Bed levels of Eglwys Nynydd reservoir. North is upwards
All colour figures use this scale, where red is highest value and blue lowest value
In the 1970's, there were problems with the water quality in the reservoir, caused by blooms of the algae Microcystis aeruginosa Kutz. emend. Elekin. The algae concentration near the surface was measured at evenly spaced points over the whole reservoir. The wind was also recorded, and its direction was from south to north.
The reservoir was modelled with a 3D CFD model, where the wind-induced currents were calculated. The velocity vectors close to the water surface are given in the figure below.
Velocity vectors in Eglwys Nynydd water reservoir at water sampling time
The algae in the reservoir responded to sunlight by rising towards the water surface. The wind-induced currents thereby caused higher algal concentration at the northern part of the lake. This was reproduced by the numerical model.
Modelled algae concentration
The input parameters needed for the model were varied in the study. The result was only slightly affected by the magnitude of the irradiance. The k-epsilon turbulence model also gave some improved result compared with a zero-equation model. The most important input parameter was the algal group diameter. The algae form colonies, and the vertical rise velocity depends very much on the diameter of the groups. The calculated maximum algal concentration varied between 200 mg/liter with a group diameter of 0.25 mm to over 900 mg/liter with a diameter of 2 mm.
This project was carried out in cooperation with Richard Hedger at Department of Geography, University of Edinburgh and Glen George at Institute of Freshwater Ecology.
The The Research Council of Norway partly sponsored the project.
Nils Reidar B. Olsen