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Mitigating Shore Erosion Along Sheltered Coasts 1 Introduction Coastlines are perpetually changing—some from natural processes—some from human activities—many from both. The frequent human response to erosion is an attempt to stabilize, or “harden,” the shoreline. Usually this is an approach that results in long-lasting consequences for the natural system, not just locally but also affecting surrounding areas. There are however many effective alternatives to hardening and depending on the selections made, the long-term consequences to the area can be positive or negative. This report reviews options available to address and mitigate1 erosion of sheltered coasts and explores why certain decisions are made regarding the choice of erosion mitigation options; provides critical information about the consequences of altering sheltered shorelines; and, provides recommendations about how to better inform decisions in the future. STUDY HISTORY Before decisions can be made concerning appropriate shoreline management strategies on sheltered coasts, several topics must be understood. These include: which natural and anthropogenic factors are responsible for land losses, and how they occur; 1 In this report, “erosion mitigation” is used to describe efforts to reduce or lessen the severity of erosion and should not be confused with mitigation of environmental damages.
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Mitigating Shore Erosion Along Sheltered Coasts why present protection techniques and planning strategies are failing, and what impacts they have had; alternative protection techniques and their best environmental design criteria; how monitoring and data collection can be used to increase the effectiveness of protection strategies on developed and underdeveloped shorelines; which planning solutions exist that protect the environment while still allowing economic development including an understanding of the potential outcome of a “no action” decision; and how to provide federal and state agencies, local officials, managers, and the private sector with a framework for collaboration to avoid ill designed solutions, and instead obtain integrated, long-term, effective shoreline management. To help address some of these questions, the U.S. Environmental Protection Agency’s (EPA)2 Global Programs Division first approached the Ocean Studies Board about developing a study on erosion mitigation options for sheltered coastlines. With additional interest and funding from the Army Corps of Engineers (USACE), the National Oceanic and Atmospheric Administration (NOAA)-Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET), and NOAA Coastal Services, the Ocean Studies Board assembled a committee of experts3 to undertake a study defined by the statement of task given in Box 1-1. SCOPE OF THE PROBLEM Throughout the coastal regions of the world there are a significant number of areas that are partially or fully protected from the high-energy regimes associated with open coastlines, such as ocean-facing beaches. The sponsors requested that this study examine sheltered coastal environments such as estuaries, bays, lagoons, mudflats, and deltaic coasts (Box 1-1). These environments may be generally characterized as lower energy coastlines, but there is no quantitative formula that covers the diversity of conditions encountered on these more protected shorelines. Sheltered shorelines, though often contiguous with the open coast, border smaller, contained, bodies of water separated from the open ocean by islands, peninsulas, reefs, or other geomorphic features. Many of the processes that govern erosion and deposition on the open coast also apply to sheltered coasts, but generally the scale at which these processes function is significantly reduced within sheltered coastal areas. Also, unlike the typically long linear features associated with open coasts, sheltered coasts exhibit characteristics that are distinctively more compartmentalized with discrete areas of the coast 2 For a list of acronym definitions, see Appendix B. 3 For committee and staff biographies, see Appendix A.
