Adaptive responses focus on protection of shores or allowing them to retreat, with subsequent loss of existing shoreline systems and structures.
Several U.S. government agencies have started to prepare for rising sea level. The U.S. Coastal Zone Management Act requires state coastal programs to address rising sea level, and a few states have modified coastal land-use policies to address rising sea level. The U.S. Army Corps of Engineers is required to consider alternative scenarios of future sea-level rise in its feasibility studies. These anticipatory measures have been implemented in part because assessments have identified measures whose costs are less than the benefits of preparation-even when future benefits are discounted by an economic rate of return.
Most assessments of North American response strategies to future sea-level rise have concluded that coastal cities will merit protection with bulkheads, dikes, and pumping systems (National Research Council, 1987; Titus et al., 1991). Bulkheads, seawalls, and rock revetments already are being used to halt erosion to protect land that is well above sea level. Dikes and pumping systems are used to protect urban areas such as New Orleans that are below sea level, and other areas that are below flood levels.
Although structural measures can protect property from rising water levels, the resulting loss of natural shorelines may have adverse environmental, recreational, and aesthetic effects. Wetland and shallow-water habitats already are being lost because protective structures prevent those systems from migrating inland. In other areas, sandy and muddy beaches are being eliminated-impairing the ability of some amphibious species to move between the water and the land and directly removing the habitat of species that inhabit these beaches. The elimination of natural beaches may harm recreational and fishing navigation by removing locations from which small craft can be launched or beached in an emergency; the loss of beaches also impairs the ability of the public to move along the shore for fishing, recreation, and other uses. In the past 15 years, the state of Maryland alone has lost the use of 500 km (300 mi) of shorelines through the issuance of permits for bulkheads and revetments (Tidal Waters Division, 1978-93).
The effects of rising sea level can be offset by elevating beaches, land surfaces, and structures as sea level rises. A key benefit of this approach is that the character of the shore is not altered. Rapidly subsiding communities such as Galveston, Texas, have used fill to raise land elevations; some authors have suggested that it will be necessary to elevate Miami because the soils are too permeable for effective pumping (e.g., Walker et al., 1989). Regulations along San Francisco Bay require projects along the shore or on newly reclaimed land to be either protected by a dike or elevated enough to accommodate accelerated sea-level rise.1
The practice of elevating land surfaces is most applicable to recreational
barrier islands, where environmental and aesthetic factors (such as natural
beaches and waterfront views) can be as important as property values and shore-protection
costs (Gibbs, 1984; Howard et al., 1985; Titus, 1990). Figure
8-9 illustrates possible responses to sea-level rise for barrier islands:
building a dike, elevating the land surface, engineering a landward retreat,
and no protection. A case study of Long Beach Island, New Jersey, concluded
that any of the three protection options would be less costly than the current
value of the threatened land (Titus, 1990). Although dikes have a lower direct
cost than elevating land and structures, the latter approach is least disruptive
to existing land uses and can be implemented gradually over time.
|Figure 8-9: Responses to sea-level rise on developed barrier islands. Lightly developed islands may have no practical choice other than the "no protection" option, which would result in ocean-side erosion and in some cases bayside inundation. Under the "engineered retreat" option, a community might tolerate ocean-side erosion but move threatened structures to newly created bayside lands, imitating the natural overwash process that occurs with narrow undeveloped islands. A more common response is likely to be to raise entire islands as well as their beaches; although the sand costs are much higher than with an engineered retreat, existing land uses can be preserved. Finally, wide urbanized islands may choose to erect seawalls and levees (dikes); the loss of beach access and waterfront views, however, make this option less feasible for recreational barrier island resorts.|
Other reports in this collection