Background of CPI

The change-potential index (CPI) for assessing susceptibility to coastal change associated with water-level change was derived from a similar methodology called the Coastal Vulnerability Index (CVI), which was designed to highlight the vulnerability of a coast to sea-level rise impacts (Thieler and Hammar-Klose, 1999). Potential impacts associated with sea-level rise include shoreline erosion, saltwater intrusion into groundwater aquifers, inundation of wetlands and estuaries, and threats to cultural and historic resources as well as infrastructure. Impacts from lake-level fall in places along the Great Lakes could include dewatering of wetland areas, decreases in harborage area, channel shoaling, stream and river mouth erosion, habitat loss, and possible reactivation of stable dune systems. A goal of the USGS/NPS cooperative project was to apply CVI methodology in a variety of sea and lake-level settings, so three Great Lakes parks were selected as locations where lake-levels are falling. Because the impacts associated with water-level rise are different from impacts associated with water-level fall, a slightly different index was developed to differentiate assessments in water-level falling areas from water-level rising areas. In this study, we considered that impacts are often evaluated from a human perspective and a human connection to the coast. Threats to infrastructure, for example, are a major concern along shorelines that are experiencing water-level rise. Alternatively, the potential subaerial exposure of polluted lake sediments may be a primary concern in areas where water-level is falling. For the purposes of this cooperative project we chose to reserve the word 'vulnerability' for shorelines that may experience loss of land, infrastructure, or natural and cultural resources as a result of sea level rise. Alternatively, for areas where water-levels are falling and associated impacts are not as well-understood or researched, we address the likelihood that coastal change may occur without assigning a subjective term such as risk, hazard, or vulnerability.

In order to apply the CPI method to an emerging lakeshore, an assumption is made that independent of the net movement of water level relative to the land surface, the variables that are most important to coastal change and shoreline evolution will be the same. That is, the six variables defined in the CPI methodology will be important for both submerging (experiencing relative water-level rise) and emerging (experiencing relative water-level fall) coastlines. Since the CPI is designed to highlight change potential based on variables that are common to almost all coasts without directly indicating a physical effect, it should be amenable to application in a variety of geologic settings. The complications with CPI or CVI methods arise after change potential or vulnerability has been determined and the possible impacts associated with water-level change are considered. Although this methodology can be applied anywhere that physical change is likely to occur as a result of changing water level, the kinds of change that may occur (i.e. exposure of polluted marine sediments, loss of wetland, erosion of river mouths, lower-groundwater levels)) are not addressed and should be considered by planners in the context of resources utilization and preservation. This report illustrates that CPI methodology can be applied along three lakeshores within the Great Lakes. The application of the data for planning purposes is a function of the nature of potential environmental change and whether such change is desirable from a resource management perspective.