Abstract
The breaking wave height is a crucial parameter for coastal studies but direct measurements constitute a difficult task due to logistical and technical constraints. This paper presents two new practical methods for estimating the breaking wave height from digital images collected by shore-based video monitoring systems. Both methods use time-exposure (Timex) images and exploit the cross-shore length ( L H s ) of the typical time-averaged signature of breaking wave foam. The first method ( H s b , v ) combines L H s and a series of video-derived parameters with the beach profile elevation to obtain the breaking wave height through an empirical formulation. The second method ( H s b , v 24 ) is based on the empirical finding that L H s can be associated with the local water depth at breaking, thus it can be used to estimate the breaking wave height without the requirement of local bathymetry. Both methods were applied and verified against field data collected at the Portuguese Atlantic coast over two days using video acquired by an online-streaming surfcam. Furthermore, H s b , v 24 was applied on coastal images acquired at four additional field sites during distinct hydrodynamic conditions, and the results were compared to a series of different wave sources. Achievements suggest that H s b , v method represents a good alternative to numerical hydrodynamic modeling when local bathymetry is available. In fact, the differences against modeled breaking wave height, ranging from 1 to 3 m at the case study, returned a root-mean-square-error of 0.2 m. The H s b , v 24 method, when applied on video data collected at five sites, assessed a normalized root-mean-square-error of 18% on average, for dataset of about 900 records and breaking wave height ranging between 0.1 and 3.8 m. These differences demonstrate the potential of H s b , v 24 in estimating breaking wave height merely using Timex images, with the main advantage of not requiring the beach profile. Both methods can be easily implemented as cost-effective tools for hydrodynamic applications in the operational coastal video systems worldwide. In addition, the methods have the potential to be coupled to the numerous other Timex applications for morphodynamic studies.
Highlights
IntroductionThe wave height at the breaking point (hereafter the breaking wave height) is an essential component in any study regarding coastal processes
The wave height at the breaking point is an essential component in any study regarding coastal processes
As a bathymetric profile is not always available, in the following, we investigate an empirical relation between the breaking wave height and the breaking pattern LHs retrievable from video imagery
Summary
The wave height at the breaking point (hereafter the breaking wave height) is an essential component in any study regarding coastal processes. When offshore waves propagate into shallower water depths, the wave steepness increases and wave profiles become skewed and asymmetric. The wave form at breaking can be characterized into four types, namely surging, collapsing, spilling, and plunging breakers, based on the Iribarren number (e.g., [4]). For surging and collapsing wave forms, the wave crest remains unbroken while the shoreward wave face breaks slightly. The wave crest spills down the face of the wave. Plunging breakers are the most energetic, with the wave crest and face completely collapsing and plunging into the wave trough. The breaking wave form, and the breaking location, is mainly determined by the beach slope and the offshore wave steepness (e.g., [5,6])
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