The average velocity of all liquid in a stream is usually what percentage of the surface velocity?

The average velocity of all liquid in a stream being approximately 85% of the surface velocity reflects a well-established understanding in fluid dynamics, particularly in the context of open channel flow. Surface velocity is the speed of fluid at the surface of the water body, significantly influenced by factors such as wind and friction.

The average velocity accounts for the entire cross-sectional area of the flow, which is generally slower than the surface velocity due to the effects of viscosity and turbulence. As fluid moves downstream, it interacts with the bed and banks of the stream, causing friction that slows down the water closer to these surfaces. Consequently, the majority of fluid particles, especially those below the surface, do not maintain the same speed as the surface particles.

This relationship between average velocity and surface velocity is crucial for environmental compliance inspectors because it aids in various calculations, including pollutant dispersion, flow estimation for ecological assessments, and designing effective water management practices. Understanding that the average velocity typically hovers around 85% of the surface velocity helps inspectors make informed assessments regarding stream health, water quality, and compliance with environmental regulations.