dike, embankment
隄 centers on a raised barrier that holds back water: a dike or embankment. Both given meanings point to the same physical structure.
隄 combines ⻖ (mound, hill) with 是, which likely contributes the sound. The mound radical suggests an earthen structure, fitting the meaning of a dike or embankment.
The left side ⻖ looks like a piled-up mound of earth, and the right side 是 suggests something upright and correct. Picture a correctly built earthen mound that holds back water: a dike.
For テイ, imagine a tea (テイ) plantation protected by a tall earthen dike. The tea plants stay dry behind the embankment: tea -> テイ.