1. HEC-RAS can perform sub-critical, supercritical, or mixed flow regime calculations all in a single execution of the program. The cross section order does not have to be reversed (as in HEC-2), the user simply presses a single button to select the computational flow regime. When in a mixed flow regime mode, HEC-RAS can also locate hydraulic jumps.
  2. HEC-RAS has the ability to perform multiple bridge and/or culvert openings at the same road crossing.
  3. At bridges, the user has the ability to use a momentum-based solution for class A, B, and C low flow. In HEC-2 the momentum equation was used for class B and C flow, and requires the trapezoidal approximation. The HEC-RAS momentum solution also takes into account friction and weight forces that HEC-2 does not.
  4. HEC-RAS can model single reaches, dendritic stream systems, or fully looped network systems. HEC-2 can only do single reaches and a limited number of tributaries (up two three stream orders).
  5. At stream junctions, HEC-RAS has the ability to perform the calculations with either an energy-based method or a momentum based method. HEC-2 only has the energy based method.
  6. HEC-RAS has the following new cross section properties not found in HEC-2: blocked ineffective flow areas; normal ineffective flow areas can be located at any station (in HEC-2 they are limited to the main channel bank stations); blocked obstructions; and specification of levees.
  7. In HEC-RAS the user can enter up to 500 points in a cross section. HEC-2 has a limit of 100.
  8. HEC-RAS has the ability to perform geometric cross section interpolation. HEC-2 interpolation is based on a ratio of the current cross section and a linear elevation adjustment.
  9. HEC-RAS has an improved flow distribution calculation routine. The new routine can subdivide the main channel as well as the overbanks, and the user has control over how many subdivisions are used. The HEC-2 flow distribution option is limited to the overbank areas and breaks at existing coordinate points.