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Mitigating Shore Erosion Along Sheltered Coasts BOX 1-1 Statement of Task The study will examine the impacts of shoreline management on sheltered coastal environments (e.g., estuaries, bays, lagoons, mudflats, deltaic coasts) and identify conventional and alternative strategies to minimize potential negative impacts to adjacent or nearby coastal resources. These impacts include: loss of intertidal and shallow water ecosystems, effects on species, and loss of public trust uses. The study will provide a framework for collaboration between different levels of government, conservancies, and property owners to aid in making decisions regarding the most appropriate alternatives for shoreline protection. In particular, the committee will address the following questions: What engineering techniques, technologies, and land management/planning measures are available to protect sheltered coastlines from erosion or inundation resulting from either natural or anthropogenically forced processes? When does the design and implementation of these measures require a distinction between natural and anthropogenic causes and how can this be achieved? What information is needed to determine where and when these measures are reliable and effective both from an engineering and a habitat perspective? What are the likely individual and cumulative impacts of shoreline protection practices or no action on sheltered coastal habitats including public and private property, and public access along the shore, locally and regionally? Over what time frame are monitoring data needed to document the effectiveness of protective coastal measures? What data are needed to predict when design criteria may be exceeded? Given current trends in erosion and inundation rates and a possible acceleration of relative sea-level rise, how can design criteria, the mix of technologies employed, and land use plans be implemented for the protection of the environment and property over the long term? encompassing a variety of geomorphic and biological resources. Chapter 2 of this report describes some of the typical physical conditions associated with sheltered coasts; relatively low velocity tidal currents and mid-to-low energy wave climates associated with a limited fetch (distance from shore to shore). These conditions promote the formation of ecological complexes (i.e., mangroves, marshes, and mudflats) that often characterize habitats on sheltered coasts and are generally not found along open coasts.4 Many of these sheltered areas are river valleys that have been drowned by rising sea-level, or drainage features that are protected by headlands or islands. Many of these semiprotected sheltered shorelines border estuaries. It is estimated 4 See “Terminology” section of this chapter for a more detailed discussion on the definition of a sheltered coast.
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Mitigating Shore Erosion Along Sheltered Coasts that in the United States alone there are 850 estuaries representing over 80 percent of the Atlantic and Gulf of Mexico coastal areas and more than 98 percent of the Virginia and Maryland coastlines (Nordstrom, 1992). Although estuaries are recognized as some of the most biologically productive areas of coastal regions, they are also increasingly popular places for people to live, work, and recreate. Estuaries are complex and dynamic systems subject to natural processes associated with wind, wave, and tidal action, which in turn cause erosion and subsequent transport and deposition of sediments along the shorelines. In addition to natural erosion, some anthropogenic activities such as recreational boating and commercial shipping can also contribute to erosion and sediment movement. When erosion occurs in the same area where human induced development exists, a “problem” is perceived and actions are taken to prevent the erosion. Historically the common practice to deal with erosion has been to use a variety of structures designed in some way to interrupt the natural processes of erosion, sediment transport and/or deposition. Such efforts may or may not be successful at preventing erosion, but most interrupt natural processes, frequently causing erosion in other areas and loss of the original habitats and other natural shoreline features. The following case studies illustrate two examples of the growing problem of eroding sheltered shorelines and the severity of how human responses have altered the function of sheltered shorelines. Mobile Bay, Alabama Mobile Bay is a shallow body of water (average depth about 3 meters [approx. 10 feet]) that empties into the Gulf of Mexico. The bay is approximately 51 kilometers (about 32 miles) long, 37 kilometers (about 23 miles) across at its widest point, and the overall length of the shoreline is approximately 160 kilometers (about 100 miles). The bay area experiences extratropical storms and is subject to hurricanes from the Gulf of Mexico. With an average flow of 1,800 cubic meters (approx. 62,000 cubic feet) per second, the Mobile Bay estuary has the fourth largest freshwater flow in the continental United States (MBNEP, 2002). In 1997, a study was conducted that investigated the effects of bulkheads on the bay shoreline (Douglas and Pickel, 1999). The study examined aerial photographs from 1955, 1974, and 1985 to document the extent of bulkheads around the bay. Aerial video and site visits were used in 1997 to document the “present conditions.” Results of the study reveal that the amount of armoring of the shoreline increased dramatically from 8 percent in 1955 to 30 percent in 1997. Table 1-1 is a summary of the length of shoreline armoring. The study also documented that the rate of armoring from 1955 to 1997 corresponded to the rate of population growth in the area. This is a strong, but not surprising indication, that development trends increase the human impacts on sheltered coastlines. Of greater concern than the amount or rate of armoring was the associated loss of intertidal habitat, roughly estimated at 4 to 8 hectares
