Texas Bureau of Economic Geology Padre Island National Seashore: A Guide to the Geology, Natural Environments, and History of a Texas Barrier Island

HURRICANES AND THEIR EFFECTS ON PADRE ISLAND

On September 4, 1961, the U.S. Weather Bureau advised that a tropical depression had formed in the Caribbean Sea just off the coast of Nicaragua. This was the first warning of one of the greatest storms of this century. One week later, the tropical depression, now grown to a full-sized hurricane designated as "Carla" crossed the Texas coastline near the small town of Port O'Connor, . . . The Gulf coast from Grand Island, Louisiana westward to the Rio Grande felt the storm to some extent, . . . Hurricane surge and wind destruction damages reached a total estimated at about 408 million dollars and disrupted all normal activities for four days in the coastal areas of some 38 counties. The hurricane winds wreaked major damage to roofs, fences, and small buildings; eroded Gulf beaches, roads, levees, and bayshores; severed communications and power supplies; stopped utility systems in cities and towns; contaminated food and water supplies; and destroyed or damaged agricultural crops through the area (U.S. Army Corps of Engineers, 1962).

It is not surprising that coastal inhabitants view hurricane season (June through October) with apprehension and fear. Hurricanes are the most devastating natural events that occur along the Gulf Coast. In addition to the threat to man and his activities, these high-energy storms can produce significant changes in natural environments (Brown and others, 1974). Hurricanes are the most destructive of all storms, primarily because of their size and intensity. A hurricane is "defined technically as a storm of tropical origin with a cyclonic wind circulation (counterclockwise in the Northern Hemisphere) of 74 mph or higher" (Dunn and Miller, 1964). Although hurricane wind velocities of up to 200 mph cannot compare with tornado winds that may exceed 300 mph, the great size of hurricanes and the length of time during which they expend their energy result in a much greater overall impact. Hurricanes often spawn tornadoes, as demonstrated by Hurricane Beulah in 1967, when more than 100 tornadoes were reported (U.S. Army Corps of Engineers, 1968).

Hurricanes can be characterized by their most destructive properties as described by Orton and Condon (1970):

Carla in 1961 wrought devastation primarily with mountainous storm tides; Beulah in 1967 inundated thousands of square miles with 20-to 30-inch rains; Celia did it all with winds.

A more detailed comparison of these three hurricanes is presented in table 1.

TABLE 1.
Characteristics of Hurricanes Carla, Beulah, and Celia, as measured at Texas stations (compiled from U.S. Army Corps of Engineers, 1962, 1968, and 1971).

*"Fastest mile" is the maximum speed of the wind sustained for a period long enough to travel a distance of 1 mile, as determined from velocities recorded at a given point.

One of the most destructive features of hurricanes is the storm tidal surge. Storm surge expends a vast amount of energy by eroding, transporting, and depositing great volumes of coastal sediment. The effects of storm surge produced by Hurricane Celia on the Texas coast are documented by McGowen and others (1970). Although the tide (known as the forerunner) rises slowly while the storm is offshore, the surge, which accompanies the hurricane as it makes landfall, is a rapid rise in water level resulting from strong onshore winds and low atmospheric pressures (Dunn and Miller, 1964). Superimposed on the elevated water level are storm waves. Because a hurricane spirals in a counterclockwise direction in the Northern Hemisphere, it develops a larger surge on its right side as it approaches the coastline. Flooding will be greater in low-lying areas to the right of the hurricane as viewed from the approaching eye of the storm.

As a hurricane approaches the mainland along the central Gulf Coast of Texas, barrier islands are the first land feature in the path of the surge. Where a well-developed fore-island dune ridge exists, such as along Big Shell Beach on central Padre Island, the barrier island provides a major line of defense for the mainland. The barrier helps to block the surge and to dissipate large amounts of wave and current energy. But the impact of the storm still leaves its mark on the island. Hayes (1967) observed that fore-island dunes were eroded landward as a result of Carla's storm surge and wave attack. Steep, gulfward-facing, wave-cut cliffs up to 10 feet high were evident along parts of the fore-island dune ridge. The normal beach profile was replaced by a broad, flat, hurricane beach. Further more, in some areas, the newly formed beach was strewn with an assortment of material plucked from offshore, from water as deep as 50 to 80 feet. The material included coarse, heavy shells and rock fragments, coral and caliche blocks, and a large Pleistocene elephant tooth. The tooth perhaps serves as a reminder of the lower stands of sea level (figs. 15a and 16) that were witnessed by prehistoric animals inhabiting the North American continent during the Pleistocene "ice ages."

