deluge, flood, vast
洪 centers on a massive overflow of water: a deluge or flood. The sense of vastness extends naturally from the idea of a flood's overwhelming scale.
洪 combines 氵 (water) with 共, which likely contributes the sound and the idea of togetherness or largeness, suggesting a great gathering of water.
Water 氵 combined with 共, which looks like a pair of hands holding something together, suggests a vast gathering of water: a flood.
For コウ, imagine a flood so vast it creates a new coast: coast -> コウ, and the water covers everything.
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flood; flooding
Hungary
heavy flooding; disastrous flood; deluge
glacial lake outburst flood
great; immense
great debt of gratitude; great obligation
flood myth
diluvial epoch
diluvial epoch
diluvium; diluvial formation
great model; great example (esp. of governing the world)