Measure and Modeling Vertical Mixing in Tropical Stratified Lakes

Lake’s mixing regime affects physical, chemical and biological processes. Each time the vertical stratification is broken an upward velocity occurs. Is this velocity strong enough to re-suspend bed material and affect water turbidity? This is a key question in water quality studies considering that turbidity is an important index in those evaluations and bed material could feedback water the water column with heavy metals, high load of nutrients or even pesticides. To respond this question turbidity measures and tools of environmental evaluation are needed. Aiming this subject, two different sizes and polymithic tropical lakes located in São Paulo - Brazil were used as study sites to identify and measure variations on the water turbidity due to mixing events and correlate them to the vertical temperature profiles. The capacity of a numerical 3D model to represent those phenomena were also assessed.

The first site considered was Billings Reservoir, 60 km² surface area and 15m depth lake situated in the surroundings of Sao Paulo City. The second one is a small pond located 80km from the first with 0.2 km² and 4.5m depth. Different sizes were chosen to be able to include the size effects on the lakes’ mixing regime. Both lakes had their temperature profile and the turbidity electronically registered in campaigns performed along a full month with time resolution of 1min and 1s, respectively.

The collected data were evaluated considering the specifics moments prior to the water columns mixing and the power of stratification were correlated to the turbidity variation. In order to estimate vertical velocities during the mixing event, a numerical 3D model, Delft3D, were previously calibrated and used with the same forcing variables of the observed events. The field date show that the relative turbidity, that is, the difference between previous and current turbidity increase when mixing events begin to happen, confirming that those events can re-suspend deposited materials. The numerical model shows that higher vertical velocities can be present in the same moments when then turbidity peaks occur, evidencing the efficient of a calibrated model to reproduce physical conditions and relations in a lake.