The federal government is in the process of updating their estimates of the 100-year storm across most of the United States. This is a good thing, because the last time a complete estimate of the 100-year storm (or more accurately, the amount of rainfall that has a 1 in 100 chance of occurring in any year) was done, it only included data up to 1960, in what is known as Technical Paper 40. In fact, the Georgia Stormwater Management Manual is based on TP 40 for the estimates of the amount of rainfall civil engineers need to design culverts, roof drains, and other water-bearing infrastructure around the state. Most roads are designed to drain a certain amount of rainfall in a given time (say, an hour) without ponding, which would otherwise increase the chance of hydroplaning and auto accidents. The design standard depends on the amount and speed of traffic on the road, with interstates being designed to the highest standards.
The concern is that the new rainfall statistics, which include a lot more data including both dry spells and floods like the Atlanta flood of 2009, show that the new value of the 100-year storm may be a lot higher than TP 40 would indicate. That means that the roads, etc. that are designed using the old standards may be a lot more prone to flooding now than they used to be, leading to an increase in accidents, roof failures, and even potential failures of dams in some cases. While I haven’t yet done the comparison for the Southeast, the Houston Chronicle posted a story this week on the impacts that this change in rainfall frequency would have on their infrastructure. You can read it here.