There are two possible explanations for this verse. The first possibility is that Tarshish, a colony of Tyre, must enjoy the freedom that comes through the destruction of Tyre. The colonies would be able to do what they wanted, like the Nile flooding its banks.
The second possibility is that without the protection of Tyre, the colonies would not be able to defend themselves and would be vulnerable to attack.
10 Cross over your land like the Nile, O daughter of Tarshish; there is no restraint anymore.