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Living with Florida's Atlantic BeachesCoastal Hazards from Amelia Island to Key West
By David M. Bush William J. Neal Norma J. Longo Kenyon C. Lindeman Deborah F. Pilkey Luciana Slomp Esteves John D. Congleton Orrin H. Pilkey
DUKE UNIVERSITY PRESSCopyright © 2004 Duke University Press
All right reserved.
Chapter OneFrom Fort Clinch to Fort Taylor: East Florida's Dynamic Coast
Vocabulary reflects attitude, and the words used in discussing the coast are often military in tone. We armor the shore and draw a line of defense in our battle with the sea. Some regard retreat in the face of sea-level rise as unacceptable. Appropriately in this context, the primary federal agency with authority over coastal activities is the U.S. Army Corps of Engineers; and among the oldest man-made structures along Florida's coast are military forts. Fort Clinch and Fort Taylor not only mark the northern and southern extremities of Florida's east coast (figs. 1.1 and 1.2), they also reflect the extremes of coastal dynamic settings from erosion to accretion, from a foundation of loose sand to solid reef rock, and from temperate climate to tropical climate. Even the way we choose to defend structures from shoreline retreat is mirrored in the history of the armoring of the shores in front of these historic forts.
Florida's oldest fort is the Castillo deSan Marcos. Completed in the late 1600s, it was located on the mainland with command of St. Augustine Inlet. Early settlements commonly located on the naturally protected mainland rather than on the open-ocean shore. But by the 1800s, forts as well as settlements were being constructed on the seaward side of Florida's barrier islands. Fort Clinch, on Amelia Island at the mouth of the St. Marys River, was begun in 1847 and, although never completed, has stood as a fixed reference point next to the changing shoreline (fig. 1.1). Long threatened by shoreline erosion, the fort illustrates how reliance on shore-hardening engineering requires additional projects to hold the shore in place. Groins were constructed seaward of the fort in 1886 and again in the 1930s, about the same time the Civilian Conservation Corps restored the fort and it became a park.
The U.S. Army Corps of Engineers built a new set of rock groins at Fort Clinch in 1992, but the erosion problem persisted. Yet another set of groins, this one utilizing the T-head design and rock-filled mattresses, was emplaced in 2000, and the cells between the groins were filled with dredged sand in 2001 (fig. 1.1). North Florida is subject to hurricanes and northeasters, large winter storms with associated wave erosion. Fort Clinch's battle with the sea is not over.
Construction of Fort Taylor on Key West began in 1845 but was not completed until 1866. Hurricanes, the bane of South Florida, were among the reasons for the delay. The fort was constructed 1,000 feet offshore, on the bedrock of a key, with walls 5 feet thick to withstand both cannonballs and waves. In 1976 the fort became the property of the Florida Park Service, and in 1985 it was opened to the public. Visitors to the park will notice that the fort is now landlocked because sedimentation has gradually filled the shallow water around the fort, aided by some dredge fill (fig. 1.2). The fresh water supply was a problem, as it is for the Keys in general, and the fort had one of the earliest desalinization plants.
Houses, hotels, and high-rises may not be looked on as forts, but if your home is your castle, and a castle is a fortress, the analogy may fit-especially if the building is sitting on the shoreline! Every building on the shoreline is subject to the same problems that beset Florida's old forts. And East Florida's shoreline is crowded with buildings that house both year-round residents and visitors by the millions.
Name a community along the coast of East Florida. What image do you associate with that community? Each community may conjure up a unique image: Ocean Drive on Miami Beach, spring break in Fort Lauderdale, beach traffic on Daytona Beach, elegant homes on Palm Beach, Cocoa Beach in the shadow of Cape Canaveral, Amelia Island's golf greens, or laidback Key West at the end of the road. Perhaps your image of choice is a glimpse of what Florida's shores looked like before wholesale development occurred: the dunes on Talbot Island, the wetlands behind Apollo State Park, the natural ecosystem of one of the national wildlife preserves, or an unbroken view of sea and beach as seen from an open stretch of Highway A1A. Collectively, Florida's shorelines are its greatest asset.
And all along the east coast, from Fort Clinch to Fort Taylor, these communities, parks, and preserves share a common trait: they are part of Nature's most dynamic system at the interface between the land and the sea. Our focus will be on this dynamism-the constant and endless change that takes place at the shore-and the consequences we face if we choose to live in conflict with such change. The coastal zone is even more dangerous today after the boom in coastal populations that has taken place over the past few decades.
