There are at least three different things happening that make it seem like there are more floods today:
The third factor is interesting. To understand how flooding can be more severe now, think about a 100-acre piece of land covered in trees. Let's say that 3 inches of rain falls on that land in a heavy downpour. Forested land is extremely good at absorbing and holding water. The leaves on the ground slow the water down and let the ground absorb it. All of that rainfall might be retained by the land and release slowly over several days.
- Over the last 50 years, the population of the United States has doubled (U.S. census), so any flood tends to affect a lot more people now. That makes it feel like the floods are more severe.
- The media seems to pile on the coverage whenever any flood happens anywhere, so you hear about floods more now than you used to.
- Floods today are more severe because of changes that people have made to the landscape.
Now imagine the same piece of land converted to a shopping mall. That 100 acres is now completely paved over. If three inches of rain falls, none of it is absorbed. So about 10 million gallons (40 million liters) of water runs straight to the nearest creek or river in a matter of a few minutes. Look around you in any developed area and you see tens of thousands of acres paved over like this. A heavy rain (from a hurricane, for example) can completely overwhelm rivers that were once adequate. The ground is not absorbing any of the water. (You do see efforts to control this sort of runoff in some cities. For example, next to some malls there are huge, dry catch basins. In a heavy rain, they fill with runoff and turn into lakes so that the water isn't immediately dumped into the river.)
The other thing that is happening today is that people now build structures in flood-prone areas and then erect levies and walls to try to keep the water out. Prior to this development, flooding rivers could spread out over wide areas when necessary, and no one noticed. Now, there is nowhere for the water to go except for the narrow, hardened channel of the river itself. The river rises higher, and if it breaches a levy it creates a major disaster.
It is not raining any more than normal. It's just that all of the rain that falls moves into a river very quickly, and the river and flood plains are much narrower than they used to be. This can create much more severe flooding.
Here are some interesting links: