Coastal Morphodynamics: Comparison
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Please note this is a comparison between Version 2 by Jessie Wu and Version 1 by Lynn Donelson Wright.

The shape of the coast and the processes that mold it change together as a complex system. There is constant feedback among the multiple components of the system, and when climate changes, all facets of the system change. Abrupt shifts to different states can also take place when certain tipping points are crossed. The coupling of rapid warming in the Arctic with melting sea ice is one example of positive feedback. Climate changes, particularly rising sea temperatures, are causing an increasing frequency of tropical storms and “compound events” such as storm surges combined with torrential rains. These events are superimposed on progressive rises in relative sea level and are anticipated to push many coastal morphodynamic systems to tipping points beyond which return to preexisting conditions is unlikely. 

  • sea level rise
  • land loss
  • coastal inundation
  • wetlands
  • estuaries
  • Arctic coasts
  • coral reefs
  • compound flooding

1. Coastal Inundation and Climate Change

Climate change is increasing the inundation of coasts, particularly low-elevation coasts, on many spatial and temporal scales. Long-term rises in global sea levels on decadal time scales as well as interannual sea level fluctuations and localized, short-lived flooding events are largely attributable to increasing sea surface temperatures. A recent comprehensive NOAA report [19][1] states, with high confidence, that “By 2050, the expected relative sea level (RSL) will cause tide and storm surge heights to increase and will lead to a shift in U.S. coastal flood regimes, with major and moderate high tide flood events occurring as frequently as moderate and minor high tide flood events occur today." “Without additional risk-reduction measures, U.S. coastal infrastructure, communities, and ecosystems will face significant consequences”. The NOAA report predicts that between now and 2050, US mean sea levels will rise an additional 0.25–0.30 m. In many localities, the relative sea level rises will be significantly greater because of land subsidence and other non-tidal effects such as reductions in the strength of the Atlantic Meridional Overturning Circulation (AMOC) [20,21][2][3]. At the present time, the global averaged rate of mean sea level rise is 3.3 mm/yr but is accelerating [19][1]. Rates of rise significantly exceed the global average in many localities for a variety of reasons. Since 1993, the rates of sea level rise over most of the western Pacific, including most of the Australian coast, have exceeded the global rate. https://soe.dcceew.gov.au/climate/environment/sea-level (accessed on 30 August 2023).
From a morphodynamic standpoint, it is not so much the global rises in mean sea level that dominate the short-term flooding of coasts and associated shoreline fluctuations as the storm surges and powerful storm waves that are superimposed on the rising seas. Fueled by increasing sea surface temperatures, compound floods involving combined storm surges and torrential rains are becoming more frequent and severe. Sea level rises are exacerbating coastal recessions by allowing the landward penetration of destructive waves [22][4]. Increasingly, new international research is being directed at the nature and impacts of complex and protracted disasters that are compounding and cascading [23][5]. A prominent recent example was the devastating impacts of Hurricane Ian on southwest Florida in September 2022. The joint probability of severe storm surges and torrential rains coinciding in U.S. coastal cities has increased significantly over the past century [24,25,26][6][7][8]. In cases where rivers are nearby, fluvial flooding further exacerbates the severity of the inundation [27][9]. A recent analysis downscaled multiple hurricane models to create a synthetic model that predicts that climate change is increasing hurricane risk for the Gulf Coast and Atlantic Coast [28][10]. The International Human Dimensions Program on Global Environmental Change points out that by midcentury, the flood risk to large coastal cities will have increased by ninefold relative to the present day. According to NOAAs Office of Coastal Management, inundation events are the dominant causes of natural-hazard-related deaths in the U.S. and are also the most frequent and costly of the natural hazards affecting the nation. The landward penetration of destructive storm waves into the floods can erode and redistribute large quantities of sediment in short periods of time, resulting in significant changes in coastal morphology. This can amplify or otherwise alter the behavior of surges.
