How Oceans and Seas Change Landscapes

Beach at Wicklow Head

This headland marks the most easterly point of County Wicklow.The seabed offshore, which is swept by strong tidal currents, has an unusual series of reefs formed by honeycomb worms. (Richard Nairn)

copyright R. Nairn

Beach at Wicklow Head
copyright R. Nairn

Beach at Wicklow Head

This headland marks the most easterly point of County Wicklow.The seabed offshore, which is swept by strong tidal currents, has an unusual series of reefs formed by honeycomb worms. (Richard Nairn)

copyright R. Nairn

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Poolbeg chimneys from Bull Island

The twin chimneys of Poolbeg are a landmark in Dublin Bay. Bull Island is next to a very busy port and harbour area.

(c) John Kennedy

Poolbeg chimneys from Bull Island
(c) John Kennedy

Poolbeg chimneys from Bull Island

The twin chimneys of Poolbeg are a landmark in Dublin Bay. Bull Island is next to a very busy port and harbour area.

(c) John Kennedy

Enlarge image

The coastal zone (i.e.the area of land surface influenced by marine processes) extends from the landward limit of tides, waves, and wind blown coastal dunes, and seaward to the point at which waves interact significantly with the seabed.The coastal zone is a dynamic component of Earth's surface, through which both marine and atmospheric processes produce rocky coasts, as well as beaches and dunes, barriers and tidal inlets, and shape deltas. Atmospheric processes including temperature, precipitation, and winds interact with marine processes, including waves, tides, water temperature and salinity to influence coastal landscape change.

Waves provide about half the energy to change the coastal landscape. Ocean waves are created by wind blowing over the ocean surface. The stronger the wind, the longer it blows and the longer the fetch, or stretch of ocean over which it blows, the larger the waves. Waves tend to be strongest in the zone of sub-polar lows centered on 40–60° north and south latitudes. The strong westerly winds produce the world's biggest waves which initially head west, and are deflected equatorward by the Coriolis effect. Given its latitude, the Irish coastline is therefore in a constant state of change.

Similarly, tides produced by the gravitational pull of the Moon and Sun acting on a rotating Earth cause a very slight bulge in the ocean, which we know as tides. These tides and the currents they generate are responsible for about 50% of the marine energy delivered to the coast. One of the most obvious ways in which oceans therefore change landscapes is the shift in shoreline between high and low tide. Two other ways in which marine processes alter coastal landscapes include the formation of deltas and beaches.

Deltas

Whilst deltas are considered a feature arising from fluvial processes, their formation can significantly alter coastal landscapes, changing a coast's appearance and form. As a river reaches the sea, in instances where tides and currents are not strong enough to transport a river's load out to sea the alluvium deposits accumulate to form new land at the river mouth. The river channel may become choked resulting in the river breaking up into smaller courses called distributaries. Moreover, depending on their location, deltas are also influenced by waves, tides, and other currents, and are therefore shaped by prevailing coastal processes. Sediment deposited at the mouth of a delta can also be transported longshore by wave action to supply beach, dune and barrier systems in a process referred to as longshore drift (i.e. movement of material along the coastline in a zig-zag manner).

An arcuate delta has a triangular shape which develops when the current is strong and straightens the delta's edge. A Birdsfoot delta has a large number of distributaries, and resembles a bird's foot as a result e.g. River Nile, Egypt. Levees may also build up adjacent to distributaries e.g. River Mississippi, U.S. Finally, an Estruarine delta is a delta that has not yet extended beyond the coastline. It consists of sediments that have been deposited in shallow water along the side of the estuary, for example the Shannon estuary.

Beaches

Beach systems are a core component of a larger scale coastal landform called barriers, which are long-term accumulations of wave, tide, and wind deposited marine sediment (usually sand) at the shore. They are comprised of a beach from which sand is transported either onshore, via wave overwash and/or wind blown sand, or alongshore, to create sand spits and move into tidal inlets.

Beaches are typically comprised of 4 zones (i.e. beach profile): swash, beach face, wrack line, and berm. The particles found in each of these zones also tend to be different. For example, finer sediment is found closer to the water (i.e. near the swash zone). This is partially because moving water continuously breaks down the particles located here. As the beach moves further inland, the particles along its surface grow in size. Along the outermost edge of the beach (i.e. beach berm), larger rocks and particles that have been washed up during storms are normally found.

Whilst erosion is usually thought to decrease the size of certain landforms, this is not always the case. For instance, erosion may actually increase the size and width of some beaches as waves deposit the eroded sediment onto the land. Additionally, beaches may experience growth in size near river deltas, where rivers carry eroded sediment to the ocean.

Wave type also plays a role in the formation of beaches. Constructive waves (i.e. those that allow the water to recede and the beach particles to stop moving between waves), produce compacted sediment. Conversely, destructive waves, are fast forming and do not allow the water to recede between waves, resulting in a near-constant state of sediment suspension in the water. Because the particles remain in the waves, rather than being deposited on the shoreline, beaches in these areas are more likely to suffer from future erosion.

Whilst beaches can grow over time, they can also recede. Beach recession can occur at a rapid rate or develop over longer periods of time. For instance, tsunamis and hurricanes are known to cause extensive destruction to beaches in a very short time. The strong waves and winds from these events transport sand and other sedimentary materials that may have taken years to accumulate. Additionally, these events remove most of the vegetation along shorelines that usually helps keep sand in place. Rising sea levels, that occur as a result of global climate change, can also cause beach recession. 

North Bull Island, Co. Dublin:

North Bull Island (Oileán an Tairbh Thuaidh ), better known as Bull Island, is an island 1 km wide and more than 5km long of dunes, scrub and marsh in Dublin Bay, and was formed as a result of human understanding and modification of coastal processes. Approximately 200 years ago longshore drift transported sand and silt southwards from the north side of Dublin Bay. This led to the development of sand banks that partially blocked the mouth of Dublin Harbour, causing difficulties for ships to dock. In response, North Bull Wall and South Bull Wall were built to extend out from both sides of Dublin Harbour. Their construction sheltered Dublin Harbour from easterly winds, and the North Bull Wall intercepted longshore drift that caused sandbanks to develop in Dublin Harbour. Overtime, longshore drift began to deposit sand and silt against the northern side of the North Bull Wall, which created the sand spit known as North Bull Island. Whilst human interactions with the coastal environment played an important role in the island's development, coastal processes have subsequently developed the spit into a 5km beach, which also acts as an important natural flood defence for northern parts of Dublin city. The island is still expanding today as processes of longshore drift continue to deposit sand along its northern-most reaches.