Search

Hurricane Fiona made landfall as an extra-tropical storm along the north shore of Prince Edward Island (PEI), Canada, in October 2022. The state of the beach and dune immediately before and after the storm was captured through the Coastie citizen science beach and dune photo monitoring initiative, as part of the global CoastSnap Community Beach Monitoring program. Coastie monitoring sites within Prince Edward Island National Park (PEINP) revealed extensive dune scarping, capturing a 12–17 m retreat of the foredune at Brackley and Cavendish Beaches. Using foredune scarp and post-storm shoreline positions, volumetric losses between 28 and 76 m3 m−1 are estimated from profiles located within the first 150 m of the stations. The average horizontal position of the projected foredune scarp position was within 2.8 m of the position identified from high accuracy unoccupied aerial system (UAS) surveys, corresponding to a mean absolute difference of 15.3 m3 m−1 or 45.3% for dune volume changes estimated from the images. Continued monitoring will yield further improvements to the volume loss estimation methodology, and insight on the timing and mechanisms of beach and dune recovery. [ABSTRACT FROM AUTHOR]

The Coastal Dune Model, used to examine controls on dune development and evolution, includes negative feedback between the wind and the evolving topography that results in a scale‐invariant beach‐dune profile. While a limited number of reference profiles appear to support the Coastal Dune Model assumption of scale‐invariance, previously published studies and conceptual models suggest that the beach‐dune profile is not scale‐invariant. Using a combination of journal articles, textbooks, and remotely sensed data from Prince Edward Island, Washington state, and Texas, this short communication finds that there is no evidence for a scale‐invariant beach‐dune profile within and between field sites. Variability in the beach‐dune profile is due to site‐specific and scale‐dependent controls on the beach and the dune that are often highlighted in the conceptual models that authors use to depict the controls on sediment supply and transport from the beach to the dune. Results highlight a need to critically evaluate the assumptions and applications of models such as the Coastal Dune Model, and to explore how and when sites can be considered representative and in support of model results. Key Messages: There is no scale‐invariant beach‐dune profile as assumed in recent models.Beach‐dune profiles vary in response to site‐specific and scale‐dependent controls on sediment supply and transport.Further study is required to determine how and when sites can be considered representative. [ABSTRACT FROM AUTHOR]

Rip currents can move unsuspecting swimmers offshore rapidly and represent a significant risk to beach users worldwide, including along the northern coast of Prince Edward Island (PEI), Canada. Although many rip currents are ephemeral and/or spatially variable in response to changes in the nearshore bar morphology and wave and tidal forcing, it is possible for rip channels to be geologically controlled and quasi-permanent in morphology, location, and flow. Several rip channels along the northern coast of PEI appear in the same location from year to year and correspond to elongated lakes, rivers, or swales behind the modern coastal dune system. Given their persistent location and alignment with back dune hydrology, ground-penetrating radar surveys were collected along Brackley and Cavendish Beaches in July 2019 to determine whether persistent rip channels are associated with now-buried river channels extending beneath the modern dunes and continuing offshore. Strong reflectors similar to V-shaped river valleys are present in alongshore transects at both beaches. These infilled valleys align with back-dune hydrology and persistent rip channels, suggesting modern rip channels are structurally controlled and maintained by antecedent geology. This link provides important guidance to beach access management and the distribution of lifesaving strategies along the affected beaches. [ABSTRACT FROM AUTHOR]

The impact of storm surge on a barrier island tends to be considered from a single cross-shore dimension, dependent on the relative elevations of the storm surge and dune crest. However, the foredune is rarely uniform and can exhibit considerable variation in height and width at a range of length scales. In this study, LiDAR data from barrier islands in Texas and Florida are used to explore how shoreline position and dune morphology vary alongshore, and to determine how this variability is altered or reinforced by storms and post-storm recovery. Wavelet analysis reveals that a power law can approximate historical shoreline change across all scales, but that storm-scale shoreline change (~ 10 years) and dune height exhibit similar scale-dependent variations at swash and surf zone scales (< 1000 m). The in-phase nature of the relationship between dune height and storm-scale shoreline change indicates that areas of greater storm-scale shoreline retreat are associated with areas of smaller dunes. It is argued that the decoupling of storm-scale and historical shoreline change at swash and surf zone scales is also associated with the alongshore redistribution of sediment and the tendency of shorelines to evolve to a more diffusive (or straight) pattern with time. The wavelet analysis of the data for post-storm dune recovery is also characterized by red noise at the smallest scales characteristic of diffusive systems, suggesting that it is possible that small-scale variations in dune height can be repaired through alongshore recovery and expansion if there is sufficient time between storms. However, the time required for dune recovery exceeds the time between storms capable of eroding and overwashing the dune. Correlation between historical shoreline retreat and the variance of the dune at swash and surf zone scales suggests that the persistence of the dune is an important control on transgression through island migration or shoreline retreat with relative sea-level rise. [ABSTRACT FROM AUTHOR]