At both long-range and event time scales, significant advances are progressively being made in modeling storm surges, storm waves, and coastal circulation. One of the simplest and lowest resolution models is NOAAs operational two-dimensional Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model. The academic community uses more accurate, unstructured grid models of coupled surge-wave effects [29][11]. Although those models yield better results than the operational, long-standing two-dimensional SLOSH model used by NOAA for several decades, SLOSH continues to be the operational model of choice because it is well accepted, fast, and does not require High Performance Computing or HPC (advanced computing) resources. NOAA has recently upgraded its Probabilistic Slosh model, named P-Surge, to version 3 [30][12]. Figure 1 shows a U.S. National Hurricane Center prediction of storm surge heights associated with landfalling Category 4 Hurricane Ian on the coast of South Florida on the U.S. Gulf of Mexico on September 28, 2022, based on output from NOAAs SLOSH storm surge model.
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Figure 1. National Hurricane Center predictions of storm surge inundation utilizing NOAA’s SLOSH model of the “maximum of maximum envelopes of water” (MOMS) associated with landfalling Category 4 Hurricane Ian on the South Florida Coast on 28 September 2022. Inundation of the red areas was predicted to exceed 9 feet (~3 m) above existing ground levels. Source: NOAA National Weather Service.
For timely, rapid forecasts of impending surges, updated P-Surge versions of SLOSH are likely to remain the operational standard for the near future. However, for anticipating potential, but not impending, future scenarios, more advanced and time-consuming models that utilize HPC resources are the current state of the art. Some of the more widely used models are described in [29,31,32,33][11][13][14][15]. Storm surge and wave modeling, together with tidal modeling, are essential for coastal erosion/deposition and coastal inundation estimates. A recent NOAA-IOOS-funded Coastal Ocean Modeling Testbed (COMT) program [29[11][15],33], involving over 20 academic institutions as well as several federal research centers, focused on improving forecasts of coastal waves, storm surges, and inundations. Included among the six models evaluated was the well-known three-dimensional, finite element Advanced Circulation (ADCIRC) [34][16] model, which was coupled with the Simulating Waves Nearshore (SWAN) wave prediction model developed by the Delft University of Technology. Additional models that may be applied to surge and wave modeling include Delft3D, WAVEWATCH III®, and GeoClaw.
The ADCIRC model was utilized by [35][17] in an assessment of the probable impacts of climate change on storm surges affecting the US coasts. That study concluded that volumes of flood waters from US Gulf Coast and Atlantic Coast storm surges are increasing and are expected to continue to increase and become more severe in the years ahead. The authors also point out that future storm surge severity and impacts will be difficult to predict. Challenges facing reliable projections of future impacts include the complex interdependence of spatially variable storm characteristics and coastal configurations linked to changing regional patterns of sea surface temperatures, which will require probabilistic assessments. The morphodynamic interdependence of nearshore morphology and the spatiotemporal variability of winds, surges, and waves represents an important area for future research and model development. Some insightful studies in eastern Australia have highlighted these interdependent relationships [36][18].

