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Comparing Tropical Cyclone and King Tide Impacts on a South Carolina Coastal Dune System
Abstract
Coastal populations face an ever-growing threat as natural hazards increase in frequency and magnitude. In South Carolina, king tides (abnormally high tides) responsible for coastal flooding have increased by 126% between 2014 and 2019. King tides present an evolving threat as sea levels rise, yet the implications for coastal dune response have not been investigated. This study compares the geomorphic impacts from king tides to two tropical cyclones on a South Carolina barrier island during four time periods (TPs): Hurricanes Florence (TP1) and Michael (TP2) and two subsequent
periods (TP3 and TP4) with king tides but devoid of tropical cyclone activity. Florence resulted in the greatest average change to the dunes, with normalized volumes (nv) equivalent to -11.6 m3/d. During Michael, the average change rate was nv=-1.5 m3/d. Dune changes during TP3 and TP4 averaged nv=-1.7 m3/d and nv=-2.3 m3/d, respectively. The
reduction rate during nonstorm conditions is indicative of the erosive potential of king tides. The erosive potential should be further investigated in conjunction with king tide magnitude and frequency. Smaller, more pervasive coastal hazards, such as king tides, should be more strongly considered, in addition to larger singular events, such as tropical cyclones, when investigating the geomorphic change of coastal dune systems.
... influences coastal dune sediment accumulations in South China (Hu et al., 2022), Australia (Tamura et al., 2018), and North America (Harris and Ellis, 2021). In Hainan Island, sediment records and historical documents show that storms have frequently hit southeastern Hainan Island over the past few thousand years (Zhou et al., 2019a;Zhou et al., 2019b), which are not only generated from the western Pacific but also from the SCS. ...
Coastal dune sediments and landforms offer a unique opportunity to understand climate change and sea level change on the centennial-millennial time scale. However, there is a paucity of chronological studies on the effects of storms on the evolution of coastal dunes along the temperate coastline of China. In this study, optically stimulated luminescence (OSL) dating of sand dunes was conducted to investigate the process and mechanism of coastal dune evolution on the southeastern coast of Hainan Island, China. The results show that the coastal dune evolution experienced three rapid accumulation periods: 28-21 ka, 14-4 ka, and 3.0 ka-present. The three rapid accumulation periods correspond to the last glacial maximum, the late Pleistocene/early-mid Holocene, and the late Holocene climatic dry-cold period, respectively. Sea level change and the East Asian winter monsoon, in conjunction with the enhanced storminess, play a key role in driving the sand dune evolution on the south-eastern coast of Hainan Island. These findings are of great significance for regional planning and coastal defense schemes.
... This area receives approximately 41% of its rain during the summer months, which includes a high number of thunderstorms or short, intense storms that contribute to spikes in surface runoff (BCDCOG 2011). More recently, fair weather or sunny day flooding caused by King Tides and rising sea levels have occurred with greater regularity and frequency, causing additional pollution loading and discharges into estuarine waterways (Harris and Ellis 2021;Román-Rivera and Ellis 2018). The Charleston peninsula (Figure 1) In relatively unaltered environments of this region, the surface soils ranging from sandy-to-loamy types and the subsurface soils ranging from loamy-to-clayey types locally influence natural infiltration and runoff patterns (Griffin et al. 2014). ...
Flooding is of great concern in fast-growing coastal communities, especially in the southeastern US, due to multiplying threats such as extreme precipitation, coastal storms, and rising sea levels. Contamination associated with stormwater runoff is often given less attention during stormwater planning and management decisions. The US EPA has long recognized that stormwater runoff is the biggest contributor to the impairment of water bodies in the US. In this study, we studied stormwater runoff contamination in a densely developed section of downtown Charleston, South Carolina, to better understand the extent of the problem and identify potential hotspots that could aid in future stormwater management decisions.
We review shoreline monitoring methodologies used by members of the South Carolina Department of Natural Resources (SCDNR) Archaeology, Geology, and Wildlife Biology teams from February 2021 to December 2022 on Pockoy Island in Charleston County, South Carolina, USA. Our project objectives were to better understand the driving forces behind the landward movement of the shoreline (transgression), to apply new understanding to the rate of shoreline erosion of the island that directly impacts the Pockoy Island Shell Ring Complex (38CH2533), and to establish best practice for future community science monitoring efforts. Each member of our team used a different shoreline monitoring methodology (a nested methodology approach). Multiple unoccupied aerial vehicle (UAV)-derived orthoimagery datasets, on-the-ground transect measurements, and Arrow Gold real-time kinematic (RTK) unit measurements have been collected monthly following significant storms or king (perigean) tide events. Moving forward, the erosion transect approach tested within this project will serve as the foundation for community science monitoring at heritage at-risk sites in South Carolina. In this article, we introduce initial efforts in establishing a community science monitoring program in South Carolina that will influence future research, land management, and policy, and we propose how our research might be adapted for other sites at risk.
