100 and 500 year floods every 5 to 10 years in the Midwest, something has to be going on. Granted some of it is from land use change, more wetlands have been drained/tiled to be farmed, constant tillage for 170 years makes the soil more compacted and less water permutation due to loss of organic matter, less permanent pasture ground to soak up excess moisture drainage areas straighten out which moves more water and lots more concrete surfaces directing water to a storm sewers that concentrate water downstream.
Columbus "discovered" America 527 years ago, and that wasn't even the mainland. The first European settlement on the mainland of North America was in 1526, and it lasted all of three months (San Miguel de Gualdape). Skipping over a couple of others that also didn't last, the first continuously occupied ("permanent") European settlement in what would become the United States was Saint Augustine, FL, in 1565. That was 454 years ago. Through the time of the American Revolution, our knowledge of the North American continent was mostly confined to the east and gulf coasts, and inland basically to the Appalachian mountains or (to a lesser extent) the Mississippi River.
Therefore, what does "500-year flood" mean? It means it's someone's guess as to the flood level that is only expected to occur once in 500 years. But, since we obviously don't have observations covering multiple 500-year windows, that's only a guess at best. At worst, it's a number made up out of thin air.
As an architect, I had training in planning, and I've been involved in zoning, wetlands, conservation, and flood plain issues for several decades. E=ven when I was first starting off, fresh out of architecture school, we were commenting that we were seeing fifty year floods at least every ten years. This was fifty years ago, in the early 1970s. What it suggests is that whatever modeling was used to establish those flood predictions was -- to put it bluntly -- wrong. Remember, "If the results don't fit the theory, it's time to revise the theory."
The fact that we are seeing "100-year" and "500-year" floods more frequently than people who had no hard data extending that far back
projected as future anticipated flood levels doesn't mean that "something has happened." It more likely means that the projections were wrong from the outset.
