Coastal Landscapes

The wider coastal landscape

The coastal landscape is changing constantly through the effects of wind, waves, currents, and tides, as you will see.

The littoral zone

The littoral zone is the area of the shoreline from the sea to the land, which is subjected to wave action. It can be divided into four subzones; backshore, foreshore, nearshore, and offshore. The littoral zone is classed as a zone rather than a line because of the effects of the waves, tides, and storms. The zone is therefore constantly changing. It is a dynamic landscape and varies because of short term (individual waves, daily tides, and seasonal storms) and long term factors (such as changes in sea levels or climate change). The subzones are:

Backshore: the area of the beach above the high tide mark. It is the area immediately adjacent to the cliff face and is only affected by wave action during major storm events.

Foreshore: the area above water level at low tide but underwater at high tide. In other words, the area within the tidal range. It is also the area commonly known as the seashore.

Nearshore: the area of shallow water beyond the low tide mark, within which friction between the seabed and the waves distort the waves sufficiently, causing them to break. Also known as the breaker zone, it extends from the foreshore to the offshore.

Offshore: the area of deeper water beyond the influence of the waves. In other words, it is the zone that extends seawards.

The diagram below illustrates the zones.

Fig. 2 - Diagram of the littoral zone

Coastal systems

A coastal landscape is generally considered to be an open system. It has input sediment brought in through various ways, and energy from waves, wind, tides, and currents. Its output includes sediment leaving through the action of waves, deposition, and transportation. Its flows/transfers refer to erosion, weathering, transportation, and deposition. Finally, it has stores/components, which are the landforms that store sediment, such as spits.

Coasts themselves are classed as high or low-energy.

A high-energy coast receives inputs from powerful destructive waves. Consequently, it has higher rates of erosion than deposition. A low-energy coast, on the other hand, is subjected to more gentle waves known as constructive waves. As a result, it has a greater rate of deposition than erosion.

Sources of energy

The interaction between the wind and the waves helps shape the coastline. Waves provide the energy for the coastal systems, tides spread that energy over a larger area, while currents spread and redistribute the energy and sediment along the coastline.

As we have seen, there are two types of waves, constructive and destructive.

• Constructive waves are low-energy waves. They have a strong swash, which carries sediment up the beach, and a weak backwash.
• Destructive waves are high-energy waves. They have a high downward force and strong backwash. They are responsible for some striking coastal features.

Fig. 3 - Constructive and destructive waves

For more information on the wider coastal landscape please refer to Wider Coastal Landscape.

Coastal classification

Coastlines can be classified according to their formation process ( primary and secondary coasts); as a result of sea-level changes (submergent and emergent coasts); and as a result of plate activity (active and passive coasts). However, it is important to remember that each classification system overlaps and complements the others.

Formation processes

• Primary coasts are formed as a result of land-based processes such as plate tectonics and erosion sedimentation.
• Secondary coasts have been significantly changed by marine processes after the sea level stabilised. This allowed erosion and deposition processes to dominate the landscape.

Sea level changes

Submergent coasts occur where the sea level rises relative to the land. This may be due to tectonic subsidence or when sea levels rise due to a glacier melt. The sea submerges (drowns) the existing coastline. This process creates the following coastal features:

• Rias coast: a drowned river valley with long fringes of water stretching a long way inland.
• Glacial coast: more commonly known as fjords: glacial valleys that have been drowned as a result of sea levels rising at the end of the last ice age.
• Concordant coastline: This is a band of differing rock types that run parallel to the coast, and the harder rock provides a barrier for the softer rock behind.

• Discordant coastline: differing rock types run perpendicular to the coast. The different resistance levels of the rocks allow for the formation of headlands and bays.

Fig. 4 - A ria is an example of a drowned coast

Plate activity

• Active coast: a coastal landscape that is characterised by mountain-building activities, such as earthquakes, volcanic activity, and tectonic motion. Active coasts are located near a plate boundary, near the tectonic activity. An example of an active coast is the rugged Pacific Coast, which is characterised by narrow beaches, steep cliffs, headlands, and sea stacks.Passive coast: They occur at the transition between the oceanic and continental crust, a non-active plate boundary. They are usually subsiding. Passive margins are characterised by wide beaches, barrier islands, and broad coastal plains. Offshore passive margins typically have a wider and flatter continental shelf and slope. Examples of passive coasts are the Atlantic and Gulf Coasts.

For more information on coastal classification, refer to Coastal Classification.

Formation of coasts

There are two main types of coasts.

Rocky / cliffed coastlines: these have cliffs varying in height from a few feet to hundreds of meters. The cliffs are made of rock. However, the hardness of the rock varies.

Coastal plains: the land gradually slopes towards the sea across an area of deposited sediment. Sand dunes and mudflats are the most common examples. If the coastline is a sandy coast it will be composed of sands, shingles, and cobbles. If it is estuarine (alluvial) it will be composed of mud (clays and silts).

