Coastal erosion
Coastal erosion is the loss of land along shorelines due to the net removal of sediments or bedrock by marine processes, including wave action, currents, tides, and wind.[1][2] This geological process, driven primarily by the energy of waves through mechanisms such as hydraulic action, abrasion, and attrition, shapes coastlines over time but varies widely in rate based on local geology, sediment supply, and exposure to hydrodynamic forces.[3][4] While erosion is a natural phenomenon that has operated throughout geological history, human interventions—such as the construction of seawalls, groins, and dams that interrupt longshore sediment transport—often exacerbate retreat in vulnerable areas by reducing natural beach nourishment.[5][6] Natural factors like storm surges and subsidence also contribute significantly, with global patterns showing that approximately 40% of coastlines experience ongoing erosion, though rates range from negligible to over 7 meters per year in high-energy environments.[7][8] The phenomenon impacts economies through property losses, infrastructure damage, and diminished coastal tourism, with affected regions facing annual costs in the billions alongside environmental degradation of habitats like dunes and wetlands.[9][10] Defining characteristics include episodic acceleration during storms versus chronic baseline retreat, prompting debates over attribution between cyclical natural variability, tectonic subsidence, and anthropogenic influences like altered sediment budgets rather than uniform sea-level drivers.[11][12] Mitigation strategies, from beach nourishment to managed realignment, highlight trade-offs in cost-effectiveness and ecological outcomes, underscoring the challenge of balancing development with dynamic coastal geomorphology.[13]Definition and Fundamental Processes
Definition and Scope
Coastal erosion constitutes the landward retreat of shorelines through the net removal of sediments, soils, bedrock, or other coastal materials by marine hydrodynamic forces, primarily waves, currents, and tides.[14] This process involves the mechanical breakdown and transport of material, leading to the reconfiguration or loss of beaches, dunes, cliffs, and barrier islands.[1] Unlike transient sediment shifts during individual events, erosion is characterized by sustained imbalance in sediment budgets, where removal exceeds supply or deposition.[15] The scope of coastal erosion encompasses both subaerial weathering—such as rain, wind, and mass wasting on exposed slopes—and marine processes like abrasion, hydraulic action, attrition, and corrosion that dominate in the intertidal and subtidal zones.[4] It affects diverse coastal morphologies worldwide, including sandy beaches, muddy estuaries, rocky headlands, and glacial coasts, though rates vary significantly by substrate resistance; unconsolidated sands erode at rates up to several meters per year, while resistant bedrock may recede at millimeters annually.[14] Erosion manifests as chronic, gradual shoreline migration or acute events triggered by storms, with global prevalence along approximately 70% of sandy shorelines exhibiting net retreat over decadal scales.[7] Distinguishing erosion from related phenomena, its scope excludes accretion (sediment buildup) and focuses on disequilibrium states where energy inputs from ocean dynamics outpace stabilizing factors like vegetation or geological uplift.[16] While naturally occurring, the process influences human settlements, ecosystems, and infrastructure within the coastal zone, typically defined as extending from the high-water mark to several kilometers inland and offshore, where sediment dynamics interconnect land and sea.[17] Empirical observations confirm its ubiquity on open-ocean coasts, with lesser incidence in sheltered bays or areas of tectonic emergence.[18]Core Physical Mechanisms
Coastal erosion arises from the interaction of hydrodynamic forces with shoreline materials, where waves, currents, and tides deliver kinetic energy that exceeds the resistance of sediment or bedrock, leading to material removal and transport. Waves constitute the primary driver, as their breaking concentrates energy dissipation at the shoreface, with typical wave heights of 1-3 meters in moderate conditions capable of eroding unconsolidated sands at rates of 0.1-1 meter per event, escalating during storms to 10-50 meters of retreat. This energy transfer follows principles of wave refraction and shoaling, where waves slow and steepen upon approaching shallower depths, amplifying shear stresses on the bed up to 10-20 N/m².