Abstract

ROCHA, M. V. L. et al. Comparative analysis between in situ measurements and numerical estimations on Cornélia Beach, Costa da Caparica, Portugal. RGCI [online]. 2012, vol.12, n.2, pp.147-157. ISSN 1646-8872.

This paper presents a comparative analysis between field data collected on Cornélia beach (Costa da Caparica, Portugal), between the 12th and the 15th of May of 2010 and numerical results obtained with the non-linear 2D COULWAVE model. The ultimate aim of this work is to evaluate the capacity and ability of the model to simulate both the measured wave climate conditions and the horizontal velocity. The COULWAVE numerical model is based on Boussinesq-type equations, obtained from depth-integration of the continuity and momentum equations, assuming a multi-layer concept, and simulates the propagation of strongly non-linear and dispersive waves, across variable-depth zones. Boundary conditions were constrained based on free-surface elevation time series measured at a 7.6 m depth (Chart datum). Free-surface elevation and cross-shore velocity time series obtained for specific points of the domain were simulated by the model using 30min-long runs, while simulations for the whole domain were only carried on for 300s, due to limited computational capacity. The numerical results are compared with free-surface elevation time series obtained with pressure sensors and horizontal velocity time series acquired with a current meter positioned near the shoreline. It is also performed a sensibility analysis of the parameter that defines the initial free surface threshold that must be exceeded for a breaking event to initiate: first, it was considered the default value for constant slope beach profiles, 0.65, and then another value, 0.35, advocated by previous authors for bar-through beach profiles. The primary focuses of the data analysis are time-domain analysis and combined statistics (BIAS, RMSE, IC) of significant wave height and period and maximum, minimum, mean and root-mean-square cross-shore horizontal velocities, aiming the validation of the numerical data. It is further presented, for an example-case, the wave energy spectra using two distinct spectral methods (Fourier and Wavelet). Overall, the model seems to be fairly capable of simulating the wave characteristics across the shoaling zone as well as the cross-shore velocity along the coast. In general, it has a greater capacity to reproduce the heights of the waves than the periods. It should be noted that there are sometimes large differences between the measured values and the values estimated by the numerical model, especially after the wave breaking. For the cross-shore velocity, the model simulates quite well the right order of magnitude of the measured values, but it has limitations in describing their behaviour, and, in general, there is an overestimation of this cross-sectional component. In the spectral domain, the model is able to represent quite well the presence of the measured higher-energy frequencies recorded, but presents difficulties representing the spectrum shape and the distribution of energy for periods over time, especially after the break. This approach aims evaluating the performance of the model on simulating the specific conditions of this particular field place, but also, in a more general way, it intends to enhance the confidence on further applications of COULWAVE to wider and longer spatial and temporal scales, which is fundamental for coastal management purposes.

Keywords : Wave propagation; Wave breaking; Nearshore currents; Numerical modeling; Cornélia beach.

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