The storm surge that is produced by hurricanes such as Carla does not always stop at the beach and fore-island area. Along low-lying segments of the island, the surge washes across, breaching the fore-island dune ridge and scouring washover channels (Scott and others, 1969) (fig. 61). Sediments eroded from the beach and adjacent dunes are spread lagoonward over the tidal flats, forming fan-shaped deposits called washover fans (figs. 61 and 62). In some areas along the southern end of the Seashore (South Section of pl. I), "pavements" composed of large shells and shell fragments can be found lagoonward of the backbeach and fore-island dunes, offering evidence of the storm surge that accompanies hurricanes such as Carla, and the subsequent effect of the wind, which deflates the sand and leaves the shell concentrated at the surface (fig. 63). Hayes (1967) discovered shells 2.5 miles landward of the beach as a result of Carla's surge.

Figure 61. Aerial photograph of washover channels along central Padre Island (pl. I, grid V-20,) that were activated by storm surge accompanying Hurricane Beulah. Photograph was taken 15 days after Hurricane Beulah crossed the south Texas coast on September 20, 1967. As Gulf waters flooded through the washover channels "C," which cut through coppice dune fields "D" and vegetated fore-island dunes "E," sediments were eroded and flushed lagoonward, depositing the washover fan at "G." As the hurricane passed, elevated waters flowed back through the washover channels and branch channels "F," transporting sediments toward the Gulf "A" and forming channel-mouth bars; note the seaward displacement of the surf zone at "B," which reflects the submerged sandbars. As normal shoreline processes resumed after the hurricane, the channels were closed and the beach reconstructed across the mouths of the channels. (Photograph courtesy of Scott and others, 1969.)

Figure 62. Sketch of washover channels and washover fans, shown in relation to other natural environments on central Padre Island. (Modified from Scott and others, 1969.) (click on image for an enlargement in a new window)

Figure 63. Natural shell "pavement" lagoonward of the fore-island dunes on Padre Island (South Section, pl. I). These shells, which include Dinocardium robustum, Mercenaria campechiensis texana, and other species, were apparently washed from the beach into the interior of the island by hurricanes and were concentrated at the surface by the wind, which removes (deflates) the finer sand, leaving the shells behind.

Although Hurricane Carla's surge height was estimated to be between 5 and 10 feet along the Gulf shore of central Padre Island, it reached more than 12 feet in the vicinity of Port O'Connor, where it made landfall (approximately 90 miles northeast of Padre Island). Where natural inlets or man-made channels (such as Mansfield Channel, pl. I) connect Gulf and bay or lagoon waters, hurricane surge may produce strong currents that dramatically raise bay and lagoon water levels. For example, a surge height of 22 feet was recorded at the head of Lavaca Bay near Port Lavaca during Hurricane Carla (U.S. Army Corps of Engineers, 1962). When the surge passes through channels and inlets, it flushes sediments bayward, commonly forming deposits at the ends of the channels. As the hurricane passes, elevated bay and lagoon waters, which are produced by surge and winds, create gulfward-flowing currents that transport sediments back through washover channels and inlets.

When the hurricane has passed, normal processes resume and, in time, a normal beach profile is restored. Longshore drift smooths irregularities in the Gulf shoreline and builds bars and beaches across the mouths of the hurricane channels. Major washover channels that were eroded below mean sea level may continue to pond water after normal near-shore processes have reconstructed the beach across the channel mouths, severing their connection with Gulf waters (pl. I, B7 photograph). Major washover channels are commonly reactivated during subsequent hurricanes. Washover-fan deposits that are deposited over the wind-tidal flats are reworked later by the wind to provide a source of sand for the formation of back-island dune fields (Boker, 1953). After a hurricane has passed, its mark remains on coastal barriers, such as Padre Island, for many years.