Determination of the wave climate for the southern brazilian shelf

Abstract

The main objective of this study is the characterization of the waveclimate in the SouthernBrazilianShelf (SBS) based on a thorough review of existing field data and on numerical modeling experiments. A quantitative knowledge about the waveclimate of this area is important to understand the mechanisms driving episodic mud bank attachments to the sandy shore, and the interaction of these banks with the flow and waves. The statistical analysis of existent data on the waveclimate throughout the SBS indicates that the predominant wave directions are 100° and 160° (E–SE), with wave heights varying between 1 and 1.50 m. The wave period varies between 6 and 14 s, with predominance of mean wave period of 8 s (sea conditions) and 12 s (swell conditions). The spectral wave model SWAN version 40.41 [Booij, N., Haagsma, I.J.G., Holthuijsen, L.H., Kieftenburg, A.T.M.M., Ris, R.C., van der Westhuysen, A.J., Zijlema, M., 2004. SWAN Cycle III Version 40.41 Users Manual, Delft University of Technology, Delft, The Netherlands, p. 118, http://fluidmechanics.tudel /swan/index.htm] is used to simulate the waveclimate for the region. Special attention is given to Cassino Beach, describing the waveclimate observed during the Cassino Experiment, carried out in 2005. The verification of the standard SWAN model was carried out based on the comparison between numerical modeling results and available data of significant wave height, peak period, mean wave direction and energy density for the period relative to February of 1998. Results showed satisfactory model predictions of significant wave height and reasonably accurate predictions of peak spectral wave period and direction. The model performance is also considered satisfactory in the representation of the waveclimate of the region when the wave spectrum has only one spectral peak, but presents limitations for bimodal wave spectrum. When two spectral peaks are observed, the SWAN model agrees with the spectral level observed in the low frequency, but underestimates the spectral level in the high-frequency band. When considering the presence of mud deposits in the area, model results predict that although the presence of mud attenuates most of the wave energy on the low frequency peak, it has a smaller effect in attenuating the wave energy on the high frequency peak. The comparison between significant wave heights calculated by the SWAN model (with and without mud) and measured with an Nortek acoustic doppler profiler (NDP) for the NDP localization point, and with data from a waverider relative to the 2005 survey for the studied period showed that all the time series have a similar pattern. Observed and calculated results without mud are in agreement, following the expected behavior. By considering the presence of mud in the whole domain, the model with mud shows a clear decrease of the significant wave height in relation to the NDP localization point. Furthermore, the significant wave height is underestimated due to the consideration of a constant mud thickness, extent, density and viscosity in the whole domain.