Species-Specific Control on Coastal Foredune Morphology along the U.S. East Coast

Abstract:

On sandy coastlines, foredunes provide protection from coastal storms, potentially sheltering low areas—including human habitat—from storm-induced elevated water level and wave erosion. The relationship between total water level and foredune height largely determines the response of sandy coastlines to storms and thus the shape and alongshore continuity of foredunes is critical in assessing coastal vulnerability. Previous work has shown that coastal dunes grow as a result of feedbacks between vegetative and sediment transport processes, and that resultant foredune morphology is species-specific. Two dune-building grasses dominate the foredunes of the U.S. East Coast; south of VA dunes are primarily vegetated by Uniola paniculata while north from VA Ammophila breviligulata is dominant. Qualitative observations suggest that dunes dominated by U. paniculata are hummocky whereas dunes dominated by A. breviligulata are long crested and contiguous. Recent observations suggest that the range of these dune-building grasses is changing (i.e., U. paniculata is advancing northward). Such a shift in the geographic range of species may lead to changes in foredune shape and thus changes in coastline vulnerability.

Using observations and numerical experiments, we explore the current and potential future role of dune grasses in determining coastal response to storms. Species-specific foredune morphology is likely a result of differences in species growth form: A. breviligulata and U. paniculata exhibit differences in their rate of lateral growth. Slower lateral growth of U. paniculata provides a potential explanation for the irregular dune crest height in the alongshore direction. We test this qualitative observation by quantitatively examining coastal foredunes morphology along several barrier islands of the U.S. East Coast dominated by A. breviligulata or U. paniculata. In addition, we expand and modify an existing numerical model of coastal dune formation (Durán and Moore, 2013) to include species-specific parameterizations for A. breviligulata and U. paniculata and compare model results to observational data. Using this new model we conduct a suite of numerical experiments to explore the change in dune morphology that may result from future changes in foredune species dominance.