Sea level rise and its impacts

The Chesapeake Bay, like all estuaries, owes its existence to global sea level rise. It was formed as the Susquehanna River valley was drowned as water poured back into ocean basins beginning about 18,000 years ago when the great ice sheets of the last glacial ice age (Wisconsin in North America) began to melt. The period between 18,000 and 12,000 years ago (late Glacial) witnessed most of the 130 m global rise in sea level that occurred as the continental ice sheets retreated worldwide. At this time, a series of ancestral Chesapeake bays transgressed across the outer and inner continental shelves from where the ancient Susquehanna River emptied directly in the Atlantic Ocean basin during the Glacial Maximum. The annual rate of sea level rise clearly varied over the centuries during this early phase, but, overall, likely averaged at least 2 cm per year (2 m or 6.6 ft per century), or

10 times the global trend of the latter half of the 20th century (see Douglas et al, 2001).

It is difficult to imagine what these proto-Chesapeake bays looked like since erosion of the shelves as sea level rose removed all but occasional traces of their sediments, even after sea level reached the inner continental shelf and the rate of global sea level rise had slowed considerably (Kraft et al, 1987). It is certain, however, that they must have appeared very different from the Chesapeake Bay we see today. For one, coastal marshes and sea grasses would have largely been absent, as the best estimates available suggest rates of sea level rise around 1 m per century may be the upper limit that they can survive. It is also almost certain that rates of shore erosion and land submergence were probably such that every decade saw wholesale changes. The paucity of preserved sediments from this period attests to the extraordinary beveling of the coastal features and sediments.

The little information that has been gleaned about ancestral Chesapeake bays of the late Glacial period gives indications of what a very rapid sea level rise - on the order of meters per century - could mean for the present bay: an impoverished system with large gaps in the range of ecosystem services that it now provides. The impacts on people around the Bay's shores (and for a considerable distance away from those past shorelines) are without historical precedent and can only be imagined.

The potential for such catastrophic change in the future cannot be considered totally out of the realm of possibility. What if outlet glaciers of the Greenland Ice Sheet become unstable, leading to rapid thinning of this major ice mass, and there is a concurrent wholesale ablation of the West Antarctic Ice Sheet (WAIS)? Though the 2007 report of Working Group I of the IPCC projects rates of rise of global sea level during this century of between 23 and 38 mm, this projection is based on largely steric (thermal or volume) expansion of ocean water down to perhaps 1000 m and includes relatively limited additions of water mass from glacial melting. The estimates also do not allow for any major melting of the largest ice masses on the planet, despite recent radar and LIDAR data suggesting much more rapid surface ablation than has been predicted (see Chapter 6). The prospect of massive melting of the largest ice masses on the planet is indeed disquieting.

At present, there is no telling whether the estimates for global sea level rise will ultimately prove far too conservative. We can, as an initial analysis, evaluate the potential impacts of a 23-38 mm (9-15 inches) rise in global sea levels for Chesapeake Bay. Over the last millennium, climatic conditions in this region have shifted from the cold of the Little Ice Age - an obvious candidate for perhaps the coldest period in the last 5000 years - to a period of warming beginning around the mid-19th century, which most scientists agree today continues to be driven increasingly by anthropogenic greenhouse gases. Though there exists no universal curve for the sea level record of the past 1000 years - unlike the 'hockey stick' for global temperatures of Mann et al (1999) - sufficient data are available (see Kearney, 2001) to portray a period when steric contraction of surface ocean waters and growth of mountain ice masses (mainly in the Northern Hemisphere) produced a flat, if periodically falling, sea level. In Chesapeake Bay, sea level stood approximately 70 cm (2.3 ft) below present about the time of the beginning of the Little Ice Age (c. 1300-1450 AD) (Kearney, 1996). Prior to about 1850 AD, sea level in the Bay rose only around 35 inches over a period going back to the early 14th century (about 500 years). Within the last 150 years, it has risen a little over 40 cm (16 inches) (Kearney, 1996), almost its rise since the first half of the past millennium. Since 1960, the trend has been particularly steep (Kearney et al, 2002). The overall sea level trend for the Chesapeake Bay for the last 1000 years is shown in Figure 7.1, and the trend since 1900 in Figure 7.2.

