See How That Major Storm Measures Up

Garrett Johnston

When a major storm hits your community, it’s only natural to wonder how it compares to a standard design storm. Was that a 100-year event (1% annual exceedance probability) or a 5-year (20% AEP)? Was it a 5-year event by peak intensity but a 100-year event by total volume? Answering these questions typically requires manual analysis of data from rain gauges or ground-truthed radar.

Freese and Nichols’ resiliency design and stormwater engineers have developed an automated GIS tool to answer these questions for communities across Texas. See below for a set of automatically-generated maps depicting the June 2, 2017, Dallas/Fort Worth storm:

The tool starts by obtaining hourly rainfall estimates from the National Weather Service. This quality-controlled data is derived from radar and rain gauges and covers the entire nation. Next, the tool compares the highest 1-, 3-, 6-, 12- and 24-hr rainfall depths with USGS depth-duration-frequency curves for Texas. These curves relate rainfall depths at any given location to a corresponding frequency, such as the 1% AEP or “100-year” storm. The tool then produces a series of color-coded maps depicting the approximate return intervals in the region of interest.

We are continuing to test and improve this tool internally. Keep an eye on this blog for updates. In the meantime, let us know if you need to evaluate a historic storm event in your area. You may contact us at / 512-617-3160.