The festival grounds were flooded with visitors from all over the country.

flooded 是动词 flood 的过去分词形式，flood 本义是"洪水泛滥"，在这里用作比喻，表示"大量涌入、充满"的意思。用 flooded 形容游客数量极多，就像洪水一样涌入场地的景象。