Search

Coastal dunes provide many ecosystem services including protection of infrastructure from wave overtopping and habitat for native species. Foredunes grow at different rates and assume different forms (i.e., short and wide to tall and narrow) depending on a range of factors including pre-existing beach and dune morphology, wind, wave, and water levels, sediment grain size, and vegetation characteristics, yet the relative importance of these factors on foredune growth is understudied. Here, we quantify foredune evolution (2016–2019) and explore the relative influence of a suite of metocean, sedimentary, and ecological factors for dune growth on three barrier islands in the U.S. North Carolina Outer Banks (Cape Lookout National Seashore). We incorporate observed and hindcast wind, wave, and water level data into the process-based, coupled beach-dune evolution model, Windsurf, to explore the relative contribution of factors likely to influence foredune growth at the annual timescale (2016–2017). Our cross-shore topographic profile observations show varied interannual foredune change rates and characteristics, including horizontal retreat and progradation at the dune toe and vertical erosion and accretion at the dune crest. Model results indicate that, of the factors explored, pre-existing morphology had the greatest influence on the type of foredune growth that occurred (i.e., incipient dune development, widening of the dune, and/or vertical accretion), which dramatically altered the final shape of the dune. Variations in wind and wave climates were associated with the relative contribution of marine- and aeolian-driven bed elevation changes and were particularly influential during storms. In addition, increases in the minimum elevation of vegetation on the dune profile (analogous to the cross-shore distance between the shoreline and established vegetation line, i.e., the vegetation limit, as coined by Duran and Moore, 2013) increased dune crest height and dune volume. Moreover, variations in the minimum vegetation elevation resulted in a larger range of dune crest elevations and dune volumes than differences in the median sand grain size. We suggest that insights into the relative influence of metocean, sedimentary, and ecological factors to dune growth can assist in the development of practical coastal management strategies. • Pre-existing beach-dune morphology had the largest influence on dune growth type. • Exposure to the dominant wave direction led to increased dune volume. • Higher minimum vegetation elevation and finer sand resulted in taller dunes. • Minimum vegetation elevation influenced dune growth more than sand grain size. [ABSTRACT FROM AUTHOR]

This paper presents a numerical model designed to simulate subaqueous cross-shore profile behavior, including response of feeder mounds and barred systems. The present model development builds on the semi-empirical model proposed by Larson et al. (2013), designed to simulate the evolution of longshore bars exposed to incident waves, as well as the exchange of material between the bar and the berm region. Here, efforts are made to expand the theory for the evolution of a single-bar to a 2-bar system, where the volumes of the individual bars (inner and outer) and their responses are modeled. In order to investigate the predictive capacity of the model, this exploratory numerical tool is first calibrated and validated against data from Duck, North Carolina, USA, where 2 bars typically appear (inner and outer). Field data derived from nearshore sand placement projects (Silver Strand State Park, California, and Cocoa Beach, Florida, USA), involving the construction of artificial longshore bars, are also employed to test the model in complex situations with diverse wave climates and typical beach profile shapes. The study presented in this paper shows that the equilibrium-based model is skilled at predicting the time-varying volume of the outer bar (ε = 0.39; NMSE = 0.24), suggesting that this morphological feature is strongly influenced by offshore wave forcing in a predictable, equilibrium-forced manner. Model skill was lower (ε = 0.51; NMSE = 0.29) when predicting the inner bar evolution at Duck, remaining questions about the predictability and the equilibrium-driven cross-shore behavior of more transient features. Model prediction of the evolution of feeder mounds (artificial bars) proved to be also successful through description of hypothetical bars characterized by zero equilibrium bar volume, leading to a good agreement with the field observations. Overall, the potential for using rather simple models to quantitatively reproduce the main trends of cross-shore volume changes in bars in a time perspective from years to decades has been demonstrated. • Simple model to predict long-term evolution (years to decades) of 2-bar systems as well as the response of feeder mounds. • Simulation of individual bar volumes exposed to incident waves following an equilibrium-forced approach. • Short calculation times while keeping the model stability. • Potential for merging with a shoreline evolution model. [ABSTRACT FROM AUTHOR]