Comprehensive Global Geography: Coastal Systems and Earth's Life Support Systems Study Guide to Life Support Systems

Factors Shaping the Coastal Landscape System as an Open System

A coastal landscape system is defined as an open system, which means it receives inputs of energy and matter, processes them, and produces outputs through a combination of geology, sea level change, and human activity. The relative significance of these factors is determined by the specific type of coastline and the temporal scale involved. Long-term factors, such as geology and wave energy, typically establish the baseline conditions or regional morphology, while short-term factors like human intervention, specific storm events, or sand dredging often determine the immediate rate and intensity of change. Geology exerts a fundamental, long-term control through lithology and structure. Lithology refers to the chemical and physical characteristics of rock types, which dictates susceptibility to geomorphic processes like erosion, weathering, and mass movement. For instance, igneous rocks such as granite possess a strong interlocking crystalline structure and are highly resistant. On a discordant coastline, where bands of alternating rock types run perpendicular to the coast, these resistant rocks form headlands while weaker rocks erode to form bays via differential erosion. An example is the Isle of Purbeck, where the cliffs absorb wave energy, and sediment accumulates in low-energy environments to form beaches like Swanage Bay. This process can lead to a positive feedback loop involving the development of caves, arches, stacks, and stumps. However, short-term events can temporarily override geological controls. In $2013$, the St Oswald’s Bay landslide, caused by heavy prolonged rainfall rather than weak lithology (as the cliffs are resistant chalk), moved large amounts of sediment into the budget and caused the cliff to retreat by $20\,m$.

Sea Level Change and Eustatic Processes

Sea level change is a critical long-term factor, specifically eustatic sea level rise triggered by the melting of ice sheets due to climate change. This rise increases water depth, allowing waves to grow higher and enhancing marine erosion processes like hydraulic action (where compressed air is forced into rock) and abrasion (where waves hurl rocks at cliffs). These processes lead to the formation of submergent landforms such as rias and estuaries. A ria is a flooded V-shaped river valley modified by fluvial and subaerial erosion, which creates a gently sloping valley side. Increased erosional power can increase the depth of the ria, facilitating more powerful waves in a positive feedback loop. Unlike short-term events, sea level change operates continuously across large spatial scales, and its cumulative impact can fundamentally transform entire systems rather than just causing temporary disequilibrium. While geology sets the baseline morphology, sea level change can override geological stability, particularly during interglacial periods, causing significant coastline retreat or advancement.

Human Influence and Shoreline Management

Human activity is another factor shaping the coastal system, though its success often depends on funding for shoreline management plans. At the Sandbanks Peninsula in Dorset, humans used $3$ to $6\,tonne$ boulders of Portland limestone to construct rock groynes along a $1\,km$ stretch of beach. This hard engineering interrupted longshore drift, causing sediment to get trapped and deposited on the updrift side, increasing beach width by $50\,m$. This helped restore dynamic equilibrium by dissipating wave energy through friction. However, human impacts are often spatially uneven; for example, while Sandbanks saw sediment growth, further along at Barton Cliffs, there was a sediment deficit downdrift, leading to continued erosion and rockfalls. Human interventions are generally considered significant only on small temporal scales and are often impermanent unless continuously repaired or repeated. They redistribute sediment rather than increase overall inputs, lacking the widespread influence of long-term physical processes like sea level change.

Geomorphic Processes and Coastal Landform Formation

Geomorphic processes include erosion, deposition, mass movement, transportation, and aeolian and fluvial processes. These are essential for the distribution of sediment and the formation of landforms, and their importance varies by environment. Marine erosion is the primary driver for cliff formation, where destructive waves use hydraulic action and abrasion to create a wave-cut notch, undercutting the rock and removing support. Weathering processes—including mechanical freeze-thaw and biological weathering—weaken the cliff top by exploiting joints and cracks. Mass movement is the final mechanism of cliff retreat, occurring when undercutting causes the overlying rock to collapse due to gravity through rockfalls or slumping. These processes are interdependent: weathering weakens rock, erosion undercuts it, and mass movement removes the debris, which is then used for further abrasion in a positive feedback loop. In low-energy environments, deposition and transportation (specifically longshore drift) are more significant. For example, at Mangawhai–Pakiri beaches in New Zealand, longshore drift maintains drift-aligned beaches and dune systems. Deposition occurs where wave energy decreases, often relying on constructive waves with a stronger swash than backwash.

