How the USGS Measured the Texas Floods of May–June 2015

The Blanco River streamgage at Wimberley after the flood of May 23–24, 2015. USGS.

The U.S. Geological Survey collects streamflow data at streamgages across the country, used for flood warning, engineering design, water management and research. In a flood these data matter most: local, State and Federal emergency managers rely on them, and the National Weather Service (NWS) uses them to forecast floods at specific places on a river.

An extraordinary spring

• Record rain: May 2015 was the wettest month on record for Texas, with a statewide average of 9.06 inches — against a long-term monthly average of 3.37 inches and a previous record of 6.66 inches (June 2004).
• City by city: of 19 selected Texas cities, 18 topped their normal May rainfall and 5 broke their May records.
• Unusually widespread: most Texas floods are local, hitting one or two river basins for a few days. In May–June 2015, flooding spread across every major river in Texas except the Rio Grande.

Streamgages where USGS crews measured discharge during May 1–June 15, 2015. USGS.

Keeping gages running

• The network: about 490 streamgages sent real-time stage and discharge data; with water-quality, reservoir and groundwater gages, 634 in all.
• How: most gages record every 15 minutes and transmit by GOES satellite every 60 minutes.
• Reliability: from May 1 to June 30, about 98 percent of possible readings were recorded, transmitted and stored in the National Water Information System.
• The missing 2 percent: mostly equipment failures and gages damaged or destroyed by floodwater.

Stage is the height of a stream's surface above a fixed reference point; it is used to compute discharge — the volume of water passing a point in a given time, usually in cubic feet per second.

The Blanco River

On the Blanco River, among the worst hit, two streamgages were destroyed, though their shelters sat well above earlier flood peaks.

• At the Wimberley gage (08171000), the river rose from a 5-foot base flow to 40 feet — about 35 feet — in just 3 hours, when the gage went under and stopped recording.
• High-water marks surveyed afterward showed the water rose 5 more feet, peaking at 44.90 feet; flood stage there is 13.0 feet.
• The next day, May 25, 2015, crews installed a temporary gage that began transmitting at once.

Recorded stage at the Blanco River at Wimberley, May 23–27, 2015. USGS.

The Wimberley gage in 2011 (left) and after the flood (right), with the new temporary gage at upper right. USGS.

Checking the ratings

At most gages, stage is converted to streamflow through a stage-discharge rating, built and checked with on-the-spot measurements of velocity, depth and width across the range of flows. Channels change, so ratings need regular checks — especially at high flow.

• More measurements: from May 1 to June 15, 2015, crews made 739 discharge measurements at 420 gages, against 590 at 388 in the same weeks of 2014, when floods were rare.
• A rare chance: Texas floods usually last 1–2 days; this long event let crews measure high flows at many gages that needed them, such as the Wichita River at Wichita Falls (07312500).

Shifts

Sediment or debris can build up on a stream's control (its bed) at low flow and scour away at high flow, changing the stage-discharge relation. If a measurement confirms the rating, it stands; if the control has changed significantly, the USGS applies temporary shift adjustments, weighing the measurement's quality, its difference from the rating, and an inspection of the channel.

A shift-adjusted stage-discharge rating. USGS.

Extensions

When a flood goes beyond the highest point on a rating, the curve must be extended — best done with measurements taken as near the peak as possible. In one example, three measurements (21, 22 and 23) extended a rating from 50,000 to about 90,000 cubic feet per second.

Extending a stage-discharge rating with flood measurements. USGS.

Setting records

Comparing a flood's peak stage and discharge with a gage's earlier record puts it in historical context. The USGS identified gages where May–June 2015 peaks beat the record for the entire period of observation; its WaterWatch site summarizes past and current peaks.

Gages where May–June 2015 peaks exceeded previous records, by length of record: less than 5 years, 5 to 10 years, and more than 10 years. USGS.

Sources

Based on U.S. Geological Survey Response to Flooding in Texas, May–June 2015, by Jeffery W. East, USGS Fact Sheet 2016–3027, U.S. Geological Survey (DOI), citing the National Weather Service, NOAA, the Office of the Texas State Climatologist, Rantz and others (1982), and Turnipseed and Sauer (2010); a work of the United States government in the public domain. The figures and photographs come from the fact sheet's PDF.