This one sounds pretty hairy scary. Thanks for keeping us posted. Stay safe, guys!!

This one has turned into a flood event. 25" and more rain coming. The authorities have asked for more rescue help from private boats.

It definitely won't go well if you don't plan for it. Which they apparently hadn't for Rita. And clearly still haven't done. You need to come up with a good plan. Then you need to practice it, by that I mean table top and other exercises for emergency management folks, first responders, etc. That helps you find out where the plan breaks. Then you need to modify your plan as appropriate. Then communicate that plan widely to residents and other stakeholders.

While getting the entire 6.5 million Houstonites out clearly isn't feasible, there are a number of things that could have (and should have) been done to get people out of the most hard hit areas.

1. Do a staged evacuation, so everyone isn't trying to leave at once. Evacuate the most vulnerable areas and populations first. That means nursing homes in low lying areas, then residents of low lying areas. Then the next most vulnerable areas, etc. These days with GIS, hydrologic data, etc, it is pretty straightforward to identify the areas most at risk.

2. Identify evacuation routes, and find ways to make them work better. Identify which roads are most likely to flood first. Plan to reverse traffic flow on inbound lanes. In other words, make all lanes on key roads and freeways outbound.

3. Communicate this planned sequence to residents and stakeholders. In today's world, hurricanes are not surprise events. They had several days to communicate which areas would evacuate first.

4. Identify safe staging areas for law enforcement and emergency personnel, so that they can remain in flooded areas to discourage looting, and rescue those who can't get out in time. Practice staging them there.

4. Make evacuation orders mandatory. That doesn't mean police drag people from homes. But studies have shown that calling an evacuation "mandatory" leads to significantly better compliance for people.

5. Have a consistent message from officials. Don't have the Texas governor saying "Evacuate!" and the Houston emergency manager saying "Shelter in Place!".

All this does require planning and practice. It also does cost a little money to create these plans and rehearse them. And recognize that no plan will cover every possibility, but it is much easier to modify an existing plan to meet unforeseen circumstances than to create a plan on the fly. Gen. Dwight Eisenhower, who successfully ran the Normandy Invasion (probably the most complex military operation of all time) put it so well:
“Plans Are Useless, But Planning Is Essential”

And for next time, consider prohibiting new single story homes.
Require that all new construction be of at least two stories, and of substantial construction.
When a structure is designed as a home for two families, require that it be divided such that each family has part of the upper floor.
Old folks homes, nursing homes and the like should be on at least two floors and have ramped access from floor to floor to facilitate moving everyone to the upper floor.

In very low lying areas, where even an upper floor would be vulnerable, prohibit new construction.

For those living in EXISTING vulnerable homes, encourage the purchase of a small and unsinkable boat, perhaps by making purchase of same tax deductible.

A few years ago I saw an article about a new style of house, developed in the Netherlands, that, while it sat on the ground, was constructed in such a way that it would float in a flood (while still centered on pylons or such).

Maybe should be looked at again, with wider application. . .

Possibly, but it sounds expensive and perhaps more applicable to places with very high population density and therefore high land prices. In other places with relatively lower population density it might be simpler to build some place else ?

some good ideas here.

I don't believe that any of America's major cities can be evacuated at the current time - not without major chaos and disastrous effects. Houston is no exception. This time they decided to "shelter in place". I'm not sure it helped, especially because a lot more rain is coming. Somewhere in America a group of city planners need to take on the challenge ... how to get a lot of people out of a city. Right now, no planners have a practical answer.

About houses ... I agree that single-story homes should have been banned. It would have been much smarter to build houses on stilts. Especially in the lowland areas. But all of that costs extra money. The explosion in city growth really equates to an explosion in city greed. Local politics are dominated by realtors and housing construction magnates. they pay off city authorities. They get themselves elected. They carve up all the land, and rake in as much $$ as they can. When a true disaster strikes ... they throw up their hands and say "Don't blame me. Nobody could have seen this coming". And then they look for more money with re-building programs.

It's no different in California. The realtors have sold housing tracts on top of the San Andreas fault. Literally, just yards from the fault line. All these houses will be bulldozed by the next great quake. The problem is widespread along a big section of the fault line. Everybody knows it's a ripoff, and the casualties will be high. But no-one ever holds them responsible. The problem is human greed, doled out in very big doses.

This is an unprecedented event. While there are certain areas here that have flooded before during extreme rainfall events and can be expected to flood every time we get lots of rain in a short period of time, this time there are areas flooding that have never flooded before.

The Harris County Flood Control District has been creating huge flood mitigation areas upstream from these problem areas over the past few years in an effort to retain water upstream and control when the water will be released in order to lessen the effects of these events.

Everything they have done has been overwhelmed completely by the estimated 15 trillion gallons of rain that have already fallen, with another 5 trillion expected to fall by the end of Wednesday.

The Latest: Harvey rainfall would fill NFL football stadiums

Unprecedented but not unanticipated. The vulnerability of the region to catastrophic flooding has been well documented many times for many years. See for example Boomtown, Flood Town. Also see Hell and High Water. And Is Texas Ready for Hurricane Harvey?.