Assessing Algal Growth Using a 2-Dimensional Hydrodynamic and Water Quality Model

One of the major sources of drinking water for Cape Town, South Africa had been suffering from blue-green algal growth. This raised concerns over the cost of treating water containing algal by-products and the potential health implications. It was believed that in addition to external nutrient loading, internal re-suspension of nutrients was also one of the key driving forces for the algal blooms. Qualitatively it was expected that algal blooms would be most evident following high wind speed events, relatively high air temperatures and low water levels.

To quantify this problem a study aimed at developing a strategy to address nuisance algal growth problems was commissioned. One of the tasks of this study was the evaluation of the in-dam nutrient dynamics. To accomplish this, a two-dimensional (2D) reservoir model was used. The model, CE-QUAL-W2, is based on the assumption that the water body shows maximum variation along its length and depth and can simulate the vertical and longitudinal distribution of velocity, constituent concentrations and temperature.

The objective of this study was to calibrate/corroborate the model for different years especially focusing on the ability of the model to reproduce the in-dam thermal characteristics and phosphate concentrations post high wind speed events.

Findings indicated that the reservoir experiences weak thermal stratification for short periods of time, but that it was easily broken down during wind events. In addition it was found that velocities generated at the bottom of the dam during high wind speed events were sufficiently large to induce re-suspension of sediments with the possible release of loosely adsorbed phosphates from the re-suspended sediments. External phosphate loading was also found to be major contributing factor in promoting algal growth in the dam.