wash face
湏 centers on the act of washing one's face. It is a rare kanji with a narrow, concrete meaning.
湏 combines 氵 (water) with 頁 (head), suggesting water applied to the head or face. The exact historical development is uncertain.
Water 氵 splashing onto a head 頁: you are washing your face.
For カイ, imagine washing your face with a special kai (shell) shaped soap: kai -> カイ, and the water runs over your head.