Coasts as Open Systems: Coasts represent dynamic interfaces where land and sea interact, functioning as open systems that receive inputs (e.g., sediment), facilitate transfers (e.g., longshore drift), store materials (e.g., beaches), and have outputs (e.g., water). This systemic view highlights the continuous exchange of energy and matter that shapes coastal environments.
Marine vs. Terrestrial Processes: Coastal processes are broadly divided into marine processes, which are water-based and occur offshore, and terrestrial processes, which are land-based and occur onshore. Both categories are essential for understanding the comprehensive forces acting upon a coastline.
Key Coastal Processes: The primary activities responsible for shaping coastal landforms include wave action, various forms of erosion, the transportation and deposition of sediment, weathering of rock materials, and mass movement events. These processes often interact in complex ways, leading to diverse coastal features.
Wave Formation and Characteristics: Waves are marine processes primarily formed by winds blowing over the sea surface, and they are responsible for eroding, transporting, and depositing material. The height and strength of a wave are determined by the fetch (distance over which wind blows), the duration of the wind, and the strength of the wind; greater values for these factors result in larger, more powerful waves.
Wave Breaking and Movement: As waves approach shallower water near the coast, friction with the seabed causes them to slow down, increase in height, and eventually crest and break. The forward movement of water up the beach is termed swash, while the return flow of water down the beach is known as backwash.
Constructive Waves: These waves are characterized by a strong swash and a weak backwash, meaning they deposit more material than they remove, thus building up beaches. They typically have a long wavelength, low wave height, and a low frequency (around 6-8 waves per minute), often leading to gently sloping beaches.
Destructive Waves: In contrast, destructive waves possess a weak swash and a strong backwash, making them effective at eroding beach material. They are characterized by a short wavelength, high wave height, and a high frequency (around 10-12 waves per minute), and are commonly associated with steeper beach profiles.
Hydraulic Action: This erosional process involves the sheer force of waves hitting the coastline, compressing air into cracks and fissures in the rock. As the wave retreats, the pressure is released, causing the trapped air to expand explosively, which can weaken and dislodge rock fragments over time.
Abrasion (Corrasion): Abrasion occurs when waves pick up sediment, such as pebbles, sand, and rocks, and hurl them against the cliff face or seabed. This constant grinding and scraping action wears away the rock, much like sandpaper, and is a highly effective form of erosion.
Attrition: Attrition is the process where rock fragments and sediment carried by waves collide with each other, gradually breaking down into smaller, smoother, and more rounded particles. While attrition does not directly erode the coastline, it produces the sand and shingle that contribute to beach formation and can be used in abrasion.
Corrosion (Solution): Corrosion, also known as solution, involves the chemical breakdown of soluble rocks by seawater, which is slightly acidic due to dissolved carbon dioxide. Rocks like limestone are particularly susceptible to this process, where minerals react with the acidic water and dissolve, weakening the rock structure.
Factors Affecting Erosion Rates: The rate of coastal erosion is influenced by several factors, including the energy of the waves (higher energy from steep, destructive waves, strong tidal currents, or rip currents increases erosion), the materials present (less resistant rocks erode faster, while large beaches absorb wave energy and reduce erosion), and the shore geometry (steep seabeds create higher, more erosive waves, while offshore bars can reduce wave energy reaching the shore).
Sources of Coastal Sediment: Material transported along the coast originates from various sources, including the erosion of cliffs, sediment moved by longshore drift, material brought in by constructive waves, and sediment discharged into the sea by rivers. This continuous supply fuels the dynamic processes of coastal change.
Modes of Sediment Transport: Once in the water, sediment is moved through four primary mechanisms: Traction involves large, heavy material being rolled or dragged along the seabed; Saltation describes smaller, lighter material bouncing or hopping along the seabed; Suspension refers to fine, light particles (like clay and silt) being carried within the water column; and Solution is the transport of dissolved minerals within the seawater.
Longshore Drift: This is a crucial process for both sediment transportation and deposition along coastlines. It occurs when waves approach the beach at an angle, driven by the prevailing wind, causing the swash to carry material up the beach at that same angle. The backwash then pulls the material straight back down the beach at a 90-degree angle due to gravity, resulting in a characteristic zig-zag movement that transports sediment progressively along the coastline.
Conditions for Deposition: Sediment deposition occurs when the energy of the waves or currents decreases, allowing suspended or transported material to settle. This can happen due to increased friction between the water and the seabed, when the water is carrying a large amount of sediment, or when waves encounter obstacles that cause them to break and lose energy.
