Quick summary. During Hurricane Harvey, a chemical plant in Baytown, Texas, was flooded and it spilled. There are more than 2,500 similar toxic chemical depots that are located in areas prone to flooding. Many plants are built near major rivers/ports for the ease of transportation, and with increased risks of flooding, we're probably going to see more chemical spills.

Floods Are Getting Worse, and 2,500 Chemical Sites Lie in the Water’s Path, New York Times, Feb 6, 2018, by HIROKO TABUCHI, NADJA POPOVICH, BLACKI MIGLIOZZI and ANDREW W. LEHREN.

This makes me think that if you live near one of these plants, you just have to bug out, possibly even when your immediate neighborhood is safe from flooding.

For those of you who cannot get the NYT article, here's a summary from Patch.com --

https://patch.com/us/across-america/2-500-toxic-sites-located-flood-prone-areas-across-us

We should avoid becoming unduly alarmed about the leakage or escape of cancer causing materials during a flood.

In the USA and other developed countries, standards are very strict and materials are classified as carcinogens when workers who have handled them daily for decades are found to be at elevated risk.
It is well for TPTB to be cautious about even small risks, when large numbers of workers are exposed.

The position in a flood is rather different, since even with increasing flooding it should still be a relatively rare and fairly brief event, not at all comparable to a lifetime of occupational exposure.

Remember that bugging out also carries risks, of auto accidents, of greater exposure than remaining indoors, and of course of your home being looted whilst unattended.