This article examines beach–dune interactions, sediment transport, and morphological connections between beach and dune systems from a process perspective. We introduce the components of beach-dune interactions from small to large temporal scales: event, storm, and geologic. The article concludes by integrating the previously discussed concepts, introducing dominant beach–dune interaction models and proposing future research prospects.
Over one-third of the Earth’s population resides or works within 200 km of the coast. The increasing threat of coastal hazards with predicted climate change will impact many global citizens. Coastal dune systems serve as a natural first line of defense against rising sea levels and coastal storms. This study investigated the volumetric changes of two dune systems on Isle of Palms, South Carolina, USA prior to and following Hurricanes Irma (2017) and Florence (2018), which impacted the island as tropical storms with different characteristics. Irma had relatively high significant wave heights and precipitation, resulting in an average 39% volumetric dune loss. During Florence, a storm where precipitation was low and winds were moderate, net volumetric dune loss averaged 3%. The primary driving force causing dune change during Irma was water (precipitation and storm surge), and during Florence, it was wind (aeolian transport). We suggest that the application of the Saffir-Simpson Hurricane Wind Scale classifications should be reconsidered because different geomorphic responses were measured, despite Irma and Florence both being designated as tropical storms. Site-specific pre- and post-storm studies of the dune morphology and site-specific meteorological measurements of the storm (wind characteristics, storm surge, precipitation) are critically needed.
The purpose of this study is to document the geomorphic evolution of a mechanical dune over approximately one year following its installation and compare it to the recovery of a natural dune following the impact of Hurricane Matthew (2016). During the study period, the dunes' integrity was tested by wave and wind events, including king tides, and a second hurricane (Irma, 2017), at the end of the study period. Prior to the impact of the second hurricane, the volumetric increase of the mechanical and natural dune was 32% and 75%, respectively, suggesting that scraping alone is not the optimal protection method. If scraping is employed, we advocate that the dune should be augmented by planting. Ideally, the storm-impacted dune should naturally recover. Post-storm vegetation regrowth was lower around the mechanical dune, which encouraged aeolian transport and dune deflation. Hurricane Irma, an extreme forcing event, substantially impacted the dunes. The natural dune was scarped and the mechanical dune was overtopped; the system was essentially left homogeneous following the hurricane. The results from this study question the current practice of sand scraping along the South Carolina coast, which occurs post-storm, emplacement along the former primary dune line, and does not include the planting of vegetation.
Coastal storms can induce significant geomorphic change often leaving behind costly erosion projects for local communities. Existing methods for quantifying geomorphic change (i.e., total stations, airborne and terrestrial LiDAR systems, drones) are often cost prohibitive. These methods often require considerable pre-planning, which often prohibits making data collection related to sudden events. The objective of this study is to introduce the applicability of repurposing existing signs as erosion pins to measure morphologic change along a barrier island. To demonstrate the applicability of this method, this novel methodology was tested along the Isle of Palms, SC after the passage of Hurricane Irma. Prior to the storm, an extensive foredune inventory was completed utilizing a GPS camera. Signs along the beach-dune system were photographed generating 70 data points and creating a comprehensive geospatial monitoring network. Post-storm, 56% of the signs remained and the foredunes experienced up to 2.5 m of erosion. The remaining signs and foredunes we re-photographed and compared in juxtaposition with pre-storm photos to measure relative geomorphic change at each data point. This method provides coastal community officials the capability to measure erosion-accretion dynamics on a limited budget utilizing existing infrastructure. While this method was tested in South Carolina, it shows promise of universal application to any coastal community.
Over the last decade the concept of "king tides" has become a common colloquial term to describe higher than normal high tides. The terminology originated in 2009, when Australia experienced their highest seasonal tides in almost 20 years. In response, a public engagement program, "The King Tides Project," was formed that has since spread globally. Discrepancies regarding the definition and frequency of king tides can be identified between the different initiatives. This communication analyzes the multifaceted and contradictory meanings of king tides and concludes by providing a holistic and comprehensive definition.
