Extraction of coastal ocean wave characteristic parameters using optical remote sensing and computer vision technologies

[摘要] ENGLISH ABSTRACT: Ocean waves are among the most important forces shaping the world's coastlines. They driveenvironmental processes and human activity that occurs within the coastal zone as well as onthe open ocean. The assessment of wave characteristics, such as wave direction, wavelength,wave period and wave velocity, are critical in understanding coastal processes that serve as abaseline for better coastal management. However, the monitoring and assessment of wavecharacteristics is challenging, given the high complexity of ocean dynamics and large spatialextent. While globally the development of ocean state forecasting models has been fruitful inrecent decades, these capabilities are only just being developed in (South) Africa and requireground truth data for model development and validation.The work presented here assesses whether optical imagery from the RapidEye satellite can beused to extract ocean wave characteristic parameters such as wave direction, wavelength,wave period and wave velocity for the purpose of further developing and validating waveforecasting models. Two techniques were adopted. The first made use of a Fourier transformto extract the directional wave spectrum. The characteristic wave parameters were thencalculated from the spectrum. The second approach made use of normalised cross correlation(NCC) to extract the phase velocity field. The techniques identified and developed weretested using RapidEye imagery on four study areas on the South African coast. Theseincluded Richards Bay, Durban, East London and Cape Point. Ground truth data in the formof wave measurements captured by wave buoys were used for validating the results.The results from the Fourier transform show a generally high agreement with the directionalspectra derived from the wave buoys. In the context of characteristic parameters extractedfrom these spectra, two of the four study sites produced highly accurate results with allparameters within 10% deviation of the wave buoy data. The results obtained from the NCCrevealed that the shoaling of ocean waves has a positive influence on the reliability ofmeasurements near the shoreline. The directions for two of the study areas had differences ofless than 10° from the wave buoy directions. It was also observed that when waves arepresent with vastly different directions the NCC approach fails. These results show a promising advance in the use of optical imagery for the monitoring andassessment of near-shore wave conditions. However, it is recommended that furtherdevelopment and validation of these techniques be undertaken before operationalization.

[发布机构] Stellenbosch University