Roger D. Shand

Automated Detection of Breaking Wave Height Using an Optical Technique

Abstract SHAND, T.D.; BAILEY, D.G., and SHAND, R.D., 2012. Automated detection of breaking wave height using an optical technique. 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 considerable 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 7%.

Determining large scale sandbar behaviour

To study the morphology of coastal sandbars and their change with time, it is necessary to obtain a sequence of maps over the period of interest. Traditional techniques such as vertical aerial photography or echo-sounding over a grid are expensive and subject to environmental constraints, especially in higher energy situations. Image processing is used to rectify elevated terrestrial images where morphological features are inferred from breaking wave patterns. As the coordinates of interest are longshore and offshore distances, the image is then warped to make the coastline straight. The sandbar crest positions within this image are detected. An error analysis shows that useful quantitative data may be obtained. The temporal evolution of the bars can be identified from time stack images.