Over 12 m**S**3 of sand per meter of beach was eroded at Debidue Island, S. C. during a minor northeast storm on September 23, 1974. Process measurements and beach surveys made before, during, and after the storm isolate the period of maximum erosion and suggest the threshold process values at the initiation of erosion. Storm processes on the 23rd included winds of 9. 1 m/s from the northeast, 6. 5 sec breakers 100 cm to 120 cm high, and average wave steepness (H//S//o/L//S//o) equal to 0. 012. The beach was 50% steeper than normal and contained well-developed cusps before the erosion, facilitating formation of a berm scarp. The bulk of erosion resulted from the undermining and slumping of the scarp at high tide. Threshold process values for Debidue Island at the onset of erosion (for tan beta ' 0. 055) are estimated to include: wind velocity 7 to 9 m/s; wave height greater than 100 cm; wave period less than 7. 0 sec. These process values are exceeded up to 14% of the time, but significant erosion will only occur a few days each year due to the influence of beach slope, beach morphology, and water level on
In mixed energy settings, tidal inlets undergo an episodic process known as shoal bypassing, whereby discrete bars are released from the ebb-tidal delta and migrate onshore. Such events can transport large volumes of sand, in the form of landward-migrating shoals, to adjacent beaches. Nine tidal inlets in South Carolina were analyzed to determine if there are predictable relationships among the volume of sand in the ebb-tidal deltas and in the individual bypassing shoals, the time interval between bypassing events, and the tidal prism. Historical photographs spanning up to 58 years yielded 221 discrete shoals at various stages of bypassing for the analysis. Building on earlier work by Kana (1995), mean shoal-bypassing event time intervals and the mean bypassing shoal volumes were found to be related to tidal prism. Larger inlets underwent shoal-bypassing events less frequently than smaller inlets, but produced larger bypassing shoal volumes. The relationship between average event interval and tidal prism was based on the equation, I = 0.046Tp + 4.56, where I is the average shoal-bypassing event interval (years) and Tp is the tidal prism (106 m3). The relationship between average shoal volume and tidal prism was based on the equation, S = 6.42Tp + 113.4, where S is the average bypassing shoal volume (103 m3). Both relationships are statistically significant. Bypassing shoals represented, on average, small volumetric percentages (0.6% to 6.6%) of their respective ebb-tidal delta volumes, yet in mesotidal, moderate wave energy settings such as South Carolina, shoal bypassing can be the single most important process contributing to a locally accreting beach.
Three types of macro-tidal beaches, higher waves, and particularly swell produces moderate gradient (1-3°), concave, planar beaches dominated by incident, though infragravity wave energy. Moderate waves and sea conditions tend to produce lower gradient (0.5°), multi-bar (ridge and runnel) topography also attributable to infragavity standing waves. As wave energy drops still further a third type results with a high tide beach fronting tide-dominated tidal flats. This latter is not a true beach but rather a transition from beach to tidal flat. -from Author
Sand transported landward from the beach and backshore by aeolian processes is the principal sediment input to coastal sand dunes and prediction of the sediment transport rate is important for determination of the dune sediment budget. This paper reviews the major controls on aeolian sediment transport on beaches and the factors controlling the amount reaching the foredune. In the first section a brief review of the atmospheric, textural and surficial factors controlling aeolian sediment entrainment and transport is presented. Predictive models developed for simple systems with uniform, steady flow and dry sand are described first. followed by an examination of factors which tend to restrict sediment supply from the surface. In the second section the prediction of sediment transport on beaches is examined in relation to the complexities of surface form, beach width and the complexities of sediment texture, moisture and cementing agents. Finally, in the third section aeolian sediment inputs are examined within the context of beach and dune sediment budgets and several scenarios for dune field development are presented.
Despite their value, coastal ecosystems are globally threatened by anthropogenic impacts, yet how these impacts are driven by economic development is not well understood. We compiled a multifaceted dataset to quantify coastal trends and examine the role of economic growth in China's coastal degradation since the 1950s. Although China's coastal population growth did not change following the 1978 economic reforms, its coastal economy increased by orders of magnitude. All 15 coastal human impacts examined increased over time, especially after the reforms. Econometric analysis revealed positive relationships between most impacts and GDP across temporal and spatial scales, often lacking dropping thresholds. These relationships generally held when influences of population growth were addressed by analyzing per capita impacts, and when population density was included as explanatory variables. Historical trends in physical and biotic indicators showed that China's coastal ecosystems changed little or slowly between the 1950s and 1978, but have degraded at accelerated rates since 1978. Thus economic growth has been the cause of accelerating human damage to China's coastal ecosystems. China's GDP per capita remains very low. Without strict conservation efforts, continuing economic growth will further degrade China's coastal ecosystems.
Near-surface airflow over a morphologically simple, vegetated 8m high foredune with a small wave cut scarp was measured during a high velocity (11-18 ms(-1)), onshore, sand-tran sporting gale event. Flow across the foredune was characterized by topographically forced accelerations up the steep stoss slope leading to a progressive increase upslope in near-surface wind speed even within the vegetation canopy. As incident wind speed approached similar to 14 ms(-1) (measured at 3m height on the crest), jet-like flow structures occurred at the foredune crest between the vegetation canopy and the 1.8m height region. Comparison of velocity profiles through time indicates that the jet structure remains consistent as velocities increase from 14 ms(-1) to 18 ms(-1). Throughout the storm period, even with a very small fetch, an echo dune formed at the scarp base. Sand in saltation bypassed the echo dune and scarp wall and was transported up the stoss slope. Fine sand was also transported in suspension across the entire dune and downwind. The long term implications for foredune development are discussed.