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Mitigating Shore Erosion Along Sheltered Coasts TABLE 1-1 Length of Shoreline Armoring Along Mobile Bay, Alabama Year Armored Shoreline Natural Shoreline 1955 12,200 meters (approx. 39,900 feet or 8%) 145,000 meters (approx. 475,600 feet or 92%) 1974 22,000 meters (approx. 72,000 feet or 14%) 135,200 meters (approx. 443,500 feet or 86%) 1985 40,200 meters (approx. 132,000 feet or 26%) 116,900 meters (approx. 383,500 feet or 74%) 1997 46,760 meters (approx. 153,400 feet or 30%) 110,400 meters (approx. 362,100 feet or 70%) SOURCE: Douglass and Pickel (1999). (approx. 10 to 20 acres) corresponding to about 10 kilometers (approx. 6 miles) of intertidal beach shoreline. Raritan Bay, New Jersey Raritan Bay extends from the mouth of New Jersey’s Raritan River to the entrance to the Atlantic Ocean, between the Verrazano Narrows to the north and Sandy Hook to the south. It measures about 22 kilometers (approx. 12 nautical miles) in length, and is about 13 kilometers (approx. 8 miles) wide at its widest point. The bay has an average depth of between 2 and 3 meters (7 and 10 feet), a tidal range of 1 to 2 meters (5 to 6 feet), and currents tend to flow east-west at about .5 to 1.5 knots. The shoreline of the bay is extensively altered by a variety of human development and stabilization efforts. The bay area is subject to frequent extratropical storms and periodic hurricanes. A 14-kilometer (approx. nine mile) segment of the New Jersey side of the Raritan Bay shoreline was examined by Jackson (1996) for the types of alteration that have historically occurred to the shoreline in the presence of development and how they affected the sandy beaches. The shoreline of the study area is characterized by differing combinations of eroding bluffs, narrow beaches, and marsh; recreational and commercial development; and shore stabilization including groins, bulkheads, beach nourishment. A variety of analytical approaches were used to evaluate changes over time of development trends, beach and marsh environments, and shoreline positions. Prior to the 19th century the area consisted largely of low bluffs and salt marsh with limited commercial and residential development. Development began in earnest in the late 19th century as a summer resort community for nearby New York City. Marsh areas were filled as homes, bulkheads, boardwalks and piers began to be constructed. Today the area is almost completely developed, predominately with year-round residences. Attempts to alter the shoreline have consisted of the use of shore parallel bulkheads and seawalls, shore perpendicular groins,
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Mitigating Shore Erosion Along Sheltered Coasts and beach fill (nourishment). The use of bulkheads and seawalls increased over time and are the most common structures found along the study area. The use of groins was common prior to the 1950s but their use has diminished and few have been constructed since the 1970s. Surprising for a bay shoreline, the use of beach fill has been used extensively, although largely locally, in the study area. It was estimated that 2.8 million m3 (approx. 3.7 million yd3) of sand has been placed on the beaches, generally for flood protection purposes. One of the key findings of the study was, “The net effect of the use of shore-parallel walls in estuaries can be significant reduction or elimination of sandy beach environments,”(Jackson, 1996). The study also concluded that the use of beach fill resulted in the most dramatic changes in physical characteristics of the shoreline areas including dune building and increased beach heights and widths. Also of interesting note, the study recognized that response to shoreline erosion is moving away from individual actions, to more geographically comprehensive, government-planned efforts. TERMINOLOGY OF SHELTERED COASTS The terms of reference of this study refer to the erosion of sheltered shorelines. However, the meaning and perception of these terms— erosion, sheltered, and shoreline—varies among the interested parties and stakeholders including homeowners, coastal zone managers, geologists, engineers, and lawyers. In an attempt to address anticipated confusion, the meanings of these and other key physical resource features, as used in this study, are discussed below. Shore Definitions of the shore, including both open and sheltered coasts, range from the single location of intersection of sea level with the land profile (the shoreline) to inclusion of the entire region affected by wave action, spanning the inner continental shelf to the upper reaches of extreme storm swash (the shore zone). Often the definition appropriate for legal applications is quite different from the most obvious definition