We don't actually have much of a model for that type of disaster.
The biggest issue are civic services, and employment.

The area damaged by Katrina was roughly the size of the UK. This wasn't all total detestation, but it was the area declared a disaster zone.
Now imagine your daily commute was from central London to Brighton if traveling by car, or Manchester if by train, with an average of the drive time to cockfosters or Greenwich. (I'm fully aware no one actually lives in central London , and traffic would be a big issue. I'm only going by the map software since I don't have a sense for drive times in the UK) Now put about ~50% of the m25 loop underwater for two weeks. Deepest about 6 meters.
All the water infrastructure is gone, and there is no power or sewer, and no food. The nearest unaffected location is belfast, and most people are evacuating to Dublin or Glasgow.
It's been a month and you have not been paid. You have been living off of charity, FEMA rations, and canned water from Budweiser (actually a really good charitable act they do: they have trucks and the ability to put enough liquid in cans and get them to a disaster area by the kiloton).
Greenwich is basically gone. Cockfosters still has two feet of water. Your employer is dead and the business will never reopen. They think they got most of the bodies out of the Thames but the authorities are still concerned about a cholera outbreak.
The government has phased out the emergency food and is now giving you vouchers you can use to buy food at stores. This will last another two months.
You have no healthcare. The military medic who gave you a once over and vaccine booster for tetanus and hepatitis is the first doctor appointment you've had in two years.
Your car is gone, and so is your home and everything you're not carrying. Your insurance company is denying they owe you anything because your home was damaged by water before it was blown away. You might get resolution in ten years.

So how do you get food? Water? How do you get your trailer in to live in? How do you make money to pay your mortgage, taxes or for medical care and the aforementioned food?
You might not even be able to sell the land given that no one wants it. You will likely declare bankruptcy and give it to the bank.

I'm not American, but I do work in safety, and the massive system failure after Katrina and how it only got worse since then is a pretty popular topic in the literature.

surely you still own the land and have permission to live on it? So worst case just stick a caravan there and live in that

The thing is, how? It's kinda like homesteading at that point, but worse. Having a place to live isn't about the square of dirt, it's about all the stuff around it. You need food, water, power and plumbing. If all that is gone, you can't really live there longterm. The kind of cascade failure is what makes the area worse than wilderness, because wilderness has at least some capacity to support people, where flattened suburbia doesn't.

Here the majority of the cost is land that you have permission to live on.

Right, but that land is valuable for reasons. Some of those reasons might include facilities, access to roads and trains, the very appealing property that it doesn't randomly flood, the community and other things.

And that's the problem. You bought the plot, and the house, for say 500k. The house the gone, and the plot is in Schrodinger's suburb, which is potentially valuable, but only if everyone rebuilds. And you already know they won't, because even if you can afford it, not everyone can. And you can't sell it, because who in their right mind would buy land that floods every few years?