Santa Rosa Island, situated along the northwestern Florida coast facing the Gulf of Mexico, is an 85-km-long wave-dominated low-lying barrier island with well-developed incipient and established dunes. In this paper, we examine the regional-scale effect on coastal dunes by a strong category 3 hurricane, Ivan, through comparison of pre- and poststorm airborne LIDAR (light detecting and ranging) surveys. On the basis of pre-Ivan LIDAR survey data, the elevation of the berm and back beach is typically 2.0 m above MSL (mean sea level). Incipient dunes range from 2.5 to 10 m above MSL, or 0.5 to 8.0 m above the surrounding beach. The hummocky dunes that developed over relic washover platforms are typically less than 4.0 m above MSL. The densely vegetated, established dune fields are composed of dunes less than 7.0 m high and intradune wetlands lying at less than 1.0 m above MSL. The entire island was severely affected by Ivan, which made landfall about 45 km to the west in September 2004. The landscape was substantially changed by Ivan. Over 70% of the incipient and hummocky dunes were destroyed, and a large portion of the low-lying wetlands was covered by washover. The degree of storm-induced morphology change depends not only on the intensity and duration of the storm but also on the antecedent morphological characteristics of the barrier island. Comparison of pre- and post-Ivan cross-island LIDAR profiles indicates that at most locations, more sand was eroded from the subaerial portion of the barrier island (e.g., beach and dune) than was deposited as washover terraces and lobes. This suggests a net sand loss to the offshore region. Evidence of sand moving alongshore related to the oblique orientation of the dunes was also identified. Under inundation regime, the subaerial sediment deficit could be accounted for by subaqueous sedimentation into the back-barrier bay.
Vegetation, in the form of cover crops, wind strips or postharvest residues is typically used as a means of managing agricultural soils prone to wind erosion. Similarly, trees and shrubs are used as windbreaks to inhibit the transport of snow or soil across roads and fields. In coastal and arid regions, vegetation is used as a means of stabilizing mobile sand surfaces. Despite this wide use of vegetation, little attention has been paid to the stabilizing role of sparse vegetation. In particular, in arid regions such as rangelands and deserts, few studies have detailed the effectiveness of vegetation in controlling wind erosion. This article reviews the relevant literature associated with the protective role of sparse vegetation on soil erosion by wind. -Authors
This paper presents a first estimate of the exposure of the world’s large port cities (population exceeding one million inhabitants
in 2005) to coastal flooding due to sea-level rise and storm surge now and in the 2070s, taking into account scenarios of
socio-economic and climate changes. The analysis suggests that about 40 million people (0.6% of the global population or roughly
1 in 10 of the total port city population in the cities considered) are currently exposed to a 1 in 100year coastal flood
event. For assets, the total value exposed in 2005 across all cities considered is estimated to be US$3,000 billion; corresponding
to around 5% of global GDP in 2005 (both measured in international USD) with USA, Japan and the Netherlands being the countries
with the highest values. By the 2070s, total population exposed could grow more than threefold due to the combined effects
of sea-level rise, subsidence, population growth and urbanisation with asset exposure increasing to more than ten times current
levels or approximately 9% of projected global GDP in this period. On the global-scale, population growth, socio-economic
growth and urbanization are the most important drivers of the overall increase in exposure particularly in developing countries,
as low-lying areas are urbanized. Climate change and subsidence can significantly exacerbate this increase in exposure. Exposure
is concentrated in a few cities: collectively Asia dominates population exposure now and in the future and also dominates
asset exposure by the 2070s. Importantly, even if the environmental or socio-economic changes were smaller than assumed here
the underlying trends would remain. This research shows the high potential benefits from risk-reduction planning and policies
at the city scale to address the issues raised by the possible growth in exposure.
This paper reviews the initiation, dynamics, geomorphology and evolution of incipient foredunes, established foredunes, and blowouts. Incipient foredunes may be initiated in a variety of ways leading to the formation of ramps, terraces or ridges depending on progradation rates, vegetation type and cover, sediment transport rates and scale of erosional processes. Vegetation cover, morphology, growth rates, and types combine with aerodynamic processes to determine dune and swale morphology. Established foredunes are classified into five morpho-ecological types, and the flow behaviour over two types is detailed. A new model of foredune evolutionary paths at various possible time scales is presented. Blowout initiation, dynamics and sediment transport, and evolution is examined. Throughout the review, the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided.
