Coastal foredunes defend against flooding and erosion and provide other important ecosystem services including wildlife habitat and carbon sequestration. Dunes form via ecomorphodynamic feedbacks between sediment and burial-tolerant vegetation, making beach grasses important in shaping foredunes. The capability of foredunes to buffer coastlines from hazards is thus determined by a combination of biological and physical factors. Here, we aim to quantify the relative roles of vegetation, sand supply, and geomorphic drivers in shaping foredune morphology along a 300-km stretch of the North Carolina (NC) Outer Banks which exhibits alongshore variations in sand supply and vegetation. We conducted surveys of dune grasses and beach-dune morphology at 90 transects from October 2016–June 2018. Key dune and beach morphometrics (e.g., foredune toe and crest elevation, foredune width, beach width), shoreline change rates (SCR; multidecadal from 1997-2016, interannual from 2016-2017), and grass density parameters (for two dominant grasses, Ammophila breviligulata and Uniola paniculata) were calculated using a combination of field surveys and airborne lidar data. Regression analyses were used to determine which variables were most strongly correlated with metrics of foredune shape over the study period. We utilized hierarchical partitioning analyses to determine the proportion of variation explained by each factor for foredune metrics.
Results/Conclusions
Foredune toe and crest elevation increased marginally over the study period, with no statistically significant change in width. Regression models provided strong evidence that foredune morphology and change metrics are correlated with multidecadal SCR (a sand supply proxy), grass density changes, and beach morphology. Foredune height was positively correlated with multidecadal SCR and negatively correlated with beach width, backshore slope, and increase in A. breviligulata density, while foredune height change was positively correlated with increase in A. breviligulata density and negatively correlated with backshore slope. Foredune width was positively correlated with multidecadal SCR, foreshore slope, and beach width and negatively correlated with increase in A. breviligulata density, while foredune width change was positively correlated with increase in A. breviligulata density and interannual SCR. Our results suggest that sand supply and beach morphology are most influential in controlling foredune height, while sand supply and grass density largely control foredune width. Our finding that A. breviligulata density change explained the greatest proportion of variation in foredune width change aligns with previous findings in Pacific Northwest dunes. Understanding the relative influence of vegetation and sand supply on coastal dune evolution is especially critical as climate-driven species range shifts may impact dune ecosystem services.