Several other historical and current factors may have attributed to the rising sea levels in the Chesapeake Bay area. The bay area was the site of impact crater 36 million years ago. The impact crater is the largest in the United States and the sixth largest in the world. The crater's effect can be seen on the James River, which abruptly jogs northeast from its southeastern flow when it encounters the crater rim. The resulting ground deformation and long term compaction of the crater fill may contribute to the rising sea levels
Another factor cited for the rise in the sea levels in the Chesapeake Bay area is land subsidence. Groundwater was pumped out to supply the increasing urban populations. As the water was removed, the ground elevation decreased, allowing the surface water to cover the sinking land. The NOAA has established stations along the bay that monitor sea level. Initially, these stations were set in place to aid navigation. But they have provided consistent data that documents the rising sea levels. Sewells Point is located east of Norfolk on the southern banks of the James River. Data from this station reveals the mean sea level trend is 4.42 millimeters/year (1.45 feet/century) with a standard error of 0.16 mm/yr based on monthly mean sea level data from 1927 to 1999. The data is relative to the 1983-2001 mean sea level datum recently established by Center for Operational Oceanographic Products and Services (CO-OPS).
While graphs of the rising sea levels document the increase, the effects of the rising sea levels are difficult to conceptualize. That is where imagery can supplement the research. Digital orthophoto quadrangles (DOQ) are computed generated images of aerial photographs in which the images are corrected to remove any distortion caused by camera angle or terrain. They are basically a georeferenced image. These images are produced by the USGS Earth Resource Observation and Science (EROS) Center located in South Dakota. These images cover the United States and are updated over a period of five years. They are produced either in black and white (B/W), native or color infrared (CIR)formats. They have a resolution of 1 meter. The 3.75-minute DOQ are available in a GeoTIFF format. The GeoTIFF format is referenced to the NAD83. Since these images are periodically updated, and are available for a relatively inexpensive costs, they provide an ideal platform for establishing a baseline image of the shoreline.
The image of Norfolk was downloaded via file transfer protocol. Once the image was obtained, it was imported into the IDRISI program using the GeoTIFF conversion module and into its three bands.
Upon inspecting the bands, the river in band 1 is flat black, indicating no reflectance. This is characteristic of the infrared band. Water absorbs the electromagnetic radiation in this frequency range and so it appears in the image as black.top
Band 1 was then modified to isolate the water from other land covers using the RECLASS module to convert all values of 0 (water) to 1 and all other values to 0.
The image was then converted to a byte/ binary image using the CONVERT module.
The GROUP module was the utilized to establish groups of water.
Once the water groups that made up the James River/Chesapeake Bay water bodies were identified, the ASSIGN module was used to reset the pixel values of the desired groups to 1, and all other groups to 0.
This allowed the main water bodies to be isolated. The AREA module was employed to determine the area of the water and land covers. The land cover 36.28 square kilometers. The water covered 11.79 square kilometers.
DOQ and other remote sensing platforms can provide a baseline image from which the various groups can correlate their data. It is important that the data are documented and the rising levels are compared to annual and seasonal fluctuations as well as tidal surges to ensure no data are distorted or used to support an invalid hypothesis.
Other remote sensing data can augment the images obtained from the DOQ. Some of these are quite expensive, are composites over time, do not have the infrared image or do not have adequate resolution. The DOQ images, while not very timely, will produce the resolution necessary at the right price. The images can also be correlated with the tides and seasonal fluctuations the characterized the tidal basin.top
Earth Resources Observation and Science, EROS, accessed May, 2008.
IDRISI GIS Software, Andres Version, Clark University, IDRISI
Larsen,C. The Chesapeake Bay: Geologic Product of Rising Sea Level, USGS PUBS accessed May, 2008.
NOAA, Application of Water Level and Datum Information to Sea Level Rise Impact Studies Impact Studies, accessed May,2008.
NOAA, Coastal Oceanographic Applications and Services of Tides And Lakes, COASTAL, accessed May, 2008
NOAA Tides and Currents, accessed May, 2008.
Poag, C. W., Coring the Chesapeake Bay Impact Crater, Geotimes January, 2004.
The Rising Tide - Cause, Effects and Planning for Rising Sea Level, Wetlands Watch, accessed May, 2008.top