Harold R. Wanless, Randall W. Parkinson, Lenore P. Tedesco
This hindsight is especially important because beginning about 1930, relative sea level has been rising along Florida's shorelines at a rate of 20 - 40 cm/100 years. This is nearly ten times the rate recorded by earlier tide gauge records and by the past several thousand years of geologic history. This increased rate of rise is already exhibiting a significant influence on Florida shorelines and wetlands and will have dramatic consequences within the next century if continued. Further increases in this rate of rise are widely predicted due to global warming, resulting from increases in atmospheric carbon dioxide, methane, and other anthropogenic gases. Higher rates of relative sea level rise will further heighten the implications of future (1) rapid retreat of south Florida shorelines on sandy beaches, mangrove swamps, salt marshes, and marl levees; (2) inundation of low-lying inland areas; and (3) loss of freshwater resources through saline intrusion into ground water. A historical perspective is helpful in appreciating the reality of these conclusions.
This chapter uses a late Holocene sea level curve with hinge points. The form of the curve is derived from the curve of Neumann (1972) for Bermuda, which closely parallels Florida. The specific nature of the curve is based on data points from south Florida by Scholl et al. (1969) and Wanless (1969 and unpublished). As can be seen in Figure 8.1, the latter stages of the Holocene sea level rise over south Florida can be divided into three stages:
MISSING THESE STAGES (will include on December 4th update)
Figure 8.2 contours the intersection of sea level with the Pleistocene limestone surface at 5500 YBP (-6.2 m), 3500 YBP (-1.5 m), and the present (0 m). These contours represent the shoreline positions had there been no accumulation of Holocene sediment over Pleistocene limestone. The shaded areas between these time-depth lines are the zones of the shelf potentially transgressed. This map of potential transgression will be evaluated more closely after the sedimentary environments have been described.
The following critical and fundamental attributes of the south Florida environment influence sedimentation:
Physiography and Coastal Landforms
Southern Florida is a partially inundated limestone platform on which Holocene carbonate, organic, and siliceous sediments of biogenic origin are forming and accumulating and to which quartzose sands are being provided by littoral transport, as well as by dissolution from limestones along the eastern and western coasts. Three shallowly inundated limestone depressions are defined by four topographically high limestone physiographic features. The depressions from west to east are the Everglades, the coastal bays, and the outer shelf depressions; the highs are the Big Cypress Ridge, the Atlantic Coastal Ridge, the Key Largo Limestone Ridge, and the shelf-edge reefal ridge (Figure 8.3), with the shelf-edge reef occurring just landward of the 20-m contour at the edge of the Straits of Florida.
The Everglades depression extends southward from Lake Okeechobee down the center of south Florida and is bounded to the west by exposed Pliocene limestone of the Big Cypress Ridge and to the east by the Atlantic Coastal Ridge, a late Pleistocene quartz sand and oolitic limestone ridge. The broad Everglades depression carries freshwater flow southward from Lake Okeechobee to the sea just west of Florida Bay. The depression is filled with Holocene freshwater peat and calcitic mud deposits. A natural coastal dam of mangrove peat and storm-levee marl limits saline intrusion in the depression to along the axis of channels such as Shark River Slough. The oolitic limestone ridge crests 26 m above sea level and extends north to south from north Miami to Homestead, where it then swings westward and is cross-cut by numerous swales, which are fossil tidal channels. These channels, largely filled with freshwater peat and quartz sand, served as conduits for freshwater flow through the ridge prior to drawdown of Everglades water levels in the early 1900's. These are still pathways for catastrophic storm discharge of fresh water.
The Coastal Bay Depression is bounded to the landward side by the Atlantic Coastal Ridge and to the seaward side by the Key Largo Limestone Ridge, a ridge of Pleistocene coral limestone of which the emergent portions form the present northern and central Florida Keys. The limestone surface beneath the coastal bays is generally less than 4 m and is shallower toward the mainland. Freshwater marshes, mangrove swamps, and coastal storm levees and flats partially fill the Coastal Bay Depression and isolate the various bay units. Florida Bay and Biscayne Bay are further subdivided by marine carbonate mud banks. Exposed limestone floors much of the bay bottom of southern Biscayne Bay, eastern Florida Bay, and the bays between. The Key Largo Limestone Ridge is now submerged along the eastern margin of central and northern Biscayne Bay. The seaward margin is defined by quartzose barrier islands to the north and a carbonate mud-bar belt (Safety Valve) seaward of central Biscayne Bay. Both cap the submerged limestone ridge.
Between the Florida Keys (Key Largo Limestone Ridge) and the shelf margin is a 5- to 10-km-wide outer shelf depression (or lagoon), commonly known as Hawk Channel, whose seaward margin is a Pleistocene reefal ridge. The shelf margin limestone ridge is now well submerged and capped with as much as 20 m of Holocene reefal sediment (Enos, 1977). The present reefal rim is well developed where seaward of an effective island barrier (the Florida Keys), but is poorly and sporadically developed where adjacent to breaks in the Florida Keys and north of Key Biscayne. In addition to the physiographic units defined by Pliocene and Pleistocene ridges, build-up of Holocene sediment generates coastlines and marine banks which define and subdivide the coastal bays and form the effective coastlines to southern and southwestern Florida.
