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.
The model setup should obey (or at least respect*) the Courant condition, which roughly means that the water should pass though one cell per time step.
Users can calculate a limiting Courant condition for their model using the smallest cell and estimating a maximum wave velocity** and then choose a fixed time step that satisfies this condition.
However, recent versions of HEC-RAS make time step selection much easier.
The Advanced Time Step Control under the Unsteady Computation Options and Tolerances includes options for dynamic 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.