Flood Risk Rising
The City Council has recognized that our flooding problem is serious and growing. In fact, they have declared that flood prevention is the city's number one priority. The growing concern is partly due to the increasing frequency of heavy rainstorms, brought about by changing weather patterns. But it is also due to rampant growth in this area, which has led to paving over a lot of the green space (such as the path of the proposed road, which is now 100% green space) that is needed to soak up the stormwater so that it won't cause flooding.
Concerns about increases in heavy rainstorms are validated by an endless stream of weather reports from many areas along the east coast of the USA, and the gulf coast. Reality is quickly outpacing rainfall predictions based on historical models. Here are some examples:
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"… Hurricane Harvey … was the third storm in just three years to bring so-called "500-year" rain …The U.S. has experienced at least 24 of these "500-year" rain events since 2010, including Hurricane Matthew in 2016. "
"... what had been considered a 100-year storm was now a more frequent 25-year storm, NOAA found. And a 500-year storm became the equivalent of a 100-year storm...."
"... in Austin ...storms previously considered 100-year-events are now 25-year events. "
"In Virginia Beach, ... Hurricane Matthew lingered over the area in 2016 to deliver a knockout punch, the third 100-year storm of the year...."
"The researchers used New York City as a test case and found …, “100-year floods” ... could more likely occur every three to 20 years. What today are New York City’s “500-year floods” —or waters that reach more than 9 feet deep — could, with climate change, occur every 25 to 240 years …"
"We have had more Cat 4 and Cat 5 landfalls in the United States from 2017 to 2021 than we did from 1963 to 2016”
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"Climate change made the record-smashing deadly 2020 Atlantic hurricane season noticeably wetter, a new study says. And it will likely make this season rainier, too, scientists said."
The pattern is clear: The reality of flooding is much worse than our historical models predict. And the relevancy is immediate: Current standards for designing stormwater management systems are misleading because they are not based on up-to-date data.
The standards are based on a database called Atlas 14 which stores vast amounts of rainfall data from historical records. Here is what a recent scientific study had to say about it:
"In 2019, researchers at the University of Wisconsin found that design guidelines based on Atlas 14 were dangerously inadequate. Looking at records from more than 900 weather stations from 1950 to 2017, they determined that 100-year storms — those with a 1 percent chance of occurring in a given year — were happening almost twice as often in the eastern half of the country as Atlas 14 predicted."
The current version of Atlas 14 was published in 2013, but only a few weather stations in Florida have entries later than 2011, and the vast majority of the data is from the last century, prior to the onset of the weather pattern changes.
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The need to update these requirements is universally recognized by the scientific community and government officials at all levels. Florida Senate Bill 712, requires the Department of Environmental Protection to update the stormwater requirements in accordance with the latest scientific information. This bill was unanimously approved (!!!) by the Florida House and Senate, and was signed by the Governor.
Unfortunately, the devil is in the details, and the process of updating the standards is complex and will take some time. But in the meantime, anyone making decisions related to stormwater design needs to bear in mind that the existing standards are obsolete in the extreme.
The point of this discussion is that, if the design just meets the current 25-year storm requirement, it will lead to flooding after the kinds of storms that are likely to actually occur.
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See relevant documents below.