The northern shore of Florida Bay is comprised of transgressive (landward eroding) to regressive (seaward expanding) coastal storm levees that are separated from the mainland mangrove coastline by a line of brackish lagoons (Figure 8.4). Depths to the limestone surface are about 1.5 m at the mainland mangrove fringe (Figure 8.5) and increase seaward to 24 m beneath the coastal storm levee. Cape Sable, at the northwestern corner of Florida Bay, is a complex of shell and sand beach ridges built in front of the relict (former) mouth of Everglades discharge. The coastal bays themselves, especially Florida Bay, are dissected by an anastomosing maze of carbonate mud and mangrove peat banks that have built nearly to sea level. Mud banks are dotted with intertidal to supratidal islands, some of which are relics from an earlier phase of mud bank nucleation on coastal storm levees and some of which are recent caps to marine mud bank deposits (Wanless and Tagett, 1989).
Along the southwestern coast of Florida, the Ten Thousand Islands form a 2-to 4-km-wide complex of mangrove islands, oyster banks, and shallow lagoons seaward of a continuous mangrove belt along the mainland shore. Depths to the limestone surface increase from about 1.5 m at the mainland shore to more than 5 m beneath the seaward margin of the Ten Thousand Islands (Figure 8.6).
Holocene Stratigraphy--Generalizations
Coastal swamp environments are transgressive on the eastern, mainland shore of Biscayne Bay and Barnes Sound. Cores from nearshore areas record a sequence from basal freshwater peat or marl, grading upward to coastal red mangrove (Rhizophora mangle) peat and capped by transgressive marine sands or muds (Wanless, 1976). The mangrove swamp coastlines on the eastern side of the northern Florida Keys are similarly erosionally transgressive, but cored sequences exhibit mangrove peat throughout their entire length.
Some mangrove peat or coastal levee deposits have accreted vertically from their inception, either as a persisting coastal environment (e.g. west shore of Card Sound; Figure 8.5) or as an island or peninsular remnant of such a coastline (e.g., the mangrove peat neck separating Card Sound and Barnes Sound; the marl levee peninsulas and the high islands in northern Florida Bay).
The sedimentary sequences beneath the northwestern coastline of Florida Bay and the Ten Thousand Islands of southwest Florida record shallowing upward sequences during the past 3200 years as rise of relative sea level slowed. In both areas, the shallowing sequences are capped by supratidal deposits. On the northern coast of Florida Bay, this is a linear emergent storm levee of carbonate marl; in the Ten Thousand Islands, this is a complex of mangrove islands capping oyster banks (Figure 8.6) (Parkinson, 1987; Perlmutter, 1982). In western Florida Bay there have also been steps of seaward progradation as storms formed a new storm levee shoreline bayward of the previous position.
The coral-algal banks on the landward part of the shelf lagoon are coarsening upward sequences, the shallower parts of which initiated 250O-3000 YBP. In the Pleistocene deep depression between the shelf margin reef tract and the Florida Keys, some sedimentation began before 5500 YBP, but the bulk of the sediment infill occurred following 5500 YBP. The seaward or outer shelf lagoon sequence is also coarsening upward, with carbonate mudstones in the base of the trough and Halimeda/bryozoan packstone at the top of the lagoonal sequence. Coral-algal packstones and grainstones dominate White Bank, a reef-derived skeletal sand bank extending into the lagoon from the reefal margin (Enos, 1977; Craig, 1983).
15,000-5500 Years Before Present
About 15,000 YBP, the end of the last major ice age was near, and sea level stood about 130 m below present level. At this time the shoreline was about 10 km seaward of the present reefal margin of southeastern Florida. In the distance to the east across the Straits of Florida were the dramatic white cliffs of the limestone plateaus of the Bahamas (now inundated). The shoreline of the Gulf of Mexico was 100 to 200 km westward (across the present continental shelf) from the southwest coast of Florida.
The great continental ice sheets began to rapidly melt 14,000-15,000 years ago. Melt waters poured back into the oceans, causing a rapid eustatic (i.e., real input of water) rise in sea level. Between about 14,000 and 9000 YBP, sea level rose from -130 m (-400 ft) (Fairbanks, 1989) to near about -20 m, at a rate of 2 m/100 years. During this extreme rate of rise, all shorelines and shallow marine deposits either eroded landward or were left behind on the rapidly deepening marine shelf. Imagine developing shoreline property when the sea was rising 30-45 cm (1-1.5 ft) every 25 years and the coastline was migrating landward at a rate of over a kilometer (0.6 mi) every hundred years.
This eustatic rate slowed somewhat between 9000 and 5500 YBP. By 5500 years ago, the rapid input of melt water dramatically slowed, as did the eustatic sea level rise. After 5500 years ago, differences in land subsidence caused important differences in relative sea level change along the coastlines (Neuman et al., 1980; Barnett, 1990). In much of the central Pacific, for example, sea level has remained relatively steady during the past 5500 years. Along the Atlantic coast of Brazil, however, sea level was nearly 2 m higher 5500 years ago than it is today. This relative lowering of sea level has been most dramatic along coastlines that were previously covered by thick glacial ice. As this ice load was removed, the land has gradually rebounded. The coastlines of Alaska, Maine, and the maritime provinces of Canada and Scandinavia are examples of places where rebounding has resulted in relative lowering of sea level.