2. Tipping Points in Coastal Systems

Physical, environmental, socioeconomic, and engineered changes in coastal areas interact, often in unforeseen ways that may involve positive feedback, leading to a tipping point at which one more small change results in a large destabilization [11,37,38,39][19][20][21][22]. At this point, the environmental system can enter a new state beyond which recovery to earlier conditions may be unlikely. Based on insights from multidisciplinary analyses of the Santa Barbara, CA, coast [36][18], it was concluded that the tipping point for serious degradation of beaches and wetlands could be crossed when sea level rises reach 0.25 m or even less. In the case of the morphodynamics of river deltas, a recent study of historical delta evolution showed that the rate of rise of relative sea level (including subsidence) tipping point for delta accretion to keep pace with submergence has been about 5 mm/yr [39][22]. Today, however, the severance of sediment supply to the coast by dams on rivers has probably lowered that critical tipping point in many cases. Once the tipping point has been exceeded, delta growth cannot keep pace with more rapid rates of SLR, and open water will replace dry land [39,40][22][23]. In a study of tidal salt marsh equilibrium [41][24], it was found that marsh growth can keep pace with rising relative sea level at rates up to 1.2 cm/yr; however, when the rate of rise exceeds that critical tipping point, subaerial marsh surfaces are replaced by tidal flats. The concept of tipping points can be applied where communities along the coast are subject to a variety of mounting pressures resulting from ongoing coastal land loss and inundation [42][25]. However, wresearchers must take care not to simply extrapolate on determinations of SLR tipping points from one study to another without understanding past and present morphodynamic processes that control morphologies and sediment budgets in the area of concern. Time lags of varying durations typically exist in shoreface adjustment of boundary conditions due to changes such as SLR and can involve long morphological-response timescales across the lower shoreface [43][26].
A somewhat less well-known but emerging potential tipping point affecting the entire Atlantic coast of North America involves the Atlantic Meridional Overturning Circulation (AMOC), of which the Gulf Stream is a part. Variability in Gulf Stream flow drives short-term coastal sea level shifts of 70 cm or more. The Gulf Stream influences coastal sea levels on Florida’s east coast and extends along the mid-Atlantic coast and northward [20,21,44][2][3][27]. Recent analyses [21,45][3][28] suggest that AMOC may be close to a tipping point for transitioning to its weak circulation mode. A major weakening or shutdown of the Gulf Stream could result in significant sea level rise along much of the Atlantic coast, particularly south of Cape Hatteras. The most recent publication on AMOC [21][3] suggests that this tipping point could be crossed at any time after 2025. Research and review papers on other far-reaching tipping points affecting significant shifts in coastal morphodynamic regimes will be important in long-range planning.

