I www.JCRonline.org SHAND, T.D.; BAILEY, D.G., and SHAND, R.D., 2012. Automated detection of breaking wave height using an optical technique. Journal of Coastal Research, 28(3), 671-682. West Palm Beach (Florida), ISSN 0749-0208.Obtaining accurate information of nearshore wave characteristics including the position and height of individual breaking waves is essential to understanding the drivers of coastal processes, for engineering design and hazard prediction. Demand for such information in real time for recreational planning and hazard assessment is also high. Remote optical techniques would offer considerahle economic and spatial coverage advantages over conventional in situ instrumentation. However, optical methods for obtaining wave height information have been slow to develop and those available remain computationally expensive and require "favourable" environmental conditions. This paper presents a relatively simple yet robust approach to detecting and quantifying breaking wave position and height across a wide surf zone using a twin video camera configuration coupled with an image time-stack analysis approach. A numerical algorithm, HbSTACK, is developed and successfully tested under the environmental conditions experienced during field trials. Errors and uncertainties may arise in both the photogrammetric transformation from pixels to real-world coordinates and in the detection of wave crest and trough positions. These errors have been assessed using both field verification of the transformation model and manually detected crest and trough locations by experienced practitioners. Errors in output wave heights were thus estimated to be less than 1%.ADDITIONAL INDEX WORDS: Wave height. Wave breaking. Surf zone. Optical detection.
Determining the largest wave height, H which can occur in water of depth, d without breaking is a fundamental reference quantity for the design of coastal structures. Current design guidelines, used to predict the ratio of breaking height to depth (H b /d), also known as the breaker index, are based on investigations which predominantly used monochromatic waves, thereby implicitly neglecting group effects. Groupiness or height modulation in wave trains is an inherent characteristic of freely propagating waves in deep water and has been shown within previous studies to induce breaker indices substantially exceeding those predicted by design guidelines. Additionally, the raw data upon which present design guidelines have been based exhibit considerable scatter. This scatter is surprising given the monochromatic and uniform nature of the laboratory waves.A physical investigation at the Water Research Laboratory using new techniques for data extraction and visualisation has yielded new insights into the shoaling and breaking processes of regular and grouped waves and revealed deficiencies in the present design techniques. Monochromatic waves trains were found to develop amplitude modulation with distance along the flume due to non-linear instabilities. These instabilities are well recognized in deep-water waves and contribute to group development and the occurrence of low-probability extreme waves. These modulations induced variation in breaking wave heights, locations and derived breaker indices. Such modulation of initially regular wave trains is proposed as a possible cause of the scatter observed in raw laboratory breaker index data.Wave group testing has revealed evolutionary cycles in local energy density during deep water propagation and that the spatial phasing of this evolution with the initiation of shoaling yielded considerably different shoaling and breaking regimes. Critically, smaller waves within the group, particularly those occurring at the front of the wave group were, at times, able to propagate into shallower water before breaking than is presently predicted by existing design guides. Causes for this discrepancy, including differences in definitions of water level and depth are investigated. However, discrepancies between observed and predicted values are found to remain. Revision to present design guidelines to directly incorporate non-linear group effects and group-induced water level variation are presented.
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