Controls of Alongshore Variable Beach Morphology on Foredune Growth

Coastal foredunes are often the first line of defense for protecting backshore infrastructure from storm-induced flooding and are an attractive form of green infrastructure due to their ability to rebuild from natural processes. However, there is an incomplete understanding of coastal dune growth mechanisms, in part due to challenges in the measurement of aeolian sediment transport and transport gradients across the full range of time scales relevant for coastal dune building (seconds to years). For example, while beach characteristics, surface moisture content, and the presence of dune grasses are all known to affect instantaneous rates of aeolian sediment transport, the relative importance of these processes on longer term dune growth is unclear. However, the recent utilization of high-frequency, high-resolution, automated morphology measurements (e.g., terrestrial lidar scanning, TLS) allows for a new means to explore subaerial landscape changes across a broad range of time and space scales.

The U.S. Army Engineer Research and Development Center's Field Research Facility (FRF) in Duck, NC, USA is a dune-backed, sandy beach where detailed morphologic and oceanographic measurements have been collected since the 1980s. Over the past two decades the dunes at the FRF have eroded almost 30 m. However, the diffusion of a nearby beach nourishment carried out in summer 2017 has recently led to rapid morphological changes, including wider beaches closer to the nourishment site. Where the beach has widened, an incipient dune formed and continues to grow. Only a few hundred meters away the beach remains narrower and the dune is still eroding. This subaerial response to the nourishment has been captured by hourly TLS scans covering a 500 m alongshore extent and quarterly scans over a larger area. Utilizing these detailed TLS datasets, in this study we explore the influence of alongshore variable beach morphology on dune dynamics. Total water levels, surface moisture, and ecological properties derived from TLS and RGB imagery provide additional insights into the mechanisms contributing to the observed alongshore variability in dune volume changes. This work provides an incremental step in improving our understanding of natural dune building processes and how those processes are modified by coastal management practices.