Observations and modeling of shear stress reduction and sediment flux within sparse dune grass canopies on managed coastal dunes

Abstract

Wind flow over coastal foredunes adapts to vegetation, resulting in spatial gradients in bed shear stresses that contribute to the formation of localized bedforms. Understanding, and having the capability to numerically predict, the distribution of sediment deposited within sparsely vegetated dune complexes is critical for quantifying the ecological, protective, and economic benefits of dune management activities. Data from wind tunnel experiments have indicated that there is a spatial lag from the canopy leading edge to a downwind location where sediment deposition first occurs. The length scale of this deposition lag is further quantified here using new field measurements of aeolian sediment transport across sparsely vegetated managed dune systems in Oregon, USA. We develop a deposition lag length scale parameter using both lab and this new field data and then incorporate this parameter into the process‐based aeolian sediment transport model, Aeolis, which also includes a new far‐field shear stress coupler. Results from numerical simulations suggest that the spatial deposition lag effect is significant for model skill in sparsely vegetated dunes. We observe with field and laboratory observations that, as canopy density increases, the length of the deposition lag decreases. As such, within the model framework the implementation of the deposition lag length does not affect the results of models of coastal dune geomorphological evolution within higher density canopies. Dune canopy density can vary due to natural (e.g., storm overwash, burial, die‐off) or anthropogenic (e.g., managed plantings, dune grading) processes.

Contemporary research in coastal dunes and aeolian processes

“…The increasing need for dune research on developed coasts was made explicit by Nordstrom (2004) in Beaches and dunes of developed coasts, and themes such as the restoration of coastal dunes have been the focus of dedicated textbooks, for example, Restoration of coastal dunes (Martínez et al, 2013). Examples of human interventions in coastal dunes in this special issue include the removal of invasive vegetation Hilgendorf et al, 2022;Konlechner & Hilton, 2022), engineered foredune notches (Nguyen et al, 2022), and intensely managed or constructed foredunes (Dickey et al, 2023;He et al, 2022;Sanromualdo-Collado et al, 2022;Walker et al, 2023).…”

“…As coastal dunes are increasingly being targeted as feasible 'Nature- The research within this special issues also reflects the increasingly sophisticated and holistic approach being taken to understanding coastal dune dynamics. For example, a range of research measured or modelled abiotic and biotic factors that impact aeolian systems including waves (Garzon et al, 2022;He et al, 2022;Ruessink et al, 2022), surface moisture (Ruessink et al, 2022), and vegetation Dickey et al, 2023;Konlechner & Hilton, 2022Kooijman et al, 2022Li et al, 2022;Sanromualdo-Collado et al, 2022). The data sources were dominated by field experiments and remote sensing, with only five of the 25 articles using data derived from experimental laboratory work or numerical models.…”

“…The increasing need for dune research on developed coasts was made explicit by Nordstrom (2004) in Beaches and dunes of developed coasts , and themes such as the restoration of coastal dunes have been the focus of dedicated textbooks, for example, Restoration of coastal dunes (Martínez et al, 2013). Examples of human interventions in coastal dunes in this special issue include the removal of invasive vegetation (Bauer et al, 2022; Hilgendorf et al, 2022; Konlechner & Hilton, 2022), engineered foredune notches (Nguyen et al, 2022), and intensely managed or constructed foredunes (Dickey et al, 2023; He et al, 2022; Sanromualdo‐Collado et al, 2022; Walker et al, 2023). Many manuscripts within the special issue that investigated the coastal dynamics of naturally forming dunes also explicitly situated the research in the context of contemporary sea‐level rise and climate change (Arce Chamorro et al, 2022; Fisher et al, 2023; Garzon et al, 2022; Hesp et al, 2022; Robin et al, 2022) or hazard mitigation (He et al, 2022).…”

“…Dickey et al (2023) found that the depositional lag length increased with decreasing vegetation canopy density. Their results highlight that the standard of practice (Raupach stress partition) used in aeolian models to parameterize shear stress reduction from vegetation cover should be used with caution for dunes with sparse vegetation canopies.…”

Skimming Flow and Sand Transport Within and Above Ammophila (Marram) Grass on a Foredune

“…This finding is supported by field experiments conducted by Hesp et al (2019) who additionally showed that in the case of taller plants (32 cm tall) near-surface flow penetration within canopies is greater and the rate of downwind flow deceleration is lower (cf. Cheng et al, 2019;Dickey et al, 2023).…”