Markku Virtanen

Three-dimensional water-quality-transport model compared with field observations

Abstract An ecological model, coupled with a hydrodynamic one, is applied to Lake Nasiselka, the southernmost sub-basin of Lake Nasijarvi, Central Finland. Its surface area is 93 km 2 , mean depth 15 m, maximum depth 61 m, and through-flow 64 m 3 /s. The transport velocities to the ecological model are computed with a three-dimensional hydrodynamic sub-model. Its results are compared with the flow velocities measured in 1981. In the ecological model the transport velocities change according to the weather conditions, through-flow and temperature stratification. The ecological mdoel computes the dissolved oxygen, total phosphorus, phytoplankton biomass, biological oxygen demand and sodium lignosulphonate taking into account their decay, settling, mutual reactions, loading and transport with water flow. The model validity is tested with extensive field observation data of the year 1982 and more sparse data of the summer 1983. The results show reasonable agreement with the measured values. The model is used to test the effects of human activities (industrial loading and through-flow regulation) on the water quality. Industrial loading affects the oxygen and algal distributions in the lake whereas the executed regulation has not appreciably changed the natural conditions in the water body.

Principles for calculation of transport and water quality in strongly regulated reservoirs

Abstract A model system has been generated for the buttom and water quality development of reservoirs during the first few decades of impounding. The model is based on 3D (three-dimensional: longitude × latitude × depth) calculation of the water currents, their variation with time and on 3D description of release, transport, degradation, settling and other processes affecting water quality and the bottom. Special attention in the model development has been paid to the effects of varying water levels and to the internal loading from the bottom. The latter is a combined result of several processes, including degradation of the former terrestial vegetation and organic matter in general, gas development and convective flows, and erosion caused by waves, currents and ice cover. Results from the Porttipahta Reservoir in Finnish Lappland are presented and compared with the field observations as an example of the model applications.