Illustrating that design isn't always 7-sigma availability, There's a shopping mall parking garage near here that is essentially the river bank. When the river runs a high flood, the waters are intended to inundate a lot of parking spaces, to avoid overly-confining those waters. And it's deep enough that some of the parked cars float. Also this parking garage replaced a surface parking lot that was doing the same thing. The second level of the parking garage is not supposed to be inundated, so the garage is a lot taller than it would have been for the same capacity. One can see flood stains on the walls of the first level.

The alternative would have been to build levees to confine the waters to a channel, and hope the channel was deep enough.