Sentence:  Deregulation of financial markets laid the groundwork for the subprime crisis in the United States, while a badly designed euro contributed to the debt crisis in Greece, and the Schengen system of open borders made it difficult to control the <WSD>flood</WSD> of refugees in Europe. 
Meaning of the word marked with <WSD> tags:  A large number or quantity of anything appearing more rapidly than can easily be dealt with.