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 fea-ture of lakes, existing mainly as a result of the wind force acting on a stratified water column. Internal seiches are im-portant 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 availabl