Several planning measures have been proposed to enable some shorelines to remain in roughly their natural state as sea level rises, rather than be replaced with structures. For the most part, these measures apply to areas that are not yet developed. They broadly fall into two categories: setbacks, which are regulations that prevent development of areas likely to be inundated, and rolling easements-which allow development today, but only with the explicit condition that the property will not be protected from rising water levels (Titus, 1997).
Setbacks currently are used to ensure that homes are safe from current flood risks. Several U.S. states currently require an additional erosion-based setback, in which new houses are set back an additional 20 to 60 times the annual erosion rate (Klarin and Hershman, 1990; Marine Law Institute et al., 1995). Eventually, however, the shore will erode to any setback line. Moreover, it is economically inefficient, and sometimes unconstitutional, to prevent the use of property now solely to avoid an adverse impact in the future (Titus, 1991).
Many of these problems are avoided with rolling easements-a planning measure in which coastal development is allowed in return for the property owner agreeing not to build structures or otherwise artificially stop the natural inland migration of wetlands and beaches. This option requires neither a specific estimate of future sea-level rise nor large public land purchases, and it is economically efficient because it does not prevent owners from using their land unless or until the sea rises enough to inundate it. The ability of the government to prevent property owners from eliminating the shore is grounded in the "public trust doctrine," under which the public has always owned tidal waters and either owned or had an access right along all intertidal beaches (Slade, 1990). If this approach were implemented in the next decade, ensuring the continued survival of natural wetland and beach shores in U.S. areas that are still undeveloped would cost approximately $400-1,200 million (Titus, 1997).
Texas common law recognizes rolling easements along its Gulf coast beaches. Maine and Rhode Island have issued regulations that prohibit structures that block the inland migration of wetlands. South Carolina's Beachfront Management Act, passed in response to the risks of a 1-ft rise in sea level, originally required setbacks along the coast, but in the aftermath of a trial court ruling that was eventually upheld by the U.S. Supreme Court (Lucas v. South Carolina Coastal Council), the statute was modified to require rolling easements in some locations2 (South Carolina Beachfront Management Act, 1988). Because Canada inherited the same common law from England as the United States, all of these approaches could be applicable to Canada if its coastal zone becomes densely developed in the next century.
Several nationwide assessments have been conducted in the United States, mostly focusing on the potential loss of wet and dry land and the cost of holding back the sea. These studies have recognized that the impact of sea-level rise ultimately depends on whether-and how-people hold back the sea; they generally estimate impacts assuming alternative policies for protecting coastal land. A rise of 50 cm would inundate 8,600-19,000 km2 (3,300-7,300 mi2) of dry land if no shores are protected and 5,700-16,000 km2 (2,200-6,100 mi2) if currently developed areas are protected (Table 8-7). The loss of coastal wetlands would be 17-43% if no shores are protected and 20-45% if currently developed areas are protected-but 38-61% if all shores were protected. These results suggest that efforts to mitigate wetland loss from sea-level rise could exempt existing development and focus on areas that are still undeveloped (Titus et al., 1991).
Studies generally estimate that the cumulative cost of a 50-cm rise in sea level through the year 2100 would be $20-200 billion; the cost of a 1-m rise would be approximately twice that amount. Titus et al. (1991) estimated that for a 50-cm rise, barrier islands could be protected by placing sand on eroding beaches and the low bay sides, at a cost of $15-81 billion; elevating houses and roads, at a cost of $29-36 billion; and protecting mainland areas with dikes and bulkheads, at a cost of $5-13 billion-for a total cost of $55-123 billion.
Yohe (1990) estimated that if shores were not protected, a 50-cm rise would inundate land and structures worth $78-188 billion; Yohe et al. (1996) estimated that the cost would only be $20 billion. Their lower estimate appears to have resulted from two differences in their study: First, rather than assuming that all developed areas would be protected, Yohe et al. assessed the value of land and structures and assumed that only areas that could be economically protected would be protected. Second, the two studies appear to make different assumptions regarding the area of developed barrier islands in the United States. The Yohe et al. (1996) analysis was based on a sample of the entire coast, which included five densely developed ocean beach resorts; Titus et al. (1991) based their estimates on an assessment by Leatherman (1989)-who examined every beach community between New Jersey and the Mexican border, as well as in California, along with one site in each of the other states.
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