Wave transformation in the surf zone

Abstract

In recent years, remote-sensing technology has become an essential tool for improving our understanding of nearshore processes. Although LiDAR (Light Detection And Ranging) scanners have been traditionally used for mapping nearshore or land areas, the application of terrestrial laser scanners to study swash zone hydrodynamics and morphodynamics was recently made possible thanks to the initiatives of few researchers. In this thesis, we explorethe use of 2D LiDAR scanners to monitor the time-varying surface elevation of breaking waves in the surf zone. The surf zone constitutes one of the most challenging environments in which to deploy instruments, due to the energetic wave conditions often found there. Hence, obtaining complete wave profile measurements at a high sampling rate represents a huge potential for better understanding wave transformation in the surf zone.In the present study, we first use data obtained from a tower-mounted LiDAR scanner deployed close to the shoreline to develop a new approach for studying inner surf zone waves at the wave-by-wave temporal scale. Waves are individually defined by extracting their crest and are then tracked until the shoreline. In combination with the Radon transform, we then apply this methodology to a numerical dataset of waves propagating and breaking in a prototype scale wave flume. This dataset illustrates the mechanism responsible for wave reflection in the nearshore: the reflected wave energy originates from the potentialenergy of the preceding swash event. The influence of these reflected waves on surf zone hydrodynamics is investigated at various temporal scales: the interactions between individual waves as well as the impact of the reflected wave field on the mean circulation in the surf zone are analysed and quantified.The most innovative dataset used in this work was collected from a nearshore pier at Saltburn-by-the-Sea, UK, where an array of three LiDAR scanners was deployed above shoaling and breaking waves. A methodology to match the three LiDAR individual datasets into a unique dataset is proposed and a technique to detect the break point from the scannermeasurements is also developed. The LiDAR scanners cover a maximum distance of about 100 meters in the cross-shore direction. This allows for an accurate description of thewave transformation at various stages: from the shoaling area, to the break point and through the inner surf zone until the runup in the swash zone. We finally present the firstdirect measurements of surface roller geometry in field conditions. These measurements in combination with a surface roller model shed light on the parameterization of energy dissipation in the inner surf zone, where waves propagate as fully developed bores.

Date of Award	6 Dec 2017
Original language	English
Awarding Institution	University of Bath
Supervisor	Chris Blenkinsopp (Supervisor) & Jun Zang (Supervisor)