Human Disruption of Geomorphic Processes

Human activity can drastically shift the balance of geomorphic processes, often making erosion more dominant. At Mangawhai–Pakiri, large-scale dredging of Holocene sand has removed approximately $170,000\,m^3$ of sediment annually for over $70\,years$. This extraction rate is roughly five times greater than natural inputs from rivers or waves. Since this sand is non-renewable Holocene material, the impact is effectively irreversible. The resulting sediment deficit leads to narrower, flatter beaches that cannot dissipate wave energy effectively, concentrating energy on the coastline and increasing marine erosion. This has led to the undercutting of foredunes, the formation of steep scarps, and loss of stabilizing vegetation. High-magnitude storm events, such as the $1978$ breach at Mangawhai Spit, exacerbate these human-induced changes by altering tidal currents and redistributing sediment within the system, often increasing deposition in harbors while intensifying instability elsewhere.

Terrestrial, Marine, and Offshore Sediment Inputs

Coastal systems receive sediment from terrestrial, marine, and offshore sources. Terrestrial sources include fluvial deposition, cliff erosion, and mass movement. Fluvial inputs are dominant in low-energy environments; for example, Farewell Spit in New Zealand receives around $2.7\,million\,tonnes$ of sediment annually from the Aorere and Tākaka Rivers. This sediment is deposited onto tidal flats and redistributed by longshore drift, allowing the spit to extend to $27\,km$. Conversely, marine and offshore sources can dominate in other environments. Chesil Beach in Dorset is primarily formed from shingle derived from offshore continental shelf stores, transported landwards during post-glacial sea level rise. Human activity also adds to the sediment budget through beach nourishment. At Sandbanks, offshore dredged sediment is added to replace material lost to longshore drift, though this requires constant repetition. Overall, terrestrial inputs are vital for systems with strong river flow, while offshore sources are more significant for landforms like Chesil Beach produced by historical sea level trends.

High-Energy vs. Low-Energy Coastlines

High-energy coastlines, such as the Isle of Purbeck, are characterized by strong prevailing winds and wave energy. The long fetch of approximately $8000\,km$ across the Atlantic creates destructive waves exceeding $2.5\,m$ in height. Geology here is the primary control; high-energy environments feature discordant and concordant coastal structures. At Lulworth Cove, a meltwater river breached the Portland limestone at the end of the last glacial period, allowing waves to erode the weaker Wealden Clay behind it to form a circular cove. Low-energy coastlines, such as Farewell Spit, are driven by sediment supply. These areas are dominated by constructive waves and high rates of deposition. At Farewell Spit, the prevailing westerly winds generate longshore drift that moves sand eastwards. While high-energy coasts are shaped by erosion and wave refraction (concentrating energy on headlands and dispersing it in bays), low-energy coasts rely on consistent sediment budgets to form features like mudflats and salt marshes. Both systems are influenced by short-term high-magnitude events like storms (e.g., Cyclone Drena in $1997$ at Farewell Spit).

The Interrelatedness of Coastal Landforms

Coastal landforms are linked through feedback loops, sediment transfers, and sequential development. In high-energy systems like Kimmeridge Bay, a negative feedback loop exists where cliff retreat creates a wave-cut platform that dissipates wave energy, slowing further erosion. At Durdle Door, cliff erosion provides sediment to the beach, which then protects the cliff from further wave attack. Relationships can also be unidirectional, where geology dictates the pattern (e.g., discordant coast headlands and bays). In low-energy systems, the interrelatedness is often stronger due to active sediment recycling. Farewell Spit’s beach–dune–mudflat system operates as a linked network: constructive waves deposit sand on the beach, winds move it to form dunes, and vegetation (like marram grass) stabilizes them. During storms, sediment is moved from dunes back to the beach. Such systems aim to maintain dynamic equilibrium through continuous seasonal transfers.