Early ecological descriptions of barrier island dune landscapes recognized the importance of biogeomorphic feedbacks and thresholds. However, these dynamics have not been formally linked to complexity theory. In this article, I develop models of dune landscape phase states, or stability domains, based on a synthesis of these prior studies and statistical analyses. Data for these analyses were obtained from compositional and topographic sampling along Sapelo Island, Georgia and South Core Banks, North Carolina. These undeveloped barrier islands are at opposite ends of a regional meteorological gradient in storm-forced overwash disturbance along the southeastern US Atlantic coast. The topography and plant functional group abundances on each island were supportive of earlier studies noting the propensity for distinct self-organizing biogeomorphic feedbacks to emerge along barrier coasts. On South Core Banks, where storm forcings of overwash are more frequent, the recovery processes initiated by burial-tolerant species may reduce topographic resistance to overwash exposure and contagion. On Sapelo Island, a greater abundance of dune-building and burial-intolerant swale species may confer a higher topographic roughness and more resistance to overwash exposure and contagion. Conceptually, the biogeomorphic modifications of overwash forcings on each island perpetuate the habitat conditions and dominant dune plant species in a weak positive feedback. To provide more empirical evidence for these models, I tested a suite of hypotheses built from the premise that the biogeomorphic interactions on each island engender unique landscape patterns. These patterns originate in each island's distinctive coupling of extrinsic overwash disturbance processes with intrinsic plant-mediated topographic modification. Via this premise, the properties proposed for each domain model were subsequently supported, through: (1) the structure of compositional variance along principal coordinate ordination axes; (2) differences in biotopographic thresholds measured in cross sections; and (3) the distinctive elevational boundaries and spatial scales over which elevation exerted its influence on species diversity. These two domain models can be used as conceptual templates for the development of dune computational simulations.
Dunes are key elements of coastal landscapes in almost every latitude. They host high levels of biodiversity and provide important benefits to society; e.g. protection against floods and erosion, or recreation. Coastal dune growth is constrained by intrinsic factors, which are critical when managing dune systems or choosing coastal dune restoration as an alternative green solution for coastal protection. Here, the evolution of a beach-dune system, characterized by a reflective coarse sandy beach and low dunes, is explored to identify the favourable and optimal conditions for dune growth in these settings. Dune growth capacity is evaluated by analysing the topographical changes observed along a coastal dune over two different temporal scales (interannual and event scale) and comparing the observations with theoretical approximations of sediment transport potentials. Observations and predictions over interannual scale document that (1) temporal variability in external conditions (wind regime) and spatial variability of estimated wind fetch length, alone, fail to explain alongshore dune growth patterns and (2) optimal conditions for dune growth occur when storms (strong winds) impact the study area, jointly with low runup levels, at zones of shoreline progradation and absence of direct human influence. Conversely, lowest values of dune accumulation are associated with areas where shoreline retreat was documented. Observations from event timescales suggest that sediment transport potential can be reached over zones with no significant signs of beach erosion, if runup levels remain low and the event duration is shorter than the time scale of sand surface depletion within the upper beach.
In many places along the U.S. East and Gulf of Mexico coasts, barrier islands are the first line of defense against extreme weather events threatening our coastlines. The trademark of these barrier islands are sand dunes that are intricately bound, from a sedimentary perspective, to the beach. Coastal storms, such as Hurricanes Matthew (2016), Irma and Maria (2017), and Florence (2018) have devastating impacts on these environments. This study investigated the volumetric changes of an anthropogenic and controlled beach-dune system on Isle of Palms, South Carolina, for approximately one year following Hurricanes Matthew (2016) and Irma (2017). This research reveals that these systems did not recover. The average loss of sand at the beach was -15.5% (nv = -0.89), whereas the dunes gained an average of 13.3% (nv = 0.79), when compared to the already diminished post-storm volumes. When considering the pre-Hurricane Irma to pre-Hurricane Florence temporal period, the recovery percentages for the anthropogenic and control dunes was -15.5% and -40.1%, respectively, suggesting a net loss of sand. Cumulative storms, such as those experienced on the coast of South Carolina and many other coasts, pose a substantial threat to the long-term viability of coastal dune systems. However, recovery at the control site in the form of incipient foredune growth is promising. This paper concludes with a list of influencing factors to dune recovery.
The coastal zone is morphologically dynamic and a range of engineering practices are used to stabilize it. The dune system serves as the first line of defense against storms and rising sea levels. Sand fences on dunes are one popular and economical restorative engineering alternative. This study evaluates the geomorphic response of dunes with and without sand fences resulting from the impact of high energy events, specifically Hurricanes Florence and Michael, which impacted South Carolina, United States in 2018. Innovative cost-effective field methods were applied to calculate dune volumes on a mechanical dune. Measurements compared a site with eleven sand fences and an unmodified control site. Dune volume decreased after Hurricane Florence for the fenced and control sites and increased after Hurricane Michael for the fenced site. There was differential geomorphic change post-storm between the control and fenced sites. Results support that sand fences are resilient under high wind conditions and are an effective strategy to aid in dune recovery and growth, suggesting that they can be installed prior to the storm season rather than being reserved as a post-storm recovery technique.