[19] [20] Key wave-induced mechanisms include hydraulic action, in which the explosive release of compressed air and water forced into rock cracks by wave impact generates cyclical pressures exceeding 2-5 MPa, fracturing bedrock over repeated cycles; and abrasion, where entrained sand and gravel particles, accelerated to velocities of 1-5 m/s, scour surfaces akin to sandblasting, removing material at rates proportional to sediment load and wave orbital velocity. Attrition complements these by fragmenting clasts through mutual collisions during transport, reducing particle size and increasing mobility, while corrosion—chemical dissolution by seawater—predominantly affects calcareous rocks, dissolving carbonates at pH-dependent rates of 0.1-1 mm/year in temperate climates. These processes are most intense on exposed, high-energy coasts, where fetch lengths over 100 km generate waves with periods of 8-12 seconds.[21] [22] Currents, including longshore drift generated by oblique wave approach, redistribute sediment parallel to the shore at speeds of 0.5-2 m/s, causing erosion downdrift of sediment sources or structures that interrupt supply, with net transport rates quantified via the CERC formula as Q = K \cdot H_b^{5/2} \cdot \sin(2\alpha_b), where H_b is breaker height and \alpha_b is angle. Rip currents and tidal flows further enhance erosion by creating localized high-velocity zones (up to 1-2 m/s) that suspend and remove fine sediments, while tidal ranges of 2-6 meters in macrotidal settings periodically shift the erosion baseline, exposing bluffs to subaerial weathering or inundating beaches. Wind-driven processes, such as aeolian transport, amplify marine erosion by deflating dry beach sands, though their contribution is secondary to hydrodynamic forces in most settings.[23] [24]Geological and Historical Context
Long-Term Geological Dynamics
Over geological timescales spanning thousands to millions of years, coastal erosion is primarily shaped by the interplay of tectonic uplift and subsidence, eustatic sea-level fluctuations, and isostatic adjustments, which collectively determine relative sea-level positions, sediment supply, and exposure to marine processes.[18][25] Tectonic forces, particularly at active margins, elevate resistant bedrock into cliffs and marine terraces, buffering short-term wave attack while facilitating long-term dissection through faulting and seismic triggers; for instance, subduction zones along the U.S. West Coast have produced uplifted terraces hundreds of meters above sea level, with average long-term uplift rates of approximately 1 mm per year over the late Quaternary (>125 ka).[18][26] In contrast, subsidence in passive margins or forearc settings, such as parts of the U.S. East Coast, promotes submergence and accelerated shoreline retreat by increasing hydrodynamic energy on low-relief substrates.[18][26] Eustatic sea-level variations, driven by glacial-interglacial cycles and changes in ocean basin volume, induce transgressive and regressive phases that migrate shorelines and redistribute sediments, often eroding coastal plains during rapid rises.[25] During the Pleistocene, glacio-eustatic oscillations of up to hundreds of meters over 10,000–100,000-year cycles exposed continental shelves during lowstands (e.g., ~120 m below present during the Last Glacial Maximum ~21,000 years ago), reducing wave-base erosion, while subsequent post-glacial rises of ~130 m over 18,000 years advanced shorelines landward by 10–20 km in areas like California, carving platforms and notching cliffs.[18][25] These fluctuations, reconstructed from proxies such as coral reefs and oxygen isotopes, control the formation of stacked coastal sequences, with highstands favoring barrier island development and transgressions enhancing basal undercutting in cliffed coasts.[25] Glacial isostatic adjustment (GIA) further modulates these dynamics by redistributing crustal loads post-deglaciation, causing uplift in formerly glaciated interiors (e.g., rates up to several mm per year in Scandinavia and Hudson Bay) that exposes ancient shorelines above modern levels and promotes progradation, while peripheral forebulge collapse induces subsidence (e.g., up to 0.5 feet per century projected in Chesapeake Bay), elevating relative sea levels and intensifying erosion on subsiding margins.[27][25] In the U.S. East Coast and Great Lakes, GIA-driven subsidence since the Holocene has compounded tectonic stability, leading to broader platforms and higher bluff recession potential from wave and groundwater processes on glacial sediments.[18][27] Long-term erosion rates in such settings, averaged over Quaternary cycles, range from 0.1–0.3 m per year in sedimentary cliffs but are modulated downward by tectonic emergence or upward by subsidence, with episodic landslides and joint-controlled block falls dominating morphology evolution.[18]Historical Erosion Patterns and Pre-Industrial Rates
