Abstract
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.
Highlights
The study of water movement in reservoirs gives us the understanding of physical, chemical and biological processes that occur in surfaces waters
There are a huge number of events that are responsible for water movements on reservoirs and lakes
Since we defined an equation to describe the generation of fundamental internal seiches mode and this mode is frequently dominant in lakes and reservoirs, we focused our attention to capture the occurrence of the first vertical baroclinic mode, n = 1 (MORTIMER, 1971)
Summary
The study of water movement in reservoirs gives us the understanding of physical, chemical and biological processes that occur in surfaces waters. The vertical movements in lakes and reservoirs have chemical and biological consequences for these ecosystems (STRASKRABA; TUNDISI; DUNCAN, 2013). As stated by Denny and Gaines (2007), internal waves are one of the most important water movements for vertical mixing on lakes and reservoirs. According to Downing et al (2006), the internal motions of small lentic water bodies are poorly studied compared to large lakes. Pannard et al (2011) showed that, internal waves in small lakes have seldom been investigated, small lentic water bodies present higher baroclinic pressure, which allows the formation of large internal waves. The large majority of basins that have been investigated to detect internal wave activity had Ld < 2.5
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