Results cont'd

diffraction region where wave height is significantly reduced or in regions of the

measurement area that are poorly illuminated, the visual signal of the wave

degrades due to lack of contrast. Figure 38 exemplifies lack of image contrast in

the diffraction region behind the breakwater structure where the wave direction

arrows are spurious. Image contrast may be improved by adjustment of the lens

iris, improved directional illumination, and use of a monochromatic camera

system. With a monochromatic camera, signal degradation may be reduced and

image contrast improved. In the Structure 3 and 4 experiments, the camera and

sources of illumination were directed along the axis of the inlet. The rear face of

waves was illuminated as they propagated through the inlet to the bay. Waves

entering the bay were refracted and diffracted, with the sharpest turning angles at

the shoulder of the inlet. In this area, wave crests approached an alignment

parallel with spectral illumination, reducing the variance in radiance between the

forward and rear faces of the wave received by the camera. Put simply, the

ability to visually distinguish between front and rear faces of the wave was

diminished.

Figure 39 shows the effect of undesired light reflection on the video analysis.

In the upper right corner of the image the reflection of the jetty structure is

observed on the water surface. The shape of the reflection deforms with

perturbations of the water surface, and the channels of the pixel array record its

visual signal. The contaminated time-history records prevent the phase

relationship between array channels to be determined accurately for the

phenomenon of interest. Shadows cast within a measurement array may also

cause this contamination of signal.

Another potential source of measurement error is array geometry. It is

assumed that processes being measured are homogeneous within the array

aperture. However, this may not be the case where large changes in direction are

occurring over a small area. Measurement array geometry may be optimized to

better resolve wave direction where waves are making large turning angles

approaching a direction perpendicular to the long axis of the array. Waves

breaking on currents and the shore also produced inhomogeneities within the

measurement array.

For each model configuration, experiment, and camera view, spectral peak

vector-mean wave direction plots are presented in Appendix Q. Due to the large

volume of analysis results generated by this investigation, S( f ) and θm( f ) are

presented in tabular form in ASCII data files compiled on a CD that accompanies

this report. It describes the naming convention and file formats for the data

contained on the CD.

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Chapter 5 Video-Based Wave Direction Measurement