A Simple Optical Method for Measuring the Statistical Distribution of Water Surface Slopes

A flash-photography technique for determining the statistical distribution of slopes of wind-created waves is described. The technique consists of taking photographs with a flash camera directed vertically downward toward the water surface. The resulting photographs are flash reflection patterns that can be readily interpreted in terms of wave slopes. Wave-slope dispersion curves, as determined from daytime flash photographs taken from a bridge about 45 feet above the water, are given for wind velocities up to 20 knots. For example, the standard deviation of the across-wind wave slopes was found to be approximately 2.5, 5, and 7.8 degrees for wind velocities of 5, 10, and 20 knots, respectively. Under the same conditions the with-wind wave-slope standard deviation was found to be 4.2, 7.5, and 10 degrees.