Rocky coasts

Coasts with a steep cliff face can be found in high-energy environments where the waves pound the face of the cliff throughout the year. A gentler cliff profile tends to indicate a low-energy environment. There are two main cliff types.

Marine erosion dominated: these cliffs tend to be steep and unvegetated, with little rock debris at the base, because any debris is quickly broken up by the waves and carried away. Cliffs that are not actively eroded away at their base by waves will have shallower, curved profiles and low relief.

Sub-aerial processes dominated: as we see in Coastal Formations, the sub-aerial processes of surface runoff, erosion and mass movement (landslides), slowly move rock and sediment downslope. Limited marine erosion means that it will not be removed.

Features found on a rocky coast:

• Headlands.

• Cliffs.
• Wave-cut platform (shoreline platform).

Coastal plains

Sandy coastlines are generally found in low-energy environments. These coasts are relatively flat, low-relief areas adjacent to the sea. These coasts are where the rate of deposition exceeds that of erosion. Dune vegetation plays a critical role in stabilising the coast and preventing erosion. Beaches are made of a gradual build-up in and across bays of either sand or shingle, or a mixture of both.

There are two types of beach:

• Swash-aligned waves break in line (parallel) with the coast. The swash and backwash move material up and down the beach.

• Drift-aligned waves break at an angle to the coast. Material is transported by longshore drift.

Features associated with a sandy coastline:

• Ripples.
• Spits and tombolos.
• Bars and barrier beaches.
• Offshore bars.

• Sand dunes.

Estuarine coastlines are found at the mouths of rivers. The flow of water from the river meets with the incoming tides and waves from the sea. This causes the water flow to virtually cease and causes the water can no longer carry its sediment in suspension.

For more information on this topic please refer to Formation of Coasts.

Geological structure

The geological structure has an important influence on the morphology and erosion rates in a coastal landscape. There are three key elements to geological structure.

• Strata: the different layers of rocks within an area, and their relation to each other.
• Deformation: the degree to which rock units have been deformed, either by tilting or folding or by tectonic activity.
• Faulting: the presence of major fractures that have moved rocks from their original position.

Strata

The strata influence geological structure by producing two dominant types of coasts.

• Concordant - Here beds, or layers, of differing rock types are folded into ridges that run parallel to the coast. The outer hard rock (ie granite) provides a protective barrier to erosion of the softer rocks (ie clays) further inland. Concordant coasts are also known as Dalmatian coasts, after the Dalmatian region of Croatia, or Pacific coasts, after the coastline of Chile in South America.

• Discordant - These occur when bands of different rock run perpendicular to the coast. The differing resistance to erosion of the rocks leads to coastlines dominated by headlands and bays. Part of the Dorset coastline, north of Durlston Head, is a clear example of a discordant coastline. Discordant coasts are also known as Atlantic coasts, after the Cork coastline in the Republic of Ireland.

Deformation and faulting

Cliff profiles are influenced by different aspects of geological structure but certain characteristics are dominant.

• Where the rock is resistant to erosion.

• The dip of the strata in relation to the coastline. This means the angle of the rock strata in relation to the horizontal. Sedimentary rocks are formed in horizontal layers but can be tilted by tectonic forces. When this is exposed on a cliff coastline it has a dramatic effect on the profile of the cliff.

• Joints are fractures in rocks created without displacement. They occur in most rocks, often in regular patterns, dividing rock strata up into blocks with a regular shape.
• Faults are major fractures in the rock created by tectonic forces, with the displacement of rocks on either side of the fault line. They represent a major weakness within the rock layer.

Fissures are much smaller cracks in the rocks, often only a few centimetres long. However, they still represent a weakness that erosion can exploit.

For more information, please refer to Geological Structure.

Lithology

Lithology refers to the physical properties of a rock. The lithology at the coast affects the speed at which it erodes or recedes.

Most cliffed coasts are not made from only one rock type. They are composite cliffs with different rock layers, sometimes from different geological periods. As a result, a complex cliff profile is produced. The resistance of the cliff to erosion will be influenced by its weakest rock type, and also its permeability.

There are three major rock types; igneous, sedimentary, and metamorphic.

• Igneous rocks like granite, basalt, and dolerite erode very slowly, at a rate of less than 1mm per year. They have few joints, and have few weaknesses, and are crystalline.
• Metamorphic rocks like slate, schist, and marble are also crystalline and resistant to erosion. They erode faster than igneous rocks, though; at the rate between 1mm-10cm per year.
• Sedimentary rocks like sandstone, limestone, and shale are clastic. As with shale, they have many bedding places and fractures and are vulnerable to erosion, from 10cm-several metres per year.

For more information, refer to Lithology and Vegetation.