Fixed vs Adaptive Time steps

The time step is the most common issue with 2D sediment models that do not run. 

Modelers usually try to run the 2D sediment model with time steps that are too large. By default, the hydraulic and sediment time steps are equal unless the user enters a Sediment Computation Multiplier larger than 1 (more on this later). There are two options for selecting a time step for the hydraulic model in HEC-RAS. The first is a fixed time step. This option uses a constant time step throughout the simulation. This option is useful for simple datasets including laboratory or analytical test cases. For most real-life cases, the user will want to use the second time step option which is the adaptive time step. In this method, the time step is adjusted so that the Courant number is always within a user-specified range. The Advanced Time Step Control under the Unsteady Computation Options and Tolerances includes options for adaptive time steps that will change during the simulation.  The option to Adjust Time Step Based on Courant is the most widely used and is becoming standard practice for 2D modeling.  Define a maximum and minimum Courant condition and then the maximum halving or doubling steps allowed from the base time step, and the model will compute the appropriate time step throughout the model

  

Modeling Note: The Computational Efficiency of Small Time Steps

It seems intuitive that smaller time steps generate larger run times.  However, HEC-RAS will iterate on each solution until it reaches an acceptable tolerance.  Iteration is computationally expensive and smaller time steps often iterate less.  Therefore, selecting a smaller time step will often reduce the number of iterations, leading to less additional run time than users often expect.  In rare cases, a smaller run time can actually speed the model up by more-than-compensating for the additional time steps by neutralizing iterations.

 * The Implicit Finite Volume solver in HEC-RAS 1D and 2D is not Courant limited.  So the Courant Condition is more of a stability guideline and time-step selection support, than the hard limit it can be in other, explicit, solvers.  Therefore, modelers sometimes set their max Courant condition closer to 2, without introducing computational issues or instabilities.

It is important to mention that the current adaptive time step method does not consider any sediment processes such as bed change in computing the time step. These will be added to HEC-RAS in future versions through additional criteria. 

Sediment Computation Multiplier

As briefly mentioned above, the user can specify a Sediment Computation Multiplier which determines the number of hydraulic time steps computed within a sediment time step. The sediment Computation Multiplier can often be used to significantly reduce model run times without significantly affected model results. For more information on the Sediment Computation Multiplier see Sediment Computation Options and Tolerances.