wash, cleanse, purge
湔 centers on using water to remove dirt or impurities: washing, cleansing, or purging something to make it clean.
湔 combines 氵 (water) with 前, which likely contributes the sound and may also suggest the idea of going forward or progressing, as in water moving forward to wash away impurities.
Water 氵 flows forward 前, washing and cleansing everything in its path. Picture a stream purging dirt as it moves ahead.
For セン, imagine a cleansing stream flowing through a thousand (千, せん) tiny channels, washing away impurities: 千 -> セン, and the water purifies.