If you clamp velocity you will get unrealistic motion for the fluid affected by the velocity as you described. The fluid will thin out as the simulation progresses as your fluid accelerates just like in reality. Flip doesn't have built-in checks to acceleration and velocity by default. Gravity has accelerating effect on the sloped water to the point where it thins out and hits a terminal velocity which can be due to faster water colliding against slower water, friction against the collision surface, internal viscosity forces when the water essentially turns in to a violent mix of air and water and in extreme cases such as a waterfall, air resistance itself.
