Profile by Constant Elevation
You can access this tool from the following:
Use the Define Profile by Constant Elevation tool to define a flat profile at a given elevation for the entire element.
Select the Define Profile by Constant Elevation icon.
When you move the cursor into the View, it is equipped with a command prompt requesting that you "Locate First Element". Data point on the element that you wish to give a constant elevation.
When prompted to "Enter Elevation", so enter the value of the desired elevation, strike the Enter key, and then data point.