Depositional Landforms (Spits, Bars, Sand Dunes)

Spits

Spits are narrow ridges of sand or shingle that extend from the coastline into the sea. They form through the process of longshore drift, where waves approach the shore at an angle, moving sediment along the coast in a zigzag pattern.

As sediment is transported by longshore drift, it accumulates where the coastline changes direction sharply or where the water becomes shallower, causing the waves to lose energy and deposit material. Over time, this builds up a spit.

The end of a spit often curves into the sea, forming a hooked end. This happens because the wind and waves change direction, pushing sediment around the end of the spit.

Behind the spit, the sheltered water is calmer, allowing fine sediment to settle and plants to grow. This leads to the formation of a salt marsh, which is an important habitat for wildlife and helps protect the coastline from erosion.

For example, Spurn Head on the Holderness Coast is a classic spit formed by longshore drift from south to north, with a hooked end shaped by prevailing winds.

Spits can grow for several kilometres and may eventually connect to another part of the coast or an island, changing the shape of the coastline.

For instance, if longshore drift moves sediment at a rate of 5000 tonnes per year, and the spit grows 100 m in length annually, the average sediment volume deposited per metre length of spit is calculated by dividing the total sediment by the length growth, assuming uniform deposition:

$\frac{5000\, \mathrm{tonnes}}{100\, \mathrm{m}} = 50\, \mathrm{tonnes\, m^{-1}}$

Bars

Bars are depositional landforms that form when a spit grows across a bay, connecting two headlands. This creates a barrier of sand or shingle across the bay.

Bars form when longshore drift deposits sediment continuously until the spit reaches the opposite side of the bay, cutting off the bay from the sea.

Behind the bar, a body of water called a lagoon forms. This lagoon is often calm and can become a valuable habitat for plants and animals.

An example of a bar is Slapton Sands in Devon, where shingle has built up across a bay, creating a barrier beach.

Bars can change coastal ecosystems by reducing tidal flow and increasing sedimentation in the lagoon.

For example, if a bay is 2 km wide and a spit grows across it at 50 m per year, it will take:

$\frac{2000\, \mathrm{m}}{50\, \mathrm{m\, year^{-1}}} = 40\, \mathrm{years}$

for the bar to fully form.

Sand Dunes

Sand dunes are hills of sand formed by wind deposition along coasts. They develop where there is a large supply of sand, strong onshore winds, and an obstacle to trap the sand.

The process starts with embryo dunes, small mounds of sand that form around obstacles like driftwood or vegetation. As more sand accumulates, these grow into mature dunes that can be several metres high.

Vegetation plays a crucial role in stabilising dunes. Plants like marram grass have deep roots that bind the sand together, preventing it from being blown away by the wind. Marram grass is especially effective because its roots grow deep and can survive in dry, sandy conditions, helping to trap and stabilise the sand.

Over time, dunes develop a series of ridges parallel to the coast, with the oldest dunes furthest inland. These provide important habitats and act as natural coastal defences against erosion and flooding.

For example, the sand dunes at Studland Bay in Dorset show a clear sequence from embryo dunes near the beach to mature dunes inland.

The height of dunes depends on wind strength and sand supply. If wind speed increases, more sand can be transported and dunes can grow taller.

For instance, if wind transports sand at a rate of 200 kg per hour and deposits it over an area of 100 m², the average sand deposition per square metre per hour is:

$\frac{200\, \mathrm{kg}}{100\, \mathrm{m^2}} = 2\, \mathrm{kg\, m^{-2}\, hour^{-1}}$