by wind direction, with prevailing winds moulding them into distinct forms.

6. As sand accumulates, hardy pioneer plant species like beach grass begin to colonise the dune. These plants help stabilise the sand by trapping it with their roots, forming a network that prevents erosion and aids in dune growth.

Types of sand dunes

Sand dunes have different shapes and you’ll be expected to explain how they form. Remember, that it’s down to the wind and the way in which the sand is affected by it.

Here are three main types of sand dunes and how they are formed:

  • Crescentic or Barchan Dunes: These dunes have a characteristic crescent or horseshoe shape, with the horns pointing downwind. They are often found in areas with limited sand supply and strong winds that consistently blow in one direction. With continuous wind in a single direction, the crescent can grow to exceptionally long lengths.
  • Linear or Seif Dunes: Seif dunes are long and linear, resembling elongated ridges. They form parallel to the wind direction and are typically found in areas with abundant sand and unidirectional winds.
  • Star or Star-shaped Dunes: Star dunes are complex formations with multiple arms radiating in different directions. They usually occur in areas where wind patterns change frequently or where winds blow from various directions.

The movement of sand dunes

Sand dunes are not static features but are constantly in motion. They migrate and evolve as a result of wind patterns and changing environmental conditions.

  • As wind patterns shift, sand dunes migrate, slowly advancing or retreating over time. This movement occurs as sand particles are transported up the windward side of the dune and deposited on the leeward side.
  • Vegetation plays an important role in stabilising dunes and preventing excessive erosion. The root systems of plants bind the sand together, reducing wind erosion and promoting dune stability.
  • Over time, as sand dunes stabilise, more complex plant communities establish themselves. This succession leads to the formation of diverse dune ecosystems that support a variety of plants and animals, specially adapted to thrive in this unique habitat.