for understanding the physics. Figure 1-1 provides a useful illustration of the complexities of defining this common term. Legal definitions of the shoreline are typically based on the location of a contour line at the elevation of mean high water or an equivalent tidal statistic. In a simple situation forced only by tides and waves on beaches with a rapid equilibrium response, this definition could be sensibly enforced. However, delineation of this boundary is complicated by the wide range of time scales over which sea level varies (from millennial glacial and interglacial climate periods to decadal climate fluctuations including El Niño to annual or seasonal storm surge signals) and by the range of time scales over which the beach responds to that forcing (interannual, annual, and single storm event). In addition, the value of
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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 1-1 Definitions of nearshore zones and legal ownership for a typical coastal zone. The common legal demarcation between private and public lands, the “shoreline,” is usually taken to be the intersection of the mean high water line with the beach profile. But temporal variation of the beach profile and even of sea level complicates this interpretation. SOURCE: Modified from National Oceanic and Atmospheric Administration (NOAA), 2001. any selected elevation statistic may be limited by the accuracy of estimates based on data availability. Local effects due to variability of wave-induced setup, local geostrophic or nonlinear shelf currents, variations in thermal expansion, among other effects, are neglected. Typically, a value is simply selected for regional application. For shores backed by sea cliffs or bluffs, the location most relevant to defining erosion may not be that of a specific elevation contour, but may instead be the location of the bluff top. To describe the physics of coastal processes requires consideration of the “shore zone,” defined as the active volume of sediment affected by wave action. However, this region can be difficult to bound, spanning from a poorly defined depth-of-no-motion on the seaward side, to some onshore boundary that might or might not include vegetated dunes and back-dune areas occasionally overtopped by extreme storm waves, or sea cliffs eroded by wave undercutting.
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Mitigating Shore Erosion Along Sheltered Coasts Erosion and Inundation Erosion and inundation both result in the landward movement of the shoreline contour. These processes occur over a full range of time scales, including short-term events (waves, tides and storms) and chronic, long-term sea-level rise. Implicit in the definition of erosion is a choice of time scale, with longer events considered erosion and shorter events variance. Thus, a landward shoreline movement that recovers prior to the next storm would not usually be considered erosion, despite the potential loss of property associated with that variation. Inundation refers to the temporary submergence of typically dry lands when there is an exceptional rise of the sea surface, and floodwaters cover the adjacent low-lying land. Because shoreline is commonly based on movement of a single contour, it is precariously sensitive to details of the dynamics that determine how shore profiles adjust to natural forcing. This can introduce legal and operational difficulties. In the 1990s, The Netherlands determined a course of action to combat the ongoing threat of erosion of its shores, making it a legal requirement to mitigate erosion beyond the shoreline location defined in a national survey of 1990. Recognizing the sensitivity problems associated with a single contour definition, they instead defined a “momentary coastline” (MCL) based on the mean shoreline location integrated between −5 meters and +3 meters (approx. −16 feet and +10 feet) from NAP (Normaal Amsterdams Peil, or Amsterdam Ordnance Datum, which is the Dutch reference for sea level) (Figure 1-2). By basing the mitigation criterion on a shore zone definition, the law became appropriately robust to short-term fluctuations. Sheltered Coasts The term sheltered coast is frequently used to describe the shorelines of estuaries and bays. These shores are considered sheltered because they typically abut smaller bodies of water with shallower depths and there is a limited distance over which waves can be generated, such that the waves are significantly less energetic than typical on an open coast with the same winds. However, there appears to be no objective criterion for the size of the body of water or the degree of energy reduction that would unequivocally distinguish a shoreline as sheltered. The shoreline forms a continuum from open to sheltered areas—in some cases a physical feature may provide a clear demarcation, but in other areas there is a gradual transition from the open to the more sheltered environments. Compared to the typically long linear nature of open coasts, however, sheltered shorelines exhibit a more irregular configuration and often display very distinct geomorphic compartments that contain a complex of resources, the mix of which may change from compartment to compartment. This variation creates conditions that are both more varied and more complex than open coasts. Shores frequently included in the “sheltered” category range from low to medium wave energy.
