During the water surface profile calculations, each of the two programs may need to calculate critical depth at a cross section if any of the following conditions occur:

  1. The supercritical flow regime has been specified by the user.
  2. The calculation of critical depth has been requested by the user.
  3. The current cross section is an external boundary cross section and critical depth must be determined to ensure the user-entered boundary condition is in the correct flow regime.
  4. The Froude number check for a subcritical profile indicates that critical depth needs to be determined to verify the flow regime of the computed water surface elevation.
  5. The program could not balance the energy equation within the specified tolerance before reaching the maximum number of iterations.

The HEC-RAS program has two methods for calculating critical depth: a "parabolic" method and a "secant" method. The HEC-2 program has one method, which is very similar to the HEC-RAS "parabolic" method. The parabolic method is computationally faster, but it is only able to locate a single minimum energy. For most cross sections there will only be one minimum on the total energy curve; therefore, the parabolic method has been set as the default method for HEC-RAS (the default method can be changed from the user interface). If the parabolic method is tried and it does not converge, then the HEC-RAS program will automatically try the secant method. The HEC-RAS version of the parabolic method calculates critical depth to a numerical accuracy of 0.01 feet, while HEC-2's version of the parabolic method calculates critical depth to a numerical accuracy of 2.5 percent of the flow depth. This, in its self, can lead to small differences in the calculation of critical depth between the two programs.

In certain situations it is possible to have more than one minimum on the total energy curve. Multiple minimums are often associated with cross sections that have breaks in the total energy curve. These breaks can occur due to very wide and flat overbanks, as well as cross sections with levees and ineffective flow areas. When the parabolic method is used on a cross section that has multiple minimums on the total energy curve, the method will converge on the first minimum that it locates. This approach can lead to incorrect estimates of critical depth, in that the returned value for critical depth may be the top of a levee or an ineffective flow elevation. When this occurs in the HEC-RAS program, the software automatically switches to the secant method. The HEC-RAS secant method is capable of finding up to three minimums on the energy versus depth curve. Whenever more than one minimum energy is found, the program selects the lowest valid minimum energy (a minimum energy at the top of a levee or ineffective flow elevation is not considered a valid critical depth solution).

Given that HEC-RAS has the capability to find multiple critical depths, and detect possible invalid answers, the final critical depth solutions between HEC-2 and HEC-RAS could be quite different. In general the critical depth answer from the HEC-RAS program will always be more accurate than HEC-2.