Fredrick Maryland is experiencing severe flooding and one home owner has flood water gushing in the house through the toilet. You can view the video from the Facebook link above. What I don’t understand is how water can come backwards up the toilet? Aren’t there traps and other things that create suction or a seal? I’m not a plumber but I thought the pipes were designed to prevent something like this.
The trap on a toilet (or any plumbing fixture) prevents sewer gas from escaping. It's a very low-pressure thing, consisting solely of a small amount of standing water filling a low spot in the drain. Any force capable of moving a little water can overcome it, including air vacuum if it's not properly vented.
In the case of this flooding phenomenon, there must be a break in the city sewer line somewhere, and water standing above it. This obviously creates pressure that will escape at any point below its surface level.
Most houses only have traps installed, which prevents sewer gasses from entering the building. Some municipalities do require backflow prevents, which prevent sewage from entering the building. They do have a couple of drawbacks, though. There are location and slope restrictions on where they'll work, which can make installation in existing construction difficult. They're also a mechanical device, which can fail. When that happens they do a bang up job of preventing sewage from leaving the building.
It's worth mentioning that combined storm and sanitary sewers used to be very popular, and are still fairly common. In some cities they're still working to disconnect downspouts from the sewer system. They are out of favor now for obvious reasons, but not every city is has separate systems for storm water yet.