Nearshore Wave Transformation Domains from Video Imagery

Abstract

Within the nearshore area, three wave transformation domains can be distinguished based on the wave properties: shoaling, surf, and swash zones. The identification of these distinct areas is relevant for understanding nearshore wave propagation properties and physical processes, as these zones can be related, for instance, to different types of sediment transport. This work presents a technique to automatically retrieve the nearshore wave transformation domains from images taken by coastal video monitoring stations. The technique exploits the pixel intensity variation of image acquisitions, and relates the pixel properties to the distinct wave characteristics. This allows the automated description of spatial and temporal extent of shoaling, surf, and swash zones. The methodology was proven to be robust, and capable of spotting the three distinct zones within the nearshore, both cross-shore and along-shore dimensions. The method can support a wide range of coastal studies, such as nearshore hydrodynamics and sediment transport. It can also allow a faster and improved application of existing video-based techniques for wave breaking height and depth-inversion, among others.

Breaking Wave Height Estimation from Timex Images: Two Methods for Coastal Video Monitoring Systems

“…The shadow created by the reduction of sunlight reflection by shoaling waves on the water surface is visible on Timestacks as a drop of pixel intensity, whereas the typical white foam created by depth-induced wave breaking appears as white pixels. Therefore, the incipient breaking point coincides with the change between dark and bright pixels of each single wave feature visible on the image [17,19,21,29,30].…”

“…On Timestack images, pixel intensity can be related to wave transformation processes in the nearshore, since the incident light reflection on the water surface varies in response to sea state [19,28]. The shadow created by the reduction of sunlight reflection by shoaling waves on the water surface is visible on Timestacks as a drop of pixel intensity, whereas the typical white foam created by depth-induced wave breaking appears as white pixels.…”

“…The video monitoring technique has been proven as a valid tool for measuring wave breaking height because the wave characteristics have a visible signature on the sea surface. Existing video-based methods for estimating breaking wave height are based on the use of Timestack images [17,18], which are obtained by sampling and concatenating a single transect of pixel from each image over a given interval of time [19]. These methodologies exploit the photogrammetric relation between the camera location and incipient breaking point visible on Timestacks [17,18].…”

“…Considering that the production of Timestacks and Brightest images is not often set routinely by the coastal video systems installed worldwide, a methodology based on the use of the most common images produced by coastal video monitoring systems, namely Time-Exposure images (hereinafter, Timex images), would be beneficial. Timex images are generated by the average of the individual images collected over a sampling period, usually 10 min [16,19]. The main characteristic of Timex is to show a time-averaged white pixel pattern at the breaking location, usually associated with the wave energy dissipation process over a bar crest and/or at the shore [22,23].…”

Assessment of a Smartphone-Based Camera System for Coastal Image Segmentation and Sargassum monitoring

“…Among the coastal video monitoring techniques, ground-sensing approaches by means of camera systems are widely used, allowing the investigation of coastal processes with high temporal and spatial resolution, with optimal accuracy. Numerous alternative or complementary uses are also pursued, with applications related to coastal hydrodynamics [1][2][3][4], morphodynamics [5][6][7], or quality monitoring (e.g., seagrass) [8,9]. Moreover, the use of low-cost devices and public resources has recently become a focus of attention and is being further explored [10][11][12].…”