I have two nephews in Houston. They are high and dry and well provisioned. Being from NOLA they know what to do. I did suspect that this is a localized, not Katrina event.
Many areas flooded that had never flooded before. We had no problems at all with last years tax day flood, or the Memorial Day flood - even though we are not far from the Brazos. But we had water about 6-8 feet from our front door before it began to recede. Parts of Cinco Ranch are still under water almost a week after the rain stopped. It may not seem like a big deal to you but it stunned a lot of us. We had prepared as much as possible - had bags ready with meds, clothes, and food - in case we needed to dash. But by Sunday our roads were flooded so we would only have been going a few doors down to a friend who has a two story home. Every neighborhood surrounding us had water too deep to drive through, and none of this area has flooded in the 13 years we have lived here. Maybe it was localized - I’m not an expert on Houston - sure seemed to be more widespread than what you are assuming.