I really wish people would get off this 'can't evacuate the whole city of 6.5 million' idea. You don't need to evacuate all of Houston. Many of those 6.5 million people are not flooded. However, you can and should try to evacuate those in the most flood prone areas. It is difficult to 'shelter in place', when your place has water up to the eaves!

Forecasters were predicting extreme record breaking rainfall amounts for the Houston area well before Harvey made landfall. Many areas in an around Houston have repeatedly flooded in the past. With that weather prediction, it was literally a no brainer that those areas that flooded in the past would flood this time. There was absolutely no reason why the authorities couldn't have tried to get people to evacuate the well known flood prone areas. However, as I noted up-thread, that takes planning, which apparently wasn't done. If even half of the people in known flood prone areas had evacuated (which would be way less than the 6.5 million number I keep hearing) it would have freed up rescue resources to concentrate on those areas which no one anticipated would flood.

Full disclosure: I'm more than a little familiar with the issues in Houston. I spent my career as a geo in the oil patch, with the bulk of that time in Alaska. However, I worked in Houston for several years, and lived west of town in Katy. Even while living in Alaska, I've also traveled down there on business on many occasions. My wife and I have ridden our mountain bikes on the levee around Barker Reservoir, the one that they are now releasing water from out of fear the dam might fail. I was there in 2001 and saw the flooding from Tropical Storm Allison.