In contrast, along the Gulf and Atlantic coasts of the United States south of Boston, 5500 years ago sea level was 5-8 m below present level. Using south Florida as an example, this relative rise in sea level appears to have occurred at a decreasing rate, with a hinge at about 3200 years ago. It is important to look at this rise more closely because it has an important bearing on forecasting the future of coastal response to sea level rise.
5500 Years Before Present
At 5500 YBP, sea level was at about -6.2 m. It had been rising at rates greater than 50 cm/100 years, but the rate of relative rise was slowing significantly. Areas that were potentially transgressed prior to 5500 YBP were, in fact, transgressed because little modern sediment build-up occurred (Figures 8.1 and 8.2). Shoreline deposits were thin, narrow, and ephemeral. Significant marine sediment bodies were accumulating only in areas of reefal growth or reef-derived sand influx on and adjacent to the southeastern shelf margin.
5500-3200 Years Before Present
Between about 5500 and 3200 years ago, relative sea level in south Florida rose from about -6 to -1 m at an average rate of about 23 cm/100 years. This rate of relative rise, although much slower than before, was still sufficient to cause rapid shoreline retreat. As seen in Figure 8.2, areas that were potentially transgressed between 5500 and 3200 YBP were, in fact, mostly transgressed because no modern sediment build-up occurred.
The coastal lagoons and estuaries of Florida were inundated during this time by repeated drowning inundation of coastal levees and the persistent retreat of mangrove swamp and cordgrass (Spartina spp.) marsh coastlines. Throughout this period these important coastal wetland communities, where present, formed only a narrow coastal band. Significantly, the character of coastal lagoons and estuaries was rapidly changing as seaward barriers were flooded or eroded, as bay depths increased, and as freshwater input and bay circulation evolved.
Biscayne Bay, a narrow channel prior to 5500 YBP, was largely inundated during this time by persistent erosional backstepping of mangrove peat and quartz sand shorelines (Figure 8.2) (Wanless, 1976). The Key Largo Limestone Ridge, which defines the seaward margin of Biscayne Bay, was extensively flooded, with only Elliott Key remaining as a significant emergent barrier to southern Biscayne Bay.
Florida Bay, emergent at 5500 YBP, was inundated to near its present configuration by 3200 YBP. This occurred through repeated erosion and overstepping of storm levee marl and peat shorelines (Figure 8.2) (Cottrell, 1987; Wanless and Tagett, 1989). Large volumes of carbonate mud and sand, recycled across outer Florida Bay with the transgressing sea, deposited several lines of coastal levees as Florida Bay was inundated. Most of these were overstepped and partially eroded but served as bathymetric features from which carbonate mud banks were subsequently initiated (Figures 8.3 and 8.4).
The Key Largo Limestone Ridge, defining the seaward margin of the coastal bays on the southeastern coast, was flooded in low areas, thus providing inlet passes between Florida Bay and the reefal shelf to the east and opening up much of the eastern margin of central Biscayne Bay.
On the southwestern coast (now the Ten Thousand Islands), Scholl (1966) and Parkinson (1987) have documented that there was only a narrow paralic mangrove band at the coast and that this was retreating landward between 5500 and 3200 YBP. Parkinson (1987) concluded that at about 3200 YBP this narrow, mangrove swamp shoreline was about at the position of the landward edge of the interior bays of the present Ten Thousand Islands (Figures 8.2 and 8.6). The limestone surface is about 1.5 m below present sea level at that position, which was sea level about 3500 years ago. Parkinson's (1987) stratigraphic and paleoecological study of core borings taken seaward of the paleoshoreline concluded that the transgressed adjacent marine environment was deepening and becoming more and more open marine between 5500 and 3200 YBP (Figure 8.6).
Significant persisting shoreline deposits were initiating only in areas such as (a) Key Biscayne on the southeast coast, where southerly littoral drift of sands from the Miami Beach area was focused along the seaward face of a Pleistocene ridge of Key Largo Limestone and Anastasia Limestone, and (b) in areas such as Sanibel Island on the southwest coast, where pre-existing topography was similarly focusing longshore drift. In some areas of southeast Florida, prolific coral reef growth was initiated and produced enough skeletal debris to keep up with relative sea level rise, but in many areas reefs were not growing upward as fast as sea level was rising (Shinn et al., 1989).
Along most of the southern Florida coast, the coastline was several kilometers landward of its present position by 3200 years ago. Had this rate of rise continued, the broad mangrove and Spartina coastal wetlands and the shallow reefal, barrier island, oyster bar, and mud bank environments, so characteristic of today's Atlantic and Gulf coastal system, would not have formed in such an extensive manner and the coastline now would have been tens of kilometers inland of its present position.
