Spatial Variability of Coastal Foredune Evolution, Part A: Timescales of Months to Years
In this study, two 50 m segments of dune on an intermediate beach were monitored monthly with a terrestrial lidar scanner. Topographic changes and vegetation coverage were quantified at high spatial resolution (0.1 m in the alongshore and cross-shore dimensions) and used to investigate dune response to a series of major tropical and extratropical storm events.
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. At the start of and throughout the study, the erosive site had steeper foredune faces with less overall vegetation coverage, and dune growth varied spatially and temporally within the site. Deposition occurred mainly at or behind the vegetated dune crest and primarily during periods with strong, oblique winds (>∼45∘ from shore normal). Minimal deposition was observed on the mostly bare-sand dune face, except where patchy vegetation was present. In contrast, the response of the accretive site was more spatially uniform, with growth focused on the heavily vegetated foredune face. 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.
terrestrial lidar; coastal foredunes; storm impacts; erosion; dune recovery; morphodynamics
Coastal dunes are frequently lauded for their effectiveness at reducing impacts during storms by protecting communities and infrastructure behind them from flooding hazards (e.g., [1,2]). In addition, the ability for coastal dunes to grow naturally, including following storm-induced erosion, provides a natural mechanism which enhances resiliency along low-lying, sandy coastal regions [3,4]. As a result, dunes are increasingly used as a form of nature-based infrastructure in many managed coastal systems (e.g., [5,6,7,8]). Understanding the hydrodynamic, aeolian, and ecological processes which contribute to accretional and erosional dune dynamics on management and engineering timescales (days to years) is therefore critical because of the numerous services that dunes provide (e.g., ).
Wave contact with the dune is common on intermediate and reflective beaches during major storms, especially during events coinciding with a large storm surge or sea-level anomalies (e.g., [10,11,12,13]). While overwash and inundation can occur when total or mean water levels overtop the dune during the most extreme storms, collisional impacts  occur frequently during moderate storm events, and result in sand being removed from the dune face and deposited on the beach or transported into the nearshore (e.g., ). The mechanisms driving dune erosion are fairly well-understood (e.g., ), and the capabilities of simulating spatio-temporal dune erosion are improving [17,18], though tuning parameters for predictive models can vary significantly along the coast .
Similarly, there has been a wide range of research focused on understanding the processes contributing to dune growth (e.g., ). Research efforts on accretional coastal dune dynamics have been conducted at many geographic locations over a variety of time scales—from milliseconds to millennia—but have primarily been focused on micro-scale aeolian transport processes on time scales of seconds to hours (e.g., [21,22,23,24]) and meso-scale foredune dynamics on the time scales of seasons to decades (e.g., [25,26,27,28]). Resulting from extensive literature on dune dynamics, the primary physical and ecological drivers of coastal foredune growth are relatively well-understood. Dunes can naturally rebuild after storms and grow through a combination of wind-driven and ecological processes. When there is sediment availability (e.g., ), sand is transported from the sub-aerial beach via the wind and deposited in the dune, leading to vertical (aggradation) and/or lateral (progradation) of the dune. For this transport to occur, a threshold wind velocity must be exceeded to initiate saltation. This threshold is dependent on both local bed grain size characteristics (e.g., grain size distribution and armoring) and moisture content. Driven in part by these supply-limiting processes, saturated sediment transport will typically not occur when the fetch length [30,31,32] is small (e.g., <20 m). The potential for aeolian sediment transport potential and dune growth is generally higher on dissipative beaches, in part because the dry beach tends to be wide (large fetch) . Recent work on dissipative beaches in the U.S. Pacific Northwest also suggests that infragravity-driven swash may deposit sand into dunes during collision regime events, adding to dune growth [17,28]. Conversely, intermediate and reflective beaches generally have coarser grained material and are narrower in width, typically resulting in smaller overall wind-driven dune growth. While these general morphodynamic trends are highlighted in conceptual models by Short and Hesp  and Psuty , the details of local dune growth depends on the details of grain size properties, beach morphology, ecological characteristics [34,35]), total water levels, groundwater dynamics , biological factors , anthropogenic influences, wind characteristics, and the antecedent dune morphology (e.g., [38,39]). These controlling factors vary significantly in the alongshore at local to regional scales. For example, the density and species of dune grass has been shown to be important for influencing the dune shape due to sand-trapping capacity—yet these characteristics vary significantly, both in space and time (e.g., ).
