Comprehensive Study Guide to Coastal Environments and Processes
Defining the Coastal Environment and Its Diversity
The coast is defined as the transition zone between the land and the sea, whereas the coastline represents the actual frontier between these two domains. The coastal environment is divided into two distinct parts: the onshore zone and the offshore zone. The onshore zone is located on the landward side of the coastline and can extend up to inland. Conversely, the offshore zone reaches as far as out to sea. Coasts are globally diverse because they are the meeting point of the land, the sea, and the atmosphere. The factors causing this diversity are categorized into four main groups: land, sea, weather and climate, and human activity.
Factors related to the land include physical processes, geology and relief, river sediment, and ecosystems. Factors related to the sea include physical processes, sea-level changes, water depth, and ecosystems. Weather and climate factors involve wind strength, wind direction, rainfall, temperature, storms and surges, and climate change. Human activity involves settlement, development, and coastal management. The physical processes that shape coastal landforms are grouped into marine (sea-related) and terrestrial (land-related) processes.
Marine Processes and Wave Dynamics
Waves are the primary drivers of marine processes, responsible for the erosion, transportation, and deposition of materials. Waves are created by wind blowing over the sea surface, where friction between the wind and the water sets the water in motion. The strength of a wave is determined by the wind's strength, the duration of time the wind has been blowing, and the fetch, which is the distance over which the wind has blown. As waves approach the coast and enter shallower water, friction with the sea bed causes the wave to tip forward and eventually break. The swash is the forward movement of water that runs up the beach until it exhausts its energy, while the backwash is the water running back down the beach under the influence of gravity.
Waves are classified into two types based on the balance between swash and backwash. Constructive waves have a long wavelength and a shallow gradient. They are characterized by a strong swash and a weak backwash. In these waves, the swash is more powerful, resulting in material being moved up the beach and left there, which is known as deposition. Destructive waves are tall waves with short wavelengths and a steep gradient. They have a weak swash and a strong backwash. In destructive waves, the backwash is more powerful, dragging material back down the beach (erosion) and transporting it along the coast.
Marine Erosion and Transportation Processes
Destructive waves are the primary cause of coastal erosion through several specific mechanisms. Hydraulic action occurs when the force of waves hitting cliffs traps and forces pockets of air into cracks and crevices, causing the rock to weaken. Abrasion (or corrasion) involves waves picking up stones and hurling them at cliffs, wearing the rock face away. Corrosion (or solution) is the process where sea water dissolves specific types of rocks. Attrition, though it does not erode the coast directly, is a process where material carried by the waves collides with other material, becoming smaller, smoother, and more rounded over time, eventually forming pebbles and sand.
Once rocks and sand are detached from a cliff, they can be moved along the coastline via longshore drift. This process occurs as waves approach the beach at an angle determined by the prevailing wind direction. The swash carries material up the beach at this angle, while the backwash pulls it straight down the beach under gravity. This creates a zig-zag movement of sediment along the shore. Lighter and smaller materials require less wave energy and are therefore transported further distances than heavier materials. Eventually, when waves lose energy, they deposit this material to create new landforms.
Terrestrial Processes and Landward Shifting
There are three main processes operating on the landward side of the coastline: weathering, erosion, and mass movement. Weathering is the breakdown of rocks in situ caused by factors such as freeze-thaw cycles, the growth of salt crystals, acid rain, and the growth of plant roots. Terrestrial erosion involves the wearing away of rocks by wind and rain. Mass movement refers to the removal of cliff-face material under the direct influence of gravity, manifesting as rock falls, slumping, or landslides.
Erosional Landforms: Headlands and Bays
The alternation of headlands and bays is the most common coastal landform, often giving coastlines an irregular appearance. The formation of these features depends on the nature of the coastal rock and its orientation relative to the sea. A concordant coast is one where the rock outcrops run parallel to the sea, typically resulting in straighter coastlines. A discordant coast occurs when rock outcrops are at right angles to the sea. In discordant coasts, weak rocks like clays and sands are easily eroded by destructive waves to form bays. More resistant, harder rocks such as chalk and limestone withstand erosion better and protrude into the sea as headlands. An example of a discordant coast is the Purbeck coast in southern England, where hard chalk and limestone form headlands like The Foreland and Durlston Head, while Studland Bay and Swanage Bay are cut into soft clays and sands.