Coastal erosion has shaped shorelines throughout the Holocene epoch, following the stabilization of sea levels around 6,000–7,000 years before present, when transgression rates slowed dramatically from earlier millennial-scale advances. During this late Holocene period of relative sea-level stability, erosion patterns were dominated by local factors including sediment supply, tectonic setting, and storm frequency, resulting in episodic retreat punctuated by periods of stability or accretion rather than uniform global degradation. Reconstructions from geological proxies, such as dated peat layers and submerged forests, indicate that many coastal systems achieved dynamic equilibrium, with erosion rates offset by sediment deposition from rivers and longshore transport, particularly in deltaic and barrier island environments.[28] Pre-industrial erosion rates, prior to significant anthropogenic influences like river damming and coastal hardening around the mid-19th century, varied widely by lithology and exposure. On rocky cliffs, cosmogenic ^{10}Be dating of shore platforms in Del Mar, California, yields average retreat rates of 5.0–12.5 cm yr^{-1} over the last two millennia, consistent with modeling that attributes this to steady wave undercutting under near-stable sea levels rising at 0.8 ± 0.3 mm yr^{-1}. In contrast, softer sedimentary coasts exhibited higher rates; for instance, analyses of late Holocene barrier island evolution in the U.S. Gulf of Mexico show localized erosion interspersed with net progradation at +2.1 m yr^{-1} since approximately 2.8 ka BP, driven by sediment fluxes exceeding wave removal until historical disruptions. Arctic permafrost coasts, even in the early historical period (1850–1950), averaged 0.9 ± 0.4 m yr^{-1}, highlighting thermal thawing as a persistent driver independent of industrial emissions.[29][28][30] Historical patterns from archival maps and chronicles, extending back to Roman and medieval eras, document chronic retreat on exposed temperate coasts, such as England's Holderness region, where approximately 3 km of land loss occurred over two millennia at rates of 1–2 m yr^{-1}, engulfing villages and prompting early defensive measures like groynes by the 16th century. These rates, derived from sequential shoreline positions and eyewitness accounts, underscore that pre-industrial erosion was often storm-amplified, with decadal bursts exceeding annual averages by factors of 10 or more, yet constrained by natural sediment replenishment absent modern interventions. Globally, compilations of cliff recession data suggest medians of 2.9 cm yr^{-1} for hard rock, 10 cm yr^{-1} for medium, and 23 cm yr^{-1} for weak substrates, reflecting intrinsic material resistance over extended pre-1850 baselines. Such variability cautions against extrapolating localized high-erosion sites to entire coastlines, as many regions maintained near-zero net change through Holocene-scale feedbacks.Causes and Driving Factors
Natural Drivers
Natural drivers of coastal erosion primarily involve hydrodynamic, meteorological, geological, and climatic processes that reshape shorelines through physical forces and sediment dynamics. Wave action serves as the dominant mechanism, where breaking waves exert shear stresses, causing abrasion of coastal materials via hydraulic pressure and quarrying, while also transporting sediments alongshore or offshore.[31] [32] In areas with high wave energy, such as exposed ocean coasts, annual erosion rates can reach 1-2 meters in unconsolidated sediments, driven by the fetch—the unobstructed distance over which wind generates waves—and wave period.[33] [31] Tides and currents further modulate erosion by altering water levels and sediment flux; macro-tidal regimes with ranges exceeding 4 meters, as in the Bay of Fundy, enhance inundation and wave attack during high tides, while longshore currents redistribute sand, leading to localized retreat where gradients in transport capacity exist.[31] [12] Storm events intensify these effects, with extratropical cyclones or hurricanes generating waves up to 10-15 meters high and storm surges elevating water levels by 2-5 meters, resulting in episodic erosion that can remove decades of sediment accumulation in hours.[34] [31] For instance, El Niño winters on the U.S. West Coast correlate with increased southerly wave directions, accelerating northern-facing beach erosion by up to 50% compared to neutral conditions.[35] Geological factors, including lithology and tectonics, determine baseline susceptibility; soft cliffs of clay or sand erode at rates 10-100 times faster than resistant bedrock, while natural subsidence from isostatic adjustment or tectonic downdropping lowers relative land elevation, amplifying marine incision.[34] [33] In tectonically active margins like California's coast, subsidence rates of 1-3 mm/year combine with wave forcing to sustain long-term retreat, independent of anthropogenic influences.[34] [36] Natural sea-level fluctuations, such as those from glacial isostatic rebound or interannual variability like ENSO, contribute variably; post-glacial eustatic rise has largely stabilized globally at under 1 mm/year, though regional deviations persist due to vertical land motion.[37] [36] These drivers interact dynamically, with wind-generated currents and tidal asymmetries creating hotspots of erosion in fetch-exposed, low-relief shores.[38]Anthropogenic Contributors
