7.4 Sea-Level Trends

The sea-level trend data set for the U.S. West Coast was derived from calculated relative sea-level trend measurements in mm/year for 16 tide-gauge stations (Woodworth 1995; Spencer and Woodworth 1993). This relative sea-level trend was calculated by a linear least-squares regression fitted to the time series of mean annual sea-level elevations for each of the 16 tide-gauge stations. Table 3 illustrates this information and Fig 5. shows the locations of the stations listed in Table 3.

To obtain a relative sea-level trend variable for the 0.25° grid cells along the West Coast lying between tide-gauge stations, the following interpolation procedure was adopted:

  1. The tide-gauge stations and the sea-level trends were plotted along a 1:2,000,000 digitized U.S. West coastline (Fig. 5).
  2. The 0.25° by 0.25° grid used in this NDP was then overlaid onto the tide-gauge stations with an ARC/INFO IDENTITY command, whereby the grid cells took on the values of the tide-gauge stations.
  3. For each coastal grid cell without data, the difference in relative sea levels was calculated between the two nearest gauge stations (i.e., occurring east and west or north and south of the given grid cell).
  4. The difference between the relative sea levels was then divided by the number of grid rows, plus one, occurring between the grid cells containing gauge stations. This value was called the slope factor.
  5. The slope factor was then multiplied by the number of grid rows from the grid cell being calculated to the nearest station (i.e., western-most or southern-most station) and added to the station's relative sea-level trend.
  6. The resultant of these five steps is the relative sea-level trend variable within the gridded data groups in this data set.

It should be noted that tide gauges measure sea-level variations in relation to a fixed benchmark on land and are therefore relative, due to vertical land movements and real changes in ocean levels. Information on long-term vertical movements along the U.S. West Coast is summarized in Appendix D. Because of active tectonism along the West Coast, which varies from place to place and can affect the relative sea-level curves, the above interpolation procedure should be used with caution. See comments in Sect. 10 and Appendix D.

The procedure for calculating the uplift or local subsidence trend variable along the U.S. West Coast differs from that used for the U.S. East and Gulf Coasts (Gornitz and Lebedeff 1987). Along the U.S. East Coast, Holocene paleosealevel indicators were used to calculate a long-term geologic trend variable (Gornitz and Seeber 1990). This geologic trend variable was then subtracted from the present relative sea-level trend (as measured by tide gauges) to provide a corrected sea-level trend variable for each 0.25° coastal grid cell. The average value of these corrected trends was used to obtain the regional eustatic trend (i.e., 1.25 mm/yr). This eustatic trend was then subtracted from the relative sea-level trend variable to yield a local subsidence trend variable for each 0.25° East Coast grid cell.

Along the U.S. Gulf Coast, Holocene paleosealevel indicators were not available, so the geologic trend variable for each 0.25° Gulf coastal grid cell was set to 0.0 for compatibility purposes between the two NDPs (i.e., NDP-043A and NDP-043B). The local subsidence factor for the Gulf Coast was calculated by assuming the global eustatic rate of sea-level rise to be 1.5 mm per year, as reported by the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al. 1990), and subtracting this rate (i.e., 1.5 mm/yr) from the relative sea-level trend variable. The resultant difference along the U.S. Gulf Coast was the uplift or subsidence trend variable for each 0.25° coastal grid cell.

Along the U.S. West Coast, Holocene paleosealevel indicators are found only in a small number of coastal marshes and bays, where they record at least a half dozen discrete seismic events (Atwater 1987; Atwater et al. 1991; and Darienzo et al. 1994) and do not yield a continuous sea-level curve. Late Quaternary (i.e., < 125,000 years) raised marine terraces, which occur along much of the West Coast,integrate the permanent deformation produced over multiple earthquake cycles. Thus, raised terrace data can be used to derive long-term geologic trends for selected data points (Appendix D, Table 1).

Because of insufficient data, there are no real data values assigned to the long-term geologic trend variable in this data set. A value of 0.0 was assigned to West coastal grid cells with a data value for the calculated relative sea-level trend variable; while a value of 9999.99 was assigned to grid cells with no data value for the calculated relative sea-level trend variable. While both values, 0.0 and 9999.99, indicate no data for the long-term geologic trend variable, a value of 0.0 serves two purposes. First, it indicates those grid cells in which the data provided in Table 1 of Appendix D may be used to calculate a long-term geologic trend variable. Secondly, it allows for compatibility among the data sets in this series of NDPs (NDP-043A, NDP-043B, and this document, NDP-043C). Consequently, the corrected sea-level trend variable within this data set contains a value that appears to be identical to the calculated relative sea-level trend variable; however, nothing has truly been corrected here [i.e., corrected sea-level trend - calculated relative sea-level trend long-term geologic trend (0.0)].

The uplift or subsidence trend variable for the U.S. West Coast was calculated by assuming a global eustatic rate of sea-level rise of 1.5 mm/yr, as reported by the IPCC (Houghton et al. 1996), and subtracting this rate (i.e., 1.5 mm/yr) from the calculated relative sea-level trend variable. The resultant difference is the local subsidence or uplift trend variable for each 0.25° coastal grid cell along the West Coast.

The local uplift or subsidence variable gives an indication of the relative vulnerability of each coastal grid cell and line segment to sea-level rise. This variable may be used to identify areas that are uplifting or subsiding faster or slower than the regional averages. It is also added to any future projected global sea-level curves, to adjust the global curve to local conditions.

The ARC/INFO IDENTITY command was used to overlay the coastal 0.25° by 0.25° grid cells onto the 1:2,000,000 digitized West coastline. The resulting relative sea-level trend, geologic trend, corrected relative sea-level trend, and local uplift or subsidence trend variables are found in the line- based data groups herein.