Luis, thank you for your thoughtful response. I guess to be more specific, I am wondering how HEC-RAS handles the dry cells in a mesh. If a majority of model runs will generally have flow limited to the main channel (within the levees), and flood plains beyond the levee will generally be dry unless modeling > Q25, does it make a difference if the model is split into separate 2D areas?
Will the model run faster and more efficiently in a low flow situation when the wetted cells are limited to the main channel and there are three separate 2D ares/meshes (i.e. one mesh for the main channel and two for either side outside of the levees) or do dry cell not affect the model run time?