open space along water
壖 centers on an open, flat area beside water: a riverside or waterside space that is clear and unused.
壖 combines 土 (earth, ground) with 需, which likely contributes the sound. The character refers to a flat earthen area by water, but the exact historical development is uncertain.
The left side 土 is earth or ground, and the right side 需 looks like rain 雨 over a surface 而. Picture a flat, open stretch of ground by the water where rain falls freely, creating an open space along the river.
For ゼン, imagine a Zen garden beside a river: Zen -> ゼン, and the open space along the water is as peaceful as a Zen garden.