dam, embankment, weir
坯 centers on earthen structures that hold back or control water: a dam, embankment, or weir. All three meanings are specific types of water-controlling earthworks.
坯 combines 土 (earth) with 丕, which likely contributes the sound. The character expresses an earthen structure for water control.
The left side is earth 土, and the right side 丕 looks like a barrier. Picture a wall of packed earth holding back a river: 坯 is a dam or embankment.
For ヒ, imagine a huge earthen dam with a sign that says 'He' (pronounced 'hee') to warn people away: He -> ヒ, and the dam keeps the water back.