Human activities have significantly altered natural sediment dynamics, exacerbating coastal erosion rates in many regions. Construction of dams and reservoirs along rivers has trapped vast quantities of sediment that would otherwise replenish coastal zones, leading to "sediment starvation" and accelerated shoreline retreat. For instance, global dam proliferation since the mid-20th century has reduced sediment delivery to deltas by up to 50% in some systems, contributing to erosion rates exceeding 10 meters per year in areas like the Mekong Delta.[39][40] In the United States, dams on major rivers such as the Mississippi and Colorado have similarly diminished downstream sediment flux by 80-90%, correlating with observed subsidence and land loss along adjacent coasts.[41] Removal of dams, as in the case of the Elwha River in Washington state, has demonstrated reversal of this effect, with post-2011 dam removal leading to sediment accretion and shoreline stabilization at rates of up to 0.6 meters per year.[40] Coastal armoring structures, including seawalls, groins, and jetties, intended to protect infrastructure, often intensify erosion by disrupting longshore sediment transport. These hard engineering interventions reflect waves directly onto adjacent beaches, scouring sand and causing downdrift erosion rates to increase by 2-5 times compared to unarmored shores.[42][43] In California, where approximately 14% of the coastline is armored, studies indicate accelerated beach narrowing and habitat loss, with erosion impacts extending hundreds of meters beyond the structures.[44] Similarly, on the U.S. Gulf Coast, armoring has contributed to unprecedented historical erosion, compounding direct human modifications like channel dredging.[28] Peer-reviewed analyses emphasize that while armoring halts bluff recession at the site, it transfers erosion burdens elsewhere, reducing overall coastal resilience without addressing underlying sediment deficits.[45] Land-use changes, such as urbanization, deforestation, and wetland drainage, further promote erosion by destabilizing soils and eliminating natural buffers. Development on dunes and removal of vegetation diminish wave energy dissipation, elevating erosion vulnerability; for example, in Mediterranean coastal zones, historical human settlement has reshaped geomorphology, with modern expansion correlating to retreat rates of 0.5-2 meters annually in built-up areas.[46] Groundwater extraction for urban and agricultural purposes induces subsidence, amplifying relative sea-level rise and erosion; in coastal cities like Jakarta, this has driven land loss at rates up to 10 cm per year.[47] Infrastructure like navigation channels and ports mirrors groin effects by interrupting sediment flow, as evidenced in global analyses showing squeezed sandy coasts with reduced adaptive capacity due to human encroachment.[48] These interventions, while providing short-term protection, underscore a causal chain where sediment interruption and habitat alteration override natural equilibrium, necessitating evidence-based reassessment of long-term efficacy.[7]Measurement, Monitoring, and Rates
Techniques and Technologies for Assessment
Assessment of coastal erosion relies on a combination of remote sensing technologies for broad-scale monitoring and ground-based methods for high-precision local measurements, enabling quantification of shoreline retreat rates and sediment volume changes over time. Remote sensing techniques, such as satellite imagery and LiDAR, facilitate repeated, cost-effective surveys across large areas, capturing temporal dynamics without direct human intervention in hazardous zones.[49] Ground-based approaches complement these by providing detailed validation data, particularly for validating remote observations against empirical site-specific erosion.[50] Integration of these data through geographic information systems (GIS) allows for spatial analysis and predictive modeling of erosion vulnerability.[51] Satellite-based remote sensing, including optical sensors from missions like Landsat and Sentinel-2, detects shoreline positions by identifying water-land interfaces through image classification and time-series analysis, achieving sub-pixel accuracy for erosion rates on the order of meters per year.[52] These methods have been applied to monitor historical changes spanning decades, such as in Campeche, Mexico, where GIS-processed satellite data revealed spatiotemporal shoreline shifts influenced by natural and human factors.