Abstract
This chapter describes the interaction between flowing water and its surroundings. Open channel hydraulics is the most complicated field in fluid mechanics. The basic principles form a complex system of nonlinear equations. For a given flow rate, there is an infinity of solutions depending upon the bed slope, boundary friction, and channel cross-sectional shape. In Nature, air-water flows are commonly encountered at waterfalls, in mountain torrents, and at wave breaking. They are also observed in aesthetical fountains and in hydraulic structures. The air–water mixing is an important re-oxygenation process, because the air bubble entrainment increases drastically the air–water interface area, hence the air–water transfer rate. Interactions between river waters and aquatic life are even more complex. Fish and aquatic species respond dynamically to river flows and changes in discharges and vegetation. Aquatic life accounts for extreme flow events ranging from long drought periods. The refraction of light by the entrained air bubbles gives the “whitish” appearance to the free surface of the flow. Natural aeration occurring at the free surface of high-velocity flows is referred to as free-surface aeration or self-aeration.
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