Weymer, B.A.; Houser, C., and Giardino, J.R., 2015. Poststorm evolution of beach-dune morphology: Padre Island National Seashore, Texas. The response and recovery of barrier islands to extreme storms and ultimately to relative sea-level rise depends on the height and extent of the foredunes relative to storm surge. The impact of storms is complicated by the tendency of dune morphology to vary alongshore at a range of spatial scales, and it is reasonable to assume that this variable response will be preserved within subsurface stratigraphy. We used ground-penetrating radar (GPR) and vibracores to investigate subsurface structures for foredunes of different heights within a 2.5-km section of beach at Padre Island National Seashore, Texas. Identical, laterally continuous radar reflectors were observed at each site at a depth of ~1.2 m and interpreted as a storm surface. Results from the smallest dune suggest that the dune experienced little net erosion during the storm but also exhibited the least recovery. The intermediate dune was completely eroded by the storm but also exhibited the greatest recovery through the migration of accretionary mounds driven by aeolian transport, leading to the development of embryo dunes forming the modern dune core. The largest dune was scoured at the base and experienced little poststorm recovery, except for beachface recovery. Thus, the response and recovery of adjacent, but morphologically distinct, dunes is quite different over the same sequence of storm activity. We suggest that the extent and form of beach-dune recovery is dependent on the impact scale of the storm and may represent a reinforced process once alongshore variations in dune height are initialized. [ABSTRACT FROM AUTHOR]

Abstract: The geomorphological impact of an extreme storm on a barrier island tends to be modeled using a single cross-shore transect and dependent only on the elevation of the storm surge relative to the height of the dune. The foredune line, however, is rarely uniform and can exhibit considerable variation in height and width alongshore at a range of length scales. The purpose of this modeling study is to determine how alongshore variations in dune height affect barrier island response to extreme storms. The MIKE21 wave and current model is used to predict the morphological response of Matagorda Peninsula, Texas in response to storm surges associated with dune scarping, washover and inundation. The extent and degree of dune-scarping, washover and shoreline erosion is predicted for each storm scenario, with respect to the base morphology of the island and low-pass filtered forms in which small-scale topographic variance is removed. Results suggest that small variations in the height of an otherwise alongshore uniform foredune act as overwash conduits and are unstable, leading to a more variable duneline that is more susceptible to change by subsequent storms. The vertical development of the washover gaps in the duneline is limited and eventually replaced by a lateral expansion that erodes adjacent dunes and leads to a more uniform island elevation. The loss of island elevation is greater for the (original) unfiltered alongshore profile, but relatively uniform duneline is the most unstable and exhibits the greatest morphological change. The different alongshore profile responses suggest that the impact of an extreme storm is sensitive to initial conditions and specifically the pre-storm variability of the crest elevation alongshore. This in turn suggests that the evolution of barrier islands is dependent on storm history until the variability in the duneline elevation reaches a maximum. Further study of barrier island response to storm sequencing with and without post-storm recovery, however, is required to understand the evolution and form for the prototype island. [Copyright &y& Elsevier]

Abstract: The height and extent of coastal dune development can exhibit considerable variability over short distances, leading to variable storm impact alongshore. The source of this variation is not well understood in general, and in many cases is not easily explained by models that ascribe dune height and development to regional variations in sediment availability and foreshore slope. In this paper, LiDAR data collected in 2000 and 2005 from South Padre Island, Texas are used to characterize the alongshore variation in dune morphology, and to identify how the variation is related to foreshore morphology and nearshore state. Results of cross-wavelet analysis suggest that foredune height exhibits a statistically significant coherence with the surf similarity parameter at length scales of ~1300 of 2300m. The resulting variation in beach state alongshore, from intermediate to dissipative, is the consequence of wave refraction and focusing by transverse ridges on the inner shelf. Dune height reaches a local maximum approximately halfway between the more intermediate and dissipative sections of the coast, where the volume of sediment in the backshore and in the foreshore bars are also at a maximum. The greater elevation of the foreshore and backshore also provides a wider fetch and potential for aeolian transport during the storm surge that tends to accompany storm winds. It is argued that the variation in dune morphology is not strictly the result of transport limitations as suggested by the fetch model, but rather depends on the alongshore coincidence of transport potential with an available supply of sediment in the backshore and on the foreshore. [Copyright &y& Elsevier]

