Water splash sounds from walking through water

Create a terrain with some dips in it, add a plane and give it the material "Generic Water". Looks nice, imagine the water laps up the gentle slopes of the terrain and dips down to a depth of 20 cm (half way up the player's calf). If the water plane Scene - Game Collision is on, the player walks on the water. If it is off, the player's foot step sound is from the terrain, not the nice splash of the water.

I have tried adding a second non-visible water plane with collision, below the surface to get the footstep splash sound back whilst allowing the visual of players walking through the water, but this does not work well for the slope transitions of the terrain. I would like a Sound FX volume that changes footsteps for any collision occurring within it, but I cannot see how to achieve this elegantly and efficiently.

If anyone has a work-around, please let me know.