Atlas 14 is a National Weather Service study of historical rainfall. We use historical rainfall data to understand our flood risk. The historical record for the previous rainfall study ended in 1994. Atlas 14 extends the rainfall data through 2017.
Before the study, we believed that, in any given year, there was a 1 percent chance of 10.2 inches of rain falling in 24 hours in Austin. This was the official definition of the 100-year storm. The Atlas 14 study shows that this amount of rainfall is now likely to occur more frequently. The new 100-year storm will be closer to 13 inches of rain in some parts of Austin. This resembles the current 500-year storm.
|Probability of Occurrence in any Given Year||Storm Level||Current Rainfall Intensity
|Atlas 14 Rainfall Intensity
|4%||25-year||7.6 inches||Up to 9 inches|
|1%||100-year||10.2 inches||Up to 13+ inches|
|0.2%||500-year||13.5 inches||Up to 19.5 inches|
Impacts of Atlas 14
Redefining the 100-year storm has far-reaching impacts.
- Flood Risk – This new rainfall information indicates that more people and property are at risk of flooding. It also indicates an increased level of risk for the people and property that we already knew had flood risk.
- Flood Insurance – We estimate that the number of buildings in the 100-year floodplain could increase from 4,000 to 7,200. Affected residents who have federally-backed mortgages will eventually have to purchase flood insurance. Those who already have flood insurance will likely see the costs go up. This will affect businesses, too. Residents and business owners in the interim 100-year floodplain should talk to their insurance agent about purchasing flood insurance now. Impacts to flood insurance will occur after FEMA approves updated floodplain maps, estimated to occur in 2023 or 2024.
- Development and Remodeling – We have revised city code to address our new understanding of the increased risk. This will minimize future flooding losses but means that regulations that limit new development in the floodplain will affect more property owners. These regulations also affect homeowners who wish to make improvements to their houses that are in the floodplain.
- Pipes and Ponds – Both public and private storm drain pipes, bridges, detention ponds and other drainage infrastructure will need to be larger to handle the new storm levels. Our existing infrastructure is not sized for the new 100-year storm. In other words, we are more likely to be faced with storms that are too big for our pipes and ponds, possibly resulting in flooding of roads, homes and businesses.
- Floodwalls – Austin's two floodwalls were designed for the current 100-year flood. Scroll down to find links to presentations about the floodwalls in the Documents section.
Drainage Criteria Manual (DCM) Revisions
We have adopted revisions to the DCM in response to the Atlas 14 rainfall study. This process included a stakeholder comment period. Refer to the links below for the revisions.
- Atlas 14 DCM Guidance Document: Brief summary of changes that incorporate Atlas 14 rainfall data.
- DCM Revisions: Updates to Sections 1 and 2 and the appendices, adopted on January 13, 2020.
- DCM Recommendations Memo: Technical memo describing the process followed to develop the new Atlas 14-based rainfall criteria for Section 2.
- DCM Rainfall Distribution Analysis Memo: Technical memo describing the process followed to develop the rainfall distribution to be used with the Atlas 14 rainfall data to calculate rainfall runoff.