6.2: What factors influence the decision whether to protect or retreat? Go Forward

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6.1.2 Retreat

The primary alternative to shore protection is commonly known as retreat (or relocation). Shore protection generally involves coastal engineering to manage the forces of nature and environmental engineering to manage environmental consequences. By contrast, retreat often emphasizes the management of human expectations, so that people do not make investments inconsistent with the eventual retreat.

A retreat can either occur as an unplanned response in the aftermath of a severe storm or as a planned response to avoid the costs or other adverse effects of shore protection. In Great Britain, an ongoing planned retreat is known as “managed realignment” (Rupp-Armstrong and Nicholls, 2007; Shih and Nicholls, 2007; UK Environment Agency, 2007; Midgley and McGlashan, 2004). An optimal retreat generally requires a longer lead time than shore protection (e.g., Yohe and Neumann, 1997; Titus, 1998; IPCC CZMS, 1992) because the economic investments in buildings and infrastructure, and human investment in businesses and communities, can have useful lifetimes of many decades or longer. Therefore, planning, regulatory, and legal mechanisms usually play a more important role in facilitating a planned retreat than for shore protection, which for most projects can be undertaken in a matter of months or years. Some retreat measures are designed to ensure that a retreat occurs in areas where shores would otherwise be protected; other measures are designed to decrease the costs of a retreat but not necessarily change the likelihood of a retreat occurring. For a comprehensive review, see Shoreline Management Technical Assistance Toolbox (NOAA, 2006). The most widely assessed and implemented measures are discussed below.

Relocating structures is possibly the most engineering-related activity involved in a retreat. The most ambitious relocation in the Mid-Atlantic during the last decade has been the landward relocation of the Cape Hatteras Lighthouse (Figure 6.8a; see also Section A1.G.4.2 in Appendix 1). More commonplace are the routine “structural moving” activities involved in relocating a house back several tens of meters within a given shorefront lot, and the removal of structures threatened by shore erosion (Figure 6.8b).

Buyout programs provide funding to compensate landowners for losses from coastal hazards by purchasing vulnerable property. In effect, these programs transfer some of the risk of sea-level rise from the property owner to the public, which pays the cost (see Chapter 12).

Conservation easements are an interest in land that allows the owner of the easement to prevent the owner of the land from developing it. Land conservation organizations have purchased non-development easements along coastal bays and Chesapeake Bay in Maryland (MALPF, 2003). In most cases, the original motivation for these purchases has been the creation of a buffer zone to protect the intertidal ecology (MDCPB, 1999; MALPF, 2003). These vacant lands also leave room for landward migration of wetlands and beaches, (NJDEP, 2006). Organizations can also create buffers specifically for the purpose of accommodating rising sea level. Blackwater Wildlife Refuge in Maryland and Gateway National Recreation Area in New York both own considerable amounts of land along the water onto which wetlands and beaches, respectively, could migrate inland.

Acquisition programs involve efforts by a government or conservation entity to obtain title to the land closest to the sea. Titles may be obtained by voluntary transactions, eminent domain, or dedication of flood-prone lands as part of a permitting process. In Barnegat Light, New Jersey and Virginia Beach, Virginia, for example, governments own substantial land along the shore between the Atlantic Ocean and the oceanside development.

Setbacks are the regulatory equivalent of conservation easements and purchase programs. The most common type of setback used to prepare for sea-level rise is the erosion-based setback, which prohibits development on land that is expected to erode within a given period of time. North Carolina requires new structures to be set back from the primary dune based on the current erosion rate times 30 years for easily moveable homes, or 60 years for large immoveable structures (see Section A1.G.4.1 in Appendix 1). Maine’s setback rule assumes a 60 centimeter (cm) rise in sea level during the next 100 years. 06-096 Code of Maine Rules §355.5(C), (2007).

Flood hazard regulations sometimes prohibit development based on elevation, rather than proximity to the shore. Aside from preventing flood damages, these elevation-based setbacks can ensure that there is room for wetlands or other intertidal habitat to migrate inland as sea level rises in areas that are vulnerable to inundation rather than wave-generated erosion. Two counties in Delaware prohibit development in the 100-year floodplain along the Delaware River and Delaware Bay (Section A1.D.2.2 in Appendix 1).

Rolling easements are regulatory mechanisms (Burka, 1974) or interests in land (Titus, 1998) that prohibit shore protection and instead allow wetlands or beaches to migrate inland as sea level rises. Rolling easements transfer some of the risk of sea-level rise from the environment or the public to the property owner (Titus, 1998). When implemented as a regulation, they are an alternative to prohibiting all development in the area at risk, which may be politically infeasible, inequitable, or a violation of the “takings clause” of the U.S. Constitution (Titus, 1998; Caldwell and Segall, 2007). When implemented as an interest in land, they are an alternative to outright purchases or conservation easements (Titus, 1998).

