7.5 Horizontal Shoreline Displacement (Erosion/Accretion)

The erosion/accretion data used in the development of the horizontal shoreline displacement data set were extracted and modified from the Coastal Erosion Information System (CEIS) developed by May et al. (1982, 1983) and Dolan et al. (1975, 1983, 1989). The CEIS data are limited in extent to coastlines that open into the ocean or large bays. The displacement data within the CEIS data base were originally obtained from over 500 individuals or organizations with records ranging in length from 20 to 165 years. The majority of the shoreline displacement measurements, however, were made from historic maps and aerial photographs that cover the U.S. West Coast for a minimum of 40 to 50 years. Most of the information was originally obtained from published reports or from regionally available high-resolution data sets (e.g., Dolan et al. 1980). Of the data within CEIS, 25% were obtained in raw form and converted into point measurements of erosion or accretion. In conducting the measurement and data compilation steps of the raw data, May et al. (1982) used the landward limit of wetted sand as the criteria for identifying the shoreline. This definition was selected because it produced the most consistent results in the photo-interpretation process.

By comparing present and past shorelines from maps, aerial photographs, and data from regional studies, May et al. (1982) were able to obtain rates of change, expressed in m/year, for coastal points on the West Coast. May et al. (1982) then averaged and extrapolated the point data into 3´ latitude by 3´ longitude grid cells (in locations with sparse data 7.5´ and 15´ grid cells were used) to minimize the problems associated with mapping errors, imprecise shoreline definitions, and poor temporal resolution within the original erosion/accretion data sources. These 3´, 7.5´, or 15´ grid cells were then overlaid onto the 0.25° grid cells used in this data base to derive the following data variables (values in m/year): minimum erosion trend, mean erosion trend, maximum erosion trend, and the number of 3´, 7.5´, or 15´ cells used in deriving the data for each 0.25° grid cell.

To transfer this information to the 0.25° grid cells used in this data set, the erosion variables were recalculated as follows:

  1. The number of 3´, 7.5´, or 15´ grid cells that occur in a given 0.25° grid cell was determined. These 3´, 7.5´, or 15´ cells were used to calculate the minimum, mean, or maximum erosion rate variables.
  2. The minimum erosion rate for a 0.25° grid cell is the minimum erosion rate found in the 3´, 7.5´, or 15´ grid cells within a 0.25° grid cell.
  3. The mean erosion rate for a 0.25° grid cell is the weighted average of the erosion rates of all 3´, 7.5´, or 15´ grid cells within a 0.25° grid cell.
  4. The maximum erosion rate for a 0.25° grid cell is the maximum erosion rate found in the 3´, 7.5´, or 15´ grid cells within a 0.25° grid cell.

Fig. 6 gives an example of how the overlay process was used to determine the number of data values from 3´, 7.5´, or 15´ grid cells used in calculating the values contained in the 0.25° grid cells distributed with this NDP.

The gridded data were then overlaid onto a 1:2,000,000 digitized U.S. West Coastline to form the line segment version of these data. Based on the length of record (from 20 to 165 years, depending on location), and the errors inherent in the data, the reported shoreline displacement trends are average values that are highly variable over time; as such, rates of change less than 0.6 m/year are not considered significant.