This review considers the role of nearshore processes and morphological change as a flexible and dynamic constraint on the supply and transport of sediment between beach and dune. It is argued that the lack of information in this regard remains a central barrier to the development of a theory of beach-dune interaction that can be translated across scales and between field sites. Existing beach-dune models do not consider how and when sediment gets transferred to the backshore where it becomes available for transport by wind. Rather, existing models largely ascribe regional variations in dune morphology to fixed constraints on beach slope and sediment budget, without explicit recognition of processes involved. Recent (process-scale) transport studies have shown that the transfer of sediment is both spatially variable and temporally intermittent as a result of transport limitations across the beachface. While these studies have identified varied controls on sediment transport and exchange, there remains a limited capacity to predict the evolution of beach-dune systems, largely because the beachface tends to be viewed as a static transport surface without regard to supply or to the changing limits to transport. Following storm erosion, dune recovery first requires that the beach recovers through the onshore migration and welding of nearshore bars, followed by accretion in the backshore to create a supply of sediment for transport by the wind. The dependence of dune recovery on the synchronization of transport events with the recovery of sediment supply in the backshore creates a strong asymmetry in dune recovery that makes barrier island susceptible to widespread erosion and breaching if a change in the frequency or grouping of storm events is capable of resetting the bar system (offshore) before the next extreme storm. [ABSTRACT FROM AUTHOR]

The article presents the study which examines the rate and alongshore variation in beachface and backshore recovery along Santa Rosa Island, Florida, after it was hit by Hurricanes Ivan, Dennis and Katrina in 2004-2005. To test whether the alongshore variation in the beach and dune recovery does vary considering its given bathymetry and island width, researchers examined the variation in island recovery with the use of light detection and ranging (LiDAR) as well as Empirical Orthogonal Function Analysis. Results reveal that island width and offshore bathymetry do characterize the alongshore variation in recovery.

During Hurricane Ivan (2004), dunes in undeveloped sections of northwest Florida were significantly reduced in height through direct wave erosion and the transfer of sediment to the backbarrier by overwash. The poststorm morphology of the island consists of a narrow beach face, multiple breach corridors, a washover terrace, and a remnant secondary dune that was further eroded in 2005 by Tropical Storm Arlene and Hurricanes Dennis and Katrina. The redevelopment of this dune will direct the response of the island to future storms, but the mechanisms of posthurricane dune recovery are unclear, particularly in this low-energy coastal environment. The recovery is further complicated by a swale that was created through a knickpoint effect on an adjacent road. The swale was reinforced when the road surface was removed and as a berm developed during the recovery of the beach face. Field observations suggest that sediment transport across the narrow beach face and berm is limited by a lag of shells and gravel from the pre-Ivan roadbed. As a consequence, sediment input to the dune is relatively minor with onshore winds, despite the presence of fans deposited during frontal storms in the swale behind the berm. The resulting sediment transport gradient across the seaward slope of the dune is more reflective of the expanding fetch than topographic acceleration and the drag imposed by the sparse vegetation. In contrast, there is a larger sediment input to the dune during offshore winds in response to the larger fetch across the washover terrace, despite the presence of a lag. It is concluded that posthurricane dune recovery in this area is strongly dependent on the deposition of sediment during frontal storms and the resulting development of a sand ramp through the swale. This will increase both the availability of sediment and the fetch for winds directly or obliquely onshore. [ABSTRACT FROM AUTHOR]

This paper examines the processes responsible for the morphodynamics of an intertidal swash bar at Skallingen, Denmark, during seven successive storms (one with a large surge of +3·02 m DNN). During this period a subtidal bar migrated landward onto the foreshore and continued to migrate across the intertidal zone as a swash bar. The onshore migration of the inner subtidal bar resulted from the erosion of sediment from the upper foreshore and dune ramp during the large storm surge that was transported seaward, causing the landward displacement of the bar through accretion on the landward slope. The magnitude and direction of suspended sediment transport within the intertidal zone, and more specifically at and close to the crest of the swash bar, varied with the ratio of both the significant (Hs) and average (Havg) wave heights to the water depth (her) at the swash bar crest (the local depth minimum). The transition between onshore and offshore suspended sediment transport was associated with the average wave of the incident distribution breaking on the swash bar crest (Havgh-1er ≈ 0·33). While the onshore-directed transport was largest at infragravity frequencies, sediment resuspension was best explained by the skewed accelerations under the surf bores. Offshore transport was dominated by the cross-shore mean currents (undertow) that developed when the significant wave of the distribution broke on the swash bar crest (Hs-1er ≈ 0·33) and weakened as the average wave of the distribution started to break at the crest (Havgh-1er ≈ 0·33) and the surf zone approached saturation. In contrast to subtidal bars, the swash bar at Skallingen exhibited a divergent behaviour with respect to the cross-shore position of the breaker zone, migrating onshore when the average wave broke seaward of the crest and migrating offshore when the average wave broke landward of the crest. [ABSTRACT FROM AUTHOR]

