Keywords:Wind forcing, Stratification, Mixing
This study shows effects of the wind forcing on a shallow lake associated with the lake size and stratification. Mixing in lakes and reservoirs are frequently evaluated by using nondimensional parameters, for example the Wedderbrun number (We). We can be estimated for a steady state wind condition or computed by using time averaged wind data, however, winds largely vary within several hours or less. Time scales associated with mixing in a shallow lake were investigated using a two dimensional (x-z) numerical simulator, SUNTANS. Several conditions of the horizontal scale of the modeled lake, stratification and wind speed were used in simulations with We ranging 0.1–10. This study employees two time scales: (1) time scale required for a steady state temperature distribution estimated from the tilt of the thermocline, Ts, and (2) time scale required for complete vertical mixing, Tmix.Tmix is computed from change in the potential energy. Numerical results showed that Ts linearly decreases with decreasing in the stratification or increasing in the horizontal distance under a same We condition. Tmix also increased when the stratification decreased or the horizontal distance increased under a same We condition. A quarter of the internal seiche period (Ti) was calculated using a stratification and a horizontal distance. Numerical results showed a linear relationship between Ti and Ts. Moreover, a quarter of Ti was fundamentally shorter than Tmix under a same stratification and horizontal distance. This implies that the lower limit of the mixing time scale is determined by a quarter of Ti.