The history of Florida's development and economy is usually described in terms of "boom" and "bust," and is always closely tied to tourism. The formula of recent and continued change includes the resounding boom of the population bomb! And along with the spiraling population come ever-increasing real estate values and all types of development to serve the growing demand.
To live with the coast rather than simply at the coast, we must understand the geology and climate as well as coastal processes and how humans interact with each factor. Knowledge is the basis of survival, and mitigating (reducing) the impact of storms and shoreline erosion is essential to reducing economic losses at the shore. What lessons can we learn from both natural history and human history?
Geology: The Basis of Environment
The great peninsula of Florida illustrates the principles that geology is a fundamental component of our environment and that all things change. Although the great age of the Earth might lead us to believe that natural change is gradual and not of concern on the human time scale, the geologic record suggests that rapid change is often the rule. The longer history of change also provides the answers to some simple questions. For example, why is Florida a relatively low-lying peninsula?
Figure 1.3 lists events pertinent to Florida through geologic history. Far back in time, the rocks deep beneath the state were once part of a much larger continent that included Africa. That ancient continental mass collided and merged with North America and other land masses to form a supercontinent, Pangea, which subsequently broke up into the present continents. A fragment of ancient Africa remained attached to North America and became the base on which the Florida Platform grew. The distinct Florida peninsular outline is the emergent portion of this much larger Florida Platform (fig. 1.4), a mass of buried limestones (carbonates) that formed in persistent shallow marine environments like those found today in the Bahama Banks and the Florida reef tract. These limestones began forming as much as 145 million years ago, and continued into Pleistocene time in South Florida.
The great carbonate bank was separated from North America by a trough, the Suwannee Channel, which effectively kept the sands and muds eroded off the Appalachian Mountains from reaching Florida (fig. 1.4). Later in time, the trough filled with sediment and the platform became attached to the mainland, allowing quartz-rich sands to move south along both the Atlantic and Gulf of Mexico shores. During the Pleistocene Ice Age the sea level fell during the times of glaciation and rose during the warmer interglacial episodes. When the sea level was lower, one can imagine that more sand was carried down the rivers from the Appalachian Mountains, and then along the coast from north to south, supplying Florida's ancestral beaches with sand. The abundant shells of marine animals also were broken up by waves to become beach sand. These two sources account for differences in Florida's natural beaches. Beaches rich in quartz sand tend to be whiter, finer grained, and firmer, like Daytona Beach in the days of beach racing (fig. 1.5). Shelly sand beaches are less firm, slightly coarser, and brownish tan. Today's beaches often don't have the same character as the original beaches because many are now artificially maintained with sands that are dredged from offshore.
During times when the sea level was rising, conditions were favorable for the formation of barrier islands-emergent sand bodies that paralleled the coast, separated from the mainland by a lagoon, marsh, or mangrove swamp. Some of these barriers migrated landward as the sea level rose, and their migration rates are well within the time frame by which we measure human history. The story of these continuing changes does not bode well for the future of coastal development.
Some of the earlier Pleistocene barrier islands welded onto the mainland or were left high and dry when the sea level fell during the next cold-climate episode. The most recent low stand of sea level was about 18,000 to 15,000 years ago at the end of the Pleistocene, when the sea level was 300 to 400 feet below its present elevation and the shoreline was out at the edge of the continental shelf. Large amounts of water were tied up in the massive glacial ice sheets (like present Antarctica and Greenland) that covered parts of North America, Europe, and Asia. The great sea-level rise that followed, known as the Holocene transgression, was one of the most significant predetermining events relative to the origin of the present shorelines. As the ice caps melted, the postglacial sea-level rise (fig. 1.6) caused the shorelines to migrate across Florida's continental shelf as much as 130 miles to their present positions.
Climatic warming and melting of those icecaps was well under way by 10,000 years ago, and the sea level was rising at a rate of 3 to 4 feet per century until 5,000 to 4,000 years ago when the rate decreased (fig. 1.6). Although scholars disagree on the details, the sea level has continued to rise since then, although at a much slower rate, perhaps on the order of 4 to 8 inches per century. This slower rate of sea-level rise across the low coastal plain favored the formation and development of new barrier islands. Most of these barriers welded onto older Pleistocene islands or the mainland shore, sometimes forming spits. The northernmost two islands of East Florida, Amelia and Little Talbot Islands, are like the Sea Islands of Georgia-composites of modern islands that have merged with Ice Age (Pleistocene) islands. Although much of the remaining coast has an artificial aspect and a different origin, the principles that control shoreline erosion, storm response, and how engineering structures impact the shore are the same all along the East Florida shore.