The dune toe is the boundary at the beach-dune interface that is important to our understanding of foredune development, storm-impact assessments, and management decisions. Although there are several morphometric based estimates of the dune toe, there is no consensus on an operational definition and whether those metrics match field observations. This study uses a crowd-source approach to compare dune toe selections and definitions among members of the Geomorphology Listserv. Respondent selections of the dune toe varied depending on the type of imagery used, the complexity of the backshore, and the self-identified expertise of the respondent. The variation by method and research background raises important questions about how landform boundaries are identified in coastal geomorphology and possibly within other areas of geomorphology. • A crowd-sourced survey asked geomorphologist to define and identify the dune toe. • Variability in identifications resulted from the image used and user expertise. • Inconsistencies in dune toe definitions are presented and discussed. • Similar assessments can be used to address other relevant issues in geomorphology. [ABSTRACT FROM AUTHOR]

Beach and dune morphology are spatially and temporally variable, changing over a broad range of scales simultaneously. Strong wind, waves, and storm surge from Hurricane Harvey substantially eroded the beach and dunes along the Texas coast, causing significant scarps and berms. This paper presents information about how anthropogenic activity, such as driving on the beach, affected the response and recover of a barrier island with regard to the post-storm resiliency along the Texas-Gulf of Mexico coast by comparing two adjacent 7 km stretches of coast: a driving section and a limited-access section. A collection of field photos, aerial imagery, and a September 2016 LiDAR-derived digital elevation model (DEM) dataset provide information on pre-storm morphology, while field photos, taken only 3 days after Hurricane Harvey made landfall, and a structure-from-motion (SfM)-derived DEM and imagery provide qualitative and quantitative information about the post-storm morphology. While beach and dune erosion in the non-driving section was restricted entirely to the beach and incipient dune system, the driving section exhibited complex patterns of erosion and deposition along the beach and the entire foredune profile was altered. Despite the hurricane making landfall north of the study site and closer to the non-driving section, beach-dune erosion and scarping was greatest in the southern section which is accessible to public vehicles. Results demonstrate that human activity affects the response and recovery of the beach-dune system along the Gulf side of the island by decreasing alongshore variability in erosion-deposition and limiting vegetative and geomorphic recovery. Unlabelled Image • Driving on the beach affects beach-dune response to and recovery following a storm. • Non-driving areas show substantial evidence of recovery within 6 months post-storm. • Driving on the beach limits incipient foredune development and recovery. • Limiting low dune recovery can reinforce the effects of framework geology. [ABSTRACT FROM AUTHOR]

Barrier islands are complex polygenetic landscapes, but contemporary models used to extract and visualize morphometric patterns tend to characterize them as two-dimensional landforms, leading to an idealized morphology in which the dune crest, dune toe, and dune heel are viewed as alongshore continuous features. A new exploratory approach to the extraction of topographic features explicitly recognizes the complex nature of barrier island morphology using surface normal vectors of the continuous surface. Complex alongshore and cross-shore ridge and swale features not visible in traditional elevation models are extracted using the directional components of the vectors. This transferable approach accounts for cross-shore and alongshore features and does not depend on a requirement of an alongshore continuous landform and provides a more objective approach to the extraction of beach and dune morphology by recognizing the polygenetic history of barrier islands. • Barrier island topography indicates the potential resiliency to climate change. • Ridge and swale morphologies highlight complex topographic structure. • Mapping the 3-dimensional structure of landforms elucidates polygenetic history. [ABSTRACT FROM AUTHOR]

The response and recovery of foredunes to storms is an important control on barrier response to sea level rise. While field studies suggest that dune height is site-specific and dependent to varying degrees on sediment supply and transport potential, the process-based Coastal Dune Model (CDM) predicts that dune height depends on the distance of the vegetation line to the shoreline (L veg) and that the beach-dune profile is scale-invariant. This study examines the intra- and inter-site variability of the beach-dune profile in relation to L veg along the north shore of Prince Edward Island (PEI), Canada. A combination of LiDAR and UAV surveys of >19,000 beach-dune profiles suggest that the ratio of dune crest elevation (C y) and distance from the shoreline above AMSL (S x) to the Crest (C x), is inversely related to cross-shore distance of L veg. The tallest dunes have the steepest seaward slopes (C y /(C x -S x)→0.45) and small L veg distances controlled by the variability of the beach-dune envelope, while the shortest dunes are associated with beaches with relatively gentle slopes (C y /(C x -S x)→0.01) and large L veg distances controlled by tide and storm inundation. Comparison to previously published beach-dune profiles reveals a similar relationship between the profile and L veg , suggesting that the morphology of the beach-dune system along the north shore of PEI and at other sites globally are reflective of their respective histories of storm activity, and do not trend towards the scale-invariant form assumed in the CDM. Further study of the contingent (or local) controls on coastal dune development and morphology are necessary to predict the evolution of coastal barriers to sea level rise and changes in storm activity. [ABSTRACT FROM AUTHOR]