What is Atlas 14?
Atlas 14 is an ongoing study used to analyze historical rainfall data in order to update statistical hypothetical rainfall events in Texas. In certain circumstances the statistical events are used to analyze waterways and help design stormwater infrastructure. Based on historical data, Atlas 14 will assign probabilities to rainfall volumes to estimate the likelihood of a rainfall event, like a 100-year storm, occurring in any given year. Since the statistical data for storms in Texas has not been updated in many years, this may cause significant changes in some regions’ projected rainfall volumes.
Commissioned by the National Oceanic and Atmospheric Administration (NOAA) and implemented by the Hydrometeorological Design Studies Center (HDSC) with the Office of Water Prediction (OWP), Atlas 14 has been completed across the majority of the United States and is currently being updated for Texas. This update will be published as the Atlas 14, Volume 11, Version 2.
How are precipitation estimates developed?
Precipitation volume data is based on rainfall intensity, duration and frequency estimates from historical gauge data across the state. Teams of scientists, mathematicians, statisticians and meteorologists from the HDSC analyze the data to help us prepare for heavy rains and possible flooding, droughts or other natural events.
What is a 100-year storm and how often is it exceeded?
To determine a 100-year storm event, gauges are set to collect and measure rainfall. To be considered a 100-year storm the amount of rainfall must meet or exceed predetermined measurements within an allotted amount of time.
The measurements required to meet the criteria for a 100-year storm vary by location. For example, in Harris County the existing criteria is just over 13 inches of rain that must fall within 24 hours or just under 11 inches in 12 hours to be considered a 100-year storm. Travis County, which is in Central Texas, has criteria that states that it must rain just over 10 inches in 24 hours and just under 8 inches in 12 hours.
The preliminary data for Atlas 14 shows an increase in the 100-year storm definition by 2-to-5 inches, which could raise intensity in Harris County to 15-to-18 inches within a 24-hour period or 13-to-16 inches within 12 hours.
Statistically, there is a 1% chance that a 100-year storm could happen in any given year.
What are potential effects of increasing the threshold of a 100-year storm?
Should the threshold for a 100-year storm increase and floodplain administrators subsequently accept the changes, FEMA floodplain maps could require updates to the 100-year floodplain. This means the floodplain boundaries may be redrawn based on the higher statistical rainfall amounts, and could mean higher insurance rates for those located in the expanded boundaries of the floodplain.
The City of Houston considered the possibility of these changes and incorporated them into the recently passed Chapter 19 Floodplain Ordinance. Along with the Chapter 19 regulation revisions, an amendment was added to the ordinance stating that if the FEMA floodplain maps within Harris County are updated, the City of Houston City Council will vote within 60 days on whether make additional changes to the floodplain regulations.
NOAA’s Atlas 14 data for the state of Texas is scheduled to be completed in October and subsequently made available to the public via an online publication. Each jurisdiction will be able to analyze the data and determine practical applications for their location through engineering reviews. For example, the Harris County Flood Control District, the reviewer of FEMA’s maps within Harris County will use the data to determine how to proceed. If adopted, Harris County will go through a process of working with FEMA to update the floodplain maps for the region.
There are local agencies who have already accepted the changes based on preliminary reviews but we recognize that the acceptance or rejection of the Atlas 14 results is at the discretion of the local agency. With that in mind, we will provide updates of any changes or potential impacts throughout Texas.
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