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Mitigating Shore Erosion Along Sheltered Coasts 4 Mitigating Eroding Sheltered Shorelines: A Trade-Off in Ecosystem Services Coastal engineering projects designed to protect the shoreline from erosion focus mainly on the need to preserve assets such as buildings, roads, rail lines, and lighthouses. The impacts of these coastal engineering projects on organisms and natural processes usually receive less attention, if any. While coastal structures may protect shorelines for a limited time, they also alter the coastal environment. Surprisingly, little attention has been paid to the ecological consequences of installing structures to mitigate shoreline erosion (Airoldi et al., 2005). This chapter focuses on the trade-off of ecosystem services associated with shoreline protection methods, i.e., the loss of ecosystem services of natural coastal communities along sheltered coasts that are being protected from further erosion and the gain of ecosystem services associated with man-made structures built to protect the shoreline from erosion. Coastal ecosystems provide a variety of marketable goods (e.g., fishes, fibers, seaweeds, crabs, sand) as well as processes (e.g., climate regulation, wave attenuation, removal of nutrients, contaminant sequestration, maintenance of biodiversity) that allow humans to thrive (NRC, 2005). These goods and ecosystem processes that benefit humankind are often referred to as ecosystem services. For example, the quality of life in coastal towns and villages is enhanced by ecosystem services such as nutrient uptake, habitat and food production provided by coastal plant communities. These services support commercial fisheries, as well as the recreational use of the shorelines and adjacent waters. Monetary values have been assigned to individual ecosystem services; (Costanza et al., 1997; but see Nature, 1998, for a comment on Costanza et al.), however, because of the challenges posed by the valuation process of ecosystem services (NRC, 2005),
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Mitigating Shore Erosion Along Sheltered Coasts this chapter brings awareness to ecosystem services lost and gained by the mitigation of shoreline erosion along sheltered coasts without attempting to incorporate specific monetary values. Natural shorelines are normally dynamic and undergo natural cycles. Vegetated shorelines experience a seasonal fluctuation in plant biomass and sandy beaches are constantly reworked by waves. These cycles allow a system to maintain equilibrium around some degree of disturbance followed by a period of recovery. Each time a natural system is disturbed, some of its ecosystem services are lost. If the level of disturbance is low in magnitude and frequency, the ecosystem and its services will recover over time. Recovery time is longer when the magnitude of the disturbance is higher. For example, a wind event may cause sediments to be resuspended in shallow waters leading to high turbidity levels no longer allowing oysters to filter. The ecosystem service of filtering water is therefore reduced or lost until turbidity levels return to acceptable levels (usually in the matter of hours). In contrast, if a hurricane buries an oyster reef, its ecosystem service of filtering water is lost until the reef is reestablished (years or even decades later). The current trend of enhanced shoreline erosion and subsequent shoreline protection (see Tables 4-1 and 4-2) leads to disturbance along sheltered shorelines TABLE 4-1 Extent of Shoreline Armored in California Year Length Percent of Coast Source Other Information 1971 42.4 km (approx. 26.4 miles) 2.4 USACE armored exclusive of breakwaters and groins 1978 100 km (approx. 62 miles) 5.7 Habel and Armstrong, 1978 “protected by engineered structures” 1985 168 km (approx. 104 miles) 9.5 Griggs and Savoy, 1985 seawalls, revetments, breakwaters 1992 208 km (approx. 129 miles) 11.8 Griggs et al., 1992 hard, engineering structures SOURCE: Griggs and Patsch (2004). TABLE 4-2 Armoring (By County) in Heavily Populated Central and Southern California Location Length of Shoreline Percent Armored northern Monterey Bay 14.4 km (approx. 8.95 miles) 77 between Carpenteria and Ventura 28.8 km (approx. 5.97 miles) 77 from Oceanside to Carlsbad 12.8 km (approx. 7.95 miles) 86 from Dana Point to San Clemente 12.8 km (approx. 7.95 miles) ~100 SOURCE: Griggs et al. (1992), reported in Griggs and Patsch (2004).
