The littoral zone

The coastal system refers to the areas where land interacts with the sea, forming a complex environment influenced by both terrestrial and marine processes. At the heart of this system is the littoral zone, which is the coastal area directly affected by wave action.

Components of the littoral zone

• Coast - The land area influenced by the sea, often including human settlements and agricultural land.
• Backshore - The region above the high tide level, typically unaffected by regular coastal processes but impacted during extreme events like storms or unusually high tides.
• Foreshore - The zone between the high tide and low tide marks, where many wave-related activities, such as erosion and deposition, occur.
• Nearshore - The shallow seawater area just beyond the low tide mark, extending slightly offshore.
• Offshore - The deeper open sea beyond the point where waves break, marking the outer boundary of the littoral zone.

This zone operates in a state of dynamic equilibrium, meaning its terrestrial and marine elements experience continuous short-term adjustments while maintaining an overall balance.

Classifications of coastlines

Coastlines undergo changes over different timescales, leading to various classifications.

Long-term coastline changes

• Emerging or submerging - Emerging coastlines form when sea levels fall relative to the land, exposing new areas; submerging coastlines occur when sea levels rise, flooding land.
• Rocky or estuarine - Rocky coastlines develop from resistant geology that withstands weathering and erosion; estuarine coastlines form in settings with softer sediments.
• Concordant or discordant - Concordant coastlines have geological strata parallel to the coastline; discordant coastlines have strata perpendicular to the coastline.

Short-term coastline changes

• Tidal range - The vertical difference between high and low tide levels. Coastlines are classified as:
  • Microtidal - Less than 2 metres.
  • Mesotidal - 2–4 metres.
  • Macrotidal - Over 4 metres.
• Retreating or advancing - Retreating coastlines lose land where erosion dominates; advancing coastlines gain land through deposition.
• High or low energy - High-energy coasts experience strong erosive forces; low-energy coasts favour deposition.
• Primary or secondary - Primary coasts are shaped mainly by land-based processes; secondary coasts are dominated by marine processes.

High-energy and low-energy coastal environments

Coastal environments vary in energy levels, as air transfers energy through wind and water transfers energy through waves, tides, and currents.

Characteristics of high-energy coasts

• They receive large, powerful destructive waves, often due to strong winds, long fetches (the distance wind blows over open water), and steeply shelving seabeds.
• Erosion typically exceeds deposition, though resistant geology can provide some protection.
• Common landforms include rocky cliffs, stacks, arches, and small sandy coves, with a sharp divide between land and sea.
• Cliffs are steep with minimal vegetation; marine cliff profiles result from wave undercutting and debris removal, while subaerial cliff profiles show gentler slopes with accumulated weathered material at the base.

Characteristics of low-energy coasts

• They experience small, gentle constructive waves, influenced by mild winds, short fetches, gentle seabed slopes, and protective features like reefs or islands.
• Deposition usually outpaces erosion, leading to coastal accretion (land expansion into the sea).
• Typical features include sandy beaches, salt marshes, estuaries, and tidal mudflats, with no cliffs directly adjacent to the beach (though cliffs may exist inland).
• The landscape has gentle relief and low elevation, blurring littoral zone boundaries; sediment arrives from rivers and offshore sources, accumulating in calm waters.

Wave formation and types

Waves form through energy transfer from wind to water. Wind friction over the sea surface creates circular water motion, forming ripples that grow into waves.

Factors influencing wave development

• Wind speed - Higher speeds generate taller, more powerful waves.
• Fetch - Longer distances allow waves to build greater height and energy.
• Seabed depth - As waves near shore, friction with the seabed slows the base, making motion more elliptical; the crest rises and collapses, causing the wave to break.

The point of impact on the beach also varies with tide levels, further altering beach shape.

Destructive waves

• They are high and steep with a circular cross-profile.
• Short wavelength leads to high frequency (10 to 14 waves per minute) and high energy.
• Strong backwash (water retreating to the sea) removes material, scouring the beach.

Constructive waves

• They are low with a long wavelength, creating an elliptical cross-profile.
• Low frequency (around 6 to 8 waves per minute) and low energy.
• Powerful swash (water advancing up the beach) deposits material higher up.

Short-term changes in wave types

• Storms can introduce more destructive waves temporarily.
• Increased wind speeds produce taller waves.

Long-term changes in wave types

• Winter profiles are steeper at the back due to destructive waves removing sediment and flattening lower areas.
• Summer profiles build taller berms from constructive waves.
• Climate change may increase storm-like conditions, leading to more destructive waves year-round.

Sediment cells and budgets

Sediment cells

Sediment cells, also known as littoral cells, are self-contained sections of coastline where sediment circulates independently, often bounded by prominent headlands. There are 11 such cells around the English and Welsh coastline. Extreme weather can occasionally allow minor sediment transfer between cells, but generally, processes in one cell do not affect others.

Components of sediment cells

Within each cell, sediment follows a cycle of inputs, transfers, and stores:

Sources (inputs):

• Rivers transport eroded material from inland.
• Cliff erosion and weathering release sediment.
• Crushed shells from marine organisms accumulate.
• Waves, tides, and currents move material from offshore deposits.
• Wind erodes small particles from sand dunes.

Transfers (flows):

• Longshore drift carries sediment along the shoreline.
• Offshore currents bring material inland.
• Aeolian processes (wind action) move fine particles.
• Tidal currents shift sediment in and out daily.

Sinks (stores):

• Flocculation binds sediment in salt marshes.
• Spits and bars form as wave energy drops.
• Wind creates sand dunes.
• Beaches accumulate material from waves and cliffs.
• Offshore bars build up underwater.

The sediment budget measures the balance of sediment entering and leaving a coastal area

• A positive sediment budget occurs when inputs exceed outputs, causing features like beaches, dunes, and spits to expand.
• A negative sediment budget happens when outputs exceed inputs, leading to erosion and land loss.