Coastal Deposition Landforms

Coastal Deposition Landforms diagram

Longshore drift is the main way sediment is transported by the sea. It happens when a wave breaks at an angle to the coast, rather than parallel (head-on) with it. This angle occurs because of prevailing winds.

Because prevailing winds are mostly from one direction, longshore drift is usually in one direction too. These angled waves wash sediment diagonally up the beach. Gravity then pulls the waves straight down the beach (backwash). As this zigzag movement occurs, the waves transport sediment all along the coastline, which is how depositional landforms can occur. Take a look at the diagram below, which shows the process of longshore drift.

Fig. 1 - longshore drift diagram

So, we've just revised a little about coastal deposition, but for more detailed information, make sure you read the explanations on Coasts, Coastal Processes, and Coastal Erosion and Deposition.

Coastal Deposition Landforms types

There are many different types of landforms created by coastal deposition, such as beaches, sand dunes, spits, and bars. Let's take a look at how some of these landforms are created.

Beaches

Beaches are the most common coastal depositional landform, and one which you will be most familiar with. They are found where the sea meets the land, and are made up of sediments that have been transported and deposited by the sea, such as sand, mud, silt, pebbles, and shingle.

Sandy beaches

Your typical sandy beach most often develops in bays. This is where low-energy, constructive waves are found, which move material up the shore and deposits it there, developing the beach.

Fig. 2 - a sandy beach

Over time, if constructive waves are dominant, the beach will build up (as more sediment is deposited), and the beach will become steeper. During the winter time, more destructive waves may become dominant, or even storm waves, which have stronger backwash, creating more gentle sloping beaches.

Pebble or rocky beaches

Pebble beaches are characterised by more powerful destructive waves. Larger sediment, like pebbles or rocks, cannot be transported so far up the beach, so it makes these beaches very steep or almost stepped. If it is stormy, the storm waves can throw heavy sediment towards the back of the beach. These beaches are also found in cliffed areas, where powerful waves erode the cliffs away.

Fig. 3 - a pebble beach

Sand dunes

On sandy beaches, you are also likely to find sand dunes. Sand dunes are big clumps of sand on the beach, usually found at the back of beaches. The material that is deposited by longshore drift, is blown around by the wind. Over obstacles, such as rocks or driftwood, sand is deposited, as a result of the wind slowing down. This forms an embryo dune. As the sand keeps building up, pioneer vegetation can grow, which actually helps to stabilise the dunes with their roots (stop them from being blown away). Foredunes are then formed.

This vegetation also forms soil from the nutrients when they die, which can cause further vegetation to grow. The sand dunes themselves become an obstacle for more sand to form around, creating more dunes in front of the first dune. Take a look at the diagram below, which shows a sand dune ecosystem.

Spits

Longshore drifts move sediments at an angle along a coastline. When the coastline comes to an end (or when the sea meets a river), wave energy slows down and deposition occurs. This can create a long stretch of sand that extends out into the sea. These are called spits.

Spits are attached to the land on one end, with the other end sticking out into the sea.

Spits continue to grow until they reach deep water, or the amount of deposition can't keep up with the amount of erosion.

Fig. 5 - Homer spit in Alaska, with human activity

Wind can cause the spit to curve around at the end. The water behind this curve is calmer, as this area is a little more sheltered. Here, mud and other sediments are deposited, developing a salt marsh.

Bars

Bars, just like spit, are associated with the process of longshore drift. Instead of jutting out into the sea, like a spit, the sand deposition actually connects two sides of the headland that surround a bay. It's essentially just a long strip of land. This is known as a bay bar. Behind this strip, a shallow lake (lagoon) forms. As deposition continues to occur in the lagoon (due to low-energy water), a salt marsh can form.

Offshore bars can also form off the coastline; destructive waves erode the sediment on a beach and drag it back towards the coastline, and deposit it, forming the offshore bar. The sediment makes it look like the seabed is raised in the middle of the water.

Coastal Deposition Landforms features

We just explored the many different types of depositional landforms formed on a coastline. Let's highlight a few features and characteristics that you might be able to squeeze into your exam answers!

• For the coast to build up, there has to be more deposition than erosion.
• There are other deposition landforms at coasts, such as mudflats, ridges, runnels, cusps and tombolos.
• Coasts are very dynamic systems; waves, tides, wind etc. all continuously shape the coastal environment. Deposition landforms change all the time.

Coastal Deposition Landforms example

As you know, case studies are important in geography. For coasts, it is no different. Let's take a look at some examples of some of the deposition landforms that we've discussed.

Dorset coast

The Dorset coast has many great examples of coastal deposition landforms. Chesil Beach is one of them. Chesil Beach is a bar, created by longshore drift, which built up a spit, that eventually joined the Isle of Portland to the mainland. Behind this bar there is The Fleet lagoon.

Fig. 6 - Chesil beach, Dorset coast

Sefton Coast

At the Sefton coast of Merseyside, which spans 20km from Liverpool to Southport, you can find the largest sand dune system in England. The coastline is characterised by a sandy beach, dunes, grasslands, and pine forest. The dunes are home to vegetation like dune heather and creeping willow, and some cool animals too, like sand lizards and dune tiger beetles!

Fig. 7 - sand dunes at Formby Point, Sefton coast