Texas Emergency Management ONLINE 2013 Vol. 60 No. 12

Flash Flood Alley

Citrus Greening

This past Halloween, following several years of extensive drought, central Texans experienced incredible overnight flash floods. Rainfall totals and flooding were at historic levels. October 2013 was the wettest October in central Texas history.

In Hays County, eight to fifteen inches of rain fell in just a few hours from Driftwood to Wimberley and in Buda and Kyle. Downstream in San Marcos, an RV park was carried away by the raging Guadalupe River.

In Travis County, the second weekend of the Austin City Limits Music Festival had to be canceled due to immense flooding at Zilker Park. Around Onion Creek in southeast Austin, over 1,100 homes had to be evacuated due to severe flooding. Eight people, including an eight-month-old boy, lost their lives as they were swept away by the fast-moving water.

The normally trickling Onion Creek became a devastating torrent, flowing at 120,000 cubic feet per second. At one point, it rose 11 feet in 15 minutes and up to 41 feet. By comparison, the Colorado River during the same time flowed at 34,330 cfps; it crested at 33 feet.

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Debris piled up near Driftwood, October 2012. Photo Mike Jones

A familiar combination of events all came together to bring the epic rainfall.

Two weeks prior, areas around Central Texas received nearly as much rain as fell on Halloween, up to 10 inches and more overnight. More rain—in normal amounts—came the next week. The watershed areas for Onion Creek along with the Comal and Guadalupe Rivers were well-saturated.

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A strong front from the northwest brought high cool air that collided with low-lying warm, humid air from the Gulf of Mexico and with the remnants of Hurricane Raymond, which moved in from the Pacific Ocean, across Mexico and into Central Texas. Weather forecasters all over the area were predicting the potential for severe weather and excessive rainfall. Flash flood watches were in place. No one, perhaps, thought there would be as much rain as there was in such a short period of time.

Unfortunately, this scenario is not uncommon to Central Texas. A very similar situation occurred on Saturday, October 17, 1998, bringing much more devastation over a much wider area. A strong upper level trough delivered high cool air that collided with the persistent low-level humid Gulf air. The remaining moisture from what had been Hurricane Madeline rolled in from the Pacific, supplying the final element for the worst flooding event in Central Texas history. Several tornadoes also were spawned by the intense storm.

Forecasters predicted a significant and possibly dangerous situation. Heavy rainfall, up to four inches, was expected all over central and southeast Texas, and a large amount of runoff into low-lying streets, creeks, and rivers was also probable.

Sixteen inches of rain fell in and around San Antonio and Seguin before the front arrived. New Braunfels received over 20 inches of rain. Austin received 10 inches of rain. Areas north of Houston also receive 15 to 20 inches of rain, as did areas east of Wharton. By the time the rain had tapered off, some areas had totaled over 22 inches of rain. A small area of Caldwell County near Lockhart received an estimated 29 inches of rain. Over the event, the National Weather Service for Austin and San Antonio issued 163 flash flood warnings. NWSO Houston/Galveston issued 60 flash flood warnings, and Corpus Christi issued 29.

Widespread flash flooding occurred from the eastern Hill Country, through Austin and San Antonio, and along the Interstate Highway 35 corridor between the two cities by Saturday night. Downstream the rivers were rapidly overflowing their banks, much faster than anyone predicted.

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Historic Flood Levels. Guadalupe River at Seguin. Photo Forrest Mims

The Guadalupe River near Seguin crested at 51.7 feet by 6:00 pm Sunday night. Flood stage is 31 feet. At Cuero, the river crested mid-Monday at 49.8 feet, 29.8 feet above flood stage. The immense flooding continued all downstream. According to the US Geological Survey, the intense rainfall far exceeded any previously known events. Flood gauges upstream from Cuero at New Braunfels, Luling and Gonzales were knocked out of service by the flood waters, but the amount of water flowing through there was estimated to be as much as three times the previously known amount.

By the time the rains ended, 31 people had died. Portions of 60 counties in Texas had been damaged by flooding, and hundreds of people forced from their homes. Twenty counties were declared federal disaster areas, and damage estimates exceeded $750 million.

SanMarcos_spillway.jpg  Spring Lake Dam 1998 Josh Millecam.jpg

Above left:  San Marcos River normal flow. Courtesy EdwardsAquifer.net
Above right:  San Marcos River October 1998 Photo Josh Millecam
Below left:  Guadalupe River, Victoria 1998. Photo NOAA
Below right:  Cuero 1998. Photo FFA

1998_moody_greens_2.jpg  FFA_Cuero_1998.jpg

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