When to use Energy vs Momentum computation mode for Steady flow analysis on HEC-RAS

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What is the difference in using the Energy vs Momentum steady flow computation mode in HEC-RAS?

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Harley Greene On

Whenever water surface passes through critical depth, the energy equation is not considered to be applicable. The energy equation is only applicable to gradually varied flow situations, and the transition from subcritical to supercritical or vice versa is a rapidly varying flow situation.

There are several instances when the transition from subcritical to supercritical and supercritical to subcritical flow can occur. These include significant changes in channel slope, bridge constrictions, drop structures and weirs, and stream junctions. In some of these instances empirical equations can be used (such as at drop structures and weirs), while at others it is necessary to apply the momentum equation in order to obtain an answer.

Within HEC-RAS, the momentum equation can be applied for the following specific problems: the occurrence of a hydraulic jump; low flow hydraulics at bridges; and stream junctions.

For instance, while modeling bridges in GeoHECRAS you have the option to choose Momentum in the Low Flow Computational Methods and you can define the pier drag coefficient.

A screenshot of GeoHECRAS for reference

Here is an article describing HEC‑RAS Bridge Low Flow Computations