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.

摘要：湖泊与水库中的内波（internal waves）所引发的垂向运动，会对这类水生生态系统产生化学与生物学层面的影响。当前绝大多数针对内波的研究均以大型湖泊为研究对象，而针对枝状水库开展此类现象研究的工作则寥寥无几。本研究旨在利用2012年5月至11月间的温度时间序列（temperature time series），识别沃索罗卡水库中的内波斜压模（baroclinic mode）。研究采用双层模型法，假设上下边界均为刚性边界；此外，由于各层内的流动均满足无旋条件，因此两层均采用势流理论（potential flow theory）。通过色散关系（dispersion relation），我们得到了浅水内波的理论周期。通过快速傅里叶变换（fast Fourier transform）得到温跃层深度温度序列的功率谱密度（power spectral density, PSD），可辅助识别频率峰值。随后将理论周期与频谱进行对比分析。在排除太阳辐射与风速干扰的严谨分析后，我们于11月观测到一处显著峰值，其对应内波周期约为8小时，与基于色散关系方程针对V1H1模的理论计算结果相符。来自西南方向的弱风激发了V1H1斜压模。频谱分析结果显示，在此次长湖盆内湖振通过后，我们观测到了高阶垂直内湖振模的形成。此外，我们还观测到V1H1模发生退化的相关迹象。