From stem to crest: Vegetation control on the maximum height, alongshore continuity, number and spacing of coastal foredunes

Foredunes—the seaward-most line of coastal dunes—arise from two-way couplings between vegetation and sediment transport processes. Model experiments and field observations are increasingly demonstrating that these ecomorphodynamic interactions shape the coastal landscape across a range of spatial and temporal scales, and dimensions. For example, in the cross-shore dimension, the maximum potential height that a coastal foredune can achieve is a function of the distance from the shoreline that dune vegetation can grow. The farther from the shoreline that vegetation becomes established, the taller the foredune can grow before steering the wind sufficiently above the beach to reduce the shear stress below the critical shear stress necessary for sand to be transported into the dune, at which point dune growth ceases. In the alongshore dimension, the relationship between the rate at which vegetation grows in the lateral direction relative to the rate at which vegetation grows in the vertical direction appears to play a role in the alongshore annealing of initially hummocky embryo dunes, likely at least partially determining whether alongshore dune morphology will tend to be hummocky or tend to be characterized by a relatively alongshore-continuous foredune ridge. Likewise, at larger spatial and temporal scales, interactions between the rate of lateral vegetation growth, as potentially controlled by dune slope, can explain the overall morphology of multiple dunes in areas where the shoreline is prograding. Where the ratio of the shoreline progradation rate to the lateral dune growth rate is large dunes tend to be smaller and widely spaced. In contrast, in areas where the ratio of the shoreline progradation rate and the lateral dune growth rate is small, dunes tend to be larger and more closely spaced, or exhibit a morphology typified by overlapping complex dune forms.