UAS Identify and Monitor Unusual Small-Scale Rhythmic Features in the Bay of Cádiz (Spain)

Lara Talavera,Javier Benavente,Laura Del Río

doi:10.3390/rs13061188

Abstract

Unusual shore-normal and barred-like rhythmic features were found in Camposoto Beach (Bay of Cádiz, SW Spain) during a monitoring program using unmanned aerial systems (UAS). They appeared in the backshore and persisted for 6 months (October 2017–March 2018). Their characteristics and possible formation mechanism were investigated analyzing: (1) UAS-derived high-resolution digital elevation models (DEMs), (2) hydrodynamic conditions, and (3) sediment samples. The results revealed that the features did not migrate spatially, that their wavelength was well predicted by the edge wave theory, and that they shared characteristics with both small-scale low-energy finger bars (e.g., geometry/appearance and amplitude) and swash cusps (e.g., wavelength, seaward circulation pattern, and finer and better sorted material in the runnels with respect to the crests). Our findings pinpoint to highly organized swash able to reach the backshore during spring tides under low-energy and accretionary conditions as well as backwash enhanced by conditions of water-saturated sediment. This study demonstrates that rhythmic features can appear under different modalities and beach locations than the ones observed up to date, and that their unusual nature may be attributed to the low spatiotemporal resolution of the traditional coastal surveying methods in comparison with novel technologies such as UAS.

Highlights

Sandy coasts host a variety of well-known morphologies generated by the action of storms
Crescentic bars, which are regularly spaced halfmoon-shaped features with horns pointing landward and embayments pointing seaward, can migrate onshore, often causing the horns to reach the shoreline and remain attached to it. This leads to the formation of transverse bar systems, known as “transverse bar and rip” (TBR), with shore-normal and/or oblique orientations [3]
The exact moment in which these rhythmic features were formed is unknown, but they were absent in the pre-summer unmanned aerial systems (UAS) survey performed on May 2017, and as mentioned above, they were later detected in the survey carried out on October 2017

Summary

Introduction

Sandy coasts host a variety of well-known morphologies generated by the action of storms (e.g., washover fans and terraces, dune scarps). Apart from these, a wide spectrum of rhythmic features and/or mesoforms can develop alongshore on sandy coasts, under lower energy conditions and in different locations within the swash and surf zones [1] This is due to the fact that sandy accumulations are constantly molded by the action of waves, an agent that presents harmonics as its main characteristic. Crescentic bars, which are regularly spaced halfmoon-shaped features with horns pointing landward and embayments pointing seaward, can migrate onshore, often causing the horns to reach the shoreline and remain attached to it This leads to the formation of transverse bar systems, known as “transverse bar and rip” (TBR), with shore-normal and/or oblique orientations [3]. Additional rhythmic features are beach cusps, defined as alternate horns and bays

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Discussion

Conclusion