Hi
I find the easiest and quickest way to create a 2D Flow Area is to create the polygon in ArcGIS, convert the polygon to a vertices point file using ET Geowizards, and calculate the coordinates at each point. Close ArcGIS and open up the point dbf file with Excel.
Next, open HecRas to the project and select Add 2D Flow Area. Draw a small square or any small shape and save it with the name you want to call your 2D Flow Area. Open the GIS Tools – “Storage Area/2D Flow Area outlines Table”. Copy and Paste the X, Y coordinates from the Excel File over the top of the coordinates in the open GIS Tools table. Click OK and when it closes, the altered 2D Flow Area will be drawn to match the ArcGIS polygon.
This works for me.