I agree with others that say your chickens will be ok. My story though not the same as you are predicting is.... We recently were involved in the floods over here that effected qld. Though we ourselves were not flooded our little town went under and was cut off. No way in or out for 8 days and no shops, no fuel etc. We swapped our eggs with neighbours or just plain gave them to people. We also usually have a few drums of fuel on hand so were able to help a few out with that too. It's times like these that make you glad you have your chickens. People love eggs so I am sure you would be able to barter your eggs or increase the value of them to pay for grain etc if needed.