The internal wave field in Sau reservoir: Observation and modeling of a third vertical mode

Water withdrawal from Mediterranean reservoirs in summer is usually very high. Because of this, stratification is often continuous and far from the typical two-layered structure, favoring the excitation of higher vertical modes. The analysis of wind, temperature, and current data from Sau reservoir (Spain) shows that the third vertical mode of the internal seiche (baroclinic mode) dominated the internal wave field at the beginning of September 2003. We used a continuous stratification two-dimensional model to calculate the period and velocity distribution of the various modes of the internal seiche, and we calculated that the period of the third vertical mode is ;24 h, which coincides with the period of the dominating winds. As a result of the resonance between the third mode and the wind, the other oscillation modes were not excited during this period. Standing internal waves, or seiches, are a ubiquitous feature of lakes, existing mainly as a result of the wind force acting on a stratified water column. Internal seiches are important for many processes in lakes and reservoirs. Part of the kinetic energy introduced at the water surface by the wind is transferred to internal seiches and becomes available for mixing. Imberger (1998) and Wüest et al. (2000) showed that most of the momentum and energy that passes through the surface mixed layer and enters the interior is transferred to internal wave motions. This energy is typically about 10% of the total wind energy input into the lake (Wüest and Lorke 2003). The vertical excursions of fluid associated with the wave motions have been shown to affect the spatial distribution of different organisms (Levy et al. 1991). The bottom currents induced by internal seiches contribute to the mixing and resuspension of sediments in the benthic boundary layer (Gloor et al. 1994). MacManus and Duck (1988) showed that seiche-induced resuspension was responsible for sediment scouring patterns observed along the sides of Loch Earn. Therefore, given that internal seiches play a significant role in a broad range of physical, chemical, and biological processes, there is a lot of interest in their assessment and evaluation. Although there are many possible mechanisms that induce seiches, like air-pressure fluctuations, earthquakes, etc., it is mainly the lake’s response to the wind forcing that causes the seiche excitation. The wind forcing in lakes is often periodic because of the periodicity in weather patterns. Wind applied to the surface can cause a surface set-up of water at the downwind end. This pressure force is balanced by the metalimnion tilting in the opposite sense—that is, downward at the downwind end (Spiegel and Imberger 1980). When 1 Corresponding author ([email protected]).