Submergence, shore erosion, storm flooding and waves

The upshot of the changes in Bay sea level rise during the last 150 years has, in many cases, been a dramatic transformation of shorelines and once distinctive islands. Clearly, the first Europeans living in the region would see a Chesapeake Bay that has been obviously changed. Physical and cultural landmarks they once knew would in many cases have disappeared; large islands once supporting plantations known for generations have vanished (Kearney and Stevenson, 1991); elsewhere, shorelines have migrated roughly 100 m (300-400 ft) landward of former positions; and former upland forests have now become marshlands (Figure 7.3 illustrates the fate of Sharps Island). In fact, it is likely that their grandchildren living in the middle 19th century were probably the first people to have witnessed the onset of changes wrought by rising sea level, though the rate of change may have been slow at first as sea level rise only really began to accelerate after 1850 AD, the conventional end of the Little Ice Age. Since then, sea level has continued to rise globally; most scientists are now in agreement that anthropogenic greenhouse warming during the 20th century has contributed significantly to this trend. In the Chesapeake Bay, rates of sea level rise are presently the highest they have been for at least 1000 years (Kearney, 1996) (see Figure 7.1).

Because the Chesapeake Bay region is subsiding (having been pushed upwards 20,000 years ago by the vast continental ice sheet to the north), it is especially vulnerable to increases in sea level. With a 'built-in' sea level rise of 1.6-2.0 mm per year (Kearney, 1996) - about equal to the present global rate (Douglas et al, 2001) - sharp upward departures in world sea level will become magnified along the shores of Chesapeake Bay. For the low-lying shorelines of the Eastern Shore of the Bay, outright submergence will be the fate of many areas. From southern Dorchester County in Maryland down to the Virginia Eastern Shore, land elevations rise almost imperceptibly away from the mean high water mark. It is not uncommon to be only a foot above mean tide level 2 km (over a mile)

Figure 7.1 Reconstructed sea level rise in the Chesapeake Bay over the last millennium from various radiocarbon dates and other data

Note: The black line shows the general trend extended through the latter half of the 21st century based on the mean sea level rise projections in the Fourth Assessment Report of the

IPCC.

landward of mean tide level. The physical expression of this nearly flat coastal profile is an extremely broad intertidal zone that is characterized by the largest marsh systems in the Bay. The 'rise to run' of such profiles can be small, 1:2500, meaning that you rise 1 unit above mean sea level for every 2500 units away from the shoreline. It is thus easy to see how vulnerable such a coast is to the possible

Figure 7.2 The Baltimore tide gauge data for sea level change in the Chesapeake Bay during the 20th century

Year

Figure 7.2 The Baltimore tide gauge data for sea level change in the Chesapeake Bay during the 20th century

Note: The superimposed lines on the general curve show the approximate mean trend for different time periods. Note the steep trend for the last 40 years. Source: Kearney et al, 2002.

global changes in sea level rise proposed in the Fourth Assessment Report of the IPCC. It is also not surprising that some of the first LIDAR surveys for detailed elevation estimates undertaken in Maryland focused on the low-lying Eastern Shore. It is here where the greatest change from an accelerated rate of sea level rise resulting from global warming will occur. There is a realistic possibility that the shoreline in some areas could migrate over 1 km (roughly a mile) from its present position (see Figure 7.4, Plate 7).

For the high cliff shorelines of the western shore and the low cliff shorelines of the northern eastern shore, greatly accelerated shore erosion will be the main consequence. Many people remain incredulous upon being told that Bay shorelines are eroding much faster than on nearby barrier islands, often a difference of meters per year for the Bay compared to a few centimeters per year for Assateague Island. The incredulity stems from the fact that observations indicate that open

Figure 7.3 Changes in Sharps Island since the 17th century

Note: The island, once a prominent feature at the mouth of the Choptank River on the Maryland Eastern Shore, was the site of a large hotel as late as 1900—1910. The photograph shows the remnants of the island in about 1950. Today, all that remains is a shoal, with only the late 19th century caisson Sharps Island Light to mark its passing. The arrow shows the island's depiction on a late 17th century map, with a symbol for a plantation. Source: Stevenson and Kearney, 1996.

coast should be eroding faster since storms there are often more ferocious and their waves much bigger than those in the estuary. But, such observations do not take into account that along the open coast the large storm waves of winter are countered by the long swell waves from the southeast in summer, which move back on shore much of the sediment eroded earlier in the year. In estuaries this does not happen. Therefore, while the waves in the Chesapeake Bay are generally much smaller, reflecting its relative shallowness and narrow width (fetch), they are much more effective at eroding the coastline.

