dam, dike, used in place names
堌 centers on an earthen barrier that holds back water: a dam or dike. It appears mainly in place names, where such structures were historically significant.
堌 combines 土 (earth) with 固 (hard, firm), suggesting a solid earthen structure. The exact historical development is uncertain, but the component choice fits the meaning of a dam or dike.
The earth radical 土 on the left and 固 (hard, solid) on the right together picture a firm earthen wall holding back water: a dam or dike.
For コ, imagine a solid earthen dam that is as hard as a 石 (こ, stone): 石 -> コ, and the dam stays firm.