washout has 3 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the channel or break produced by erosion of relatively soft soil by water
“it was several days after the storm before they could repair the washout and open the road”
the erosive process of washing away soil or gravel by water (as from a roadway)
“from the house they watched the washout of their newly seeded lawn by the water”
Synonyms, antonyms and the broader and narrower relations for washout come from Princeton University’s WordNet. Full sources and licences.