Evolution of the Upper Texas Coast
John Anderson likes to joke that he studies the Antarctic in the winter and the upper Gulf Coast in the summer, but what he sees happening along the Gulf Coast is no laughing matter.
Anderson, the W. Maurice Ewing Chair in Oceanography and professor of Earth science, has spent two decades studying the Texas coastline and continental shelf, and he put what he has learned in “The Formation and Future of the Upper Texas Coast: A Geologist Answers Questions About Sand, Storms and Living by the Sea” (Texas A&M University Press, 2007). In this richly illustrated book, he sets out to answer fundamental questions about coastal evolution, natural processes that affect the coastline and how human development can be managed to help preserve it.
Want to know what happens to the sand that erodes from Texas beaches or if beach erosion can be stopped? Can a hurricane have positive impacts? How much development can the coast stand? How severe is the possibility of extensive coastal flooding due to rising sea levels?
The answers to these questions and others might be surprising. Sand, for example, does not erode from beaches — some of it is washed down the coast by currents that run parallel to the shoreline, some collects in tidal deltas or behind man-made barriers such as jetties, and some is layered over by river silt. Beaches merely seem like they’re eroding due to rising sea levels and the second-worst coastal subsidence in the United States — both exacerbated by decreasing infusions of river sand due to up-stream dams.
“Along our low gradient Texas and Louisiana coastlines, an annual rise in relative sea level of between 1/16 and 1/8 inch per year results in an average of three feet to five feet of coastal retreat.”
Equally fascinating is Anderson’s recounting of the geological history of the upper Texas coast. Did you know that, at the end of the last Ice Age, the Texas shoreline was located approximately 80 miles farther out in the Gulf than it is today? At the time, Galveston Bay was a broad valley carved by the San Jacinto and Trinity rivers — a valley that was about 170 feet deep. Between 14,000 and 5,000 years ago, the shoreline moved landward as much as 60 feet a year in some locations, flooding the valley, which then filled with sediment. Galveston Island didn’t exist until the end of that period, and Bolivar Peninsula didn’t form until 2,500 years ago.
Anderson’s projections into the future are a little more frightening. While the geophysical interactions involved in relative sea level rise are complex — a combination of rising sea levels and land subsidence — and well explained in the book, the outlook is not rosy. “Along our low gradient Texas and Louisiana coastlines,” Anderson wrote, “an annual rise in relative sea level of between 1/16 and 1/8 inch per year results in an average of three feet to five feet of coastal retreat.” This will subject significant portions of Galveston Island and much of southern Chambers County, which is below five feet in elevation, to flooding in the next century or two.
Aerial and satellite photos, maps and charts augment Anderson’s text, giving dramatic evidence that the landscape, which we tend to view as eternal, is, in fact, more ephemeral than we’d like to admit.