In laboratory experiments, records of the water surface elevation are usually obtained using electrical wave height meters which measure the change in conductivity through a wire. This technique works very well but has the drawback that many wave height meters are necessary when a large number of points must be sampled. Large arrays of wave height meters are expensive and moreover, since the measuring rods and support frames are intrusive in the water, the measurements may become affected in areas with large densities of instruments.
Alternatively, video observations may be used to overcome these disadvantages and provide information for a large area of the basin at a high spatial and temporal resolution. This is possible due to the fact that a linear relationship exists between the water surface slope and the cameradetected irradiance. This relationship is only dependent upon the location in the basin and not on the wave conditions. This relationship is also thus fixed for a given experimental set-up and only needs to be measured once. Hence, given a time series of the irradiance in a camera-pixel, it is possible to recover the corresponding surface elevation and, via the normal post-processing, to compute energy-density spectra and wave statistics.
In the current project, two video observation techniques ("coincident illumination" and "spatial illumination") have been utilised. The advantage offered by these techniques when compared with a great deal of other techniques is that a large area of the basin surface may be monitored since the camera angle is oblique to the water surface. Another advantage is that camera and lights may be placed on the beach and other locations, and do not need to be positioned above the water surface.
In the case of regular waves, a good agreement is obtained between the wave height as measured by a wave gauge and the wave height reconstructed using the irradiance. In the case of irregular and breaking waves, the energy spectra of the directly measured water surface and reconstructed surface show a reasonable agreement. In addition, the presence of white-capping does not appear to have a negative influence on the reconstruction of the water surface. Financed by (a.o.) Ministry of: Transport, Public Works and Water Management. |
