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The dynamics of vertical mixing and the occurrence of basin-scale internal waves (internal seiches) in lakes and reservoirs are often classified and described based on the force balance of wind shear and horizontal pressure gradients resulting from wind-generated currents (the Wedderburn number). The classification schemes consider specific time scales that are derived from a simplified vertical density distribution, a rectangular basin shape, and a constant water depth. Using field measurements and numerical simulations with a validated hydrodynamic model, we analyzed the transfer of energy from wind to the internal seiche field in a small reservoir. Our results demonstrate that the basin shape has a strong influence on the energy dissipation and on the transfer of energy to high-frequency internal waves, thereby attenuating the generation of basin-scale internal seiches. Most of the energy loss of the internal seiche occurs at the sloping boundary, where the internal seiche is susceptible to shoaling and breaking. These findings suggest that the Wedderburn number can be used to predict the occurrence of internal seiche activity in continuously stratified systems. As the Wedderburn number and derived mixing classifications are widely applied also for the interpretation of observed ecological and biogeochemical processes, its application to basins with sloping bathymetry and complex shape should be critically scrutinized, and deviations from predicted dynamics, including the presence of hotspots of turbulent mixing, should be considered.

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.Keywords: Internal seiches; Two-layer method modeling. RESUMOOs 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 grande pico de frequência durante o mês de Novembro, mês que apresentou maior estratificação térmica. O pico identificado como uma onda interna de periodicidade de 8 horas condiz com o período teórico de uma onda V1H1, obtido a partir do modelo. Ventos de baixa intensidade (≈ 1 m/s) predominantemente direcionados à nordeste excitaram uma onda V1H1. De acordo com os espectros de potência, após a passagem desta onda foram verificadas ondas internas de modos baroclínicos superiores e uma possível degeneração da onda...

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