3. Coastal Erosion and Shoreline Transgression

Coastal morphologic changes typically involve erosion in some places and accretion in other places. In the classic literature on beach behavior, it was understood that, typically, during storms or other high-energy events, waves move large volumes of beach and surf zone sand seaward to depths of 20 m or more in short periods of time. After the event subsides, the sands are slowly returned to the beach by low, long-period swells. This volatility is cyclic and seldom involves permanent land loss unless the section of coast is, for natural or human-induced reasons, starved of sediment. Climate change now has the potential to cause irreversible recession in many coastal regions and is displacing whole communities in many cases [46[29][30],47], particularly on muddy coasts and deltaic coasts. The superimposition of high waves and storm surges during tropical and extra-tropical storms can cause landward translations of the surf zone, often well into seaside communities, as was recently the case when Hurricane Ian made landfall near Ft. Myers, Florida. Even where no net-long recession is apparent, increased storm severity and surge intensity cause increases in the distance inland that storms penetrate.
As storms become more intense and sea level rises, eroded sediments are transported offshore to greater depths beyond the reach of fair-weather waves and may not return to the beach or may return more slowly than in the past. As is known from historic and geologic studies, shoreline recession can extend over broad lengths of coast. Sand barriers have evolved during the Holocene (and in places in previous interglacials) because of marine transgressions associated with post-glacial SLR. A well-documented example is the Danish North Sea coast, where shoreline recession took place at rates of SLR of 8.6 mm/yr. [48][31]. Progradation occurred when abundant sediment became available, even with a declining rate of SLR. However, on other coasts, a rate of SLR as little as 2 mm/yr was seen as the threshold value between coastal transgression and regression [49][32]. A recent comprehensive analysis of the coasts of the UK and Ireland [50][33] showed that a large proportion of that extensive coast is eroding because of a combination of rising sea levels, reduced sediment supply, and human interference. Such erosion has been ongoing for centuries in some sections, given its glacial and post-glacial history, affecting land and seabed sediment supplies and types. Another recent UK-based study concluded that erosion is not limited to sandy or soft sediment coasts but is also impacting rocky coasts because sea level rise is allowing storm waves to reach the cliffs more often [51][34].
In contrast to the conclusions reported in [22][4], a recent analysis of the observed behavior of Australian beach systems concludes that many have been stable for several decades and may not be threatened by climate change [52][35]. From a notable recent analysis of a 50-year time series of monthly beach surveys of a high-energy beach system on Australia’s New South Wales Coast [53][36], it was concluded that while there has been no long-term net coastal recession, beach fluctuations that occur reflect patterns of storminess and the capacity of the beach to recover in post-storm periods. Consistent with these Australian studies, a recent analysis of global trends in changing wave intensity and related shoreline behavior [54][37] reports that “Over the past 30 + years, wresearchers show that there have been clear changes in waves and storm surge at global scale. The data, however, does not show an unequivocal linkage between trends in wave and storm surge climate and sandy shoreline recession/progression.” Understanding the reasons for these results could provide insights relevant to future adaptation. In addition, the role of global atmospheric systems such as ENSO in influencing large regional variability in coastal erosion and accretion, as recently documented along the Pacific Rim, must be factored into any understanding of shoreline variability [55][38].
A multi-scale Probabilistic Coastline Recession (PCR) model was applied by [22][4] to swell-dominated Narrabeen Beach near Sydney, Australia, and storm-dominated Noordwijk aan Zee Strand, The Netherlands. The model estimates the magnitude of coastline recession caused by the combination of sea level rise, storm waves, and storm surge over a prolonged period (~100 years). The results suggested that sea level rise likely plays the dominant role in the long-term recession of both types of beach regimes. Long-term changes in wave climate were predicted to have only marginal impacts on recession. It should be pointed out, however, that this model-based conclusion may not apply to coasts such as the US Gulf Coast and East Coast, where tropical cyclones are becoming larger and more intense and storm surges are becoming higher and reaching farther inland [43][26]. A contrasting conclusion from the Australian east coast resulted from an evaluation of changes in wave climate over a centennial time scale in relation to sediment sources and sinks and provides a range of challenges in understanding the interaction of processes leading to future shoreline transgression [36][18]. More on this subject follows in the next subsection.
Narrow barrier islands, backed by open-water lagoons or tidal wetlands, are common along much of the U.S. Atlantic and Gulf coasts, the Arctic Ocean coast, and the coasts of the North Sea, particularly the Netherlands. The Outer Banks of North Carolina are probably the best-known barrier island chains. These environments are extremely vulnerable to increased storm intensities and sea level rises. An analytical model for predicting the impact of storms on barrier islands was recently developed in the Netherlands [56][39], and the predictions for the case of the impact of Hurricane Sandy on the US Atlantic compared well to the Delft 3D predictions but poorly to the observed outcomes. It was concluded that anthropogenically developed coasts do not behave as models predict. Landward transgressions of barrier islands involve more than wave and storm surge breaching. Physical and morphologic factors interact with numerous ecological factors to control barrier island stability [57][40]. Recent analyses of interannual sea level fluctuations caused by variability in the AMOC and Gulf Stream [45][28] contribute to episodic barrier island transgressions along the U.S. Atlantic coast.
Numerical Modeling of coastal morphodynamic changes is essential to informed planning for the future. Fortunately, there are numerous relatively sophisticated community models available. Some of these are briefly described by [58][41]. For long range climate change planning, a decadal-scale model of long-term coastal evolution that assimilates data from routine monitoring has been developed by [59][42]. It is very important to sustain monitoring programs in such a way that the data collected is accessible and thus used to test models in different coastal settings. Here, the application of technologies to use satellite imagery has dramatically increased the potential to repeatedly measure shoreline change at a very high resolution [55][38]. The Community Surface Dynamics Modeling System (CSDMS), maintained at the University of Colorado, Boulder (https://csdms.colorado.edu/wiki/Coastal_models) (accessed on 2 September 2023), is an accessible source for a total of 94 open-source numerical models for predicting coastal processes, including erosion, accretion, and sediment transport. Predicting the future responses of shores and coastal lands to climate change and rising seas will require the coupling of morphodynamic, ecological, physical, engineering, and socioeconomic models along with observational time series of morphological behavior.

