Shore Protection and Retreat: How sustainable?

6.5 How Sustainable are Shore Protection and Retreat?

Coastal communities were designed and built without recognition of rising sea level. Thus, people in areas without shore protection will have to flood-proof structures (see Section 9.7.2), implement shore protection, (Section 6.1.1) or plan a retreat (Section 6.1.2). Those who inhabit areas with shore protection are potentially vulnerable as well. Are the known approaches to shore protection and retreat sustainable? That is: can they be maintained for the foreseeable future?

Most shore protection structures are designed for current sea level and may not accommodate a significant rise. Seawalls (Kyper and Sorenson, 1985; NRC, 1987), bulkheads (Sorenson et al., 1984.), dikes, (NRC, 1987), sewers (Wilcoxen, 1986), and drainage systems (Titus et al., 1987) are designed based on the waves, water levels, and rainfall experienced in the past. If conditions exceed what the designers expect, disaster can result—especially when sea level rises above the level of the land surface. The failure of dikes protecting land below sea level resulted in the deaths of approximately 1800 people in the Netherlands in a 1953 storm (Roos and Jonkman, 2006), and more than 1000 people in the New Orleans area from Hurricane Katrina in 2005 (Knabb et al., 2005). A dike along the Industrial Canal in New Orleans which failed during Katrina had been designed for sea level approximately 60 cm lower than today, because designers did not account for the land subsidence during the previous 50 years (Interagency Performance Evaluation Taskforce, 2006).

One option is to design structures for future conditions. Depending on the incremental cost of designing for higher sea level compared with the cost of rebuilding later, it may be economically rational to build in a safety factor today to account for future conditions, such as higher and wider shore protection structures (see Section 10.5). But doing so is not always practical. Costs generally rise more than proportionately with higher water levels8. Project managers would generally be reluctant to overdesign a structure for today’s conditions (Schmeltz, 1984). Moreover, aesthetic factors such as loss of waterfront views or preservation of historic structures (e.g., Charleston Battery in South Carolina, see Figure 6.10) can also make people reluctant to build a dike or seawall higher than what is needed today.

Figure 6.10. Historic homes along the Charleston Battery. Charleston, South Carolina (April 2004). [Photo source: ©James G. Titus, used with permission]

6.5.1 Is “Business as Usual” Shore Protection Sustainable?

Public officials and property owners in densely developed recreational communities along the mid-Atlantic coast generally expect governmental actions to stabilize shores. But no one has assessed the cost and availability of sand through beach nourishment even if required sand is proportional to sea-level rise, which previous assessments of the cost of sea-level rise have assumed (e.g., U.S. EPA, 1989; Leatherman, 1989; Titus et al., 1991). The prospects of barrier island disintegration and segmentation examined in Chapter 3 would require much more sand to stabilize the shore. Maintaining the shore may at first seem to require only the simple augmentation of sand along a visible beach, but over a century or so other parts of the coastal environment would capture increasing amounts of sand to maintain elevation relative to the sea. In effect, beach nourishment would indirectly elevate those areas as well (by replacing sand from the beach that is transported to raise those areas), including the ocean floor immediately offshore, tidal deltas, and eventually back-barrier bay bottoms and the bay sides of barrier islands. Similarly, along armored shores in urban areas, land that is barely above sea level today would become farther and farther below sea level, increasing the costs of shore protection and setting up greater potential disasters in the event of a dike failure. It is not possible to forecast whether these costs will be greater than what future generations will choose to bear. But in those few cases where previous generations have bequeathed this generation with substantial communities below sea level, a painful involuntary relocation has sometimes occurred after severe storms (e.g., New Orleans after Katrina).

