Reducing Coastal Risk on the East and Gulf Coasts (2014) / Chapter Skim
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3 Performance of Coastal Risk Reduction Strategies
3 Performance of Coastal Risk Reduction Strategies
Pages 69-110
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From page 69... ...
This chapter discusses the various coastal risk reduction strategies and reviews what is known about their proven performance, including their economic costs and benefits and environmental effects. Isolating the economic value of coastal risk reduction projects in comparative studies of protected and unprotected areas is difficult to accomplish because of lack of available data.
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From page 70... ...
Most of the Corps efforts related to coastal risk mitigation within the last two decades have focused on beachfront areas, with a heavy reliance on beach nourishment as the primary means of coastal risk reduction (Figure 1-9; Appendix B)
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From page 71... ...
. The results of a telephone survey of coastal managers conducted by a group of researchers in South Carolina offer evidence that many states use strategies to reduce the consequences of a coastal storm, such as hazard zoning, building elevation, land purchase, or setbacks (see Table 3-1; London et al., 2009)
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From page 72... ...
or hard structural strategies. Natural and nature-based approaches to coastal risk reduction include using or expanding dunes, salt marsh, mangroves, reefs, and seagrass to mitigate flooding and erosion associated with wave action or storm surge.
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From page 73... ...
Respondents were the same states listed in Table 3-1. systems as an explicit focus of the coastal risk reduction project (termed "ecosystem restoration" in Appendix B)
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From page 74... ...
74 FIGURE 3-2 Various components of the multiple lines of defense approach for Louisiana. SOURCE: Reprinted, with permission, from Pontchartrain Basin Foundation.
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From page 75... ...
are built to reduce coastal risks to infrastructure where the natural beaches and dunes have been eliminated or significantly restricted and where other risk reduction options are prevented by lack of space or sediment. These hard structures are also intended to hold the shoreline position by reducing wave attack, storm surge, and associated erosion of landward areas.
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From page 76... ...
76 REDUCING COASTAL RISK ON THE EAST AND GULF COASTS FIGURE 3-3 Types of shore-parallel structural risk reduction strategies. SOURCE: Douglass and Krolak (2008)
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From page 77... ...
In 1904, the originally specified length of the FIGURE 3-4 Relic seawall in Bay Head, New Jersey, that was buried beneath a dune and uncovered during Hurricane Sandy. R02656 Fig 3-4.eps SOURCE: Photo courtesy of Jennifer Irish.
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From page 78... ...
According to Dean (1986) , it is likely that coastal armoring will cause an increase in erosion on adjacent unprotected beaches because hard structures prevent the protected beach from providing sediment to the rest of the coastal system.
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From page 79... ...
Coastal armoring placed in an True Coastal armoring is designed to protect area of existing erosional stress the upland, but does not prevent erosion will cause the beaches fronting of the beach profile seaward of the the armoring to diminish. armoring.
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From page 80... ...
-- for example, building rip-rap revetments rather than vertical walls. Great potential exists for altering designs to reduce hard structures' environmental impacts and enhance habitat value, but more
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From page 81... ...
Levees and Floodwalls Unlike seawalls, coastal levees in the United States are generally not designed to resist the direct attack of high-energy waves. Instead, levees are often located landward of large areas of salt marsh that dissipate wave energy or along rivers or waterways inland from the coast to reducing flooding associated with coastal storm surge.
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From page 82... ...
. However, because coastal levees and floodwalls can be designed to withstand varying degrees of storm surge and wave attack, costs will vary widely with design specifications.
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From page 83... ...
Levees also prevent the delivery of waterborne sediment to formerly inundated areas, contributing to subsidence and reducing the potential to keep pace with sea level rise. Levees in the coastal environment have little direct environmental value, but there is increasing interest in modifying the design of coastal risk reduction features to increase ecological functions and reduce the overall impacts (Day et al., 2000)
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From page 84... ...
. Economic Costs and Documented Returns to Date The cost and maintenance requirements of storm surge barriers are likely to limit them to intensively developed areas, where they are or would be key elements in risk reduction strategies (Jiabi et al., 2013; Walsh and Miskewitz, 2013)
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From page 85... ...
They can also have positive effects as part of coastal risk reduction projects, assuming that there is enough space for a sufficiently functional natural system between the coastal hazard and the area to be protected. Truly natural landforms are most likely to effectively provide risk reduction when used in concert with nonstructural land-use approaches or for portions of the shoreline dedicated to conservation or restoration, because the inherent dynamism of natural processes (see Box 3-1)
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From page 86... ...
These "nature-based" approaches primarily include beach nourishment; dune building; and conservation, creation, or restoration of salt marsh, seagrass, mangroves, and oyster and coral reefs. The sections below focus mostly on these human-modified nature-based systems and their role in coastal risk reduction rather than on truly natural approaches, although these are mentioned where appropriate.
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From page 87... ...
Overall Effectiveness for Reducing Flood Damage Beach nourishment projects provide fill sediment to counteract and/or repair erosion while increasing the distance of coastal infrastructure from the surf and swash zones and providing space and a source of sand that favor formation and protection of dunes. The coastal risk reduction value of dunes is in their height and volume.
