Hey RAS Users.  HEC just put out this very informative document that helps and guides the modeler with the decision between building a steady versus unsteady model, as well as when to go with a 1D, 2D, or 3D model.

Modeler Application Guidance for Steady vs Unsteady, and 1D vs 2D vs 3D Hydraulic Modeling.

written by Gary Brunner, P.E., D.WRE; Gaurav Savant, Ph.D., P.E.; and Ronald E. Heath.

HEC goes into detail about what information is required to know before making the decision between steady/unsteady and 1D, 2D, and 3D.  Both knowledge about the objectives of the modeling effort as well as what the data requirements are for each of these modeling options.  There’s a thorough discussion of the differences, both in theory and in practical application, between steady flow and unsteady flow modeling.  There’s a chapter that compares and contrasts 1D and 2D hydraulic modeling, and another chapter that does the same between 2D and 3D modeling.

If you are a HEC-RAS modeler, this document needs to be on your (virtual) bookshelf.

You can download it from the HEC website here:

https://www.hec.usace.army.mil/publications/TrainingDocuments/TD-41.pdf