Elucidating Coastal Foredune Ecomorphodynamics in the U.S. Pacific Northwest via Bayesian Networks

Abstract

As sea level rise and possible changes in storminess threaten coastal communities and infrastructure, the capacity for foredunes to provide protection depends on their geomorphology, which is determined by interactions between physical beach processes and vegetation. Here we use descriptive Bayesian network analyses to examine how sediment supply, beach characteristics, and two species of beachgrass (Ammophila arenaria and Ammophila breviligulata) alter foredune morphology and patterns of sand accretion on U.S. Pacific Northwest foredunes. We show that sediment supply and beach type primarily determine foredune morphology. Beachgrass density also influences foredune shape, but its effects differ among species: increasing density of A. arenaria was associated with steeper sloping dunes, whereas increasing density of A. breviligulata was associated with wider, more shallow sloping dunes. An examination of the change in foredune morphology over a 2-year period found sand accretion was most strongly influenced by species-specific patterns of vegetation growth and beach type. Specifically, A. breviligulata exhibited more lateral growth, resulting in greater sand accretion at the seaward margin of the foredune. In contrast, A. arenaria exhibited little lateral growth, resulting in comparatively more sand accretion near the foredune crest. Consequently, growth form-generated sand accretion patterns resulted in steep, narrow A. arenaria-dominated foredunes and shallow-sloping, wider A. breviligulata-dominated foredunes. These results illustrate that vegetation density and patterns of growth influence foredune morphology and its changes over time.

Species-Specific Functional Morphology of Four US Atlantic Coast Dune Grasses: Biogeographic Implications for Dune Shape and Coastal Protection

“…In particular, steeper sloped beaches, positive shoreline change rates, and higher shoot densities of both beachgrass species were all associated with taller and wider dunes. In addition, Biel et al [19] found that species-specific differences in sand capture of the two beachgrasses explained additional variability in dune shape, as had been hypothesized in other studies [11,12]. Ammophila breviligulata, which has less dense but thicker shoots, exhibited more lateral growth, resulting in greater sand deposition and widening of the foredune at the seaward margin.…”

“…This feedback process builds dunes, reducing vulnerability to coastal flooding and erosion [6,7], providing habitat for wildlife [8], and serving to sequester carbon [9]. One area of focus has centered on the role of species-specific biophysical feedbacks of dune plants in determining dune geomorphology [4,5,[10][11][12][13][14][15][16][17][18][19]. These studies show that vegetation functional morphology and its interaction with windblown sand can contribute to the variability in dune geomorphology at local to landscape level scales.…”

“…Wind moves sand from the beach to the backshore where vegetation plays a pivotal role in nucleating dunes; for example, Kuriyama et al [21] found that plant cover as low as 28% reduced sand transport by 95%. Once vegetation captures sand, a positive feedback between plant growth and accretion occurs, eventually resulting in foredunes that vary in size and shape [5,[10][11][12][13]18,19,22,23].…”

“…A recent study of US Pacific Northwest dunes explored the relative role of geological factors and the invasion of two grass species (European beachgrass Ammophila arenaria and American beachgrass A. breviligulata) in determining foredune variability along a 500-km stretch of coastline [19]. Using a Bayesian network analysis that compared foredunes at 20 sites, they found that~50% of the variability in foredune crest height was explained by beach geomorphology, specifically backshore slope and shoreline change rate, both of which contribute to sand availability to the foredune.…”

“…In addition, dune grass species differ in the average number of shoots produced per rhizome, a factor that varies from a single shoot to multiple shoots. These two features, when combined, produce species-specific differences in aboveground shoot density and spacing (i.e., from evenly spaced to clumped) and differences in the relative amount of lateral versus vertical spread across the foredune, all of which directly affect sand capture and dune shape [4,5,[10][11][12]18,19,24]. Overall, these studies show that sparse, evenly spaced, and/or laterally spreading growth forms produce shorter and wider dunes compared to dense, clumped, and/or vertical growing forms, which create taller, steeper, and narrower foredunes.…”