Variations in Sea Level

Tide staff at Solomons Island, MD

Tide gauges around the Chesapeake Bay indicate that the relative sea level in the Bay is rising at twice the average global rate of 1.8 mm per year [Douglas, 1991]. Such rates point to both shore erosion and marshland pond development as likely factors in wetlands loss around the Bay. As part of this project, a network of continuously operating Global Positioning System (GPS) receivers will be deployed to the Chesapeake Bay area. GPS is a very precise but simple tool for determining locations. Using GPS, it will be possible to directly measure changes in elevation at tide gauges and other points around the Chesapeake Bay. These measurements can then be combined with tide gauges data, and together give a more accurate estimate of sea level changes for the Chesapeake Bay and the potential threat of this change to wetlands around the Bay.

The following topics will be discussed:

The tide gauge record for the Atlantic Coast of North America

Text contributed by Bruce Douglas

The tide gauge records shown above were selected to provide global coverage, avoid tectonic plate boundaries and have a long history. The Baltimore tide gauge, which is on the Chesapeake Bay, is emphasized using a different color. Note how similar the plots for Portland, ME and Baltimore, MD are and the noticably faster rise compared to the other tide gauges.
Figure provided by Bruce Douglas (NODC) and John Lilibridge (GL)

For the last century, the global level of the sea appears to have risen at an average rate of nearly two mm/yr. However, in any given region, the apparent rate of rise can vary considerably from the long term global value. Geographical and temporal variations from the long-term mean value occur from a variety of causes such as interdecadal fluctuations of ocean density and circulation, continuing isostatic adjustment of the land level from the last deglaciation, subsidence due to the extraction of underground fluids, and others. The middle Atlantic region of the U.S. east coast gives a good illustration of this phenomenon.

A similar plot just for tide gauges along the east coast of North America. Tide gauges on the Chesapeake Bay are again emphasized using a different color. Note the similarity of all these plots.
Figure provided by Bruce Douglas (NODC) and John Lilibridge (GL)

The Table below shows the relative sea level rise in mm per year for long-term tide gauges in the Chesapeake Bay region. Trends for 1930-1990 and 1970-1990 are displayed. It is obvious at a glance that there is a great difference between the regional rate of sea level rise during the two periods.


     Sea Level Station     Trend 1970-90    Trend 1930-90
   _________________________________________________________
     PORTSMOUTH                 4.3 mm/year     3.8 mm/year
     HAMPTON ROADS              1.1             4.2 
     GLOUCESTER POINT          -1.5             (*)
     CAMBRIDGE                 -3.2             3.2 
     WASHINGTON DC             -1.8             3.0 
     SOLOMONS ISLAND            2.0             3.3 
     ANNAPOLIS                 -0.3             3.5  
     BALTIMORE                 -0.4             3.2
     KIPTOPEKE BEACH            0.5             (*)
     LEWES                     -0.5             3.3 
     PHILADELPHIA              -0.5             2.7 
   _________________________________________________________
     AVERAGE                    0.0 mm/yr       3.4 mm/yr

     (*) Data not available for the entire period 1930-60

The interval 1930-1990 is long enough at 60 years to establish that the middle Atlantic region has a systematically higher rate of sea level rise than the long term global average of nearly 2 mm per year. The approximately 1.5 mm/yr extra rise for the region comes from a general sinking of the area into the ocean. Postglacial rebound, that is, readjustment (sinking in this area) of land elevations since the retreat of the glaciers at the end of the last ice age, is the cause. The overall global rise of sea level adds to the effect of land subsidence in the Chesapeake area to produce an unusually high rate of long-term local sea level rise relative to the global average.

The period 1970-1990 tells a very different story. The sinking of the land from postglacial rebound is still going on, but the regional ocean circulation and density structure has produced a temporary fall of sea level in the zone that has entirely offset the effect of the subsidence due to postglacial rebound. Thus for now, the net change of sea level in the middle Atlantic area is zero. Of course this situation will not last. The nearby ocean will inevitably recover, and even overshoot, its long term rate of sea level rise in the area, producing at some time in the future (probably in the next few decades) a rate of rise that exceeds the long term average rate for the region.