Wind itself, which is the primary driver of aeolian sediment transport, can also be highly spatially variable. On a local scale, wind variability is driven in part by topographic effects on the flow field, including steering across the dune face [41,42]. For sediment to ultimately be transported from the beach to the dune, there must be a cross-shore component of the wind vector—otherwise, sediment is transported down the coast or into the ocean. Topographically-induced gradients in the wind field and bed shear stress reduction from dune grasses combine to produce local gradients in the transport field, and result in bed deposition or erosion [35,43,44].
Despite the dominant drivers of dune growth processes being qualitatively understood, there are complex eco-morphodynamic feedbacks, scale-dependent processes, and aggregation effects that have precluded accurate simulation of dune evolution across the range of timescales relevant for coastal management (days to years) (e.g., [20,38,45,46]). This represents a gap in the ability to apply research to coastal management problems where coastal foredunes are used as an asset in developing resilient coastlines . Research gaps at intermediate scales, where the aggregation of short time- (<seasonal) and space- (<1 m) scale hydrodynamic, meteorological, and ecological processes are all important drivers of coastal landscape change, contribute to this relatively poor predictive capability of coastal dune evolution. However, as managed dunes become recognized as important features for coastal protection , there is an increasing need to synthesize the relevant environmental, morphologic, and ecological controls on their evolution across the continuum of temporal and spatial scales relevant for coastal management. Improved predictive capabilities of dune erosion, growth, and recovery are critical for the effective management of coastlines throughout the world, particularly in the context of changing environmental forcings (e.g., sea-level rise, changes in storminess) and increased anthropogenic pressures on coastal resources (e.g., ).
Limits in data collection techniques have contributed to limitations in understanding the spatio-temporal deposition patterns in dunes at these relevant scales. Historically, aeolian transport processes have been characterized by short (<week) in-situ field studies, ecological processes by infrequent measurement campaigns, and meso-scale dune dynamics through sparse transect-based topography surveys  or from intermittent airborne lidar surveys [50,51]. Data frequency and sparseness both pose issues for bridging scales. Recently, high-resolution terrestrial lidar scanners have been utilized to explore beach and dune dynamics at high spatial resolution (centimeters) and, in some cases, with high frequency (e.g., [52,53,54,55,56]). These high-density data provide an opportunity to expand the scientific understanding of complex dune eco-morphodynamics that was not previously attainable.
In this paper, we investigate dune evolution at the U.S. Army Engineer Research and Development Center’s Field Research Facility (FRF) in Duck, NC, located on the barrier islands of North Carolina’s Outer Banks. High-resolution terrestrial lidar data are used to explore morphodynamic controls on alongshore variable dune growth patterns. Specifically, we use ∼monthly lidar scans to quantify topographic evolution and vegetation coverage at two 50 m alongshore stretches of dune within the FRF property. Observed topographic and volumetric responses are compared to forcing conditions (waves, water levels, and wind speeds) in each inter-survey period to gain insights into the drivers of alongshore variability in dune growth patterns. In this manuscript, Section 2 provides an overview of the FRF field site, and Section 3 provides details on the instruments, datasets, and methods used to characterize controls on dune evolution at the study site. Section 4 presents the results of the data analysis. The discussion and conclusions are given in Section 5 and Section 6, respectively. This work was conducted as a complementary study to that of Palmsten and Brodie , which focuses on the longer term (multi-decadal) beach and dune dynamics at the FRF.
