Wave Characteristics As Revealed By Aerial Photography
Authors: McClenan C.M. , Harris D. Lee 
Published: October 2008

Aerial photos of coastal regions generally show complex but well organized wave patterns. It appears that the concept of a "random sea" may be the result of making most wave observations from too Iowan elevation, and thus with a too-limited field of view. The photos reveal several commonly overlooked wave phenomena.

Introduction

- The most common sources of information about waves are:

- visual observations from shipboard or shore,

- mathematical analyses, and

- instrument records that provide a time history of the water surface elevation or of the pressure due to waves.

These sources do not provide the dependable information about wave direction needed for the solution of many engineering problems. Records from an array of instruments and mathematical analyses, combined with assumptions about the two-dimensional wave pattern may provide a unique solution to the problem of determining the direction of wave propagation. The validity of the solution, however, depends on the .validity of the assumptions.

Aerial photography can provide the clearest available information about the validity of many common assumptions used in the analyses of wave records, whether for determining wave directions or for the solution of other engineering problems.

IMPORTANCE OF ELEVATION IN WAVE PHOTOGRAPHY

A few examples of wave photos will show their value as a source of information about waves. Fig. 1, from Harris,5 is an enlargement of a portion of a photo taken with a hand-held camera aboard Apollo 7, at an elevation of about 100 miles.

The diagonal lines are waves, generated by a hurricane about 1,400 miles distant from this scene. The waves appear to be monochromatic and long crested as assumed in the most elementary wave theory.

Fig. 2 was obtained from an airplane at an elevation of 65,000 ft (19,800 m) near Huntington Beach, Calif. Two distinct wave trains may be identified. In deep water these intercept with an angle near 90 degrees. In shallow water both wave trains show refraction and the crests are more nearly parallel.

From these two examples one might suspect that photos of the sea tend to display more regularity as the elevation of the camera, and thereby the field of view, is increased. The next two photos have been selected to illustrate this point. These photos were obtained near an oil well platform a few miles offshore from the Corpus Christi Water Exchange Pass, Tex. Fig. 3 was obtained from an elevation of 500 ft. The term "confused sea" appears to provide an accurate description. Fig. 4, which includes the area of Fig. 3, was obtained a few minutes later from an elevation of 5,000ft. This tendency for photographs of the sea to display more regularity of the surface waves as the elevation of the camera is increased, displayed by these two photographs, is also noted by Kinsman.6

Until about 1950, most reports of actual wave conditions were expressed in terms of significant wave height, period and direction as though the real sea could be adequately represented as a uniform monochromatic wave field similar to Fig. 1.