Hello,
I was rushed to create a HEC-RAS model for a very large watershed (200,000+ acres of ungaged land) and evaluated a historical 100-yr storm. Due to time constraints, I modeled the watershed in an entirely 2D, rain on grid format. (Which, although it is technically time consuming to run, minimizes the amount of time you need to spend setting up your model, and allowed me to work on other things while my model was running.) The big draw for me for using a 2D format is that I did not need to draw thousands of cross sections. The results were a success; by validation tests for the design storm, the results are conservative, and agree within 1′ of error with the high water marks the project area. If you would like to know how I did this, let me know.
-Lauren