Is it generally appropriate to use an ineffective flow area below an obstruction based on a 4:1 expansion rate? Velocity is 3-4 fps.
There is a PDF file with instructions related to the Flow Area and the way it should be used. The file is more like an user guide. It provides various information about the procedure available in this application. I recommend you download the PDF to find out more. There is also a part titled Ineffective Flow Area on page 38.
PDF file: http://www.hec.usace.army.mil/software/hec-ras/documentation/HEC-RAS_4.1_Applications_Guide.pdf