I have a riser which is just above an inline structure (dam) to supplement primary spillway. I have modeled riser in Hydrocad to obtain stage-discharge curve, I now want to enter as a dam gate in HEC-RAS. How??
You'll want to use User Defined Curves for your gate type to make use of the rating curve you computed in Hydrocad. First enter in a gate of any size/shape. The size and shape is inconsequential, since you'll be using a rating curve. Make sure that the invert elevation is correct though. Then, select Gate type as User Defined Curves. Then enter in your family of rating curves. It's a family of curves because you must enter in a rating curve for each gate opening. If you just have a overflow riser, then you only need to put in one opening (this too, can be anything-just make sure that you use the same gate opening when you set up your gate boundary condition in the unsteady flow editor). More detail is presented here: http://hecrasmodel.blogspot.com/2013/12/lateral-structure-coefficients.html