Definition of Weathering: Weathering is the in-situ breakdown of rocks and minerals at or near the Earth's surface, meaning it does not involve the movement of material. This distinguishes it from erosion, which always includes the transport of weathered material.
Sub-aerial Weathering: This term specifically refers to coastal weathering processes that are not directly caused by the action of the sea, but rather by atmospheric and biological agents. Sub-aerial weathering plays a crucial role in weakening cliffs, making them more susceptible to marine erosion and mass movement.
Mechanical Weathering: This type of weathering physically breaks rocks into smaller pieces without changing their chemical composition. Key examples include freeze-thaw weathering, where water in rock cracks freezes, expands, and exerts pressure, eventually splitting the rock, and salt weathering, where salt crystals grow in cracks as seawater evaporates, causing similar expansive forces.
Chemical Weathering: Chemical weathering involves the decomposition of rocks through chemical reactions, altering their mineral composition. A common example is carbonation, where slightly acidic rainwater (due to dissolved carbon dioxide) reacts with minerals in rocks like limestone, dissolving them. The rate of chemical weathering is influenced by rock type and temperature.
Biological Weathering: This process involves the breakdown of rocks by living organisms. Plant roots growing into cracks can exert pressure and widen them, while tiny organisms like bacteria and algae can produce chemicals that dissolve rock surfaces. Burrowing animals can also destabilize ground, contributing to rock breakdown.
Definition of Mass Movement: Mass movement refers to the downhill movement of rock, soil, and regolith under the direct influence of gravity. It is a significant terrestrial process that reshapes coastal cliffs and slopes, often triggered or exacerbated by heavy rainfall which increases the weight and reduces the stability of the material.
Factors Influencing Mass Movement: The type and rate of mass movement are influenced by several factors, including the angle of the slope (steeper slopes lead to faster movement), the nature of the regolith (loose, unconsolidated material is more prone to movement), the amount and type of vegetation (roots can stabilize slopes, but heavy vegetation can add weight), the amount of water (saturation reduces friction and adds weight), the type and structure of the rock, human activity (e.g., construction, deforestation), and climate.
Types of Mass Movement: Different forms of mass movement are distinguished by their speed and mechanism:
Soil Creep: This is the slowest form of mass movement, typically occurring at speeds less than 1 cm per year. It involves individual soil particles being lifted perpendicular to the slope (e.g., by freezing, wetting, or heating) and then falling straight down under gravity, resulting in a gradual downhill migration.
Flow: Flows occur on slopes between 5° and 15°, often after the soil has become saturated with water, leading to a viscous, fluid-like movement of material. Speeds can range from 1 to 15 km per year, and vegetation can be carried along with the moving soil.
Slide: A slide involves a coherent mass of material moving downslope along a distinct plane of weakness, maintaining its integrity until it reaches the bottom. This 'en masse' movement can be rapid or slow, depending on the slope and material.
Fall: Falls are rapid movements of rock fragments from steep slopes or cliffs, often initiated by extreme weathering (like freeze-thaw), heavy rainfall, earthquakes, or drying out of soil. The material detaches and plunges freely downwards.
Slump (Rotational Slip): Common on weaker rock types like clay, especially when saturated and heavy, slumping involves a large area of land moving down a slope in one piece along a curved slip plane. This rotational movement leaves behind a distinctive curved surface at the top of the slump.
Differentiate Wave Types: A common exam question involves distinguishing between constructive and destructive waves. Focus on their key characteristics: swash/backwash strength, wavelength, height, frequency, and their impact on beach profiles (building vs. eroding).
Understand Erosion Mechanisms: Be able to clearly define and provide examples for each type of erosion: Hydraulic action (force of water), Abrasion (rock-on-rock/cliff impact), Attrition (rock-on-rock wear), and Corrosion (chemical dissolution). Pay special attention to the subtle differences between abrasion and attrition.
Weathering vs. Erosion: A critical distinction to master is that weathering is the in-situ breakdown of rock without movement, while erosion involves both breakdown and subsequent transportation of material. Weathering often weakens rock, making it more susceptible to erosion.
Longshore Drift Mechanism: Practice explaining longshore drift step-by-step, including the role of prevailing wind, angled swash, and perpendicular backwash, and how this results in net sediment movement along the coast. A well-annotated diagram can significantly enhance your explanation.
Interconnectedness of Processes: Recognize that coastal processes are not isolated; for example, weathering weakens cliffs, making them more vulnerable to mass movement and marine erosion. Similarly, wave energy influences both erosion and deposition, and sediment transport links these processes.