Barrier/lagoon systems occupy a significant part of the world's coast. They are diverse in size, morphology, geological and oceanographic setting, and morphodynamic behaviour. Understanding the behaviour of barriers at 101 to 102 year and 101 to 102 km scales (mesoscale) is an important scientific and societal goal, not least because of the preponderance of intensive coastal development in a time of global climate change. Such understanding presents significant challenges. Challenges in describing mesoscale system behaviour relate largely to the incomplete evidence base of (i) morphological change in system components, (ii) dynamic and internal forcing factors (drivers) and (iii) geological constraints. These shortcomings curtail the development of baseline datasets against which to test models. Understanding observed changes and thereby predicting future behavioural patterns demands assumptions and simplifications regarding the linkages between initial state, dynamic drivers, system feedbacks and a multiplicity of geological constraints that are often location-specific.
The record of mesoscale change is improving with the acquisition of long-term morphological datasets. Advances in technology and chronological control mean that geological investigations can now provide decadal to century-scale temporal resolution of morphological change. In addition, exploratory modelling is improving understanding of the influence of various dynamic and geological factors.
Straightforward linking of dynamic forcing and response is seldom able to account for observed mesoscale behaviour. Geological factors exert a significant or even dominant control on barrier behaviour at decadal to century timescales. Whereas these geological controls can be quantified to some extent by detailed investigations of contemporary barrier/lagoon morphology and constituent materials, underlying geology and topography and sediment supply, in all but a few locations, such data are absent. This sets an unavoidable constraint on efforts to quantitatively predict the future behaviour of barrier systems, which are strongly site-specific in terms of their geological setting and morphology. Geological controls exist in a network of interactions that individually and collectively influence mesoscale barrier behaviour. Dominant, first-order controls are:
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Basement slope;
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Basement irregularity and erodibility;
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External sediment supply;
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Orientation; and
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Shoreline lithification (beachrock and aeolianite)
An important intermediate level of geological control is exerted by shoreface morphology. Shorefaces are themselves influenced by underlying geological factors, but they are dynamic at longer timescales than barriers. Geological influences are in most cases unquantified and are usually disregarded when conceptualizing and modelling barrier evolution. Consideration of the geological influences is, however, essential in efforts to predict future behaviour at mesoscale (management) timescales.
Coastal foredune growth is typically associated with aeolian sediment transport processes while foredune erosion is associated with destructive marine processes. New datasets collected at a high energy, dissipative beach suggest that total water levels in the collision regime can cause dunes to accrete ‐ requiring a paradigm shift away from considering collisional wave impacts as unconditionally erosional. From morphologic change datasets, it is estimated that marine processes explain between 9% and 38% of annual dune growth with aeolian processes accounting for the remaining 62% to 91%. The largest wind‐driven dune growth occurs during the winter, in response to high wind velocities, but out of phase with summertime beach growth via intertidal sandbar welding. The lack of synchronization between maximum beach sediment supply and wind‐driven dune growth indicates that aeolian transport at this site is primarily transport, rather than supply, limited, likely due to a lack of fetch limitations.
The severe surge damage to the South Carolina shoreline during Hurricane Hugo was a result of high winds, a fast coast-normal hurricane track across a wide continental shelf, landfall near high tide, and anthropogenic alterations along the shoreline. Structural damage was a function of location with respect to the eye, elevation, exposure, foundation type, and bracing. Acting on the many surge lessons of Hugo can greatly decrease damage when a Category-4 storm inevitably hits a much more urbanized shoreline, such as those in New Jersey or on Long Island, New York, in the future. -from Authors
A new scale is proposed that categorizes impacts to natural barrier islands resulting from tropical and extra-tropical storms. The proposed scale is fundamentally different than existing storm-related scales in that the coupling between forcing processes and the geometry of the coast is explicitly included. Four regimes, representing different levels of impact, are defined. Within each regime, patterns and relative magnitudes of net erosion and accretion are argued to be unique. The borders between regimes represent thresholds defining where processes and magnitudes of impacts change dramatically. Impact level 1 is the 'swash' regime describing a storm where runup is confined to the foreshore. The foreshore typically erodes during the storm and recovers following the storm; hence, there is no net change. Impact level 2 is the 'collision' regime describing a storm where the wave runup exceeds the threshold of the base of the foredune ridge. Swash impacts the dune forcing net erosion. Impact level 3 is the 'overwash' regime describing a storm where wave runup overtops the berm or, if present, the foredune ridge. The associated net landward sand transport contributes to net migration of the barrier landward. Impact level 4 is the 'inundation' regime describing a storm where the storm surge is sufficient to completely and continuously submerge the barrier island. Sand undergoes net landward transport over the barrier island; limited evidence suggests the quantities and distance of transport are much greater than what occurs during the 'overwash' regime.
