Flood Country

Forrest M. Mims III

While the consequences of Hurricane Katrina continue to unfold, it's important for residents along the entire Gulf and Atlantic Coasts to be aware that even more powerful storms have battered the United States. In 1980, for example, Hurricane Allen, struck the Texas coast. Allen was one of only a few category 5 hurricanes to strike the United States.

Compared with Katrina, Hurricane Allen caused minimal property damage. That's because it struck the coast in a largely unpopulated region between Corpus Christi and Brownsville. Its legacy would be very different had it struck a major city.

Hurricanes and tropical storms can cause flooding hundreds of miles from where they make landfall. On October 17, 1998, Hurricane Madeline was churning the Pacific Ocean off the west coast of Mexico. A giant stream of moisture extended from Madeline across Central Texas.

At the same time, a front was approaching Central Texas from the west while moist Gulf air was approaching from the East.

These three weather events merged together and dropped as much as 56 cm (22 in) of rain over Central Texas in less than a day. Some areas received the worst flooding they have experienced since records were kept. What happened next provides an important lesson to many other areas subject to hurricanes.

Massive property destruction occurred along the Guadalupe River between New Braunfels and Victoria. The flow of the river at Cuero was three times greater than the highest ever recorded.

The Guadalupe River crested at 11.7 meters (38.5 feet) at Lake McQueeney. Many homes were badly damaged during the flood. Some were washed away.

People enjoy living along rivers, and many believe that the 1998 flood was so rare that it may be a century or more before another such flood. The historical record is less optimistic.

The Guadalupe-Blanco River Authority web site in Texas has considerable flood information on its web site (www.gbra.org). Some other States and countries have similar information, as does the U.S. Geological Survey. The Guadalupe reached 11.7 m (38.5 ft) at Lake McQueeny during the 1998 flood. But that is not the record, for the river reached 12 m (39.5 ft) in 1913.

The normal flow of the Guadalupe was exceeded 19 time between 1913 and 2004. If these floods are plotted on a graph, a downward trend is evident. This may be due to the construction of a dam on the Guadalupe River that forms Canyon Lake.

The big exception is the 1972 flood, when the Guadalupe reached 11 m (36.6 ft) at Lake McQueeney. Major rainfall between Canyon Lake and New Braunfels was the key reason for this great flood.

Rivers like the Guadalupe are found throughout hurricane country. They have flooded thousands of homes in the past. They will flood thousands of homes in the future.

The same lesson applies to hurricanes themselves. Hurricanes are beyond human control. They will form every summer and fall and will make landfall where they are guided by the steering currents of the atmosphere.

Forrest M. Mims III and his science are featured online at www.forrestmims.org.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.