National Assessment of Coastal Vulnerability to Sea-Level Rise: Preliminary Results for the U.S. Pacific Coast
The data variables underlying the Coastal Vulnerability Index (C.V.I.) show variability at several spatial scales. The rate of sea-level rise and tide range vary over a spatial scale of ~100 km. For sea-level rise, this variability represents the large-scale patterns of isostasy and tectonism along the Pacific continental margin of North America (Peltier, 1996; Komar and Shih, 1993). Changes in tide range generally reflect changes in the configuration of the continental shelf as a whole (e.g., continental shelf width).
A second group of variables, consisting of geomorphology and wave height, vary on a ~10 km spatial scale that reflects primarily the alongshore changes in environments and distribution of energy in the coastal system. For example, the high energy coast of Washington, Oregon and northern California stands in sharp contrast to the lower-energy coast of southern California.
The shoreline erosion/accretion rates vary on a spatial scale equal to the minimum size of our grid, which is 3-minutes or ~5 km. It is this variable that generally adds the greatest variation to the C.V.I. values. As described above, however, erosion rates are the variable in our data set that is the least well-documented. On the U.S. Pacific coast, the erosion rate variable is of great importance because shoreline erosion and cliff retreat rates are controlled to a large extent by the cliff material, as well as geologic structure such as jointing and faulting (Komar and Shih, 1993) which can vary significantly over this spatial scale. The effect of well-documented erosion rate data on C.V.I. rankings is visible in the area to the north of Tillamook Head where the variation in erosion rates is reflected in the variation of the C.V.I.
To highlight the nature of the C.V.I. and its underlying data,
different index variables from two geographic regions are presented
on the next two pages: