riprap
An underwater bank seen as a danger to shipping.
countable, uncountable
They hold on, spray flying, the ceaseless racket of water impacting on rock all but swallowing them, the river pitching dizzily, snags and riprap raking like claws at the bottom of the boat.
Rocks or other materials used to shore up an embankment, deter or prevent erosion, guide shipping, or serve as a temporary mooring.
countable, uncountable
Where it comes from
Apparently a reduplication (with vowel dissimilation) of rap (“blow, stroke”).
Adapted from the Wiktionary entry riprap (CC BY-SA 4.0).