River models for transport of matter and heat

Abstract

Insufficient quality of water-supply may become the imiting factor for growth and development particularly in highly industrialized regions. A deterioration of water quality may result, aside from natural sources, from excessive municipal or industrial waste-water or cooling water discharges. Assessment of the environmental impact of such discharges requires adequate knowledge of the mixing and transport processes to which the introduced substances are subjected. Such phenomena are investigated in hydraulic models. A decisive distinction between mixing- and conventional models is the requirement to simulate turbulent transport processes correctly, which necessitates an exact reproduction of the local velocity distribution in the model. This results in more sophisticated requirements for model similarity. The main task of mixing models including intake- or outlet structures is usually the determination of the effluent concentration field in the water body, which is a prerequisite for the evaluation of possible negative consequences on the river ecology and on other water users located downstream. The model experiment gives answers to the question of how changes in the design of the outlet structure can influence the mixing pattern. Such questions are primarily important for large rivers and reservoirs, where incomplete mixing and stratification are likely to occur. For small rivers or creeks, the main problem is longitudinal dispersion, since cross-sectional mixing is quickly achieved because of the large ratio of effluent- to river flow rate. Similarly, the effect of very small waste-water discharges is easily evaluated as long as they do not produce a noticeable disturbance of the river flow.