Examining the Table more closely, we see that several sites (Portsmouth, Solomons Island) still do show substantial positive values of sea level rise for the last 20 years. It is possible that these sites are subject to land subsidence beyond the regional value caused by postglacial rebound. A typical cause of such localized subsidence is groundwater extraction, which in fact has occurred in both areas at an ever increasing rate in recent decades. If this is indeed the case, when the ocean returns to something like its average behavior, very high and damaging rates of relative sea level rise (5 to 7+ mm/yr) will be observed at these locations for possibly many decades. Because of the observed rate, Solomon's Island was chosen as the first permanent GPS site for this project.

An excessive rate of sea level rise of a few mm/yr might at first glance appear to be of little consequence. But the slope of the coastal plain in the region is very shallow, being in many places less than 1 part in 1000. Thus a five mm rise of sea level in one year translates into a five or more meter loss of land! And this rate of loss can persist for decades.

Local effects make the tide gauge data difficult to interpret

It is important to remember that tide gauges are primarily a navigation aid. As such, many of these tide gauges are located near ports and other growing population centers. In turn, it is possible that these sites are subject to local land subsidence caused by ground water extraction; an activity known to be increasing around the Chesapeake Bay in recent decades [Gornitz and Seeber, 1990; Holdal and Morrison, 1974]. Changes in the elevation of the land on which the tide gauges rest would also appear as a changes in the relative sea level.

Tide gauges measure sea level changes relative to the land on which the the tide gauge rests. By itself, a tide gauge cannot tell the difference between local crustal motion and sea level changes. The following figures illustrate this point. The dashed red line is at a constant height in all figures. In the first figure, the water level is fixed but the land has fallen.

Crustal Motion

In the second figure, the land is stable but the water level has risen.

Rising Sea Level

A real rise in the relative sea level has occurred in both figures but the causes are completely different.

Land subsidence that results in a large rise of relative sea level and concomitant flooding of coastal areas has long plagued many communities. Houston, Galveston, and parts of Louisiana have had to undertake major programs to deal with these problems. It is urgent to monitor the land elevation at sites in the Chesapeake region by geodetic means as early as possible to see if similar problems are destined to occur in this area. In this way it will be possible to determine if groundwater extraction associated with commercial development and agricultural practices is having a deleterious effect on relative sea level rise and will hasten the loss of precious coastal lands and islands.

The effects of rising sea levels on marshlands

During episodes of gradual sea level increase, salt marshes can keep pace with the rising water levels by backfilling, that is by trapping sediments and their own organic detritus in the water column [Leatherman, 1991]. This raises the bottom, offsetting the rise in water level, and giving the plant species time to adjust to the change by moving progressively landward. However, if the sea level rises significantly faster than the rate at which the marsh can respond, the marsh will drown and be lost.

A more probable and catastrophic mechanism of marsh loss from increasing sea levels is the formation of interior marsh ponds. If these ponds form, the provide additional boundaries where sediment can be washed away destroying the base from which the plants grow. These shallow-water bodies enlarge and coalesce in response to sea level rise resulting in rapid coastal submergence at the expense of marsh vegetation [Orson et al., 1985].

A striking example is shown in the images above taken at the Blackwater National Wildlife Refuge in Maryland. The darker areas are the open water; the lighter are vegetation. Note both the growth in number and size of the interior marsh ponds.

Indirectly, sea level rise will exert other pressures on coastal marshlands. In response to a rising sea level, bulkheads will be built to control shoreline erosion. As the marshes attempt to migrate inland in response to sea level rise, they will literally be squeezed out of existence. New patterns of building and agriculture; conserving and clearing; planting and dredging will influence the evolution of the Chesapeake Bay marshlands.

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