Cliffs, Platforms, and Headland Features
Most headlands are edged by cliffs. When destructive waves break repeatedly against the base of a cliff, they undercut it to form a wave-cut notch. This undercutting weakens the rock above, eventually leading to its collapse. As the cliff face retreats while maintaining its steep profile, a gently sloping wave-cut platform is left at the base.
Headlands are subject to continuous attack by waves, particularly at points of weakness like faults or joints. Erosion, primarily through hydraulic action and abrasion, opens these weaknesses into caves. If erosion continues or if two caves meet from opposite sides of a headland, an arch is formed. Continued erosion widens the arch and undercuts its pillars until the roof collapses, leaving an isolated pillar of rock called a stack. Further erosion at the base causes the stack to collapse into a stump, which may only be visible at low tide. Old Harry Rocks in southern England is a classic example of these features, occurring at the end of the chalk headland between Swanage Bay and Studland Bay.
Depositional Landforms: Beaches and Spits
Depositional landforms occur where the accumulation of mud, sand, and shingle exceeds the rate at which waves can remove them, typically in areas dominated by constructive waves. Beaches are the most common depositional feature, forming between the storm-beach area and low-tide marks. Beach material comes from offshore sea beds, river estuaries (which contribute mud and silt to sheltered bays), and longshore drift from other coastal areas.
Specific beach features include:
Storm beaches: Located at the top end of the beach, consisting of large material thrown up by storms.
Berms: A series of small ridges marking the average high-tide marks.
Beach cusps: Semi-circular depressions formed by swash and backwash.
Ridges and runnels: Features on the lower beach; runnels are channels that may contain pools of water. In general, finer beach material results in a more gently sloping beach profile.
Spits are long, narrow beaches of sand or shingle attached to the land at one end, extending across a bay or estuary where the coastline changes direction. They are formed by longshore drift. At river estuaries, spits cause rivers to drop sediment, leading to the formation of salt marshes on the landward side. Prevailing winds may cause the end of the spit to curve toward the land. If a spit grows across a bay to link two headlands, it becomes a bar, trapping water behind it to form a lagoon. This requires a gently sloping beach and the absence of a large river. An example of a bar and lagoon is found at Sotavento in Fuerteventura, Canary Islands.
Tombolos, Cuspate Forelands, and Sand Dunes
A tombolo is a spit that grows seaward until it joins the mainland to an island, such as the long tombolo connecting Sharp Island to Kiu Tau in Hong Kong. Cuspate forelands are triangular accumulations of sand and shingle extending seaward, likely formed by the collision of longshore drift from two different directions.
Coastal sand dunes are formed when dry sand from a beach is blown inland by the wind. They develop best with a wide beach, plenty of sand, and an onshore prevailing wind. Sand accumulates around obstacles like driftwood to form embryo dunes. Over time, these grow into foredunes and eventually a series of ridges parallel to the coast. Older, landward ridges are colonized by vegetation like sea twitch and sea couch grass, which stabilizes the dunes. This sequence of colonization and stabilization is called plant succession, eventually leading to dune heath in temperate areas.
Factors Affecting Coastal Environments
Geology is a major influence on coastal shape. Hard rocks result in high, steep, rugged cliffs with boulders at the base. Soft rocks like clays result in lower, less steep cliffs with smoother faces and evidence of slumping, with sand or mud at the base. Geology affects the coast in plan view (headlands and bays) and vertically (cliff height and profile).
Vegetation protects and preserves coastal landforms, provided the plants can survive high salt levels in air and soil. Sea-level changes also play a critical role. Currently, global warming is causing rising sea levels, drowning low-lying coasts. Historically, sea levels fell during the Ice Age as water was locked in ice sheets and rose when they melted. Submerged coastlines feature rias (drowned river valleys) and fjords (drowned glacial valleys, common in Norway). Emerging coastlines feature raised beaches (wave-cut platforms higher than current sea level) and relict cliffs with ancient caves and arches, as seen in Scotland.
