rising water
滕 centers on water surging or welling up, specifically rising water. It is a rare kanji with a narrow, concrete meaning.
滕 is composed of 水 (water) on the bottom and a phonetic element above, likely related to rising or surging. The exact historical development is uncertain.
The bottom part 氺 is water, and the upper part looks like a container with a lid. Picture water bubbling up and lifting the lid: rising water.
For トウ, imagine a tall tower (タワー) being engulfed by rising water: tower -> トウ, and the water surges up around it.