rip-rap or riprap
n loose broken stones, used to form a foundation on soft ground or under water, or in the construction of revetments and embankments; a foundation formed of these.
[From an obs word imit of the sound of repeated blows]
Well perhaps sarumite - though with stretch of turbulent water being a RIP and to grab being RAP to reach the answer of loose stones RIPRAP, don't think it is too bad a cryptic clue myself.