Calculating Erosion Rates: Using Long-Term Data to Increase Data Confidenceby Mark Crowell, Federal Emergency Management Agency, Washington, United States,
Michael K. Buckley, Federal Emergency Management Agency, Washington, United States,
Abstract: Computer cartographic techniques were used to produce historical shoreline change maps for two study areas: (1) an eight kilometer section of Calvert County, Maryland; and (2) an eight kilometer section of Cape May County, New Jersey. The shorelines were digitized from historical Nation Ocean Service (NOS) T-sheets, and historical and current aerial photography. Erosion rates and concomitant uncertainty ranges (i.e., ± erosion rate error bars) were calculated from these data sets using pairs of endpoint shorelines spanning several different time-frames. For each time-frame, the total number of erosion rates falling within the error bars was tallied. The results demonstrate that: (1) the uncertainty range is usually less when calculating erosion rates from long-term (greater than 60 years) endpoint data, than when calculating rates from short- to medium-term (less than 60 years) endpoint data. This is true even though historical (pre-1930) T-sheets may have a greater potential for shoreline location error than do more modern data (such as post-1930 T-sheets and air photos); and (2) the percent of erosion rate data falling within the uncertainty range increases dramatically as time-frames are decreased. In summary, the confidence level of long-term erosion rate data will usually be much greater than the confidence level of short- to medium-term data.
Subject Headings: Erosion | Shoreline | Mapping | Uncertainty principles | Aerial photography | Ocean engineering | Shoals | Confidence intervals | North America | United States | New Jersey | Maryland
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