I've just purchased the bottom composition module and can see from the documentation it does both hardness and roughness?

My Problem: I want to be able to identify rubble patches in a (usually soft) muddy river, the brisbane river to be exact.

I've had a quick look at the peak, e1 and e2 returns. I guess I'm looking at a bit of specific advice on how I would go about mapping these rocky/rubble patches.