I must rewrite the follwing passage into passive:
They have to take new measures to prevent more flooding.
We use must when "it's our intention" and "have to" - when we can do nothing with the task or current situation and someone else is making the decision for us.
Since that, it would be better to say:
New measures must be taken to prevent more flooding.
instead of:
New measures have to be taken to prevent more flooding.