Pre- and post-storm aerial videotape surveys were made along 51 km of the barrier island coast of South Carolina from Garden City to Folly Beach. Before Hugo the shoreline from the beach landward to, and including the first row of development, was classified as dune field (45%), bulldozed dune ridge (25%), revetment (14%), bulkhead (12%), vegetated washover terrace (3%), and beach only (1%). After the storm, 80% of the shoreline was classified as washover sheet, and 5% as washover fan. The only areas that were not overwashed were sections of very high dune field (13%) and large bulldozed dune ridge (2%). Our most important observations can be summarized as follows. 1) Provided the dune field was not submerged, the minimum width of dune field required to survive Hurricane Hugo, was 30 m. 2) Two types of dunes survived the storm: those high enough to prevent being overwashed and wide enough to prevent being completely eroded. 3) All bulkheads and revetments were overtopped, and wave activity was carried inland to the first and succeeding rows of development. 4) Fifty percent of all buildings completely destroyed or removed from their foundations were fronted by a combination of dry beaches less than 3 m wide and dune fields less than 15 m wide. -from Authors
The 245-page report lays out the approaches, assumptions, processes, and parameters scientists are using in the study. It offers decision-makers a mechanism to identify options that can better achieve core human development and sustainability goals and better understand the trade-offs in decisions about development and the environment.
Beach erosion is one of the most significant impacts of coastal storms because beach width is a measure of coastal vulnerability for beachfront houses. Storm tide, wave energy, and duration are three major factors determining storm erosion potential. Several authors have proposed erosion indexes for large storms in terms of storm intensity measured by wind speed or wave energy and duration, but the role of the storm tide has not been fully incorporated into previous analyses. It has been found that the erosion potential of severe nor'easters is more dependent on storm tide than wave energy and duration. Thus, we propose a quantitative index of storm erosion potential that includes storm tide based on hourly water level measurements. Our storm erosion potential index (SEPI) is the sum of the products of hourly storm surge and corresponding storm tide water levels, and it correlates well with observed erosion.
Beaches and coastal dunes are dynamic geomorphic systems that respond to process forcing over a broad spectrum of spatial and temporal scales. At all scales, there are potential suites of interaction between system forms and processes, and the mechanisms of interaction are stressed in this article. At micro-scales, this interaction is formalized through the concept of morphodynamics, and deterministic and probabilistic approaches are used to model sediment transport and landform development over time scales of hours to months and space scales of metres to kilometres. Meso-scale interactions are conceptualized using a sediment-budget approach. Nine characteristic environments comprise the beach and dune sediment-budget ensemble, representing a classification scheme for reciprocating coastal systems. -Authors
The relative importance in geomorphic processes of extreme or catastrophic events and more frequent events of smaller magnitude can be measured in terms of (1) the relative amounts of "work" done on the landscape and (2) in terms of the formation of specific features of the landscape. For many processes, above the level of competence, the rate of movement of material can be expressed as a power function of some stress, as for example, shear stress. Because the frequency distributions of the magnitudes of many natural events, such as floods, rainfall, and wind speeds, approximate log-normal distributions, the product of frequency and rate, a measure of the work performed by events having different frequencies and magnitudes will attain a maximum. The frequency at which this maximum occurs provides a measure of the level at which the largest portion of the total work is accomplished. Analysis of records of sediment transported by rivers indicates that the largest portion of the total load is carried by flows which occur on the average once or twice each year. As the variability of the flow increases and hence as the size of the drainage basin decreases, a larger percentage of the total load is carried by less frequent flows. In many basins 90 per cent of the sediment is removed by storm discharges which recur at least once every five years. Transport of sand and dust by wind in general follows the same laws. The extreme velocities associated with infrequent events are compensated for by their rarity, and it is found that the greatest bulk of sediment is transported by more moderate events. Many rivers are competent to erode both bed and banks during moderate flows. Observations of natural channels suggest that the channel shape as well as the dimensions of meandering rivers appear to be associated with flows at or near the bankfull stage. The fact that the bankfull stage recurs on the average once every year or two years indicates that these features of many alluvial rivers are controlled by these more frequent flows rather than by the rarer events of catastrophic magnitude. Because the equilibrium form of wind-blown dunes and of wave-formed beaches is quite unstable, the frequency of the events responsible for their form is less clearly definable. However, dune form and orientation are determined by both wind velocity and frequency. Similarly, a hypothetical example suggests that beach slope oscillates about a mean value related in part to wave characteristics generated by winds of moderate speed. Where stresses generated by frequent events are incompetent to transport available materials, less frequent ones of greater magnitude are obviously required. Closer observation of many geomorphic processes is required before the relative importance of different processes and of events of differing magnitude and frequency in the formation of given features of the landscape can be adequately evaluated.
