Thanks to Harvey, my rentals located in 500 year flood zone are now flooded. What are the things that I need to do now? 

Mitigate damage as soon as it is safe to do so. Coordinate with your insurer, who will likely decline. Get in touch with your local FEMA representatives to see what assistance might be available.

Both FEMA and the CDC have information on their web sites about how to deal with the damage.

@Shu Zhu   How deep was the water?  Were they occupied?  How many units do you have?  If you have the money and don't have to wait for insurance move very quickly because there is going to be a major shortage of housing?

If you do not have the money you should be able to get an SBA low-interest loan via FEMA to rebuild.