Click on the link to see a map of the area detailed.
From the Austin American-Statesman
http://www.statesman.com/news/conten...05edwards.html
Central Texas lives up to 'flash flood alley' reputation
Edwards Plateau topography leads to 'rain bombs'
By Asher Price
AMERICAN-STATESMAN STAFF
Thursday, July 05, 2007
More than 19 inches of rain drenched Marble Falls on June 27, an epic storm that added to Central Texas' reputation as "flash flood alley."
Marble Falls, which saw nearly eight months' worth of rain in eight hours, became the latest in the list of towns on the Edwards Plateau that have found themselves in the bull's-eye of what meteorologists call "rain bombs" — some of the most intense rains recorded in the United States.
The plateau is the enormous limestone table that rises just west of Austin. Specifically, it is the rise itself, known as the Balcones Escarpment, that has had a hand in some of Texas', and the world's, most intense rainfalls, according to a 1986 report by the U.S. Geological Survey.
New Braunfels saw 10 inches of rain in an hour in 1972. The town of D'Hanis, in Medina County, got at least 22 inches of rain in 2 hours and 45 minutes on May 31, 1935. And in 1921, 32 inches dropped on Thrall, in Williamson County, in 12 hours, causing 215 deaths and $19 million in property damage.
Bob Rose, a meteorologist with the Lower Colorado River Authority, said he doesn't think the escarpment triggered the June 27 deluge.
"This was a cluster of thunderstorms that stalled in one location," he said. "It was more of just a stationary system."
But historically, National Weather Service forecasters have said that large flood-producing storms occur more frequently over the Edwards Plateau and its escarpment than over any other area in the United States east of the 105th meridian, which runs north-south near El Paso.
The escarpment, which marks the southern and eastern edges of the Hill Country, cuts across South Central Texas about 140 miles inland from the Gulf of Mexico.
"The escarpment gives some additional lift (to the air), and that leads to more intense rain and longer-lasting rain," said Larry Eblen, a meteorologist with the Weather Service.
According to a 2003 U.S. Geological Survey study: "The escarpment lies at right angles to the general direction of winds from the Gulf. Moisture-laden air is cooled as it rises up the slopes, causing condensation and subsequent precipitation along the escarpment."
Why don't these "rain bombs" drop more often?
"You have to get the conditions just right," said retired Geological Survey hydrologist Raymond Slade Jr., who co-authored the study on the 215 "major" and 41 "catastrophic" storms and floods in Texas from 1853 to 2002. As many as half of both types of storms were over the Edwards Plateau, Slade said.
Those conditions can include a low pressure area drifting next to the escarpment.
"Just like water likes to flow from a high point to a low point, air flows from high pressure to low pressure," Slade said.
The topography of the Edwards Plateau plays a role in the area's flash flood alley nickname. The Hill Country's slopes and shallow soil translate into fast-moving, often dangerous water draining toward lakes, rivers and streams, according to hydrologists.
Or, as NASA put it in a 1986 "Geomorphology from Space" report that describes Central Texas as if it were the lunar surface: "Among the geomorphic factors that enhance runoff concentration from rains in this region are the thin soils, relatively high local relief, steep hillslopes, relatively impermeable bedrock, and high drainage densities."
Linear Mode
