3D numerical modeling of Microcystis distribution in a water reservoir

A 3D computational fluid dynamics program was used to calculate the wind-in duced accumulation of phytoplankton in Eglwys Nynydd, a water supply reserv oir in Wales. The computational fluid dynamics model solved the Navier-Stok es equations for the water velocities using the SIMPLE method to calculate the pressure. Two turbulence models were tested: a zero-equation model and the k-epsilon model. An unstructured nonorthogonal 3D grid with hexahedral cells was used. The distribution of the blue-green algae Microcystis was ca lculated by solving the transient convection-diffusion equation for phytopl ankton concentration. based on the modeled flow field. The numerical model included algorithms for calculating the growth rate of phytoplankton and si mulating the response of the algae to changes in underwater light intensity . The model was validated by comparing the horizontal distribution patterns produced by simulation with those recorded during a field survey of surfac e concentrations. The results demonstrated reasonable agreement, particular ly when using the k-epsilon turbulence model. The main parameter affecting the results was the effective diameter of the Microcystis colonies.