The Global Importance of the Water Cycle

Water is a finite resource fundamental for climate regulation, human survival, and economic development. The oceans, the largest store in the hydrosphere, moderate global temperatures by absorbing and releasing heat. Water vapor is the most abundant greenhouse gas, contributing to a natural greenhouse effect that keeps the Earth approximately $15\,^\circ C$ warmer than it would be otherwise. Biologically, water constitutes $60\%$ of the human body and $65$ to $95\%$ of most living organisms, acting as a medium for metabolic reactions, nutrient transport, and thermoregulation through sweating (where hydrogen bonds break to remove heat). Economically, agriculture consumes $70$ to $85\%$ of global freshwater for irrigation. In low-income countries (LIDCs), water is essential for hydroelectric power and hygiene; access to safe water is a determinant of social development, as contaminated supplies spread diseases like cholera. Flora also rely on water as a reactant in photosynthesis and to maintain cell turgidity for structural support. In some ecosystems, water availability determines global biomes; for example, the Amazon supports biodiversity through consistent rainfall and high primary productivity.

The Global Importance of the Carbon Cycle

Carbon is the basis of all life, with humans being $18\%$ carbon-based lifeforms. Carbohydrates, proteins, and lipids are all carbon compounds. It is cycled through photosynthesis (where plants convert $CO_2$ into glucose) and respiration. The Amazon Rainforest acts as a significant global carbon sink and a site for oxygen production. Economically, fossil fuels (coal, oil, gas) provide about $80\%$ of global energy, driving industrialization and the petrochemical industry (plastics, pharmaceuticals). Carbon also regulates climate; $CO_2$ and methane ($CH_4$) trap heat to keep Earth habitable. However, mismanagement leads to the enhanced greenhouse effect. The long-term carbon cycle stores carbon in rocks and oceans over millions of years, while the short-term cycle moves carbon between organisms and the atmosphere. Carbon and water cycles are interdependent; for example, increased $CO_2$ raises global temperatures, leading to permafrost thaw (releasing more carbon) and increased ocean evaporation (adding water vapor, another greenhouse gas, to the atmosphere).

Impacts on Tropical Rainforests and Arctic Tundra

Human activities like deforestation in tropical rainforests disrupt both cycles. Removing the canopy reduces interception and evapotranspiration, increasing surface runoff and flood risk while weakening convectional rainfall. Deforestation also releases $CO_2$ through burning and reduces photosynthetic capacity; in the Amazon, clearing can reduce carbon storage by $30$ to $60\%$. Management strategies like REDD+ (Reducing Emissions from Deforestation and Forest Degradation) provide financial incentives for conservation. In the Arctic tundra, the water cycle is controlled by permafrost and low temperatures. Fossil fuel extraction and infrastructure (pipelines, roads) disrupt permafrost, leading to thermokarst lakes and altering drainage. Arctic amplification causes the tundra to shift from a carbon sink to a carbon source as thawing permafrost releases methane and $CO_2$. Positive feedback loops occur where warming leads to more thaw and more greenhouse gas release.

Management of Water and Carbon Cycles

Efforts to manage human disruption include international agreements like the Paris Climate Agreement, which targets limiting temperature rise to below $2\,^\circ C$ and achieving net zero by $2050$. Other strategies include afforestation, wetland restoration, and improving drainage basin planning. Wetland restoration is effective because wetlands act as major water stores that increase infiltration and evapotranspiration. Afforestation increases carbon sequestration and interception. However, these strategies face challenges such as economic dependence on fossil fuels, illegal logging, and the irreversibility of certain feedback loops like permafrost thaw. While management can mitigate impacts locally and regionally, global-scale changes are difficult to reverse entirely due to the large volume of accumulated atmospheric carbon and the persistent pressure of economic development.