In North Texas, the Houston Metro, and Central Texas, much of the drainage infrastructure design is driven by the 100-year, 24-hour design parameters. In September of last year, the results of the Atlas 14 study were released to the public. Since that time, regulating entities in Southeast and Central Texas have been forced to make significant changes to their design criteria to align with the new rainfall data models. North Texas, however, is a bit different as the results of the Atlas 14 study did not produce significant statistical differences from the current rainfall data.
Many municipalities and cities in North Texas have relatively recent engineering criteria manuals or have adopted parts of the North Central Texas Council of Government’s iSWM Technical Manuals (Revised in 2014). The hydrology manuals include rainfall data, which is derived from USGS Depth-Duration Frequency of Precipitation for Texas (published in 1998), Hydro-35 (published in 1977), and TP-40 (published in 1961). Prior to Atlas 14, TP-40 was relied on to determine drainage design criteria throughout most of the state, including Southeast and Central Texas.
The City of Dallas’s Drainage Design Manual, which has not adopted iSWM, uses Hydro-35 data for durations less than 60 minutes and TP-40 criteria for durations over 60 minutes. The below table contains the approximate percent change in rainfall data from the iSWM Hydrology Technical Manual to Atlas 14 for various durations in Dallas, Denton, and Tarrant Counties.
In North Texas, the resulting Atlas 14 data does not differ significantly for the 100-year, 24-hour design storm. If adopted, Atlas 14 rainfall data in North Texas is unlikely to significantly affect stormwater infrastructure design. NCTCOG is looking at the Atlas 14 data to see if or when they may need to update their manuals to incorporate the newest data set.
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