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From page 88... ...
. Federal Insurance Administration claims for damage caused by storm surge and wave attack and overwash resulting from Hurricanes Bertha and Fran revealed far less damage to structures in locations protected by USACE beach nourishment projects than in adjacent unprotected locations (USACE, 2000a)
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From page 89... ...
The economic benefits of beach nourishment extend well beyond coastal risk reduction. Houston (2013)
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From page 90... ...
argues that adding the economic returns of beach nourishment projects for tourism to returns for coastal risk reduction makes a compelling case for nourishment as an economic investment. Viewpoints of the value of beach nourishment depend on whether the observer has primary interest in damage reduction, perceived retention or enhancement of property values, recreation, or environmental benefits (Camfield, 1993; Nordstrom, 2005)
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From page 91... ...
. The long-term, cumulative ecological implications of repeated burial from large-scale nourishment projects are still unknown (Lindeman and Snyder, 1999; Posey and Alphin, 2002; Speybroeck et al., 2006)
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From page 92... ...
often creates a vertical scarp that restricts movement of fauna and FIGURE 3-5 Beach nourishment scenarios for coastal risk reduction and implications for beach function. Nourishing a beach at the height of a natural berm (C)
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From page 93... ...
. The process of offshore sediment extraction to support beach nourishment projects also has important ecological impacts.
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From page 94... ...
. Current and Potential Environmental Benefits Beach nourishment also has potential environmental benefits, although some would necessitate changes to existing practices and design to achieve.
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. Most coastal risk reduction projects include artificially constructed dunes built by machines or established using fences and vegetation plantings.
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From page 96... ...
. Nature-based approaches, such as restored or enhanced seagrass, salt marsh and mangrove habitats, and oyster or coral reefs, can reduce coastal erosion and wave damage and augment other structural and/or nonstructural coastal risk reduction strategies.
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From page 97... ...
. Wave attenuation and shoreline stabilization are the primary coastal risk reduction benefits of salt marshes, although the quantitative effects are not fully understood.
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From page 98... ...
. The study was conducted during a period of high tropical storm activity, and results suggest that oyster reefs may help mitigate erosion from routine wave heights, but that oyster reefs are quickly overtopped during storms and are not effective at dealing with higher storm surge and wave heights common in tropical storms.
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From page 99... ...
The authors concluded that wave height reductions were similar to or exceeded the benefits of constructed low-crested detached breakwaters, at a lower median cost. Although coral reefs are likely to have minimal effects for reducing storm surge (see previous discussion on oyster reefs)
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From page 100... ...
occurring at the seaward edge. However, as the water hits the resistance of the vegetation, it can cause an increase in water levels in front of the mangroves, increasing the height of the storm surge there.
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From page 101... ...
, contribute to wave attenuation, and retain and stabilize sediments in shallow coastal areas (NRC, 2007)
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From page 102... ...
. Estimates of benefits are widely ranging, and little economic data are available on documented cost savings from nature-based coastal risk reduction features from prior storm events.
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From page 103... ...
Federal, state, and local governments and nongovernmental organizations, such as land trusts or conservancies, work to preserve natural lands through purchase or donations of land or through conservation easements. However, in most states, land and habitat conservation has only recently been seriously considered as part of coastal risk reduction strategies.
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From page 104... ...
The grants were administered under the three main federal programs that supported building design modifications and nonstructural hazard mitigation during this period: the Hazard Mitigation Grant Program, Project Impact, and the Flood Mitigation Assistance Program (see Chapter 2)
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From page 105... ...
. Instead, this study found that state mitigation policy has a stronger influence on inclusion of land-use policies in hazard mitigation plans aimed at avoiding flood hazard areas.
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From page 106... ...
. REGIONAL AND NATIONAL IMPLICATIONS OF SAND MANAGEMENT Coastal risk reduction involves significant cost, whether by naturebased strategies such as beach nourishment and wetlands restoration, or hard structures such as levees, seawalls, and storm surge barriers.
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From page 107... ...
sand and gravel resources are plentiful, the availability of high-quality beach sand near shorelines could become scarce, which could increase the costs of future beach nourishment projects. In most instances, development along the landward edge prevents beaches from naturally migrating landward in response to rising sea levels.
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From page 108... ...
The U.S. Geological Survey, in conjunction with several other institutions, has built usSEABED,8 a database of offshore sediment data for the entire United States that can be used to map possible sand regions for beach nourishment projects.
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From page 109... ...
Beach nourishment and dune-building projects for coastal risk reduction can be designed to provide increased ecological value. Beachfill projects provide some level of risk reduction for coastal infrastructure from erosion, flooding, and wave attack and may reduce the likelihood of forming new inlets.
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From page 110... ...
Coupling nature-based approaches with hard structures to buffer the structures against wave attack provides an effective coastal risk reduction strategy if space allows. Strategies that reduce the consequences of coastal storms, such as hazard zoning, building elevation, land purchase, and setbacks, have high documented benefit-cost ratios, but they are given less attention by the federal government and are viewed as difficult to implement by states.
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