flood, overflowing
沔 centers on water rising beyond its normal bounds: a flood or an overflowing state. The two given meanings are closely linked as different expressions of the same idea of excess water.
沔 combines 氵 (water) with 丏, which likely contributes the sound. The exact historical development of 丏 is uncertain, but the character is associated with water overflowing.
Water 氵 on the left and a shape 丏 that looks like a barrier being overwhelmed: imagine a dam or levee failing as water floods over it.
For ベン, picture a bent barrier giving way under the flood: bent -> ベン, and the water overflows.
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