8.3.7.2.3. Protecting natural shorelines by allowing shores to retreat
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.
8.3.7.2.4. National assessments of adaptive responses
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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