[53] LiDAR, particularly airborne topographic LiDAR, generates high-resolution digital elevation models (DEMs) with vertical accuracies of 10-15 cm, quantifying cliff-top retreat and beach volume loss by differencing sequential surveys; for instance, NOAA's coastal LiDAR datasets support erosion assessments along U.S. shorelines by mapping subtle topographic changes post-storm events.[54][55] Unmanned aerial vehicles (UAVs) equipped with photogrammetry extend these capabilities to finer resolutions, producing orthomosaics and DEMs for near-real-time monitoring of dynamic beaches.[50] Ground-based techniques include erosion pins—metal rods inserted into sediment or cliffs to measure retreat distances manually or via repeat surveys—and differential GPS for beach profiling, which tracks cross-shore elevation changes with centimeter-level precision to compute sediment budgets.[56] Terrestrial photogrammetry and structure-from-motion multi-view stereo (SfM-MVS) methods reconstruct 3D models from ground-based photographs, enabling erosion volume calculations on cliffs with accuracies rivaling LiDAR, as demonstrated in studies of coastal bluffs where sub-centimeter changes were detected over seasonal cycles.[50] Bathymetric surveys using multibeam sonar complement these by assessing nearshore profile adjustments that drive onshore erosion.[57] GIS-based modeling integrates multi-source data for comprehensive assessments, employing tools like the Digital Shoreline Analysis System (DSAS) to calculate long-term erosion rates from transects perpendicular to shorelines, incorporating variables such as wave exposure and sediment supply.[52] Vulnerability indices derived in GIS, such as those weighting geomorphology, elevation, and historical rates, predict future risks; a 2021 study in Brazil used GIS to map mangrove-adjacent erosion vulnerability, revealing high-risk zones with retreat rates exceeding 5 m/year.[51] Numerical models, calibrated with observed data, simulate hydrodynamic forcing on erosion processes, though their accuracy depends on high-fidelity input from remote and in-situ measurements.[53] These approaches collectively enable evidence-based management, with ongoing advancements in machine learning enhancing automated shoreline extraction from imagery.[58]Empirical Data on Erosion Rates and Trends
Global assessments of sandy beaches indicate that approximately 24% are eroding at rates exceeding 0.5 meters per year based on satellite observations from 1984 to 2016.[59] Over a similar period (1984–2015), satellite data reveal a net loss of about 14,000 km² of coastal land worldwide, with erosion exceeding accretion in 13% of analyzed transects by more than 50 meters, compared to 8% showing comparable accretion.[60] These changes are concentrated in regions like Asia, where over 50% of global coastal alterations occur, including substantial erosion along Pacific Asia and southern American coasts.[60] For coastal cliffs, a database compiling 1,530 sites from peer-reviewed measurements reports median recession rates varying by rock resistance: 2.9 cm per year for hard rocks, 10 cm per year for medium rocks, and 23 cm per year for weak rocks. Rates can reach up to 85 cm per year in the 83rd percentile for weak rocks, with rock strength emerging as the primary control rather than marine forcing in many cases. In California, long-term (120-year) data show 40% of beaches eroding, rising to 66% in short-term (25-year) analyses, while statewide cliff retreat averages 0.3 meters per year over 70 years, with higher rates in northern regions due to landslides.[61] Regional trends highlight variability and accelerations in specific areas. In the U.S. Gulf Coast, particularly south Louisiana, wetlands have declined by about 25%, or 5,000 km², from 1932 to 2016, driven by subsidence and sediment deficits.[28] Arctic coastal permafrost erosion rates have increased by 80% to 160% at monitored sites since baseline periods, with mean annual rates now exceeding historical values amid warming.[62] On U.S. Southeast barrier islands, recession rates of 7.6 meters per year are common, with episodes reaching 15 meters per year.[8] Despite these erosional hotspots, recent satellite surveys suggest a slight net growth in global sandy beach area over the past three decades, though with increasing variability tied to climate oscillations like El Niño.[63]| Region/Coast Type | Key Erosion Metric | Period | Source |
|---|---|---|---|
| Global Sandy Beaches | 24% eroding >0.5 m/yr | 1984–2016 | [59] |
| Global Coasts | Net land loss 14,000 km² | 1984–2015 | [60] |
| Coastal Cliffs (Weak Rocks) | Median 23 cm/yr | Various | |
| California Beaches | 66% eroding (short-term) | Recent 25 yrs | [61] |
| U.S. Gulf Wetlands | 5,000 km² loss (~25%) | 1932–2016 | [28] |
| Arctic Permafrost Coasts | 80–160% rate increase | Recent vs historical | [62] |