well rope
綆 centers on a rope or cord used for drawing water from a well. The meaning is specific to a well rope.
綆 combines 糸 (thread) with 更, which likely contributes a phonetic element. The character is associated with a rope for drawing water, but the exact historical development is uncertain.
The left side 糸 suggests thread or rope, and the right side 更 can be thought of as 'again'. Imagine a rope that is used again and again to pull water from a well.
For コウ, picture a well rope coiled like a 'coil' (コイル, koiru) of rope: coil -> コウ, and the rope is used to draw water.