Atlas 14 is the nationwide study of rainfall intensities, led by the National Oceanic and Atmospheric Administration (NOAA) with additional input from other federal, state, and local entities. Historical rainfall data was collected from 3,900 water gauges and is beneficial in understanding rainfall events and flood risk. Atlas 14 assigns the statistical probable occurrence of 100-year storms and other rainfall frequency events.
Atlas 14 is segmented into various regions, with Texas denoted as Volume 11. Texas rainfall data is now available through 2017, and includes data from Hurricane Harvey. The Texas/Volume 11 data, which was released in October 2018, clearly displays an increase in rainfall across a band from Del Rio to Austin over to Houston, as well as Corpus Christi and San Antonio. Central Texas and East Texas are the areas with the most increase in rainfall. The northern part of the state was not affected.
Local governments throughout the impacted areas are currently updating their regulations to address this new data and the new risk it presents. The following is a summary of changes currently underway in the Austin, San Antonio, and Houston areas.
Impact to Central Texas/Austin
With the new study data, a 100-year storm for Austin more closely resembles the previously estimated 500-year storm. Austin’s 100-year storm data increased from approximately 10.2 inches in a 24-hour period to nearly 13 inches (an approximate 30% increase).
While approximately 4,000 buildings were located in the previous 100-year floodplain, that number increased to about 7,200 due to the increased size of floodplain areas in the Atlas 14 study (an 80% increase).
“Our understanding of flood risk in Central Texas has changed,” says Division Manager for the City of Austin, Watershed Protection Department, Watershed Engineering Division Kevin Shunk. “Homeowners, renters, and the real estate community need to understand how Atlas 14 may affect their lives and business. The City of Austin is proposing new floodplain management regulations to protect our community from the impacts of flooding.”
Atlas 14 has revealed more homes and business at risk of flooding than previously realized, which impacts the ability to develop, remodel, or redevelop property. The need for and cost of flood insurance is likely to increase once FEMA maps are updated accordingly. In addition, floodplains will need to be restudied. The City of Austin has created a website, atxfloodpro.com, which allows the review of properties and potential Atlas 14 impact.
In response to the Atlas 14 findings, the City of Austin is developing and implementing a three-step process to handle the latest data changes:
- Step 1: Land development code amendments include reclassifying the current 500-year floodplain lines as the 100-year floodplain to limit construction of new buildings in areas of known flood risk; creating a redevelopment exception for those who are redeveloping a property in a floodplain area in Austin; expanding the Colorado River exception, since the Colorado River watershed for the Austin area was not impacted by the new study data; and increasing the freeboard or finished floor elevation requirements from one foot to two feet.
- Step 2: Drainage criteria manual revisions will update the requirements for the design of appropriately sized stormwater infrastructure, such as storm drainpipes, inlets, ditches, and detention ponds. These updates, which are used primarily by engineers, are in addition to the first round of land development code ordinance revisions.
- Step 3: Floodplain study and mapping updates will take two to three years to complete and will provide maps and data to FEMA for flood insurance map updates.
The City of Austin’s projected timeline for these changes are expected to be as follows:
- Summer 2019: Provide updated draft ordinance for the land devel- opment changes.
- October to December 2019: Public hearings at boards and commissions and City Council to approve the draft ordinance.
- Summer 2019: Start Drainage Criteria Manual updates.
- 2019 End of Year: Pass new rules and ordinance changes for the previous steps
- 2019 to 2021: Remap Austin floodplains.
- 2022: Update FEMA map.
Other Central Texas entities are following a similar plan of action in response to Atlas 14.
Impact for San Antonio and Houston
“It is important to utilize the best available data for floodplain management,” says Jacob Powell, storm water engineering manager, floodplain management department for the City of San Antonio. “San Antonio was able to strengthen its already high design standards by translating the Atlas 14 data into a usable format for engineers while maintaining continuity in the development process.”
In April 2019, San Antonio adopted new drainage rules/criteria, which include five precipitation zones across the city ranging from 11.15 inches to 12.87 inches. Overall implementation has gone smoothly, and there is no grandfathering to the old data. Designs created using the old data may be accepted only if there is no impact downstream; the design would be deemed out of compliance but would be approved. San Antonio is also open to review special conditions on a case-by-case basis (especially relating to phased projects), and is currently updating the FEMA maps to reflect the new data.
Houston had the largest impact of any area in Texas, with an increase of more than five inches in the 24-hour 100-year rainfall in some areas. The entire Houston area is updating development codes in response to Atlas 14. For example:
- The City of Houston, Harris County, and Fort Bend County are updating their design storm intensities and depths to Atlas 14.
- The City of Houston, Harris County, and Montgomery County, like Austin, are using the 500-year event data as the new 100-year data until the FEMA maps are updated.
- The City of Houston will follow Harris County’s lead and expects to have three regional zones of rainfall depths to be used in watershed studies. Additionally, new more stringent criteria were added for the design of storm sewer systems. New rules were adopted in July 2019.
- Smaller cities and area communities are following the same general processes as the counties.
So far, notable implications in Houston include increased precipitation depths, which cause an increase in runoff rates that result in increased detention rates. Previously constructed outfall systems—the way water is discharged from a detention pond back into the existing system such as a storm sewer pipe, channel, or stream—are often found to be inadequate to serve the drainage areas for which they were designed. This in turn requires additional mitigation to be provided or supplemental capacity to be added. There is also an increase in storm sewer sizing, along with about a 10% increase in supply costs.
Like Austin, Houston has increased the freeboard—the difference between the 100-year flood elevation and the finish floor elevation of a structure—to two feet, which impacts design in relation to Americans with Disabilities Act (ADA) grading requirements and other compliance. With Houston’s flat terrain, the freeboard increase is a significant design challenge to overcome. Also, areas of Houston are now requiring the mitigation of the 500-year floodplain and not just 100-year floodplain, which increases the cost, time, and study of projects.
Questions to Ask About Potential Effects on Property and Development Sites
Now that you have a basic understanding of what government agencies are doing to respond to this new data, what do you need to do when evaluating a development opportunity in and around the areas impacted by Atlas 14?
The impact of Atlas 14 needs to be studied during the early stages of a development opportunity. It is important to consider the following and perhaps ask your team the following:
- Is the due diligence/development team aware of Atlas 14?
- Is there an existing floodplain on the property?
- With Atlas 14, will all or part of the property be considered to be located in a floodplain?
- Where is the project’s drainage outfall?
- Can the drainage outfall handle post-Atlas 14 rainfall events?
- Is the project phased? If so, how does the phasing impact the overall drainage of the project?
- Have the freeboard requirements increased and if so, how will the extra freeboard impact the grading/ADA and lot grading?
- For a phased project, should an updated flood study be conducted?
- Are you doing redevelopment? If so, is the redevelopment in or near a floodplain? If yes, meet with the city or other permitting entity to get clarification on their specific compliance.
- Should flood insurance be evaluated now while the property is not in a floodplain or later when the FEMA maps are updated?
- If you have a portfolio of properties, how will Atlas 14 impact each? Has value been eroded and risk increased?
The data from Atlas 14 will expand areas with high potential for flooding and thus limit development. This increases risk for developing in these areas. With proper understanding and preparation, future land development transactions can be completed successfully and in a timely way, while also allowing for safe development.