3200-60 Years Before Present
About 3200 YBP, relative sea level rise appears to have slowed dramatically; for south Florida the rise averaged only about 4 cm/100 years (equal to less than 2 in./100 years) between 3200 years ago and near the present. This much slower rate of relative rise during the last meter of inundation permitted many coastlines to stabilize or begin expanding seaward and many shallow marine environments to build upward to sea level. The rate of landward migration of barrier islands dramatically slowed, and some stabilized or began growing seaward.
The retreating mangrove, Spartina, and marl levees at the margins of coastal bays stabilized at about 3200 YBP and subsequently have been prograding seaward, especially in Florida Bay and the Ten Thousand Islands (Figures 8.2, 8.3, and 8.4). The low-energy southern and southwestern coastline of Florida prograded several kilometers seaward as mud banks and oyster bars shallowed the marine bottom to sea level, permitting mangroves to take hold. The maze of mud banks within and marginal to Florida Bay also has mostly built up to sea level and set the stage for mangrove colonization.
The marl levees forming the northern margin of Florida Bay stabilized and, in the vicinity of Flamingo and Cape Sable, have been accreting seaward. The shorelines of Card Sound were stabilized by mangroves and, with the continued very gradual rise of sea level, have accreted upward to produce a marginal mangrove peat "dam." This dam has permitted a landward-expanding freshwater marsh to form behind (Figure 8.5). Although this dam is cut by natural channels and sloughs (such as Shark River Slough and Taylor Slough), there is sufficient freshwater head during much of the year to inhibit saline intrusion, and during the dry season saline intrusion is limited to areas near these sloughs.
In the Ten Thousand Islands, the mainland mangrove shoreline stabilized about 3200 YBP and has accreted vertically and expanded seaward since. In addition, the shallow nearshore environment became colonized by oyster banks which in turn were colonized by mangroves. This process has gradually extended this mangrove coastline 2-3 km seaward (Figure 8.6).
Importantly, this period of slowed relative sea level rise permitted coral reefs seaward of the Keys to catch up to sea level, except where water exchange with coastal bays through newly flooded inlets caused reef demise (Ball, 1967). Had the pre-3200 YBP rate of rise of 23 cm/100 years continued, the shallow reefs, sand and mud banks, barrier islands, oyster bars, and paralic (marginal marine) swamp environments and deposits seen today would not have formed in such an extensive manner. These broad coastal environments are all the product of the very slow sea level rise (4 cm/100 years) during the past 3200 years. Only along the sediment-starved shorelines of southwestern Biscayne Bay and northeastern Florida Bay did mangrove shorelines continue to gradually retreat.
All in all, over the last 3200 years the Gulf and Atlantic coastal and nearshore marine environments have been shallowing and the coastlines stable or expanding. Around the southern tip of Florida, an extensive natural coastal dam was built during this time by the coastal mangrove peats and storm levee marls. This wetland "dam" separates the landward freshwater environments from the sea.
Hindsight Uncertainties
Recent research from regressive coastlines (Brazil) and from intensely studied transgressive coastlines (Netherlands) indicates that the late Holocene contains a series of high-frequency sea level oscillations (either a rapid lowering followed by a rapid rise or stillstand followed by a rapid rise, 200-400 years in duration and 0.5-2 m in amplitude) superimposed on the overall trend of sea level.
In several parts of the world, relative sea level has been dropping for the past 5000-6000 years. Such a relative lowering has occurred in Brazil and has produced broad regressive strandplains of prograding beach ridges, emergent lagoons, and elevated skeletal encrustations of intertidal and subtidal marine organisms on rocky coasts (Martin et al., 1985; Dominguez, 1987). Radiocarbon dating of these lagoonal peats, shells, and intertidal skeletal encrustations indicates that there have been two or more high-frequency sea level oscillations during the past 5000 years (Dominguez et al., 1987). In Brazil, high-frequency sea level oscillations occurred about 4100-3800 YBP and 2900-2500 YBP (Martin et al., 1985; Dominguez, 1987). They appear to have had a duration of 200-400 years and an amplitude of 1-2 m. The oscillations consist of a rapid lowering followed by a rapid rise of sea level.
Morphostratigraphic reconstructions of Brazilian strandplain deposits indicate that high-frequency sea level oscillations have caused direct and predictable responses in the coastal system. The response on the strandplain coasts (coasts with seaward prograding sets of beach ridges) is strandplain progradation followed by a transgressive barrier island migration phase (Dominguez, 1987; Dominguez et al., 1987). These high-frequency oscillations occur within an overall gradual lowering of relative sea level between 5500 YBP and the present. The high-frequency sea level oscillations are visible in the Brazilian situation because the overall regression is leaving behind a visible, exposed, prograding strandplain record of the late Holocene.