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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 1-2 The definition of the open-coast shoreline used in The Netherlands is called the Momentary Coastline (MCL) and is found as the total area of beach sediment lying between −5 meters and +3 meters (approx. −16 feet and +10 feet) from NAP divided by 8m. This quantity integrates detailed profile fluctuations that would otherwise confuse estimation based on a more traditional shoreline location. NOTE: H = Height between dune foot and mean low water [m]; A = Momentary Coastline Zone [m2]; B = A/2H = Momentary Coastline position [m]; C = Distance dune foot to reference [m]; XMCL = B+C = Momentary Coastline position + Distance dune foot to reference [m]. SOURCE: van Koningsveld and Mulderc, 2004. Courtesy of the Coastal Education and Research Foundation. Instead of high energy waves, strong currents, storm events, or large boat wakes may be the primary drivers of sediment mobilization and transport on sheltered coasts. In lower energy environments, retention of smaller grained sediments creates mudflats and supports the growth of subtidal and intertidal vegetation. Overall, sheltered coasts describe a much greater diversity of conditions than found on open ocean coasts, requiring more site-specific approaches for managing erosion. Figure 1-3 illustrates the variety of shoreline configurations and environments found on sheltered coasts. Many studies have focused on mitigating erosion on open ocean shores that are exposed to constant wave and current forcing. Thus, a focus of our early discussion was on defining those aspects of sheltered shores for which the physics and mitigation strategies were not simply scaled-down versions of their
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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 1-3 Aerial image of Lower Machodoc Creek on the Potomac River in Westmoreland County, Virginia, showing the irregularity and diversity of shore types. The insets are six typical shoreline profiles around sheltered coasts. SOURCE: Aerial image courtesy of the Commonwealth of Virginia.
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Mitigating Shore Erosion Along Sheltered Coasts Insets as referred to on previous page. open-ocean counterparts. Compared to their open-ocean counterparts, waves on sheltered shores are typically shorter, steeper and more episodic, with fluid energy often insufficient for sediment mobilization for extended periods of time. Thus, morphologies sculpted during storms often persist through subsequent calm periods as relict features. Local surge and currents generally play larger relative roles compared to waves in shaping sheltered shores. Sheltered coasts consist of various combinations of geomorphic settings, or features, such as unconsolidated upland bluffs, dunes, beaches, intertidal (e.g., marsh, mangroves) and subtidal (e.g., macroalgae, seagrasses) vegetation, tidal flats, and sandbars. The definition of the three major geomorphic categories (beaches and dunes; bluffs; and mudflats and vegetated communities) are described below. The location and extent of any of these features is dependent on site-specific conditions. While some sheltered shorelines include hard rock outcrops, the erosion of features such as rock cliffs or shore platforms on sheltered coasts is considered a slow process and one unlikely to result in the need for the protective shoreline measures that are the focus of this study.
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Mitigating Shore Erosion Along Sheltered Coasts Beaches and Dunes The shorelines most commonly used for recreational purposes (e.g., sunbathing, walking, swimming, fishing) are those characterized by sandy beaches. Beaches are accumulations of any type of unconsolidated material that can be transported by wind or waves, from fine sand to cobbles. Beaches extend landward from the Mean Low Water Line to the place where there is a change in material or physiographic form, or to the line of permanent vegetation. In some cases, wind transported sediment will be deposited and accumulate along the backshore and form dunes. The morphology of beaches and dunes is dependent on the interactions of the available sediment supply with the energy from waves, tides, and wind. Beaches are very dynamic systems. Storms have steep short waves that tend to lower and flatten beaches. Berms can be removed, the upper beach is eroded and dunes are cut as the beach adjusts to a lower volume of sand. The sand removed from the subaerial part of the system during storms accumulates just below low tide level to produce a wider flatter beach with a nearshore bar that allows storm wave energy to be dissipated. Under nonstorm conditions the fair-weather waves bring the sand in the bar back to the beach and wind actions restores the dunes. Such changes are frequently seasonal and result in a continually changing beach-dune form. Bluffs Coastal bluffs (also commonly referred to as banks) are elevated landforms composed of partially consolidated and unconsolidated sediments, typically sands, gravel and/or clays that are generally located landward of a beach or marsh. As sea level rises, waves will attack the bluff face and toe of the slope, gradually undercutting the bluff and causing sections of it to fall away. The erosion rate is dependent upon the degree