In the past I worked for the National Weather Service in the Office of Hydrology, determining the magnitude of 100-year rainfalls. A 100-year-rainfall is the amount of rain that has a 1 in 100 chance of occurrence in a given year. Note that it does not mean that it can only happen once in a hundred years, which is a common misconception.
The values for a 100-year storm are calculated assuming that the climate is stationary–that is, it has no trends one way or the other. However, observations of rainfall over the last 100 years have shown that instead, heavy rains (those of approximately 2 inches or more in a day) are increasing across the US. In the Southeast, they have increased by an estimated 27%, in the Southwest less and in the Northeast, more. The consequences for this trend are concerning for our older infrastructure, which is all designed to meet the amounts of rain calculated using stationary statistics.
So, for example, a bridge or culvert that was designed to withstand a 100-year rainfall using the old numbers now might only be able to withstand something like an 80-year storm, which occurs more frequently. (In Georgia, the current Stormwater Management Manual used by civil engineers is based on rainfall statistics which were published in 1961.) In fact, any structure designed for a particular rainfall amount (say, a manure lagoon) is also likely to feel the heavy rainfall more frequently than in the past, with a greater chance of failure than if the climate was really stationary. Flooding is also being enhanced by increasing urbanization, which paves over land that previously could absorb the heavy rainfall, making the streams more variable and letting more water flow down to the sea instead of replenishing the groundwater.
The result of the newer calculations is causing changes to infrastructure that include things like raising the heights of bridges to help prevent catastrophic losses like flooding due to bridges that are too low and which may act like a dam across a flooded river. Scott McFetridge of Associated Press had an interesting article this weekend which describes this problem and steps that some governments are taking to reduce the threat. You can read it at https://onlineathens.com/national-news/2017-06-10/climate-change-raises-new-risk-are-inland-bridges-too-low.