On barrier islands along the open coast, the Bruun Rule provides a useful estimate of how much shore erosion is likely to occur with sea level rise. Zhang et al (2002) demonstrate that the Bruun Rule predicts a 1:150 ratio for recession of the shoreline from sea level rise. In essence, a 1 m rise in sea level would result in a 450 m (about 1500 ft) retreat. The Fourth Assessment mean expected increase

Figure 7.4 Mean elevations above mean sea level for the Chesapeake Bay region, showing the likelihood that areas will be flooded for different levels of sea level rise induced by global warming (see Plate 7 for color version)

Source: Courtesy of J. G. Titus. See also Titus and Richmond (2001).

of 17 cm, adjusted for subsidence in the Bay region, thus suggests that Assateague Island could sustain about 42 m (about 139 ft) of shoreline retreat this century from erosion, regardless of submergence.

While this is astounding enough, it is likely that the same 1 m rise would produce a substantially greater amount of shoreline retreat from erosion in Chesapeake Bay. Unfortunately, the Bruun Rule simply does not work in estuaries, or along coasts composed of materials other than sand - both of which characterize the Bay. However, if the figures of 30-200 m of retreat from erosion in the Chesapeake Bay since 1850 AD, with an approximate rise of 0.3 m, are scaled up for a rise of 37 cm (the mean IPCC estimate plus mean subsidence rate of 2 mm yr-1 for the Chesapeake Bay), erosion of 37-240 m (122-792 ft) could occur in the Bay.

The heightened rates of shore erosion that will occur with accelerated sea level rise in the Chesapeake Bay will not only threaten structures within the potential zone of erosion, but will also make flooding and wave damage to property close to the shoreline much more likely, even though the properties are now relatively secure from storm impacts. Potentially damaging storms include not only hurricanes and nor'easters that make landfall within the Bay region, but 'backdoor' storms that come ashore in the eastern Gulf of Mexico and transverse up the spine of the Appalachians, passing out to the Atlantic Ocean across the Chesapeake Bay. Such storms are far more common in the Bay than people realize (Stevenson and Kearney, 2005). In fact, the flood of record within the last 40 years in the estuary resulted from Tropical Storm Agnes in 1972, a large Gulf of Mexico hurricane that moved up the Appalachians into the Susquehanna River drainage basin; it dumped so much rain that the flood wave eventually submerged downtown Norfolk under 1.8 m (6 ft) of water (US Army Corps of Engineers, 1990).

An additional large liability of becoming ever closer to the shoreline as erosion occurs is to come within striking distance of waves. At the very least, being within the zone of wave impacts will increase flooding risk as a result of both how far floodwaters penetrate landward and increasing water depth. Two processes are involved: wave run-up and wave set-up. Wave run-up is by far the most important, allowing storm waves to reach farther landward and even overtop shore protection structures like bulkheads. However, it is the power of large waves that is devastating. Large waves are one of nature's most powerful phenomena, exerting forces both due to their mass (hydrostatic force) and momentum (hydrodynamic force). The power of large waves was indelibly demonstrated by the Galveston Hurricane of 1900 when 9-12 m (30-40 ft) waves came crashing over Galveston, probably moving close to 50 miles per hour. The storm surge of around 5 m (17 ft) facilitated the waves reaching the island, although by itself, it would not have caused the tremendous destruction and loss of life produced by the storm, because the rise in sea level was only about 1.5 m (5 ft) higher than average elevation of the center of Galveston (see Figures 7.5 and 7.6).

Figure 7.5 Hearst newspaper reconstruction of Galveston Island about 3 pm, 8 August 1900

Source: Courtesy of J. G. Titus. See also Titus and Richmond (2001).

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