In contrast, the late Holocene sea level of the Gulf and Atlantic coasts of the United States has continued to have a relative rise: transgressing, inundating, eroding, and obscuring the previous deposits. Deposits that may have been initiated or formed during brief stillstands or sea level lowerings are mostly modified and their origin at least somewhat obscured by the following rise in sea level and associated transgression. To date, most coastal and shallow-marine sediment bodies along the Atlantic, Gulf, and Pacific coasts of the United States have been thought to initiate (1) in response to a gradually decelerating rate of sea level rise or (2) at hinges in the decreasing rate of sea level rise. DePratter and Howard (1981), studying the Georgia coast, offered geological and archeological evidence that there was at least one significant lowering of sea level within the late Holocene transgression sometime between 3000 and 2400 YBP. They use radiocarbon dates of in-place tree stumps to suggest that sea level dropped from -1.5 m to about -4 m below present level during this period.
Detailed work in northwestern Europe (Morner, 1980) and the Netherlands (van de Plassche, 1982) showed that the late Holocene has been a period of continued relative sea level rise at a decelerating rate. Their research, however, also suggested that there are high-frequency sea level oscillations superimposed on this decelerating rise. In some curves, these oscillations are illustrated as relative sea level lowerings (Morner, 1980) and in others as periods of slowed rise or stillstand (van de Plassche, 1982). Some authors, however, caution that it is presently extremely difficult to recognize minor cycles within the overall Holocene (Kidson, 1986).
The coastal and associated environments of northwestern Florida Bay are individually and collectively very sensitive recorders of environmental change. There is a suggestion that a regression in sea level occurred between 3000 and 2400 YBP. The rapid coastal progradation following this period demonstrates that the coastal system had large volumes of sediment toward the end of the potential sea level oscillation.
The core borings and radiocarbon dates that have been obtained from the coastal complex adjacent to northwestern Florida Bay and in the Cape Sable area indicate that the surficial strandplain on Cape Sable was well into its regressive phase of sedimentation at 2200 YBP and had completed forming (as a quite rapid burst of prograding beach ridge sedimentation) by about 1200 YBP (Roberts et al., 1977). This burst of regressive sedimentation is what would be expected following a rapid transgression, as sediment bodies in disequilibrium following sea level rise are reworked and sediment is transferred to more stable sites. Cross sections by Roberts et al. (1977) across Cape Sable and across the seaward shore levee on northwest Florida Bay suggest that this progradation took place across a surface 1.5 to 2.0 m below present sea level.
The stratigraphic sequences of Roberts et al. (1977) through Cape Sable and those of Spackman et al. (1964) through environments adjacent to Whitewater Bay are shown as shallowing upward sequences, but the stratigraphic sequences and radiocarbon dates provided are very compatible with a regression between 2900 and 2500 YBP. In fact, freshwater calcitic mud dates in that interval occur well below the smoothed sea level curve of Scholl et al. (Scholl et al., 1969; Scholl, 1964a, 1964b).
The cross sections of Roberts et al. (1977) are very similar to those of Parkinson (1987) for the Ten Thousand Islands area farther west on the southwest Florida coast. Both record a halt in the landward migration of coastal environments before 3000 years ago, but find that most of the coastal progradation and marine shallowing has occurred since 2500 YBP.
Tide gauge records from south Florida document a dramatic increase in the rate of relative sea level rise beginning in about 1930 (Wanless, 1982,1989). For Key West, this increase is from a prior rate equivalent to about 3 cm/100 years to a post-1932 rate equivalent to about 38 cm/100 years (Figure 8.7). The tide gauge records for Miami and Naples (which begin in the 1930s and the 1950s, respectively) show comparable trends and rates during the years since 1932.
Similar increases beginning in about 1930 are recorded in tide gauges along the Atlantic, Gulf, and Pacific coasts of the United States (Wanless, 1989). Analyzing tide gauge records throughout the world, Peltier and Tushingham (1989) concluded that there has been a global eustatic rise in sea level of 2.4 mm yr-1 since 1920 and that at least 75% of that rise must be explained by glacier and ice sheet melting. This six- to tenfold increase in the rate of relative sea level rise in southern Florida has persisted for 60 years and has already resulted in a relative rise of over 20 cm. This has initiated dramatic changes in the character of coastal wetlands and estuaries and has set the stage for dramatic (storm-driven) future modifications in coastal deposits.
The rate of relative sea level rise since 1930 is faster than the rise that occurred between 5500 and 3200 YBP. The lesson from that period is that, with few exceptions, all types of coastlines steadily eroded and retreated landward. Because this landward backstepping tends to occur in storm-driven steps, an immediate response to this recent increase should not be expected. Thus, it should not be surprising that this recent rise is accurately expressed by the upward migration of oysters and barnacles on the seawalls, but is not yet extensively shown by dramatic shore retreat.
Future Coastal Response
The simple conclusion is that if relative sea level rise continues at its present rate, the sandy, mangrove, and levee coasts of south Florida will erode at an accelerated rate, low-lying freshwater wetlands will become saline, and increased saltwater intrusion will diminish freshwater resources. The lesson from the geologic record is that any prolonged rate at or faster than 23 cm/100 years will lead to rapid and complete coastal erosion. This erosion includes landward migration and storm dissection of sandy barrier islands, erosion of coastal wetlands at a faster rate than they are expanding landward, and deepening and increased marine influence in coastal bays and estuaries. Coastlines along northeastern Florida Bay, which have demonstrated instability during the past 3000 years, should respond first and most dramatically.
