Wind-induced internal seiches in Vossoroca reservoir, PR, Brazil

ABSTRACT The vertical movements caused by internal waves in lakes and reservoirs have chemical and biological consequences for these ecosystems. The vast majority of studies that investigate internal waves are conducted on large lakes. There are just few researches that investigate this phenomenon on dendritic reservoirs. The purpose of this research was to identify internal waves (baroclinic mode) in the Vossoroca reservoir by using temperature time series recorded between May to November 2012. A two-layer method was used which considered rigid upper and lower boundaries. Moreover, the potential flow theory was used for both layers since the flow within each layer was considered irrotational. From the dispersion relation, we obtained the theoretical shallow internal wave period. The power spectral density (PSD) of temperature series of thermocline depth, provided by fast Fourier transform, helped in the identification on the frequency peak. Subsequently, the theoretical period was compared with the frequency spectra. Using a careful analysis (excluding the interference of solar radiation and intensity of wind), we observed a clear peak in November due to an internal wave with period around 8 hours, which matched the theoretical calculation from the dispersion relation equation for V1H1 mode. Weak winds from southwest excited a V1H1 baroclinic mode. According to spectral analysis, after the passage of this long-basin internal seiches, we identified the formation of higher vertical internal seiche modes. In addition, we observe indications of V1H1 mode degeneration. RESUMO Os transportes verticais de nutrientes, compostos químicos e microrganismos causados pela presença de ondas internas causam um grande efeito no estado trófico de reservatórios. Poucas pesquisas buscam verificar a ocorrência de ondas internas em reservatórios, sistemas muitas vezes mais dendrítico que lagos. O objetivo desta pesquisa foi identificar ondas internas no reservatório do Vosorroca (Paraná), utilizando, para isso, séries de temperatura da coluna de água coletados entre os meses de Agosto e Novembro de 2012. Foi utilizado o método de duas camadas para a identificação da periodicidade das ondas internas. Através da relação de dispersão foram obtidos os períodos teóricos das ondas internas em águas rasas. As densidades espectrais de potência da série de temperatura na profundidade média da termoclina auxiliaram na identificação dos picos de frequências. Foi possível observar um gra