Analysis of Beach Cusp Formation and Evolution Using High‐Frequency 3D Lidar Scans

Abstract

AbstractIn this study, beach cusp characteristics were explored using 15 months of 3D lidar scans collected hourly at the Field Research Facility in Duck, NC. Fourier analyses were performed on lidar‐derived beach elevation contours to generate spatial cusp spectra. Active cusp events were identified on the basis of the location and magnitude of each spectrum's peak and used to evaluate conditions during cusp formation and evolution. Cusps primarily developed during times with normally‐incident, long period, low energy wave conditions with low frequency spread, and reflective beach conditions. The upper and lower beaches often exhibited different behavior and morphologies, with persistent upper‐beach cusps lasting days to months and dynamic lower‐beach cusps evolving over individual tidal cycles. At times, beaches exhibiting multiple cusp systems reverted to a single cusp system extending over the entire beach when the high‐tide waterline reached the upper‐beach cusps, with the location and spacing of the resulting lower‐beach cusps controlled by the upper‐beach cusps. These observations are consistent with a “morphological coupling” hypothesis proposing that hydrodynamic‐morphodynamic feedbacks between the swash and upper‐beach cusps can result in the formation of lower‐beach cusps with a related wavelength as the tide falls. However, there were also times when the high‐tide waterline reached the upper‐beach cusps that did not result in a unified beach state. These results suggest that while morphological coupling is often an important factor in controlling the development of new lower‐beach cusps, this coupling cannot initiate cusp formation in hydrodynamic conditions outside those favorable for cusp activity.