Hydraulics as physics

Stream characteristics are derived from the basic physics of flowing water. Fluid mechanics is an old science with well-established physical relationships. Typically, simple empirical equations are used that do not account for all the variability that occurs in the flow. An example is Bernoulli’s equation for balancing flow depth, velocity, and pressure. In this case, the flow must be considered steady. If it is important to assess how flow depth, velocity, and/or pressure change over time, Bernoulli’s equation by itself will not be sufficient.

The assumption that flow velocity is generally downstream in direction is also a common simplification in the analysis of streamflow. Real streams have many eddies where the flow circulates horizontally. Streams also have areas of upwelling, roiling, and vertical circulation. While designers commonly make use of an average velocity at a given cross section, the actual velocities in the plane of a cross section vary markedly from top to bottom, side to side, and in direction, varying with time and three-dimensional space.

Water surface profile analyses generally assume a constant flow elevation across a given cross section. Real streams, however, super-elevate their water surfaces in curved channel sections and may set up significant surface wave patterns that defy prediction. Finally, hydraulic analyses often assume that water flows against a fixed boundary. Real streams actually readjust their bed and banks constantly, move significant amounts of sediment, and transport unpredictable amounts of natural or humanmade debris.

It is, therefore, important to understand the limitations and restrictions of any equations before using them to obtain necessary information.