Particle system grounded

I’ve created an effect much like this: Unreal Engine 4 Tutorial - Shockwave Effect in Cascade - YouTube
Yet my ground isn’t flat like the one in the video, and I would like the particles of the shockwave to be on the ground, always.

I tried using collision (with and without a velocity of down), an on low slopes it does work,
but on high slopes the particles seems to go way off up (I’m guessing due to the collision response)

How can I do such a thing anyway?