C. F. Keil and Franz Delitzsch on Isaiah 23:10

Isa 23:10

The consequence of the fall of Tyre is, that the colonies achieve their independence, Tartessus being mentioned by way of example. “Overflow thy land like the Nile, O daughter of Tarshish! No girdle restrains thee any longer.” The girdle (mēzach) is the supremacy of Tyre, which has hitherto restrained all independent action on the part of the colony. Now they no longer need to wait in the harbour for the ships of the mother city, no longer to dig in the mines as her tributaries for silver and other metals. The colonial territory is their own freehold now, and they can spread themselves over it like the Nile when it passes beyond its banks and overflows the land. Koppe has already given this as the meaning of Isa 23:10.