Most retreat policies are designed for current rates of sea-level rise and would not necessarily accommodate a significant acceleration in the rate of sea-level rise. Erosion-based setbacks along ocean shores generally require homes to be set back from the primary dune by a distance equal to the annual erosion rate times a number of years intended to represent the economic lifetime of the structure (e.g., in North Carolina, 60 years times the erosion rate for large buildings; see Section A1.G.1 in Appendix 1). If sea-level rise accelerates and increases the erosion rate, then the buildings will not have been protected for the presumed economic lifetimes. Yet larger setback distances may not be practicable if they exceed the depth of buildable lots. Moreover, erosion-based setback policies generally do not articulate what will happen once shore erosion consumes the setback. The retreat policies followed by organizations that manage undeveloped land for conservation purposes may account for foreseeable erosion, but not for the consequences of an accelerated erosion that consumes the entire coastal unit.

6.5.2 Sustainable Shore Protection May Require Regional Coordination

Regional Sediment Management is a strategy for managing sand as a resource (NRC, 2007). The strategy recognizes that coastal engineering projects have regional impacts on sediment transport processes and availability. This approach includes:

Conservation and management of sediments along the shore and immediate offshore areas, viewing sand as a resource;

Attempt to design with nature, understanding sediment movement in a region and the interrelationships of projects and management actions;

Conceptual and programmatic connections among all activities that involve sediment in a region (e.g., navigation channel maintenance, flood and storm damage reduction, ecosystem restoration and protection, beneficial uses of dredged material); sediment more efficiently;

Connections between existing and new projects to use sediment more efficiently;

Improved program effectiveness through collaborative partnerships between agencies; and

Overcoming institutional barriers to efficient management (Martin, 2002).

The Philadelphia and New York Districts of the U.S. Army Corps of Engineers have a joint effort at regional sediment management for the Atlantic coast of New Jersey (USACE, 2008b). By understanding sediment sources, losses, and transport, how people have altered the natural flow, and ways to work with natural dynamics, more effective responses to rising sea level are possible.

One possible way to promote better regional sediment management would be the development of a set of “best sediment management practices”. Previously, standard practices have been identified to minimize the runoff of harmful sediment into estuaries (NJDEP, 2004; City of Santa Cruz, 2007). A similar set of practices for managing sediments along shores could help reduce the environmental and economic costs of shore protection, without requiring each project to conduct a regional sediment management study.

6.5.3 Either Shore Protection or a Failure to Plan Can Limit the Flexibility of Future Generations

The economic feasibility of sustained shore protection as sea level rises is unknown, as is the political and social feasibility of a planned retreat away from the shore. The absence of a comprehensive long-term shoreline plan often leaves property owners with the assumption that the existing development can and should be maintained. Property-specific shoreline armoring and small beach nourishment projects further reinforce the expectation that the existing shoreline will be maintained indefinitely, often seeming to justify additional investments by property owners in more expensive dwellings (especially if there is a through-road parallel to the shore).

Shore protection generally limits flexibility more than retreat. Once shore protection starts, retreat can be very difficult to enact because the protection influences expectations and encourages investments, which in turn increases the economic justification for continued shore protection. A policy of retreat can be more easily replaced with a policy of shore protection because people do not make substantial investments on the assumption that the shore will retreat. This is not to say that all dikes and seawalls would be maintained and enlarged indefinitely if sea level continues to rise. Nevertheless, the abandonment of floodprone communities rarely (if ever) occurs because of the potential vulnerability or cost of flood protection, but rather in the aftermath of a flood disaster (e.g., Missouri State Emergency Management Agency, 1995).

6.4: What are the societal consequences of retreat and shore protection?

Part III: Preparing for Sea Level Rise Go Forward

For previous reports focused on the implications of rising sea level, go to More Sea Level Rise Reports.

| Climate Change for Kids | Shore Protection and Retreat | Coastal Elevation Maps | Related Links

Shore Protection and Retreat: How sustainable?

Related Links

6.5 How Sustainable are Shore Protection and Retreat?

6.5.1 Is “Business as Usual” Shore Protection Sustainable?

6.5.2 Sustainable Shore Protection May Require Regional Coordination

6.5.3 Either Shore Protection or a Failure to Plan Can Limit the Flexibility of Future Generations

Local Navigation