Investigating the natural evolution of a multilevel beach cusp system using UAV photogrammetry and numerical modelling

Identifying the morphodynamic evolution of the swash zone requires the analysis of key coastal processes responsible for its change. Sandy beaches, and the features prevalent on their beachface (e.g., beach cusps), are naturally dynamic systems and require intense monitoring campaigns to truly quantify their evolution. However, collecting data in the swash zone can be challenging; breaking waves, strong nearshore currents, and wave run-up can make sampling difficult. This study uses a composite modelling technique to achieve a better understanding of the swash zone by examining beach cusp characteristics prevalent on the beachface. In particular, beach cusp evolution has been examined over a six-month period, on the beach in Long Strand, Co. Cork, Ireland. The study site is unique in the world as it has a three-tiered beach cusp system on its beachface, which allowed to fill some gaps currently still in place regarding beach cusp evolution. Field observations were carried out with an Unmanned Aerial Vehicle (UAV) and critical cusp parameters are reported, including cusp spacing, cusp elevation, cusp depth, and cusp amplitude. So far, such an extensive dataset has not previously been reported in the literature from a single site. The evolution of the different cusp parameters is then linked with the hydrodynamics in the study area, and new prediction formulas are proposed for the different cusp parameters, linking the horizontal swash excursion with the cusp parameters. The lower beach level cusps (1 < z < 2.5 m Irish Transverse Mercator [ITM]) changed with every high tide and appeared when the surf-similarity parameter - x 0 < 1.55. These cusps had a mean cusp spacing of lc,mean = 11.09 m, which is closely linked with the predictions of the self-organisation theory (p < 0.05). In contrast, the mid beach level cusps (2.5 < z < 3.5 m ITM) are less dynamic compared to the lower beach level cusps and can persist between tidal cycles. They had a mean cusp spacing of lc,mean = 18.17 m. The upper beach level cusps (approximately z = 6 m ITM) are above astronomical tide levels and have a mean cusp spacing of lc,mean = 40.26 m. They did not change significantly over the survey period due to lack of major storm events. In addition, the morphodynamic evolution of the swash zone was investigated using the MIKE 3 software, something that has not been examined before. MIKE 3 is able to accurately model swash motion, but the model failed to reproduce observed cusp spacing and other cusp parameters as reported from the study area. As such, the outcomes of this study can be used as a starting point to increase the capabilities of MIKE 3