Steep streams are very difficult to model with an unsteady flow model in general. Modeling a dam break flood wave through a steep stream system is even more difficult. Steep streams tend to have very high velocities and rapid changes in depth, area, and velocity, which make it more challenging to obtain a stable model solution through these areas.
The default solution methodology for the 1D unsteady flow routing option within HEC-RAS is generally for gradually varied flow. Areas of rapidly varied flow, such as flow profiles transitioning from subcritical to supercritical flow, and hydraulic jumps, tend to cause the 1D solution scheme to have difficulties in remaining stable. Additionally, the assumption of a hydrostatic flow distribution may not be valid. As Froude number approaches 1.0 (critical depth), the inertial terms of the St. Venant equations and their derivatives tend to cause model instabilities (generally in rapid flow areas the derivatives are over estimated). However, the HEC-RAS software does have an option to run the 1D solution scheme in a mixed flow regime mode, which allows it to solve through these types of flow transitions.

Manning's n Values. If you are running the software in the default mode (mixed flow option not turned on), and if the program goes down to critical depth at a cross section, the changes in area, depth, and velocity are very high. This sharp increase in the water surface slope will often cause the program to overestimate the depth at the next cross section upstream, and possibly underestimate the depth at the next cross section downstream (or even the one that went to critical depth the previous time step). One solution to this problem is to increase the Manning's n value in the area where the program is first going to critical depth and in the steeper portions of the reach. This will force the solution to a subcritical answer and allow it to continue with the run. It is common for people to underestimate the magnitude of the Manning's roughness coefficient for steep streams. Additionally, it is common to have pool and riffle sequences in steep streams. In a pool and riffle sequence, Manning's n values will often be higher in the steeper riffle areas, and lower in the flatter pool areas. This level of detail for modifying Manning's n values is often not done, and can be a contributor to the instability of the model.

Mixed Flow Regime Option. If you feel that the true water surface should go to critical depth, or even to an extended supercritical flow regime, then the mixed flow regime option should be turned on when using 1D river reaches to model steep areas. In order to solve the stability problem for a mixed flow regime system, Dr. Danny Fread (Fread, 1986) developed a methodology called the "Local Partial Inertia Technique" (LPI). The LPI method has been adapted to HEC-RAS as an option for solving mixed flow regime problems when using the unsteady flow analysis portion of HEC-RAS. This methodology applies a reduction factor to the two inertia terms in the momentum equation as the Froude number goes towards a user defined threshold.

The default values for the methodology are FT = 1.0 (Froude number threshold) and m = 4 (exponent). When the Froude number is greater than the threshold value, the factor is set to zero. The user can change both the Froude number threshold and the exponent. As you increase the value of both the threshold and the exponent, you decrease stability but increase accuracy. As you decrease the value of the threshold and/or the exponent, you increase stability but decrease accuracy. To learn more about the Mixed Flow Regime option in HEC-RAS, please see the HEC-RAS User's Manual.

Increased Base Flow. Another solution to the problem of flow going from subcritical to supercritical flow, and back again, is to increase the base flow in the hydrographs, as well as the base flows used for computing the initial conditions. Increased base flow will often dampen out any water surfaces going towards or through critical depth due to low flows that are in a pool riffle sequence.

Modified Puls Routing. HEC-RAS has an option that will allow the user to define any portion of a model to be solved with the Modified Puls routing method instead of the full unsteady flow equations. This allows the user to define problem areas, such as very steep reaches, as Modified Puls routing reaches. A Modified Puls routing reach can be defined at the upstream end of a HEC-RAS river reach, at the downstream end, in the middle of a reach, or even defined for the entire reach. The computations are performed in conjunction with the unsteady flow equations on a time step by time step basis. Additionally, reaches that are defined as Modified Puls reaches can contain bridges, culverts, and even lateral structures. The hydraulics of these structure types are accounted for during the Modified Puls routing. To use this option, please review the HEC-RAS User's Manual.

2D Flow Areas. The new 2D Flow Area option in HEC-RAS allows user to model areas with either the Full Saint Venant equations in two-dimensions, or the Diffusion Wave form of the equations in two-dimensions. The new 2D solver uses a finite volume solution algorithm, which can handle subcritical, supercritical, and mixed flow regime (including hydraulic jumps), much more robustly then the current 1D finite difference solution scheme. This makes it very easy to use 2D flow areas to model steep streams.