Just 9.9" of rain did this in Houston April 2016. The are now expecting 20"-40" from this storm. The continued long duration of heavy rain is of great concern.....
On the amwx board one of the mets said he had no words for the predicted rainfall. Biblical doesn’t being to cover 60” of rain over 5 days.
Every few years when a big storm hits Houston the flooding is worse because each time there is more concrete, more roofs, more built up ground for pads, less area for the water to spread and more run-off in less surface area and deeper.
You just can’t fix incessant urban sprawl and no zoning. The flood district and detention ponds just don’t cut the mustard and never would with rain such as is expected.