Floods are a liquid disaster caused by heavy precipitation or training storms which drop their moisture over the same spot over and over again. Slow-moving storms and saturated soils can contribute to the volume of water that comes down the river channels. In some parts of the country, rain falling on snow makes rapid snowmelt join with the liquid precipitation to make bigger events. Humans have tried to minimize the damage from floods by building flood control structures such as channels, dams and levees to protect their properties. However, these structures, combined with land use changes that pave over porous soil, can end up making the floods worse. This article in Scientific American magazine describes how humans’ efforts to tame the Mississippi River have ended up making the floods worse than they were before.
We can see similar effects in the Southeast. A bridge washed out along the creek behind my house in 2004 with rainfall from Hurricane Ivan due to building and increases in pavement upstream. With increases in the heaviest rain events (more than 2 inches in 24 hours) increasing by 27% over the last half century, flooding is expected to become more extreme even if none of the other changes in things like land use occur.