Observations of aeolian transport in coastal areas have focused on short-term experiments because of limitations imposed by instrumentation. This paper uses a case study at Greenwich Dunes, Prince Edward Island National Park, Canada, to analyze how sediment transport takes place at the beach over periods of weeks to months. A monitoring station provided hourly time series of vegetation cover, shoreline position, fetch distances, surficial moisture content, presence of ice and snow, wind speed and direction and transport processes over nine months. Analysis shows that high wind speeds may not generate any net transport into the dunes because of the limitations imposed by snow/ice cover, moisture, and short fetch distances. Despite extreme winds during intense storms, such events often lead to wave scarping rather than aeolian sediment input to the foredunes. When sediment was transported on the beach, the magnitude was regulated by a combination of factors including: angle of wind approach, fetch distance, moisture content, and duration of the wind event. In particular, angle of wind approach (and therefore fetch distance) may demote a high magnitude wind event with strong transport potential to one with no transport at all, which poses challenges for predicting the effects of individual storms over the course of several months. A significant proportion of sediment delivery to the foredunes was associated with wind events of low to medium magnitude. It is suggested here that large magnitude wind events have low probabilities of resulting in transport towards the foredune because factors such as wave inundation play an increasing role in preventing sediment movement across the beach. This has implications for modelling and management, and highlights differences between the magnitude and frequency of aeolian transport events in the coastal environment compared to those in deserts and to fluvial sediment transport.
The famous Cape Hatteras Lighthouse is threatened with destruction by an eroding coastline. Recent attempts to control the
erosion have reduced but not stopped it The natural erosion trend for 41 km of coast from Rodanthe to Cape Hatteras was determined,
based on 94 years of survey records from 1852 to 1946 At the lighthouse, the natural erosion rate is 7 5 m/yr In 2005, if
no further human interference with coastal processes occurs, 190 m of coast will have eroded since 1980, leaving the shoreline
nearly 90 m west of the lighthouse Considering the expensive effort being undertaken to protect the lighthouse from destruction,
an inventory of property along the Atlantic coast should be made, before other similar projects are initiated We can afford
to protect only the most valuable property.
Santa Rosa Island is an 85 km-long, wave-dominated low-lying barrier island situated along the northwestern Florida coast, facing the Gulf of Mexico. The entire island was severely impacted by Ivan, a strong category 3 hurricane that made landfall about 45 km to the west in September of 2004. Ten months later in July of 2005, Dennis, another category 3 hurricane, made landfall about 30 km east of the western tip of the island. Santa Rosa Island is characterized by well-developed but relatively low dunefields, described in this paper as incipient and established dunes, based on the presence of grassy and woody types of vegetation, respectively. The dunes were severely eroded by the two hurricanes. This paper investigates the factors controlling the regional-scale destruction and survival of the dunefields.Dune survival is controlled by: 1) hurricane characteristics, including intensity, duration, and frequency, and 2) morphological parameters including width of the barrier island, height and width of the dunefields, vegetation type, distance of the dunes to the ocean, and continuity of the dunefields. Three processes of dune destruction are described including, from most to least severe, inundation, overwash, and scarping. The interaction of all the above factors determines the different dune responses to the storm impacts. In general, the extensive and densely woody vegetated dunefields near the bay-side shoreline survived the storms, while the discontinuous dunes with grassy vegetation near the Gulf shoreline were almost completely destroyed.
The critical relationships between beach and nearshore parameters and the erosion/accretion status of frontal dunes have been investigated using LIDAR data, complemented by data obtained from conventional topographic and bathymetric surveys, at five dune systems in England and Wales. Morphometric parameters calculated from the LIDAR data demonstrate the presence of a general relationship between beach morphology, dune morphology and erosion/accretion. Eroding dunes were found to be associated with narrower, steeper beaches whilst accreting dunes were associated with wider, low-angle beaches. However, the critical beach width/mean slope angle separating eroding dunes from stable or accreting dunes was found to vary between sites. At any given site, changes in beach width/slope over time can be used to provide an ‘early warning’ of changes in frontal dune erosion/accretion behaviour.
In order to determine the effects of the continental shelf bathymetry on coastal geomorphology, a series of wave-refraction diagrams were generated for the S.C. coast from Bull Island to the Isle of Palms REFRAC, a computerized wave-refraction program developed for this study, generates refraction diagrams which delineate the patterns of longshore variation in wave energy. A variety of input wave conditions were used in REFRAC to model the various possible wave conditions existing in nature. The results indicate that the offshore bathymetry does partially control the coastal geomorphology by creating zones of potential erosion and deposition and by influencing the direction of sediment transport. -from US Govt Reports Announcements, 7, 1981
Dunes-Their function and design
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