rapids
湍 centers on fast, turbulent water: rapids in a stream or river. The meaning is tightly focused on rushing water.
湍 combines 氵 (water) with 耑, which likely contributes the sound and the idea of a rushing, pointed flow. The exact historical development of 耑 is uncertain, but the water radical clearly anchors the meaning to rapids.
Water 氵 rushes over a pointed, mountain-like shape 耑, creating fast rapids. Picture a stream crashing over sharp rocks.
For タン, imagine a tan-colored rapid churning with muddy water: tan -> タン, and the water rushes through.