The tenfold increase in the rate of sea level rise during the past 60 years and the projection for continuation of that rise must be considered a minimum. All indications are that there will be a further dramatic increase in the rate of sea level rise within the next century in response to global warming, caused by the buildup of carbon dioxide and other gasses in the atmosphere. The U.S. Environmental Protection Agency, for example, forecasts that by the year 2100, there will be an additional eustatic rise in sea level of 55-335 cm (average 150-210 cm).
For south Florida, all this could be turned into a simple forecast for the future, except for two problems: the limestone topography of south Florida is complicated and the capability for rapid upward growth of its natural coastal dam of mangrove peat and levee marl is not certain. The Everglades depression is just that-a low-lying swale between the limestone and quartz sand Atlantic Coastal Ridge (on which Fort Lauderdale, Miami, and Homestead are situated) and the limestone ridge associated with the Big Cypress Swamp. The limestone surface is less than 1 m above sea level in much of the southern Everglades drainage basin. Additionally, both the Atlantic Coastal Ridge and the Big Cypress Swamp Ridge are dissected by numerous peat-filled swales or channels. If the coastal dam is eroded back or overstepped, these swales and the Everglades depression itself may become saline. As this happens, remaining fresh groundwater resources will be further stressed, and undoubtedly there will be accentuated saltwater intrusion beneath the remaining upland areas. The swales and channels dissecting the ridges will encourage saltwater intrusion, probably by landward jumps as individual ridge segments are effectively breached.
An important difference between the present phase of accelerated sea level rise and that of 5500 to 3200 YBP is that the present follows a prolonged period of growth of coastal sediment bodies creating large coastal and nearshore sediment reservoirs. For example, the volume of mangrove peat, levee marls, and lagoonal muds spread across a coastal complex several kilometers wide at the northern margin of Florida Bay, even if unstable, will take time to rework.
Using the knowledge available, the following forecasts can be made:
1. With a continued rate of relative rise of sea level of about 30 cm/100 years, nearly all south Florida coastlines will become erosional. The mangrove coastline should continue to build upward, maintaining a coastal dam, but the seaward mangrove margin will be eroded. The landward margin of the saline wetlands will migrate landward; coastal bays will become deeper and more saline, and breaching of new inlets through the Florida Keys will further diminish reef growth.
2. With the rate of relative sea level rise increased to 60 cm/100 years, the coastal marl and wetland dam will become overstepped in numerous places, causing rapid landward encroachment of the sea into the Everglades wetlands. In association, there will be rapid loss of transitional and freshwater habitats, extensive saltwater encroachment, and increased water levels in the adjacent freshwater wetlands. Physical erosion of the transgressed coastal peats and marls will increase turbidity and nutrient levels to the coastal bays. Erosion of mud banks within and opening of inlets into the coastal bays will occur. Bay waters will become less restricted but more turbid, and the bay bottoms will become deeper, with significant changes resulting in benthic marine communities. Recall that minimum U.S. Environmental Protection Agency forecasts are for 55 cm in the next 110 years in addition to the current rate (30 to 40 cm/100 years).
3. Rates of relative sea level rise greater than 90 cm/100 years will produce catastrophic inundation of southern Florida, loss of coastal wetlands, and loss of freshwater resources.
Role of Catastrophic Events Large catastrophic hurricanes cause the real changes in shoreline configuration and loss of coastal wetlands; sea level rise just sets the stage.
Hurricanes As south Florida had not been subjected to a hurricane event between 1965 and 1991, the opportunity for major storm modification had not occurred until very recently (see discussion of Hurricane Andrew). Each area in south Florida should be affected by hurricane-force winds once every 7.5 years (Neumann et al., 1978). Hurricanes can affect shorelines of all exposures and can significantly affect interior lakes and ponds.
The Great Labor Day Hurricane of 1935 and Hurricane Donna in 1960 swept across Florida Bay and the Everglades, causing major modifications (Craighead, 1964). These storms caused significant loss of exposed mangrove shoreline erosion of the margins of interior lagoons and ponds, and major erosion of mangrove flats adjacent to deeply penetrating channel systems (Figures 8.8 and 8.9). Much of these eroded areas are now permanently marine. With higher sea levels, these steps of storm erosion will become greater and greater as storm frequency, intensity and level of inundation increase significantly, with global warming the tropical Atlantic should warm, encouraging hurricanes to become more frequent and more intense (Barron, 1988).
Effect of Hurricane Andrew on Coastal Evolution
Hurricane Andrew swept across south Florida on August 24, 1992. The storm moved east to west across south Florida, with the center of the eye crossing Elliott Key, Turkey Point nuclear power plant, Homestead, and the Everglades at Lostman's River. The eye contracted to about 20 km (12 mi) in diameter as the storm approached shore, and the general winds in the eye wall increased to about 235 km hr-1 (145 mph). There were, however numerous vortices within the eye wall that generated winds at or above 320 km hr-1 (200 mph). The storm moved onto and across south Florida extremely rapidly, at a forward velocity of 35 km hr-1 (22 mph). As a result, the coastal environment was subjected to intense but very brief wind and stormwaves, currents, and surge.