2. Field Site
The FRF is an oceanographic research facility located on the Northern Outer Banks barrier islands of North Carolina, USA, focusing on the collection of oceanographic and morphologic data. The narrow (1 km wide) barrier island is situated between the Atlantic Ocean to the east, and the Currituck Sound to the west (Figure 1a). The facility includes 1 km of a sandy, dune-backed, micro-tidal, intermediate beach with fine to coarse sand , and an average foreshore slope of 1:12 . The surfzone is often characterized by one or two sandbars that are frequently longshore-periodic , and nearshore morphology is reflective of the recent storm history . The tide range is roughly 1 m, and 0 m NAVD88 is approximately equal to mean sea level (MSL). A local coordinate system for the facility is used in this analysis in which the positive x-axis is aligned with the FRF research pier and directed offshore, and the positive y-axis points 18 degrees west of true north along the beach (Figure 1), with NAVD88 defined as the vertical datum. During the late summer and fall, the region is frequently impacted by tropical low-pressure systems that move north along the U.S. Atlantic East coast. During the winter, cold Arctic air meets the warmer coastal waters of the Gulf Stream off of the North Carolina coast, causing the formation of strong low-pressure systems, called Nor’Easters that track north towards New England. Both types of storm systems can produce storm surge, large waves, and strong winds. The Nor’Easters are characterized by strong winds out of the Northeast, whereas wind direction during the tropical storms (e.g., hurricanes) can be more varied, depending on the storm track. Resulting from the aggregation of wave-driven processes during both calm and energetic conditions, annual net longshore sediment transport along this section of coast is to the south . A large, prominent dune was constructed (fenced and planted) along the entirety of the Outer Banks barrier islands in the 1930s and 40s . The dunes along the FRF property have been unmanaged since then, and their morphology has been measured regularly since 1981 (See Palmsten and Brodie ). During storms, waves frequently impact the dune along the property, but have rarely overtopped the dune since 1980.
Figure 1. Overview of field sites. (a) Aerial imagery of the Field Research Facility and regional inset map. The north dune field site is highlighted in red, and the south dune field site in blue. The location of the anemometer used in this study is shown by the white star. (b,c) Example high-resolution ortho-imagery with the approximate dune crest (solid) and dune toe (dashed) position indicated, and (d,e) digital elevation models (colors show elevation in m, NAVD88) of the North and South sites, respectively.
Palmsten and Brodie  quantified dune morphology evolution over 25 years along the property, identifying significant differences in the evolution of the dunes on the northern and southern property in the last decade. A similar spatial history of shoreline evolution at the FRF is described in Pianca et al. . These works indicate that while the beach-dune system throughout the FRF property grew for the early part of the observational morphologic dataset, which started in the 1980s, the northern end of the property began experiencing significant erosion in the early 2000s. The dunes on the north side of the property retreated 25 m from 2003 to 2014 (Figure 2a). During this same time period, the southern dunes continued to grow, with a prominent foredune developing from 1990 to 2014 that resulted in the abandonment of the former active foredune (Figure 2b). Dune volume changes were statistically correlated with sub-aerial beach and surf-zone volumes, suggesting that spatial variability in local beach characteristics in the alongshore played a role in the alongshore variable evolution of the dune complex at multi-year time-scales. However, these observations did not have a high enough temporal resolution to identify conclusive relationships to forcing conditions .
Figure 2. Field Research Facility (FRF) long-term dune evolution. Elevation (m, NAVD88) versus cross-shore position through time (colors increase in time from 1980 (blue) to 2014 (red)) for a cross-shore transect through: (a) the northern study site at y = 869 m, and (b) the southern study site at y = 183 m. Profile locations are dictated by standard FRF Profile sample locations.
In this work, we examine two 50 m sections of the dune system at the northern and southern ends of the FRF property (Figure 1), and utilize detailed terrestrial lidar observations to quantify monthly evolution of the two dune systems over a two-year period. The northern study site was located between y = 885 and y = 935 m in the alongshore, and the southern study site was located between y = 142 and y = 192 m in the alongshore, where distances are expressed in the local coordinate system. Henceforth, the study sites will be referred to as the “north dune” and “south dune”, respectively.