TERRESTRIAL PHOTOGRAMMETRIC MEASUREMENTS OF BREAKING WAVES AND LONGSHORE CURRENTS IN THE NEARSHORE ZONE

Joseph W. Maresca, Erwin Seibel

Abstract


We conducted a study to determine the feasibility of shore-based, oblique photographic monitoring of breaking waves, water levels, and currents within the surf zone. The purpose of this paper is to describe a new method of oblique single-image and stereoscopic-image analysis, the potential errors, and the types of measurements that can be made in the surf zone. Examples of application are presented to demonstrate the technique. Sophisticated photographic equipment is not required to collect, analyze, and interpret the data. The analysis and error discussions are directed toward the problems encountered using common equipment. Vertical images from aircraft, helicopters, and balloons have been used in the past to study shoreline changes, directional ocean-wave spectra,8 and longshore currents.3 Oblique images taken from the bridge of a ship have been successfully used to measure whitecap coverage under different wind speeds.4 Terrestrial oblique images have been used to study longshore currents,5 ice-ridge formation and breakup,6'7 and beach changes .8 Oblique images, taken with a 35-mm single-lens reflex camera from an elevated point such as a bluff, are particularly suitable for the measurement of breaking waves, water level, beach run-up, and current in the surf zone under storm conditions. In contrast to other techniques of monitoring the surf zone, the photographic technique described in this paper is simple to install, reliable, accurate, and inexpensive. It can be used in all weather conditions, and the analysis of the images is simple. Both stereoscopic and single oblique images can be analyzed, depending on the specific needs and existing environmental conditions. Since the scale of an oblique photograph changes with increasing distance from the camera, the technique is limited in range to about 250 m for a cliff approximately 8 m above the mean water level. Accuracies to within 1% in the horizontal plane and better than 10% in the vertical plane are achievable at this distance.

Keywords


breaking waves; longshore current; neashore zone; terrestrial photogrammetric measurements

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