Unfortunately, this unique hydrology in Flash Flood Alley has produced a tragic history of flooding events.Ī tropical storm resulted in extreme rainfall in South Texas beginning Sept. “Combine that with the fact that there’s increased impervious cover around cities and suburban areas - all of these things work together, almost in perfect combination to result in extreme floods.” A tragic history “And, much of the rural landscape is overgrazed,” Earl said. When it rains, it just rushes into the streams and you get really intense increases in the amount of flow in the stream.”Ĭlay-rich soil types in the region are another contributing factor because once they are wet, clay soils have low infiltration and high runoff. “That’s hydrologically efficient drainage. “It is fluvially dissected, which means that when it rains, the water doesn’t sit there - it runs off into the streams,” Earl said. Along the escarpment and in areas just north and west of it, almost the entire landscape is sloped. Such terrain is created by the Balcones Fault zone, expressed on the surface by the Balcones Escarpment, which “goes through the heart of Texas,” Mace said. “The Hill Country is karst terrain, so it’s limestone that tends to erode in beautiful ways, but along with that beauty you get thin soils, hard surfaces and steep hills, and that all serves to funnel rainfall very quickly into restricted valleys,” Mace said. Robert MaceĬombined with its propensity for intense rainfall, the region’s rocky topography makes it flood-prone. The transition between the Gulf Coastal Plain and the Hill Country is recognized nationally as a place where topographic changes cause these intense, localized floods. And, in the 1998 flood, one station had 19 inches in 24 hours and a number of stations had 16-18 inches in 24 hours.” “In 1935, 22 inches in less than three hours fell near Uvalde. “In 1921, just on the other side of Austin, 39 inches fell in 24 hours,” Earl said. “The transition between the Gulf Coastal Plain and the Hill Country is recognized nationally as a place where topographic changes cause these intense, localized floods,” he said. Robert Mace, TWDB deputy executive administrator for water science and conservation. This is called an orographic effect, where a change in elevation causes moisture-laden winds to deflect upwards and cool, resulting in rainfall, said Dr. “That effect gets concentrated over the Balcones Escarpment, and if very slow-moving frontal systems come through, such as what happened with the 2015 Memorial Day storms, then this constant stream of moisture from the Gulf will produce many inches of rainfall over a short period.” “The rising air condenses and that creates rainfall,” Fernando said. Additionally, the Balcones Escarpment’s hilly terrain acts as a “ramp” for the fronts and “enhances what was already taking place between the two air masses,” she said. When warm and cool air masses combine, it results in instability as the warm air rises above the cool air. Nelun Fernando, hydrologist at the Texas Water Development Board (TWDB). Over Texas, these moist, warm air masses from the Gulf collide with cool air masses from the north and moisture flow from the Pacific, said Dr. The Memorial Day 2015 flood left widespread damage along the Blanco River. High rainfall intensities are common in the region because there’s an infinite source of moist air from the Gulf of Mexico, he said. Earl, who joined the department in 1991, has studied flooding hazards for decades and has experienced numerous floods in San Marcos. Richard Earl, professor in Texas State University’s Department of Geography. “The region has some of the highest flood discharge per unit area of a drainage basin in the country,” said Dr. Major flash floods are common along the Balcones Escarpment because of two factors prevalent in the region, according to experts: intense rainfall events and efficient drainage off the landscape. Following the curve of the Balcones Escarpment through Texas’ middle - from Waco south to Uvalde - Flash Flood Alley’s weather and landscape distinctively work together to produce rapid flood events. Those rivers run through Flash Flood Alley, one of the most flood-prone regions on the continent. Those same rivers can tell another story about the Hill Country, however. For many Texans, the Hill Country is not just a region but a way of life: beautiful vistas of rocky hillsides, small towns with live music and quaint festivals, and, of course, hot summer days spent diving into spring-fed swimming holes or floating down iconic rivers.
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