This was the first hurricane to impact south Florida in 26 years and affords an opportunity to consider some of the predictions of storm-generated modifications during a time of rising sea level. Two environments will be considered--beaches and mangrove coastlines.
Sandy Coastlines
Key Biscayne (on the east coast) and Highland Beach (on the west coast) were the only beaches which received the full force of the storm. Initial overflights of the area led some scientists to conclude that there was little erosion of the Key Biscayne shoreline. Key Biscayne received a building north-to-south longshore current followed by a brief strong onshore surge which increased southward from 2 to 4 m (6 to 12 ft). The longshore current first swept a large volume of nearshore sand to the south. The onshore surge then swept a large volume of sand landward from the beach onto the island and reprofiled the beach into a very gently sloping storm ramp which intersected sea level at about the same position as before the storm. Since the storm, prevailing easterlies, tropical waves, and winter storm waves and swells have dramatically steepened the beach face and resulted in 20-30 m (65-100 ft) of erosion to an initially 60- to 80-m-wide (195- to 260-ft) beach (renourished in 1986). This is a dramatic stepping back of this shoreline.
The storm exited Florida along the broad mangrove forest between Cape Sable and Everglades City in Everglades National Park. Here, the south side eye wall, although moving offshore at more than 33 km hr-1 (>20 mph), generated a brief but intense onshore surge that reached +4 m (13 ft). This surge generated 5 to 15 m (15 to 50 ft) of landward migration of the narrow quartz sand and coarse shell beach ridge on Highland Beach and left a very gently sloping beach profile similar to Key Biscayne. The onshore surge swept a carbonate storm layer through the broad mangrove swamp and washed an organic and leaf-litter layer into the interior bays. The swamp storm layer was commonly capped with stranded dead fish, frequently in densities of more than one fish per square meter.
As the storm passed, the impounded surge water, which had built up in the broad mangrove swamp, surged back seaward, dissecting Highland Beach with more than 20 ebb channels, each with an ebb-tidal delta spread across the shallow seaward platform. This breaching has severely weakened the beach ridge as an effective shoreline barrier, separating the interior mangrove swamp from wave and tidal processes of the marine environment. (Hurricane Donna caused similir flooding in 1960, but not as extensive channel breaching.) The ebb surge also created thick mud deltas from side creeks feeding into the large penetrating tidal channels.
Mangrove Coastlines
Over 28,329 ha (70,000 acres) of mangrove forest was destroyed by Hurricane Andrew. Most of this destruction was by wind stress on the taller forests. The onshore surge, reaching 5.2 m (16.9 ft) on the east coast and about 4 m (13 ft) on the west coast, also flattened many trees (Figure 8.10). The storm surge and waves, however, caused less than 15 m (50 ft) of erosion at the shoreline. Again, at first glance, the hurricane appeared to have caused little erosion to the shoreline environment.
The 28,000 ha of flattened tall forest was mostly adjacent to the coastline or tidal channels. In these areas, 90-100% of the trees (20-30 m (65 to 100 ft] in height) were either uprooted or broken at the lower trunk and in either case killed (Figure 8.10). Although former storm layers are covered with coarse charcoal (the next step in the taphonomy of this forest), in times of rising sea level, the widespread flattening of mangrove forests can be recognized as a step in transgression. It is very likely that much of this downed forest will convert to a marine environment. The once flat swamp surface is now tortuously rugged with over 1.5 m (5 ft) of relief from widespread uprooting. This has resulted in stagnant ponds and supratidal patches. Over the next 5 to 10 years, marine processes will modify portions of the downed mangrove community substrate and may inhibit recolonization by mangrove seedlings.
Similar to Andrew, Hurricane Donna in 1960 caused some coastline erosion, but the main damage was destruction of the interior mangrove forest. Hurricane Donna generated an onshore surge that penetrated far into many of the tidal channel complexes of northern Cape Sable. This surge flattened the mangroves in a 50- to 100-m-wide band (165- to 325-ft) adjacent to the channels and extending 1-2 km (0.61-1.2 mi) into the swamp. Since that storm, these flattened mangrove forests have evolved into a deepening intertidal to subtidal environment in which mangrove seedlings have not been able to recolonize (Figure 8.11), and marine burrowing and erosion processes are gradually removing and deepening the peat substrate. The present rapid rise of sea level is helping to assure that the stormeroded areas are not recolonized as mangrove swamp, but evolve into deepening, expanding bays.
Hurricane Andrew and Hurricane Donna thus show that the historical loss of wetland habitat and evolution to marine conditions is primarily due to the loss of interior wetland damaged by hurricane winds and surges. By watching only for coastline setback, the initiation of a major transgression may be overlooked.
Hurricanes are causing significant changes in coastal environments. With an increase in the rate of sea level rise, these steps can be expected to become more dramatic and less reversible.
