Spatial Variability of Coastal Foredune Evolution, Part A: Timescales of Months to Years

Abstract

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 monitoring of coastal foredune evolution in managed and unmanaged systems

“…As other researchers have noted (e.g. Walker et al ., ; Brodie et al ., ), transport to dunes is complex and has been observed from alongshore wind directions, challenging this formulation. Furthermore, topographic steering may even contribute to transport to the dune from offshore wind directions, making predictions challenging.…”

“…Finally, events were summed over each survey interval, and potential aeolian transport Q I was converted from kilograms per metre (kg m −1 ) to cubic metres per metre (m 3 m −1 ) using the average bulk density of sand, 1590 kg m −3 (Keijsers et al ., ). Since dune growth at this site has previously been observed to occur during time periods with highly oblique winds (Brodie et al ., ), we calculated two versions of Q I , one that accounted for wind direction [Equation ] and one that removed offshore winds but did not account for wind direction [Equation ].…”

“…Frequent terrestrial LiDAR scanning (TLS) has been used recently to study beach‐dune morphodynamics and can help resolve small‐ and large‐scale dynamics, including storm response and recovery (e.g. Montreuil, ; Schubert et al ., ; Fairley et al ., ; Fabbri et al ., ; Smith et al ., ; Telling et al ., ; Brodie et al ., ).…”

Evaluating Collection Parameters for Mobile Lidar Surveys in Vegetated Beach-Dune Settings

“…Past research in coastal systems has used MTL to monitor beach nourishment performance (e.g., Pietro et al 2008), cliff erosion, and regional scale subaerial beach and dune morphology (Donker et al 2018;Spore et al 2019). Fixed station (tripod-based) terrestrial lidar scanning (FTL) has also widely been utilized for providing valuable new insights on coastal dune dynamics on sub-meter spatial scales and at time scales of months to years (Brodie et al 2019;Conery et al 2019;Danchenkov et al 2019). However, the ability to accurately quantify dune evolution at these same temporal scales utilizing MTL systems, which can cover much larger spatial extents than FTL scanning, would enable significant advancements in the ability to effectively monitor coastal dune evolution at sub-decimeter scale.…”

“…Even over sub-meter scales, both the ecological and morphological properties of the dune differ substantially (e.g. Brodie et al 2019).…”