We don't have synonyms for flushboard yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are flashboard and fuseboard.
(n) :
Alternative form of flashboard (“board placed upon a milldam”). [(US) A board placed temporarily upon a dam, river, stream, etc. (typically within a permanent frame) to raise the water above its usual level.]
(n) :
Alternative form of flashboard (“board placed upon a milldam”). [(US) A board placed temporarily upon a dam, river, stream, etc. (typically within a permanent frame) to raise the water above its usual level.]