dredging, cleaning out
渫 centers on removing unwanted material from a waterway or container: dredging a channel or cleaning something out by flushing or scooping.
渫 combines water 氵 with 枼, which likely contributes the sound and may relate to the idea of separating or sifting, fitting the action of dredging or cleaning out a waterway.
Water 氵 beside 枼 (a shape suggesting leaves or thin plates) gives the image of water flowing through a sieve-like structure, scooping out debris and cleaning the channel.
For セツ, imagine a set of sieves used to dredge a river: set -> セツ, and the water is cleaned out.