NOAA Atlas 14: Precipitation-Frequency Atlas

What is the National Oceanic and Atmospheric Administration (NOAA) Atlas 14: Precipitation-Frequency Atlas of the United States; Volume 11: Texas?

The National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) is pursuing a program to update precipitation-frequency estimates for the United States and its territories. Because Texas has lagged other regions, the Water Management Section of U.S. Army Corps of Engineers - Fort Worth District has managed a project to update NOAA Atlas 14 for the State of Texas. The outcome of this project vastly improves precipitation-frequency data in terms of both the period of record and station density, statistical techniques, and approach to spatial interpolation that accounts for variation in terrain.  This webpage is intended to provide background on NOAA Atlas 14 Volume 11 Version 2: Texas and how it is intended to be used for the future study and design of drainage conveyances within our community.

How will NOAA Atlas 14 data be used?

The purpose of NOAA Atlas 14 is to determine annual exceedance probabilities (AEP) and average recurrence intervals (ARI) for precipitation durations ranging from 5 minutes to 60 days and for ARIs from 1 to 1,000 years.  After researching historical precipitation data, point estimates of AEPs and ARIs were developed and spatially interpolated.  The study results published as Version 2.0 of Volume 11 of NOAA Atlas 14 are in the form of a web-based publication available online through NOAA NWS (https://hdsc.nws.noaa.gov/hdsc/pfds/index.html).  The application includes a Precipitation Frequency Data Server, electronic maps, trend analyses results, seasonal distribution charts, probabilistic rainfall distributions over time, and other detailed documentation.  

NOAA Atlas 14; Volume 11: Texas will assist in developing hydrologic studies with the best available precipitation frequency data and support flood risk studies.  Various state agencies, such as the Texas Department of Transportation (TxDOT), will use the precipitation-frequency estimates acquired in this study to assist in the design of highways, culverts, bridges, and other transportation structures. These precipitation-frequency data may also be used to design storm water facilities, estimate the volume of detentions basins, and size detention basin outlet structures.

The 100-year flood or base flood is the surface water flow in a watershed with a 1% probability of occurring in any given year. NOAA Atlas 14; Volume 11: Texas updates the 100-year storm frequency for our area to 13 inches, an increase of 3 inches above the current referenced historic event.  In addition, the update shifts the probability tables so that the previous 10-inch rain event becomes a 25- to 50-year rain event (that is, a 2-4% probability of occurring in any given year).  While flooding in Central Texas is riverine based, there’s more that goes into a 100-year, 50-year, or 25-year flood determination than precipitation.  Factors like watershed size and shape; length of the storm; and the soil and ground surface conditions all play a role in flood determination.

The updated rainfall data indicate storm systems are more intense than previously thought.  The new data can be used to perform flood studies to evaluate whether there is a greater probability of out-of-bank flows and flash flooding with these larger storms. The data can also be used to plan for drainage infrastructure improvements and storm water management systems to address any identified problem areas.  Potential investments could include resizing detention basins, installation of new storm drain systems, upsizing drainage pipes, dredging channels, and increasing structural elevations.

What are our current flood damage prevention standards?

As an National Flood Insurance Program participating community, the City must enforce against current regulatory effective special flood hazards; however the City must also recognize preliminary special flood hazard modeling as "best available data" for planning purposes.  Under the City’s current Flood Damage Prevention Ordinance, very limited construction is allowed in a FEMA regulatory special flood hazard area.  If allowed, the lowest finished floor elevation of a structure must be 2 feet above the regulatory base flood elevation.  But because the City must defend the preliminary flood plain delineations, calculated finished floor elevations will be slightly higher.

What are the planned changes? 

Web publication of NOAA Atlas 14 Volume 11 was completed in September 2018.  Supporting documentation will be published in December 2018.  These updated precipitation-frequency data sets will provide a public benefit in the form of more accurately designed drainage facilities.  Because the City of Buda has adopted City of Austin drainage design criteria under our Unified Development Code, it is anticipated that the City will also adopt the proposed Code changes outlined in the attached summary.

Once the data are fully published, the City will have new data for many forms of drainage study and design.  An evaluation will be required that entails running new hydrologic and hydraulic models using approved criteria to define changes (if any) in the base floodplain and base flood elevation (100-year recurrent storm event).  These delineations can then be compared against the current Hays County FEMA Flood insurance Study (FIS) and Flood Insurance Rate Maps (FIRMs) (with a 9/2/2005 regulatory effective date) AND the preliminary Hays County FEMA Updated FIS and Digital FIRMs (with 4/7/2017 release date).  It should be noted that the impact of some of the more recent "beyond the historical norm" rain events were captured in the Updated FIS modeling process for the Onion Creek Basin.

Additional Resources
COA Atlas 14 Briefing

TxDOT Atlas 14 Briefing

USACE-Fort Worth District Briefing