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Mitigating Shore Erosion Along Sheltered Coasts no longer allowing the natural system to become reestablished to its original state. Instead, a new equilibrium is reached altering the ecosystem services provided. In some highly populated areas, 75 to 100 percent of the shoreline is armored (Table 4-2) and the long-term ecological consequences are unknown. Practices commonly implemented to mitigate shoreline erosion (e.g., beach nourishment; construction of seawalls, breakwaters, and groins) not only introduce a certain level of disturbance changing the equilibrium of the coastal ecosystem but usually also introduce new substrate such as concrete, rock, wood, or coarser sand. As a result, organisms that can benefit from this new substrate may appear in the ecosystem. Therefore, as an ecosystem service of the original (eroding) system is lost (e.g., habitat provided by fallen trees), an ecosystem service of the plant and animal community taking advantage of the new substrate may be gained. We refer to this as a “trade-off” in ecosystem services. The alteration of a small section of the shoreline via the construction of a single structure to control erosion at a specific site alters the ecosystem services in the immediate area. The construction of many structures in relative close proximity to each other within a body of water may lead to the shift of ecosystem services of the entire ecosystem. Therefore, the cumulative effect of shore stabilizing structures on ecosystem services is also considered in this chapter. ECOSYSTEM SERVICES PROVIDED BY NATURAL COASTAL SYSTEMS This consideration of coastal systems focuses on those that are subject to erosion on sheltered shorelines. These are either constructional coastal features such as beaches, dunes, mudflats, and vegetated communities (both intertidal and subtidal), and erosional landforms such as bluffs, which contribute sediment to sheltered coasts. 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. Beaches and Dunes Ecosystem Services For dunes and beaches (see Figure 4-1 and Chapter 1 for description), the ecosystem services provided depend on the structure and local environmental factors such as climate, salinity, turbidity, and wave energy to which they are exposed. Ecosystem services commonly listed for dunes and beaches include:
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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 4-1 Conceptual diagram of a beach emphasizing its importance along sheltered coasts and outlining processes that occur on beaches. Habitat These sheltered coastal environments provide habitat for a variety of organisms. The shallow refuge areas of shoreface environments provide suitable conditions for pupping by some shark species. Some turtle species nest on upper beaches and within low dune areas in bays and estuaries—an important consideration if erosion mitigation measures are proposed because many populations are endangered or threatened. The subaerial environments can also provide important habitat for mammals, including rabbits and small rodents (Van Aarde et al., 1996), as well as for nesting birds (Lafferty, 2001). Nutrient Uptake Vegetation on the upper beach and in dunes is frequently nutrient limited. Many dune soils are deficient in macronutrients though levels of calcium may be high. Experimental work shows that addition of various combinations of nutrients to dune soils results in growth of different combinations of dune plants, especially grasses (Packham and Willis, 1997). In general, nutrient deficiency promotes a diverse plant community. Grazing by herbivores (especially where focused on some of the nutrient-fixing dune plants) can also impact nutrient availability. Sandy substrates such as those found in beaches are quite effective in nutrient cycling (Rasheed et al., 2003; Ehrenhauss et al., 2004); therefore, any alteration
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Mitigating Shore Erosion Along Sheltered Coasts to the substrate such as accumulation of fine particles landward of a breakwater will affect this process. Food Production Beaches support an extensive trophic structure, mainly in the form of infauna (animals that live in the sediment), from bacteria and microalgae to molluscs, crustaceans, and shorebirds. Wave Attenuation The role of beaches and dunes in protecting interior areas from wave attack is very dependent on magnitude of wave attack. Beach form can adjust to dissipate wave energy provided there is sufficient sediment supply. Such adjustment has been well documented in response to seasonal changes in wave conditions (Wright et al., 1979). On sheltered shores, boat-wake wave energy may also be seasonal and interaction between these and the wind-wave regime, especially in areas of limited sediment supply, can limit the ability of the beach to adjust. Sediment Stabilization For beaches and dunes, the provision of this ecosystem service is very dependent on presence of vegetation (see below). Nutrient limitation can result in low vegetative cover in dunes and active erosion of beaches along sheltered shorelines indicates that this service is impaired in many areas. Recreation This is one of the most socially important services provided by beaches and dunes. Many sheltered shorelines, especially in estuaries, are close to urban areas, and large numbers of people take advantage of beaches and public access opportunities. Pressure from offroad vehicles as well as continual trampling by foot traffic can lead to blow outs (amphitheater-shaped arenas where the wind has carved away the dunes) on dunes (Hesp, 2002). Raw Materials Excavation of sand and gravel from beaches and dunes for aggregate has been a problem on some open coasts (Borges et al., 2002) but is likely not an issue for sheltered shores where the volume of material available is lower and inefficiently renewed due to lower rates of sediment transport.