Winter Storms
Each winter, 30 to 40 cold fronts pass through south Florida. Effective erosional wind waves are from the north and northeast following frontal passage (Wanless et al., 1989). Winter storms thus cause persistent erosion of north- and east-facing shorelines if there is not an adequate supply of sediment. The north- and east-facing shores of islands in Florida Bay, of bay margins, and of the smaller brackish lakes in the southern Everglades complex all tend to be erosional. With increased rates of sea level rise, the rates of erosion will increase.
Response
There is little that can be done at a local, state, or national level to prevent future relative rises of sea level. Proper education and legislation are necessary to adopt wetland and coastal management strategies that will accommodate the inevitable modifications to the environment.
A dike cannot be built around south Florida to keep the rising water level at bay, because the sand and limestone substrate is much too porous. Planning is the only way to minimize impact. Some of the key planning elements are as follows.
First and foremost is the need for planning and programs to protect freshwater surface and groundwater resources-in terms of both availability and quality. What effect will a 0.5-, 1-, or 2-m sea level rise, with various scenarios for coastal retreat and wetland evolution, have on freshwater resources? Are these areas presently being zoned and managed to assure future availability and quality of water?
Regional and local zoning and policy for residential and commercial development, waste dumps, wetland management, and coastal modification should be completely re-evaluated in light of various 50- to 200-year scenarios for relative sea level rise and coastal retreat. Building codes, stormwater discharge, and flooding management should be similarly re-evaluated. It must be emphasized that many low-lying inland areas of south Florida will be as seriously impacted by a 1-m sea level rise as will the coastal zone.
It will be necessary to carefully evaluate projected changes in coastal circulation, salinity, environments, and marine habitats that will result from various projected rates of sea level rise and then to re-evaluate the economic future of the various marine resources and development programs. Any forecast planning must be based on coastal response forecasts rooted in firm knowledge of how coastal environments will, in fact, respond and interact. This is presently inadequately understood, as are the details of sea level for the past several thousand years, on which hindsight knowledge for forecasting will be based.
The cost effectiveness of the various shore protection/management programs for sandy shorelines needs careful re-evaluation in light of the present increased rates of relative sea level rise, especially beach nourishment versus relocation.
It is important not to overreact to short-term (less than decade scale) fluctuations in relative sea level or climate. Most importantly, however, it is time to begin obtaining the necessary background information from which to make useful forecasts and to begin serious re-evaluation of coastal regulations, management, and policy. This is not yet a crisis, and planning can be done in a logical progression and rational atmosphere. In 30 years, that may not be possible.
The Past
The broad coastal wetlands and the broad freshwater marsh of south Florida are a result of the very slow relative rise of sea level during the past 3200 years. Prior to that time, relative sea level was rising at a rate of 30-50 cm/100 years (3-5 mm yr-1)--too fast for coastal swamp communities to dominate the south Florida coastal environment. Beginning about 3200 years ago, this relative sea level rise slowed to less than 4 cm/100 years (<0.4 mm yr-1) This slowed rate permitted shallow marine sediments and organic coastlines to build upward. As shallow marine sediments caught up with sea level, coastal mangrove swamps prograded seaward across them. The resulting broad, low-gradient coastal swamp has provided a natural barrier to marine waters and has permitted the freshwater environments of the Everglades to spread seaward behind and on top of the coastal swamp deposits. Swamp coastlines continued to erode only in inner Florida Bay and on the southwest coast of Biscayne Bay.
The Present
Tide gauges throughout the United States record a dramatic increase in the rate of relative sea level rise beginning about 1930. In the following 60 years, the relative sea level rise for south Florida has averaged 30-40 cm/100 years (3-4 mm yr-1).This rate is nearly 10 times that of the past 3200 years. As a result of this increased rate of rise, the following changes are taking place in the coastal wetland environments: (1) relative sea level has risen 18-24 cm; (2) marine waters have encroached significantly landward, setting back freshwater marsh communities; (3) surficial sea water has encroached further landward seasonally and during storms, affecting freshwater communities and soils; (4) a setting has been created for major hurricanes to cause increased erosion and saltwater invasion; and (5) marine waters can penetrate further landward in natural and artificial channels, causing increased saltwater intrusion into the ground waters.
The Future
If the historical rate of sea level rise for the past 60 years continues (and all forecasts are that the relative rate of rise will be much greater), the coastal mangrove swamp will erode at an increasing rate, both along the shoreface and along penetrating channels and creeks. The erosion will occur in dramatic hurricane pulses. This will narrow and dissect the protective coastal swamp that presently serves as a barrier between marine waters and the freshwater marshes. If future rates of relative sea level rise are much faster than at present, marine waters can be expected to penetrate far into freshwater environments as tidal sheet flow. In addition, storm erosion will be increased further. Because the limestone substrate to the main axis of the Everglades (Shark River Slough) lies near or below present sea level as far north as Alligator Alley, there is the strong potential for catastrophic loss of freshwater wetlands and diminished fresh groundwater resources during the next 100~200 years.
This chapter is adapted from articles by Wanless and Parkinson (1989) and Wanless (1989). Research funding is in part from National Science Foundation Grants EAR-77-13707 and EAR-92-24480 and National Park Service Grant 5280-2-0990. The authors thank National Ocean Survey for providing tide gauge data.
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