The purpose of a rolling easement is to align the property owner’s expectations with the dynamic nature of the shore (Titus, 1998). If retreat is the eventual objective, property owners can more efficiently prepare for that eventuality if they expect it than if it takes them by surprise (Yohe et al., 1996; Yohe and Neumann, 1997). Preventing development in the area at risk through setbacks, conservations easements, and land purchases can also be effective—but such restrictions could be costly if applied to thousands of square kilometers of valuable coastal lands (Titus, 1991). Because rolling easements allow development but preclude shore protection, they are most appropriate for areas where preventing development is not feasible and shore protection is unsustainable. Conversely, rolling easements are not useful in areas where shore protection or preventing development are preferred outcomes.

Rolling easements were recognized by the common law along portions of the Texas Gulf Coast (Feinman v. State; Matcha v. Mattox) and reaffirmed by the Texas Open Beaches Act, Tex. Nat. Res. Code Ann. §§ 61.001-.178 (West 1978 & Supp. 1998), with the key purpose being to preserve the public right to traverse the shore. Massachusetts and Rhode Island prohibit shoreline armoring along some estuarine shores so that ecosystems can migrate inland, and several states limit armoring along ocean shores (see Chapter 11). Rolling easements can also be implemented as a type of conservation easement, purchased by government agencies or conservancies from willing sellers, or dedicated as part of a planning review process (Titus, 1998); but to date, rolling easements have only been implemented by regulation.

Density restrictions allow some development but limit densities near the shore. In most cases, the primary motivation has been to reduce pollution runoff into estuaries; but they also can facilitate a retreat by decreasing the number of structures potentially lost if shores retreat. Maryland limits development to one home per 8.1 hectares (20 acres) within 305 meters (m) (1000 feet [ft]) of the shore in most coastal areas (see Section A1.F.2.1 in Appendix 1). In areas without public sewer systems, zoning regulations often restrict densities (e.g., Accomack County, 2008; U.S. EPA, 1989).

Size limitations also allow development but limit the intensity of the development placed at risk. Small structures are relocated more easily than large structures. North Carolina limits the size of new commercial or multi-family residential buildings to 464 square meters (sq m) (5,000 square feet [sq ft]) in the area that would be subject to shore erosion during the next 60 years given the current rate of shore erosion, or within 36.6 m (120 ft) of the shore, whichever is farther inland 15A NCAC 07H. 0305-0306. The required setback for single-family homes and smaller commercial structures is half as great (see Section A1.G.4 in Appendix 1 for details). Maine’s Sand Dune Rules prohibit structures taller than 10.7 (35 ft) or with a “footprint” greater than 232 sq m (2,500 sq ft) in all areas that are potentially vulnerable to a 60 cm rise in sea level. 06-096 Code of Maine Rules §355 (5) (D) (2007)..

6.1.3 Combinations of Shore Protection and Retreat

Although shore protection and retreat are fundamentally different responses to sea-level rise, strategies with elements of both approaches are possible. In most cases, a given parcel of land at a particular time is either being protected or not—but a strategy can vary with both time and place, or hedge against uncertainty about the eventual course of action.

Time. Sometimes a community switches from retreat to protection. It is common to allow shores to retreat as long as only vacant land is lost, but to erect shore protection structures once homes or other buildings are threatened. Setbacks make it more likely that an eroding shore will be allowed to retreat (Beatley et al., 2002; NRC, 1987; NOAA, 2007); once the shore erosion reaches the setback line, the economics of shore protection are similar to what they would have been without the setback. Conversely, protection can switch to retreat. Property owners sometimes erect low-cost shore protection that extends the lifetimes of their property, but ultimately fails in a storm (e.g., geotextile sandbags, shown in Figure 6.7b). Increasing environmental implications or costs of shore protection may also motivate a switch from protection to retreat (see Section 6.5). To minimize economic and human impacts, retreat policies based on rolling easements can be designed to take effect 50 to 100 years hence, until then protection might be allowed (Titus, 1998).

Place. Different responses operate on different scales. In general, a project to retreat or protect a given parcel will usually have effects on other parcels. For example, sand provided to an open stretch of ocean beach will be transported along the shore a significant distance by waves and currents; hence, beach nourishment along the ocean coast generally involves at least a few kilometers of shoreline or an entire island. Along estuaries, however, sands are not transported as far—especially when the shoreline has an indentation—so estuarine shore protection can operate on a smaller scale. Shoreline armoring that protects one parcel may cause adjacent shores to erode or accrete. Nevertheless, along tidal creeks and other areas with small waves, it is often feasible to protect one home with a hard structure, while allowing an adjacent vacant lot to erode. In areas with low density zoning, it may be possible to protect the land immediately surrounding a home while the rest of the lot converts to marsh, mudflat, or shallow water habitat.

Uncertainty. Some responses to sea-level rise may be appropriate in communities whose eventual status is unknown. Floodproofing homes (see Chapter 9), elevating evacuation routes, and improving drainage systems can provide cost-effective protection from flooding in the short term, whether or not a given neighborhood will eventually be protected or become subjected to tidal inundation. A setback can reduce hazards whether or not a shore protection project will eventually be implemented.

Hybrid Shore Protection

6.2: What factors influence the decision whether to protect or retreat? Go Forward

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

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