How to create Curve Line with Gradient (slope) for extrusion

I need to model a complex profile with gradient. 

From the top view, the alignment line have various curves and its not straight. 
From the side view, the alignment line is straight and sloping downwards at 2% gradient. The percentage doesn't matter the key question here is the sloping downwards while maintaining the top view profile. 

For illustration purpose, i made a comparison showing what happen when its a straight line with gradient.