Microscale and Mesoscale Aeolian Processes of Sandy Coastal Foredunes from Background to Extreme Conditions

Aeolian transport affects beach and foredune pre-storm morphologies, which directly contribute to storm responses. However, significant spatiotemporal variation exists within beach-dune systems regarding how biotic and abiotic factors affect topography. There are multiple metrics for quantifying topographic change, with varying pros and cons, but understanding how a system changes across spatiotemporal scales relative to varying forcings is necessary to accurately model and more effectively manage these systems. Beach and foredune micro- and mesoscale elevation changes (Δz) were quantified remotely and in situ across a mid-Atlantic coastal system. The microscale field collections consisted of 27 repeat measurements of 73 elevation pins located in vegetated, transitional, and unvegetated foredune microhabitats over three years (2015 to 2018) during seasonal, event-based, and background wind-condition collections. Unoccupied aerial System (UAS) surveys were collected to link microscale point Δz to mesoscale topographic change. Microscale measurements highlight how Δz varies more pre- to post-event than seasonally or monthly, but regardless of collection type (i.e., seasonal, monthly, or event-based), there was lower Δz in the vegetated areas than in the associated unvegetated and partially vegetated microhabitats. Despite lower Δz values per pin measurement, over the study duration, vegetated pins had a net elevation increase of ≈20 cm, whereas transitional and unvegetated microhabitats had much lower change, near-zero net gain. These results support vegetated microhabitats being more stable and having better sediment retention than unvegetated and transitional areas. Comparatively, mesoscale UAS surfaces typically overestimated Δz, such that variation stemming from vegetation across microhabitats was obscured. However, these data highlight larger mesoscale habitat impacts that cannot be determined from point measurements regarding volumetric change and feature mapping. Changes in features, such as beach access paths, that are associated with increased dynamism are quantifiable using mesoscale remote sensing methods rather than microscale methods. Regardless of the metric, maintaining baseline data is critical for assessing what is captured and missed across spatiotemporal scales and is necessary for understanding the contributors to heterogeneous topographic change in sandy coastal foredunes.

Spatial variability of coastal foredune evolution, part A : timescales of months to years

Coastal foredunes are topographically high features that can reduce vulnerability to storm-related flooding hazards. While the dominant aeolian, hydrodynamic, and ecological processes leading to dune growth and erosion are fairly well-understood, predictive capabilities of spatial variations in dune evolution on management and engineering timescales (days to years) remain relatively poor. In this work, monthly high-resolution terrestrial lidar scans were used to quantify topographic and vegetation changes over a 2.5 year period along a micro-tidal intermediate beach and dune. Three-dimensional topographic changes to the coastal landscape were used to investigate the relative importance of environmental, ecological, and morphological factors in controlling spatial and temporal variability in foredune growth patterns at two 50 m alongshore stretches of coast. Despite being separated by only 700 m in the alongshore, the two sites evolved differently over the study period. The northern dune retreated landward and lost volume, whereas the southern dune prograded and vertically accreted. The largest differences in dune response between the two sections of dunes occurred during the fall storm season, when each of the systems’ geomorphic and ecological properties modulated dune growth patterns. These findings highlight the complex eco-morphodynamic feedback controlling dune dynamics across a range of spatial scales.

Literature-based latitudinal distribution and possible range shifts of two US east coast dune grass species (Uniola paniculataandAmmophila breviligulata)

Previous work on the US Atlantic coast has generally shown that coastal foredunes are dominated by two dune grass species,Ammophila breviligulata(American beachgrass) andUniola paniculata(sea oats). From Virginia northward,A. breviligulatadominates, whileU. paniculatais the dominant grass south of Virginia. Previous work suggests that these grasses influence the shape of coastal foredunes in species-specific ways, and that they respond differently to environmental stressors; thus, it is important to know which species dominates a given dune system. The range boundaries of these two species remains unclear given the lack of comprehensive surveys. In an attempt to determine these boundaries, we conducted a literature survey of 98 studies that either stated the range limits and/or included field-based studies/observations of the two grass species. We then produced an interactive map that summarizes the locations of the surveyed papers and books. The literature review suggests that the current southern range limit forA. breviligulatais Cape Fear, NC, and the northern range limit forU. paniculatais Assateague Island, on the Maryland and Virginia border. Our data suggest a northward expansion ofU. paniculata,possibly associated with warming trends observed near the northern range limit in Painter, VA. In contrast, the data regarding a range shift forA. breviligulataremain inconclusive. We also compare our literature-based map with geolocated records from the Global Biodiversity Information Facility and iNaturalist research grade crowd-sourced observations. We intend for our literature-based map to aid coastal researchers who are interested in the dynamics of these two species and the potential for their ranges to shift as a result of climate change.

