Spatial and Temporal Spectra of Swash Motions Estimated by Multiple Synchronized UAVs

Abstract

It is important to understand the characteristics of swash in terms of disaster mitigation and coastal management, as it has great impacts on coastal structures, beach processes and coastal zone management. Although many studies focused on the temporal variation of the swash motion as the importance of infra-gravity waves is recognized, the spatial characteristics remain unknown probably because of difficulty in the observation. In this study, the swash motion on 2.87 km sandy beach in Miyazaki, Japan was captured in videos by three synchronized Unmanned Aerial Vehicles (UAVs). The UAVs were situated in 850 m interval, shooting bird’s-eye video. The videos were synchronized and projected orthographically to detect the edges of the swash using the luminance difference. Spatiotemporal profile of swash on the beach was obtained for 2.87 km and 823 s. With spectral analysis, longer periods of run-up fluctuation (14 s, 25 s and 43 s) were observed, whereas the predominant swell period is about 10s. The intensity of those waves fluctuates in the longshore direction. The spatial fluctuation of intensity of infra-gravity waves appears to correlate with the width of surf zone, which implies a resonant interaction mechanism between the topography and infra-gravity waves within the surf zone.