of consolidation; rocky bluffs are not readily eroded by the forces that characterize sheltered coasts. Bluffs are also subject to erosion from other natural and anthropogenic events. Natural events include wind that strips vegetation and loosens sediment and groundwater seepage that undermines the vertical integrity of the structure; this is exacerbated in cold climates where freeze/thaw cycles speed disintegration of a cliff face. People contribute to bluff erosion when they build on top of bluffs and create places for water to enter and destabilize the system, they trample the vegetation that supports the steep cliff and sometimes mine the cliff for building material. Bluff erosion provides a sediment source for other coastal features, such as the fronting beaches. Mudflats and Vegetated Communities Mudflats are intertidal areas with relatively fine sediment that can be vegetated by plant communities (marshes or mangroves) or be barren in which case
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Mitigating Shore Erosion Along Sheltered Coasts they are colonized by microscopic plant communities (microalgae) and bacteria. While marshes and mangroves are found in the intertidal area where they are regularly flooded during every high tide, seagrasses and seaweed occur in the subtidal area where they are submerged most of the time, except for extreme low tides when the plants in the shallowest areas may become exposed to air for a brief period of time. Temperature zone determines if marshes or mangroves will be found in the intertidal: Mangroves occur in warmer climates (tropical, warm temperate) while marshes are found in cooler climates (temperate). Substrate type determines if seagrasses or seaweed will be found in the subtidal: seagrasses dominate soft (sandy and muddy) substrates while seaweed dominate on hard (rock) substrates. These plant communities as well as the mudflats support a highly diverse and productive number of associated animals. Therefore, techniques to mitigate shoreline erosion that change the substrate characteristics will lead to changes in the associated plant and animal communities. STUDY ORGANIZATION The committee met three times during the course of the study. The first meeting, held in Washington, DC, in June 2005, provided the committee with an opportunity to discuss the background and study expectations with representatives from the sponsor agencies; Environmental Protection Agency, National Oceanic and Atmospheric Administration-Cooperative Institute for Coastal and Estuarine Environmental Technology, and the U.S. Army Corps of Engineers. In addition, the committee developed plans for a workshop that was subsequently held in Seattle, WA, in October 2005. The purpose of the workshop was to provide the committee with additional background information, largely focused on an analysis of options available to mitigate erosion of sheltered coasts. In planning this activity, the committee decided not to limit the discussion to marine or estuarine areas, but to include experts from the Great Lakes. The rationale is that if the conditions leading to erosion are comparable in these bodies of water, then the issues arising with efforts to mitigate erosion will be similar. Additionally, the Great Lakes are recognized as subject to the Federal Coastal Zone Management Act (16 U.S.C. 1450 et seq) and with the exception of Illinois, the adjoining states have federally approved Coastal Management Programs containing many of the authorities and mechanisms in place to address the recommendations of this report. The workshop explored the geomorphic settings of sheltered coasts as well as how various erosion measures affect those settings. The workshop brought together approximately 32 professionals with such diverse expertise as: state and federal regulatory matters, science, engineering, land use planning, and legal issues. The participants came from around the continental United States and provided expertise on the range of erosion problems in various coastal regions. The complete agenda and participants list for the workshop are available in Appendix C.
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Mitigating Shore Erosion Along Sheltered Coasts The report is organized to address the issues outlined in the charge to the committee described above. Chapter 2 explores the physical processes of shoreline erosion and the erosion mitigation strategies used to address those processes. Techniques used to address shoreline erosion are discussed in Chapter 3. This chapter identifies four broad categories of options: Land use regulation and management; Vegetative stabilization; Hardened structures (armoring the shoreline); and Trapping or adding sediment. Chapter 4 covers ecosystem services and values and how they are affected by shore erosion and mitigation measures, including living and nonliving components of the coastal habitats and the impacts of the most common structures installed to prevent erosion (revetments, breakwaters, seawalls, groins, and pilings). Chapter 5 describes the various regulatory, engineering, esthetic, and financial considerations that contribute to the decision-making process for mitigating erosion. This is followed in Chapter 6 by a description of a new shoreline management framework that synthesizes the committee’s major findings and recommendations.
Representative terms from entire chapter: