The SWAN wave model is widely used in coastal waters and the main focus of this work is on its application in a harbor. Its last released version -SWAN 40.51 -includes an approximation to compute diffraction, however, so far there are few published works that discuss this matter. The performance of the model is therefore investigated in a harbor where reflection and diffraction play a relevant role. To assess its estimates, a phase-resolving Boussinesq wave model is employed as well, together with measurements carried out at a small-scale model of the area behind the breakwater. For irregular, short-crested waves with broad directional spreading, the importance of diffraction is relatively small. On the other hand, reflection of the incident waves is significant, increasing the energy inside the harbor. Nevertheless, the SWAN model does not achieve convergence when it is set to compute diffraction and reflection simultaneously. It is concluded that, for situations typically encountered in harbors, with irregular waves near reflective obstacles, the model should be set without the diffraction option.

ABSTRACT. The SWAN wave model is widely used in coastal waters, and the main focus of this work is on its application in a harbour. A recently released version of SWAN now includes an approximation to compute diffraction, however there are few published works that discuss this matter. The performance of the model is therefore investigated in a harbour where reflection and diffraction play a relevant role. To assess its estimates, a phase-resolving Boussinesq wave model is employed as well, together with measurements carried on at a small-scale model of the area behind the breakwater. For irregular, short-crested waves with broad directional spreading, the importance of diffraction is relatively small. On the other hand, reflection of the incident waves is significant, increasing the energy inside the harbour. Nevertheless, when the SWAN model is set to compute diffraction and reflection simultaneously, it does not achieve convergence. It is concluded that for situations typically encountered in harbours, with irregular waves near reflective obstacles, the model should be used without diffraction.Keywords: wind waves, SWAN 40.51 wave model, wave reflection and diffraction.RESUMO. O modelo de geração e propagação de ondas SWANé amplamente utilizado pela comunidade científica para obtenção de estimativas do espectro de ondas emáreas costeiras, lagos e estuários. O cálculo da difração não era representado no modelo tornando as simulações em regiões próximas a obstáculos ou portos menos precisas. Recentemente, uma aproximação para o cálculo da difração foi implementada, porém poucos trabalhos publicados até o momento abordam esta questão. Seu desempenhoé então investigado em um terminal portuário, uma região onde difração e reflexão são importantes. Para validar suas estimativas, um modelo baseado nas equações de Boussinesqé empregado juntamente com medições realizadas em um modelo em escala reduzida do porto. Para ondas irregulares, com espalhamento direcional mais amplo, a importância da difraçãoé comparativamente menor. A reflexão das ondas incidentes, por sua vez,é significativa aumentando a energia dentro do porto. Contudo, os cálculos do modelo não convergem quando SWANé configurado para computar difração e reflexão simultaneamente. Conclui-se que, para situações tipicamente encontradas em portos, com ondas irregulares próximas a obstáculos refletivos, a presente versão do SWAN deve ser configurada sem a opção de cálculo da difração.Palavras-chave: ondas geradas pelo vento, modelo de ondas SWAN 40.51, reflexão e difração de ondas.