A hundred years ago, there were 11 natural inlets along the approximately 365-mile East Florida coast between the Georgia state line and Key Biscayne. Today, the same reach has 21 active inlets, although only 5 (Nassau Sound, Boca Chica, Matanzas, Norris Cut, and Bear Cut) are natural, unjettied inlets (figs. 1.7, 1.8, and 1.9). Inlets are more common south of Cape Canaveral, with 14 of the state's 21, including what were once natural inlets at Jupiter, Boca Raton, and Hillsboro (these were later jettied; figs. 1.8 and 1.9). The other inlets were dredged or in many cases blasted through rock, mostly in the 1920s. Almost all are now routinely dredged to maintain artificial channels. A century ago, before these artificial inlets were cut, the equivalent of a 150-mile-long barrier island extended on either side of Cape Canaveral, from Ponce de Leon Inlet south of Daytona Beach to Jupiter Inlet near Palm Beach. This barrier island was the longest in the world, longer than 135-mile-long Padre Island in South Texas.
Palm Beach, the "wealthiest barrier island in America," once sat in front of a freshwater body called Lake Worth. In 1917, Lake Worth Inlet was opened by mule, drag pan, and dynamite. When South Lake Worth Inlet was opened in 1927, Palm Beach became an island.
Lagoons of several origins back the East Florida islands. Behind some islands are freshwater marshes. At one time it was possible to wade through the marshes to get to the islands, albeit with some difficulty. Other open-water lagoons have river or lake names, reflecting their origins. Examples include the Halifax River behind Daytona Beach, the Indian River behind Melbourne Beach, Lake Worth behind Palm Beach, and Lake Mable, which became the Port Everglades turning basin near Fort Lauderdale. Today the Intracoastal Waterway is behind the entire coast and sometimes constitutes the only reason for the designation "island."
Most people would not call the southeastern Florida shore a rocky coast, but in fact, this coast has a foundation of rock covered with a thin layer of sand (see appendix C, ref. 56). Underlying many of the barrier islands and most of the beaches is coquina rock or limestone, hard Pleistocene rock (Anastasia Formation) originally deposited as barrier islands or coral reefs and believed to have formed during the 120,000-year sea-level high. So the current barrier island chain coincides with the ancient barrier island chain. Due to the shallow rocky substrate, the Intracoastal Waterway behind the islands was blasted rather than dredged for much of its length. According to Florida coastal geologist Charles Finkl, the typical thickness of sand over the rock is only 3 to 6 feet. Hutchinson Island, Singer Island, Hillsboro Beach, Fort Lauderdale Beach, Dania Beach, and Miami Beach are examples of rock-cored islands. On Palm Beach a ridge of coral reef limestone forms the spine of the island; it can be seen in a local road cut on the island.
The facts that the post-Pleistocene sea-level rise probably reduced sand delivery from the more northerly sources and that Florida's rivers neither drain high uplands nor carry sand to the coast have serious coastal management implications. Given that the continental shelf has very limited sand reserves, the barrier islands depend largely on the reworking of their own sands, and to a lesser degree on the erosion of shelly sands from coquina rock units such as the Anastasia Formation. Because there are relatively few inlets along the East Florida coast, sands derived from inlet-associated tidal deltas are of only local importance. In effect, the existence of East Florida's beaches depends on limited sand resources of ancient origin-that is, fossil sand. The Atlantic's waves and currents are reworking deposits from earlier times. This geology dictates prudent sand management.
The geologic record illustrates that climate and sea level are in a constant state of flux, and whether one accepts the premise of the greenhouse effect or not, the climate and the level of the sea are changing over both the short and long terms. We need to go back only to previous interglacial warm periods to find that the sea level was 150 feet higher than at present, inundating well over half of the present land area of Florida-an inundation likely to be repeated in Florida's not-so-distant future (fig. 1.10). The geologic lesson is that sea level falls and rises, sediment supplies change, and we should not expect the shorelines to remain as they are today.
Short-term events can have effects just as important as long-term events. Hurricanes and smaller storms, and even the day-to-day wind and waves, reshape beaches, dunes, shorelines, and inlets, obliterating buildings that have their foundations in such mobile features (fig. 1.11). Repeated storms leave platted lots behind in the surf while accreting new ephemeral shores in your former fishing spot!
Excerpted from Living with Florida's Atlantic Beaches by David M. Bush William J. Neal Norma J. Longo Kenyon C. Lindeman Deborah F. Pilkey Luciana Slomp Esteves John D. Congleton Orrin H. Pilkey Copyright © 2004 by Duke University Press. Excerpted by permission.
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