dégravoyer

A narrow technical verb from building and public-works vocabulary: to wash away or strip the gravel bed beneath a wall, pilings, or a streambed, typically by the action of running water. A specialist's word, not one most French speakers use or even recognize.

• Le courant a fini par dégravoyer les fondations du pont. — The current eventually washed away the gravel bed under the bridge's foundations.
• Il faut surveiller les berges pour éviter qu'elles ne se dégravoient. — The banks need to be monitored so they don't get eroded down to bare rock.
• Les ouvriers ont dégravoyé le lit de la rivière avant les travaux. — The workers cleared the gravel from the riverbed before the works began.

A specialist term from hydraulics and civil engineering, not conversational vocabulary — a learner is unlikely to need it outside a technical text.