Coastal Processes

Summary

  • Various coastal processes lead to the formation of coastal landscapes
  • Landscape processes are mainly grouped into three
  • Erosion, transportation and deposition are the main coastal processes
  • Solution, attrition, hydraulic action and abrasion are the major forms of erosion in coastal landscapes
  • Cliffs, headlands and bays are the major features of coastal landscapes resulting from erosion
  • The longshore drift plays a big role in coastal material transportation and deposition
  • Bars, tombolos and lagoons are the major features of coastal deposition

Coastal landscapes develop due to the combined effects of transportation, erosion and deposition processes. Each process has unique features identifiable on an OS map. The three major coastal processes include erosion, transportation and deposition.

1. Erosion

Sea forces change coastal landscapes with the wind being the major source of waves. The time wind takes to blow, wind speed and the fetch (distance the sea has covered) influence wave sizes. Erosion worsens as the waves becomes stronger.

Erosion along the Coastline

The four erosional coastal landscape processes include the following:

  • Attrition: Waves carry pebbles and rocks, smashing them into other processes. The rocks eventually wear and becomes rounder, smaller and smoother.
  • Hydraulic Action: Wave forces use hydraulic action to crash against cliffs and shores. Waves become so powerful then forces air into the wall cracks. Action of the sea compresses the cliffs and blows air into the cracks to relieve pressure. The action results in disintegration of the rocks.
  • Solution: Also known as solution, corrosion occurs when seawater chemicals dissolve rock minerals and thus breaking them up.
  • Abrasion: Also known as corrasion, abrasion occurs when sea waves sweep away pebbles and rocks, leading to the rocks wearing off over time as they hit cliffs.

Attributes of Coastal Erosion

Erosion of the original headland shape into a coastal landscape involved various processes as follows:

  • Abrasion, hydraulic action, solution and attrition result from sea waves, eroding lines of weaknesses on rocks. The headland is undercut all round.
  • The lines of weaknesses enlarge into small sea caves.
  • Caves widen and deepen all around the headland, eventually cutting into it to create an arch.
  • Rocks on the arch top lose support as the arch enlarges, collapsing into a stack.
  • Erosion of the stack into a stump
  • Eventual disappearance of the stump
  • Headland retreats and gently slopes at the bottom of the cliff retreating through a wave-cut platform.

Cliffs

Cliffs are common along the coastline. Weathering weakens the top of cliff faces, leading to the breakage of certain parts. Water hits the bottom of the rock cliff through hydraulic action, causing erosion. Materials in the sea water causes the cliff to wear off through abrasion. Water acids react with the rocks they get in contact with, the solution eventually eroding the rocks.

Coastal Cliffs

As a result of all the coastal processes, a wave cut notch develops at the bottom of the cliff. It is then eroded further backwards. The cliff eventually falls into the sea when it can no longer support the weight at the top, increasing the force of gravity. The sea backwash washes all the materials on the face of the cliff. At this point, the process ends and begins again, repeating the cycle.

The force of waves reduces over time as the rock left at the bottom of the cliff becomes smaller. What remains on the coastal landscape is a wave cut platform leaning towards the bottom of the new cliff.

Headland and Bay

Coastal areas with various forms of hard and soft rock bands are likely to form headlands and bays. They are discordant to the coast, meeting it at right angles. Soft rocks such as clay erode faster into bays that eventually form sandy beaches. On the other hand, harder rocks such as chalk erode slowly, creating headlands that jut from the land into the sea. It occurs in a process known as differential erosion.

Headland and Bay

The sea causes various processes that result into erosion. For instance, abrasion occurs when waves throw rocks and other sea materials against the coastline, further causing erosion. During hydraulic action, waves hit the coastline and the resulting water force causes erosion. Chemical erosion can also erode the coastline. When sea water acids react with rocks such as limestone or chalk, the solution causes erosion.