4. Coral Reefs and Reef Islands

Coral reefs have traditionally served as the natural protectors of tropical coasts, and they have proved to be highly resilient. As with any future projections of potential change to the morphologies of coastal landforms in the new climate era, the inherited nature of landforms and their evolution should form part of the analysis. A diversity of Holocene reef studies in different tropical oceans have indicated varying reef growth strategies in relation to sea level rise: some kept up; others could not as the rate of SLR declined, stabilized, or accelerated [110,111][43][44]. Further understanding of the geological and geographical variability of reef systems will assist in deciphering how coral reefs will respond to future SLR. Several recent studies based on morphodynamic principles and observations have challenged the assumption that increased flooding due to SLR will automatically render reef islands uninhabitable within decades [110,112][43][45]. For instance, results have shown that the magnitude of island change from a range of locations over the past 50 years is not unprecedented compared with paleo-dynamic evidence that has defined large-scale changes in island dimension, shape, and beach levels since island formation c. 1500 years ago. The authors argue convincingly that their results highlight the value of a multi-temporal methodological approach to gain a deeper understanding of the dynamic trajectories of reef islands to assist in the development of adaptation strategies for the people of these islands. While coral reefs and reef islands may tolerate rising sea levels, rising sea temperatures are proving to be more problematic. Rising temperatures are causing the bleaching of coral’s symbiotic microalgae, the source of coral nourishment, and this eventually leads to coral death. For some island nations such as the Maldives, narrow, fringing reefs offer the only protection from rising seas. Elsewhere, offshore reefs are highly effective in dissipating storm waves, thereby sheltering the shores that lie behind [113][46], but the degradation of reefs allows more energetic waves to reach the hinterland shores and communities [114][47]. Today, rising sea surface temperatures are having devastating impacts on coral reefs worldwide [115,116][48][49]. Australia’s Great Barrier Reef (GBR) is probably the most prominent example [116,117][49][50]. In the month of July 2023, sea temperatures in the Florida Keys reached 38.4 °C, causing extensive coral bleaching and the death of a local reef (Sombrero Key). In 2016, roughly 20% of the GBR experienced bleaching when the temperature reached 29.1 °C [116][49]. With ocean temperatures around GBR expected to rise by an additional 1–2 °C by 2030, new management approaches to conserve this unique marine ecosystem have been launched in a program involving a partnership between marine scientists and indigenous people [118][51]. Strategies for restoring reefs include raising new stocks of heat-tolerant coral in aquaculture facilities and transplanting those stocks onto reefs. Similar coral restoration programs are underway elsewhere, including Florida [119][52].

5. Built and Natural Protective Infrastructure

Coastal landscapes are increasingly anthropogenic as the necessity for protection against rising seas, storms, and floods continues to grow along with increasing urbanization [120,121][53][54]. Traditionally, hard-engineered structures such as seawalls, levees, and dikes have been relied on, and the U.S. Army Corps of Engineers has led the development of engineered protections in the US. However, the Netherlands has, for many decades, led the world in keeping the sea out of their cities and communities. Following the catastrophe of Hurricane Katrina, the U.S. Army Corps of Engineers completed a $14.5B flood protection system surrounding New Orleans intended to withstand a 100-year flood event. This system consists of levees, a storm surge barrier, and high-volume pumps [122][55]. This protection is limited to the city of New Orleans. The rest of coastal Louisiana is losing land and wetlands at a phenomenal rate, and addressing this problem is the focus of extensive and highly complex coastal restoration programs [95,123][56][57]. Coastal protection agencies are increasingly turning to nature-based alternatives to engineered structures [124,125,126,127,128][58][59][60][61][62]. Wetland restoration, planting of mangrove forests, diversion of sediment-bearing river channels, nurturing coral reefs, and abandoning “grey infrastructure” (which often tends to work against natural infrastructure)—while breakwaters and seawalls often tend to exacerbate rather than stop the offshore loss of beach sand, an intermediate approach, termed “living shorelines,” may prove useful in offering resilience to adverse impacts of hard structures. It involves a combination of intermittently spaced minimalist hard structures and natural wetlands, or mangroves [128][62]. Of course, one of the best “nature-based” approaches is to simply allow the natural erosion/accretion cycles to proceed without structural impedance and not permit construction within a certain distance from the active zone. The iconic coastal geologist and environmentalist, Orin Pilkey, has spent many decades advocating retreat as the best alternative to harmful engineering practices. However, in many cases, especially on densely populated urban coasts, the relocation of entire communities is simply not feasible [42,63,69][25][63][64]. Nature-based adaptations can also deliver ecological, recreational, and other services. However, as pointed out in [42][25], “for these benefits to fully manifest, wresearchers must first understand how design and decision optima are influenced by consideration of diverse, uncertain ecosystem services." Unfortunately, in an increasing number of cases, managed retreat is the only feasible solution, even though it is always the last resort. Coastal management challenges in areas where there are conflicts over future use have been outlined in a recent paper, including legal and social issues that arise, especially in the management of contested beach areas [129][65]. An emerging, but less well acknowledged, strategy involves deploying the approaches long used by indigenous people, including Native Americans and Indigenous Australians [130,131][66][67]. The Indigenous Australians have occupied and adapted to their natural environment for over 60,000 years, and many of the coastal marine ecosystems are sacred to them [131][67]. Notably, the concept of private “ownership” of natural environments does not exist within ancient indigenous cultures.

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