Sediment Transport

The role of currents, tides, and drifts in sediment transport

Let’s look at how currents, tides, and longshore drift affect sediment transport.

Currents

Coastal currents are water masses in motion. They are found between the coastline and the edge of the continental shelf. They have two components: alongshore (or parallel to the coast) and cross-shore (or perpendicular to the coast). The currents are driven by winds or initiated by differences in water density, temperature, or salinity. Rip currents on the beach transport sediment a few metres out to sea for a few hours when the wind blows directly onshore.

Tides

Tides are changes in the sea level produced by the gravitational pull of the moon and the sun. The incoming and ebbing tide can create tidal currents in the nearshore and offshore zones that transport sediment.

Longshore drift

Longshore drift, also known as littoral drift, moves material along the shore by wave action. It usually occurs in one direction (based on the wind), and the waves approach the beach at an angle. The swash carries material up and along the beach at an angle of around 45 degrees, while the backwash carries material down the beach at right angles (due to gravity). Over time, this creates a net shift of material along the coast. Longshore drift contributes to the formation of a range of depositional landforms, such as spits and onshore bars.

Fig. 1 - This diagram illustrates the process of longshore drift

Longshore drift and sediment cells

Longshore drift plays a critical role in coastal sediment cells and sediment transport. Coasts can be split into sections called sediment cells. For example, the coastline of England and Wales is divided into 11 primary sediment cells, with sub-cells within each primary cell. These cells occur along the coastline and nearshore area, where material movement is mainly self-contained. They are considered a closed coastal sub-system because their boundaries are formed by major headlands or large estuaries, which direct the movement of the sediment within the cell. The sediment is largely recycled within them rather than having significant new inputs or outputs.

Sediment cells are a dynamic system. This means that sediment is consistently generated in the source region, transported via the transfer region, and deposited in the sink region.

• Inputs are where sediment is generated (cliffs or eroded sand dunes). It can also come from offshore bars and river systems.
• Outputs are areas where deposition is dominant.
• Transfers are areas where longshore drift and offshore currents move sediment along the coast, assisted by drift-aligned beaches and parts of dunes and salt marshes.
• Although most sediment stays in the cell, changes in wind direction and movements of ocean currents affect some of the sediment under high-energy conditions. This will cause some sediment to move offshore into long-term ocean floor sediment stores.

Sediment transport and coastal deposition

During sediment transportation, waves sometimes lose energy, and when they do, sediment will drop out and begin to settle. This is known as coastal deposition.

Deposition is likely to occur when there is little or reduced wind, waves enter an area of shallow water, the swash is greater than the backwash, waves enter a sheltered area (a cove or bay), a river or estuary flowing into the sea reduces wave energy, there is a good supply of material, and the wave energy is unable to transport the amount of material within it.

Coastal landforms are created when (a) there are lots of constructive waves and (b) the accumulation of sand and shingle is greater than what is removed.

Landforms of coastal deposition

Let’s see what types of landforms created by coastal deposition exist.

Beaches

This is the most frequent temporary depositional feature along the coastline. Deposits of sand, shingle, pebbles, mud, and silt found in the foreshore and backshore form beaches. This happens because the swash is stronger than the backwash, so the material moves up the beach. Beaches protect the coast from erosion. Sandy beaches are created in sheltered bays, while waves throwing massive boulders and large pebbles up the beach during storms form storm beaches.

Bayhead beaches

Bayhead beaches form along swash-aligned coastlines where wave refraction spreads wave energy around the bay perimeter. Here the waves can break at 90 degrees to the shoreline, moving sediment into a bay and forming a bayhead beach.

Tombolos

Tombolos are linear ridges of sand and shingle connecting an offshore island to the mainland coastline. Wave refraction around the island creates an area of calmer water where deposition occurs between the island and the coast. Sometimes oppositional longshore currents are involved, in which case the depositional feature is similar to a spit.

Fig. 2 - St Ninian’s Isle tombolo, Shetland, Scotland

Spits

These are unstable, narrow, long stretches of sand exiting into the sea. They are joined to the mainland at one end. Spits form on drift-aligned coastlines where the coastline changes direction due to a bay or a river mouth. Where the spit turns, longshore drift moves material along the coastline. Wave refraction and currents cause the wave’s energy to disperse, which leads to deposition forming on the seabed. Over time, the amount of sediment deposited is exposed, extending the beach into the sea as a spit.

Three factors determine the length of a spit: secondary currents causing erosion, the flow of a river, and wave action.

Double spits

Double spits are where two spits extend in opposite directions from both sides of the bay towards the middle. Double spits are formed when longshore drift works in different directions on opposite sides of the bay. Rising sea levels, which drive ridges of material onshore from the offshore zone, can also form double spits. A barrier beach driven across a bay creates a bar (Haff coastlines), but a strong exiting river current may breach the bar to form a double spit.

Hooked/Recurved spits

Hooked/recurved spits are spits with ends that are curved landward, forming a bay or inlet as a result. Wave refraction around the distal end transports and deposits sediment over a short distance in the landward direction. Wind and wavefronts are at an opposite angle to the prevailing wind, generating short periods of longshore drift in the landward direction.

Cuspate forelands

Cuspate forelands are low-lying, triangular-shaped headlands extending out from a shoreline formed from deposited sediment. They are created when longshore drift currents from opposite directions meet at the boundary of two sediment cells. Sediment is deposited by both currents, creating a triangular-shaped area. They extend from a few metres to several kilometres.

Fig. 3 - Diagram showing coastal and oceanic landforms

Bars

A bar is a ridge of sand or shingle that joins two headlands on either side of a bay. It is formed by longshore drift transporting sediment along the coastline. A lagoon, formed by trapped water, is usually created behind the bar.

Offshore bars

Offshore bars are ridges of sand or shingle in the offshore zone. They are created from sediment deposited at the offshore and nearshore zone boundary.

They are also known as breakpoint bars because the offshore/nearshore boundary is where waves begin to break. Occasionally they are exposed by the neap tide (when the tide is at its minimum).

Sand dunes

Sand dunes are small ridges of sand found at the top of a beach, beyond the reach of waves. They are created by the wind blowing up the beach. Larger pieces of sediment will rest against obstacles, creating a ridge. At the same time, smaller particles will settle on the far side of the ridge, forming an embryo dune.

Fig. 4 - Sand dunes are a classic example of sediment transport

After a while, the pile of material becomes too steep and unstable and collapses, allowing smaller particles to fall down the other side. This continues until a stable angle is achieved (so the sand stops slipping). The sand becomes an obstacle itself, and more dunes may form in front of it through the same process. The stronger the wind, the higher the dunes.

The material is transported in the following ways:

• Suspension is when sand is picked up and carried within the wind. Approximately 1% of the movement of sand is carried in this way.
• Saltation is when grains of sand bounce along the beach as they are picked up and dropped by the wind. 95% of sand movement results from saltation.
• Creep is when sand grains collide and push each other along other grains. 4% of transportation is by creep.

Salt Marsh

A salt marsh is also a coastal salt marsh or tidal marsh. It is located in a sheltered part of the coastline between the land and open saltwater or brackish water, which is regularly flooded by tides. Here, mud and silt can be deposited at a greater rate than transportation due to the waves’ lack of energy. Often salt marshes are found on the inside of a coastal spit.