2. Coastal Transportation and Deposition

The strength of waves affect how transportation of materials in the sea occurs, including the angle at which they hit the shoreline. The direction from which the coastal winds blow is important.

The Longshore Drift

During the longshore drift, sand or pebble materials move from point A to point B as the beach swashes up. The direction of the wind and the waves determine the angle at which the movement occurs. The particles then move from point B to point C down the beach through backwash waves and gravity.

Longshore Drift

The process repeats several times, moving the materials farther down the shoreline in a process known as the longshore drift. When the process continues and pebbles or sand particles move along the coastline in large numbers, spits, a depositional feature may result.

Attributes of Coastal Deposition

The longshore drift form depositional features such as bars, spits and tombolos. The direction of waves and secondary wind carves the edge of spits as waves hit them from the second yet different direction. As a result, sandspits feature hook-like or curved features on the edges. Many hooks can form over time. Spits form and shelter calmer water, which eventually develop into a lagoon, salt marsh then dry land.

Tombolo

A spit can connect an island to the mainland, forming a tombolo. Material deposition along the coastlines result in the formation of spits. The longshore drift moves materials in the sea water along the shoreline. The depositions move up at an angle onto the beaches, swashing onto the shoreline at an angle of 45 degrees. During backwash, it moves back into the sea at a right angle to the coastal landscape.

Tombolo

The materials constantly move along the shoreline via the repetitive process. The longshore drift continues to occur in the presence of a river estuary or when the direction of the shoreline changes. As a result, the materials are deposited along the coastline, forming a long thin strip known as a spit off the coast. The feature becomes a tombolo if the material is deposited towards an island, connecting it to the mainland.

Bars

Gaps in the coastland that contain water result in the formation of bars, natural hollows or bays in the coastland. The longshore drift and swash moves materials across the bay front. The materials move up at an angle of 45 degrees onto the beaches. The materials move back into the sea during backwash at a right angle. Therefore, the sea materials constantly move along the shoreline during the process.

Bar

The materials are eventually deposited on the other end of the bay, creating a strip that seals off the water in the bay. A lagoon is then formed in the area behind the newly-created bar.

Lagoons

Bars trap water to create lagoons often fed by streams flowing into them. Streams that flow faster might destroy the sand bar, diverting into the bay.

Lagoon
References:
  1. International Conference on Physical Coastal Processes, Management and Engineering, Brebbia, C. A., Benassai, G., & Rodríguez, G. R. (2009). Coastal processes. Southampton: WIT.
  2. International Conference on Physical Coastal Processes, Management and Engineering, Benassai, G., Brebbia, C. A., & Rodriguez, G. (2011). Coastal processes: II. Southampton, U.K: WIT Press.
  3. Komar, P. D. (2018). Handbook of Coastal Processes and Erosion. Milton: CRC Press.
  4. Masselink, G., Hughes, M. G., & Knight, J. (2014). Introduction to coastal processes and geomorphology.
  5. Davidson-Arnott, R. (2010). Introduction to coastal processes and geomorphology. Cambridge: Cambridge University Press.
IMAGE SOURCES:
  1. Erosion along the Coastline – Brocken Inaglory, Bluff erosion in Pacifica 2, CC BY-SA 3.0
  2. Coastal Cliffs – Raki_Man, Coastal Cliffs – panoramio (1), CC BY 3.0
  3. Headland and Bay – Gypsy Denise, Australia headland, Murnane Bay, Childers Cove, Bay of Islands Coastal Park159b, CC BY-SA 4.0
  4. Longshore Drift – Yefi, Longshore drift, marked as public domain, more details on Wikimedia Commons
  5. Tombolo – Mike Pennington, St Ninian’s Isle tombolo – geograph.org.uk – 346681, CC BY-SA 2.0
  6. Bar – Thewellman, StoneLagoonBar, marked as public domain, more details on Wikimedia Commons
  7. Lagoon – Tim b (Wildroo), Blackmans Lagoon – panoramio, CC BY-SA 3.0