HEC-RAS is currently capable of performing 1D water surface profile calculations for steady gradually varied flow in natural or constructed channels. Subcritical, supercritical, and mixed flow regime water surface profiles can be calculated. Topics discussed in this section include: equations for basic profile calculations; cross section subdivision for conveyance calculations; composite Manning's n for the main channel; evaluation of the mean kinetic energy head (velocity weighting coefficient alpha); friction loss evaluation; contraction and expansion losses; computational procedure; critical depth determination; applications of the momentum equation; air entrainment in high velocity streams; and limitations of the steady flow model.