The federal study, known as Atlas 14—the first study of its kind since the 1960s—proved Austin’s flood plains and flood risk were greater than what the city understood. In response, City Council approved heavier regulations within the flood plains. Now, no dwelling units can be added within the flood plain. For new structures, or redeveloped structures, the floorboards must be raised at least 2 feet above the flood plain and they cannot create more runoff onto, or flood risk for, surrounding properties.
“We think this is a significant step in responding to the flood risk in the city,” said Kevin Shunk, flood plain administrator with Austin’s Watershed Protection Department.
Atlas 14 found a roughly 33% increase in the amount of rain that could fall in a 24-hour period. As a result, the properties experiencing greater flood risk are those in close proximity to the city’s creeks, such as Shoal, Waller, and East and West Bouldin, since more intense rain means a wider spread of creek overflow.
Flood plains are governed by rainfall intensity and the likelihood that a storm with certain rainfall intensity could happen in a given year. A 25-year storm has a 4% chance of occurring in a year; a 100-year storm has a 1% chance of occurring in a year; and a 500-year storm has a 0.2% chance.
Before Atlas 14’s findings, a 25-year storm brought 7.5 inches of rain in 24 hours; a 100-year storm brought 10.2 inches of rain in 24 hours; and a 500-year storm brought 13.5 inches. Atlas 14’s findings changed all of this. A 25-year storm would now bring 2.5 more inches of rain. From a regulatory perspective, the significant change is that properties formerly in the 500-year flood zone are now in the 100-year zone, meaning a storm that drops more than 13 inches of rain in 24 hours is five times more likely to hit in a year than previously understood.
Cities regulate based on the 100-year storm floodplain. Flood insurance is also federally required based on whether a property falls within the 100-year storm floodplain. The results of Atlas 14 found the number of structures in Austin within the 100-year storm floodplain increased from 4,000 to 7,200. The 3,200 properties new to the flood plain will have to get flood insurance and will have to abide by the city’s new and stricter development regulations.
The new floodplain restrictions have followed a journey of their own, Shunk told Community Impact Newspaper. The restrictions were supposed to come to council in November 2018. They were then postponed to January, February, April and October 2019 before finally making the agenda on Nov. 14. The postponements were mostly to conduct additional outreach and seek more stakeholder input, Shunk said earlier this year.
Community Impact Newspaper did a deep dive on the findings of Atlas 14 and the impact on Austin in its April 2019 Austin editions. You can find that story here.