Human activities also impact the coast. Settlement is densest in coastal lowlands. Economic development includes agriculture, industry, and the construction of ports for trade and fishing. Tourism exploits sandy beaches and scenery (e.g., the Jurassic Coast World Heritage Site in England). Coastal management uses structures like sea walls and groynes to control erosion and deposition. Energy industries utilize the coast for wind farms and nuclear or thermal power stations.
Coastal Ecosystems: Coral Reefs and Mangroves
Coastal ecosystems are rich in biodiversity. Coral reefs are marine ecosystems built by living organisms and consist of calcium carbonate deposits. Their distribution is controlled by temperature (minimum , best between and ), light (requiring shallow water), water depth (usually less than ), and salinity. Locally, they need well-oxygenated saltwater (strong wave action), limited exposure to air, and clear water free of sediment. High biodiversity is found in South-East Asia and the Great Barrier Reef in Australia.
Mangroves are common in tropical areas, mostly within latitude of the equator. They grow in the intertidal zone and are adapted to changing salinity, heat, and mud. Mangroves have complex root systems: some have snorkel-like roots to take in air from mud, while others have prop roots or buttresses for stability. Their roots trap sediment, building new land, while floating seeds allow them to colonize new areas. South-East Asia contains the highest mangrove biodiversity, with individual trees reaching over in height.
Coastal Ecosystems: Salt Marshes and Sand Dunes
Salt marshes develop in sheltered intertidal zones like estuaries and bay heads. In estuaries, the water is brackish. Salinity and flooding frequency determine the plant and animal types. The low marsh floods twice daily, while the high marsh floods only during storms or high tides. Common plants include salicornia (pioneer), sea manna grass, and sea lavender. Meandering creeks drain tidal water, slowing energy and allowing silt and mud deposition, which causes the marsh to extend seawards.
In sand dunes, plants like marram grass, sea couch grass, and lyme grass must cope with salinity, lack of moisture in draining sand, wind, and submergence by blown sand. As conditions improve, more complex vegetation like gorse, bracken, and eventually trees like oak or pine (climax vegetation) appear. The wet depressions between dunes are called dune slacks.
Nutrient Cycling and Food Webs
Coastal ecosystems function via nutrient cycling between three stores: the nutrient source (soil/mud/sand), biomass (living matter), and litter (dead matter). In mangroves, salt marshes, and dunes, the nutrient cycle is similar to terrestrial ones. In coral reefs, the seawater acts as the 'soil,' providing abiotic nutrients from ocean currents and rivers. Tiny algae within the coral convert sunlight into energy. The biomass includes the coral polyps, seaweeds, fish, and invertebrates. Litter consists of dead coral, fish, and seaweeds, which enriches the seawater with nutrients as it decomposes.
Coral reefs support a complex food web consisting of:
Producers: Plankton and ocean plants.
Primary Consumers (Herbivores): Organisms that eat producers.
Secondary Consumers (Carnivores): Organisms that eat herbivores.
Scavengers and Decomposers: Bacteria, fungi, and detritus feeders that break down dead material.
Questions & Discussion
Q: How does mass movement differ from erosion? Mass movement is the removal of material specifically under the influence of gravity (e.g., landslides), whereas erosion is the wearing away of material by agents like wind, rain, or waves.
Q: What is the difference between a concordant and a discordant coast? A concordant coast has rock layers running parallel to the sea, leading to straighter shorelines. A discordant coast has rock layers at right angles to the sea, leading to headlands and bays.
Q: Why do smaller or lighter materials move further during longshore drift? Smaller materials require less wave energy to remain in transport, allowing them to be carried greater distances before the wave's energy is insufficient to hold them.
Q: What is the significance of the difference between spring and neap tides for salt marshes? Spring tides have a higher range and flood more of the marsh (including the high marsh), while neap tides have a smaller range and may only affect the lower zones. This frequency of flooding dictates the zonation of salt-tolerant plants.
Q: What is the difference between a food web's producers and its consumers? Producers (like algae and plants) create their own energy from sunlight, while consumers must eat other organisms to obtain energy.**