mound, grave, ridge in field
壠 centers on a raised earthen shape: a mound, a burial grave, or a ridge in a field. All meanings share the idea of earth piled up into a distinct form.
壠 combines 土 (earth) with 龍, which likely contributes the sound. The character expresses an earthen mound or ridge.
The left side is earth 土, and the right side is a dragon 龍. Picture a dragon curled up into a long earthen mound, like a ridge in a field or a grave.
For ロウ, imagine a low, long mound shaped like a dragon: low -> ロウ, and the dragon's body forms the ridge.