embankment, dam
壩 centers on a barrier that holds back water: an embankment or dam. Both meanings describe the same physical structure.
壩 combines 土 (earth) with 霸, which likely contributes the sound and may relate to a barrier or dam. The exact historical development is uncertain.
The left side is earth 土, and the right side 霸 looks like a massive structure. Picture a huge earthen wall built across a river to form a dam.
For ハ, imagine a dam holding back a huge 'ha' of water: the sound of rushing water 'ha!' cues ハ while the earth barrier holds firm.