If there is no clean water available, for flood victims, is it permissible for them to use flood water for their ablution?
If possible, try your best to find mutlak water for ablution, however, if it cannot be found, it is permissible to use flood water for it is considered pure. This is based on the statement of Syeikh ‘Abdullah bin ‘Abd al-Rahman Bafadhal:
وَلَا يضر تغير بمكث وتراب وطحلب وَمَا فِي مقره وممره
“It isn’t an issue of the change of colour of the water for it isn’t moving in a long period of time due to dust, moss and whatever stays in stagnant and flowing water.” 
This is also mentioned by Dr Muhammad al-Zuhaili:
“If the water has changed in terms of its colour, smell and feel, whether it is due to the long period of it being stagnant, dust or dirt, moss, Sulphur and anything hard for it to be protected from, then it remained pure and purifying. However, if the water has already been used for ablution, bathing and cleaning najis, then it is no longer in its pure and purifying state.” 
If a person is certain that the flood water is mixed with a lot of najis, then it cannot be used for ablution. But if it is just a presumption or doubt, then it doesn’t change the purity of the flood water to be used for ablution. This is following the Islamic legal maxim:
اليَقِين لَا يُزَالُ بِالشَّكّ
“Certainty cannot be abrogated by syak (doubt).”
Thus, the use of floodwater for ablution is permissible according to syara’ if other mutlak water cannot be found. Wallahua’lam.
 Al-Muqaddimah al-Hadramiyyah, 21
 Al-Mu’tamad fi Fiqh al Syafie, 1/38