Floods are among the most frequent and costly natural disasters in the United States. Flash floods — sudden, high-water events in a local area — are especially hard for emergency managers to predict and warn about, and a lack of data can slow recovery afterwards.
Central Texas is hilly country known as "flash flood alley": intense rain, thin soils and steep terrain together can cost lives and destroy property. In July 2002 a flash flood killed 12 people in central Texas, one of them in Utopia, a town on the east bank of the Sabinal River in a flood-prone area along the Balcones Escarpment. On July 5, 2002, the U.S. Geological Survey (USGS) streamgage on the Sabinal River near Sabinal (08198000) recorded a peak flow of 108,000 cubic feet per second, at a stream stage — or gage height — of 33.74 feet. In a typical year, the median daily flow there is only about 23 cubic feet per second.
Building the toolset
In 2021 the USGS, working with the Bandera County River Authority and Groundwater District and the Texas Water Development Board, built a flood warning toolset for the Sabinal River near Utopia, following earlier USGS flood work on the Medina River in Bandera County. It has three parts:
- a new USGS streamgage, Sabinal River at Utopia (08197970), 13 miles upstream of the Sabinal gage;
- a hydraulic model of the river near Utopia;
- an online library of digital flood-inundation maps tied to the stage at the Utopia gage.
Figure 1. The study area, the extent of the hydraulic model, the study reaches and the nearest USGS streamgages. Credit: U.S. Geological Survey.
The streamgage
The Utopia gage measures and records stream stage every 5 minutes and sends the data by satellite every hour to the USGS National Water Information System and the National Water Dashboard, where anyone can see them. As of May 2022 it could not yet report continuous flow: it had only been running since January 15, 2020, through a long dry spell. Once enough flow measurements cover the full range of conditions, a rating curve — the relation between stage and flow at a cross-section of the stream — will be built. For this study, a synthetic rating curve was derived with a regional regression method.
Figure 2. The new streamgage on the Sabinal River at Utopia. Photograph by Namjeong Choi, U.S. Geological Survey, February 26, 2020.
The model and the maps
The hydraulic model, built with the U.S. Army Corps of Engineers' Hydrologic Engineering Center River Analysis System, covers a 10-mile stretch of the Sabinal River and a 7-mile stretch of the West Sabinal River near Utopia. It draws on high-resolution elevation data, land cover data, field surveys of structures and a channel cross-section, and the synthetic rating curve.
From it came a library of 35 flood-inundation maps showing the areas likely to flood at stages from 11 to 28 feet at the Utopia gage, in half-foot steps. The top value, 28 feet, is the estimated peak stage at Utopia in the flood of July 5, 2002.
Getting the information out
The maps are online through the Interagency Flood Risk Management initiative's Flood Decision Support Toolbox, which shows flood extents and water depths for various stages at selected USGS streamgages. Through the USGS WaterAlert service, people can also sign up for email or text alerts when a streamgage's stage or flow passes a level they choose.
Together, the maps and the live data from the Utopia gage are meant to help people take their own precautions, and to give emergency managers the tools to run flood operations and recovery efficiently.
Sources
Based on "Flood Warning Toolset for the Sabinal River Near Utopia, Texas," by Namjeong Choi, U.S. Geological Survey Fact Sheet 2023–3001 (version 2.0, September 2023), prepared with the Bandera County River Authority and Groundwater District and the Texas Water Development Board, published by the U.S. Geological Survey; rewritten in hubnx's own words. It is based on USGS Scientific Investigations Report 2023–5001.
- Figures 1 and 2 come from the fact sheet's PDF: https://pubs.usgs.gov/fs/2023/3001/fs20233001.pdf
- The fact sheet's Figure 3, an example flood-inundation map that includes building outlines from Microsoft, is not reproduced.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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