Lateral vegetation growth rates exert control on coastal foredune hummockiness and coalescing time

Coastal foredunes form along sandy, low-sloped coastlines and range in shape from continuous dune ridges to hummocky features, which are characterized by alongshore-variable dune crest elevations. Initially scattered dune-building plants and species that grow slowly in the lateral direction have been implicated as a cause of foredune hummockiness. Our goal in this work is to explore how the initial configuration of vegetation and vegetation growth characteristics control the development of hummocky coastal dunes including the maximum hummockiness of a given dune field. We find that given sufficient time and absent external forcing, hummocky foredunes coalesce to form continuous dune ridges. Model results yield a predictive rule for the timescale of coalescing and the height of the coalesced dune that depends on initial plant dispersal and two parameters that control the lateral and vertical growth of vegetation, respectively. Our findings agree with previous observational and conceptual work – whether or not hummockiness will be maintained depends on the timescale of coalescing relative to the recurrence interval of high-water events that reset dune building in low areas between hummocks. Additionally, our model reproduces the observed tendency for foredunes to be hummocky along the southeast coast of the US where lateral vegetation growth rates are slower and thus coalescing times are likely longer.

Vegetation controls on maximum coastal foredune hummockiness and annealing time

Coastal foredunes serve as a primary defence against storms and high water events. Dune morphology can determine how a dune functions as a barrier to storm impacts. Hummocky foredunes – those with alongshore variability in dune crest elevation – may have a greater potential for breaching during storm events compared to continuous foredune ridges. Initially scattered dune-building plants and species that grow slowly in the lateral direction have been implicated as causes of foredune hummockiness. Our goal in this work is to understand the causes and dynamics of hummocky foredunes by examining vegetation characteristics. Using a numerical model, we explore how the initial configuration of vegetation and vegetation growth characteristics set the development and evolution of hummocky coastal dunes including the maximum hummockiness of a given dune field. We find that given sufficient time and absent external forcing, hummocky foredunes anneal to form continuous dune ridges. We develop a predictive rule for the timescale of annealing that depends on initial plant dispersal and two parameters that control the lateral and vertical growth of vegetation, respectively. Our findings suggest that whether or not hummockiness will be maintained depends on the time scale of annealing relative to the recurrence interval of high water events that reset dune-building in the low areas between hummocks. This relation explains the tendency for foredunes to be hummocky along the southeast coast of the U.S. where lateral vegetation growth rates, and thus annealing times, are likely longer.

Coastal foredune evolution: the relative influence of vegetation and sand supply in the US Pacific Northwest

Biophysical feedbacks between vegetation and sediment are important for forming and modifying landscape features and their ecosystem services. These feedbacks are especially important where landscape features differ in their provision of ecosystem services. For example, the shape of coastal foredunes, a product of both physical and biological forces, determines their ability to protect communities from rising seas and changing patterns of storminess. Here we assessed how sand supply and changes in vegetation over interannual (3 year) and decadal (21 year) scales influenced foredune shape along 100 km of coastline in the US Pacific Northwest. Across 21 years, vegetation switched from one congeneric non-native beachgrass to another ( Ammophila arenaria to A. breviligulata ) while sand supply rates were positive. At interannual timescales, sand supply rates explained the majority of change in foredune height (64–69%) and width (56–80%). However, at decadal scales, change in vegetation explained the majority of the change in foredune width (62–68%), whereas sand supply rates explained most of the change in foredune height (88–90%). In areas with lower shoreline change rates (±2 m yr −1 ), the change in vegetation explained the majority of decadal changes in foredune width (56–57%) and height (59–76%). Foredune shape directly impacts coastal protection, thus our findings are pertinent to coastal management given pressures of development and climate change.