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Mitigating Shore Erosion Along Sheltered Coasts The Impact of Shoreline Stabilizing Structures Erosion of beaches and dunes is a natural process and part of the dynamics of coastal areas. Eroding beaches and dunes still provide many of the ecosystem services described above. Fallen trees along the shoreline also serve as additional habitat for terrestrial and aquatic organisms due to the structural complexity they bring to the environment. The provision of sand by eroding dunes also benefits species that require sandy substrates (see case study mentioned later in this chapter). In most instances shoreline structures are placed in order to address local erosion and protect private property, in some cases preventing the loss of an ecosystem service (e.g., beach access to nesting turtles). However, the placement of erosion control measures frequently have unintended consequences in reducing the ecosystem services provided both by proximal and distant beaches and dunes. Beaches Some shore stabilizing structures will prevent further erosion of sandy shorelines, maintaining the beach habitat and other services. However, when structures interrupt longshore sediment movements, they may result in loss of beach ecosystem services in downdrift locations, such as turtle nesting, recreational opportunities, and wave energy dissipation. This effect is well illustrated in a number of shoreline studies (e.g., Pilkey and Wright, 1988). Dunes Sheltered coast dunes which are presently vegetated most likely formed at an earlier time (e.g., Pleistocene); vegetation takes place later in the successional development of dunes. With sea-level rise, these dunes erode and serve as a source of sediment. Therefore, structures that protect the shoreline from eroding may cut the supply of sediment. In contrast, along sheltered shorelines where dunes are unvegetated and still being formed, their formation is the result of aeolian movement of sand from beaches. Therefore, structures that maintain the beach also protect dunes. When bulkheads or revetments are placed high in the intertidal, or even in supratidal locations, to limit erosion, the shoreline remains fixed even as the beach erodes away and some aspects of dune habitat will degrade. Mudflats and Vegetated Communities Marshes, mangroves, seagrass beds, and macroalgae stands, as well as mudflats, support highly diverse and productive communities of associated animals (See Figure 4-2 and Chapter 1 for description). Therefore, techniques to mitigate shoreline erosion that change the substrate characteristics will lead to changes
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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 4-2 Conceptual diagram of a mudflat (top), mangrove forest (bottom), and salt marsh (next page) emphasizing their importance along sheltered coasts and outlining processes commonly observed in these communities.
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Mitigating Shore Erosion Along Sheltered Coasts in the associated plant and animal communities. Ecosystem services of mudflats and vegetated communities include: Habitat Marshes, mangroves, seagrasses and macroalgae serve as habitat for a large diversity of organisms from bacteria to mammals (e.g., sea otters in kelp beds). Often the organisms found in coastal plant communities support large local fisheries, such as the scallop fisheries in New England (Heck et al., 1995) and the crab fisheries in the Chesapeake Bay (Ryer et al., 1990). Mudflats also serve as habitat for a variety of infauna, including molluscs and crustaceans. Nutrient Uptake Coastal plant communities and their associated epiphytes (microalgae on their leaf or root surface) are quite effective in removing nutrients from the water column. Excessive nutrients in the water column can lead to increased turbidity due to the proliferation of phytoplankton and cause the loss of benthic vegetation such as seagrasses. Marshes and mangroves remove nutrients from land runoff before it reaches coastal waters, and seagrasses further reduce nutrient availability.
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Mitigating Shore Erosion Along Sheltered Coasts Food Production Coastal plant communities fuel the food web in shallow waters. Macroalgae are often directly consumed by organisms, such as sea urchins and herbivore fishes. In contrast, marshes, mangroves, and seagrasses serve as an indirect food source in the form of detritus. The detritus particles are not only consumed within the specific plant communities but are also exported to adjacent unvegetated areas, such as mudflats. Mudflats are well known to attract migratory birds due to the food they provide (Zhao et al., 2004). Wave Attenuation Intertidal vegetation presents a barrier to water flow, providing some wave attenuation. This can be important for protection of the coastal area as seen during the tsunami of December 2004 in the Indian Ocean. In villages protected by a fringe of mangroves, houses were less damaged than in areas where the mangroves were cut (Badola and Hussain, 2005; Danielsen et al., 2005). It has also been speculated that the impact of Hurricane Katrina on Louisiana would have been reduced if local marshes were still relatively intact. Even when unvegetated, mudflats contribute to wave attenuation via their gentle slope, often over extensive areas. Sediment Stabilization Wave attenuation (described above) leads to sediment stabilization and less damage to the area colonized by coastal plant communities during storm events. Maintenance of Biodiversity Coastal plant communities host a variety of organisms directly on their leaves and roots. As a result, the diversity in these vegetated systems is much higher than in unvegetated areas. Even so, mudflats host a diverse animal community, especially those associated to the sediments. Recreation Marshes and seagrass beds are well known as hunting and fishing grounds, respectively. Ecotours are offered through mangrove forests and scuba diving in kelp forests (macroalgae) is a tourist attraction on the West Coast of the United States. Bird-watchers are also attracted to mudflats where migratory birds feed.
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Mitigating Shore Erosion Along Sheltered Coasts Production of Raw Materials In developing countries, wood from mangroves is used as a fuel for cooking and marsh plants are a source of roofing material. Macroalgae supply phycocolloids used in many products, such as ice cream, tooth paste, fertilizers, etc. The Impact of Shoreline Stabilizing Structures on Mudflats and Vegetated Communities Surprisingly, little is known about the impact of erosion control structures on adjacent natural communities. The exception is the protective effect of breakwaters and sills on marshes. If plant communities are allowed to erode, most of the ecosystem services they provide will be lost. Marshes Natural coastal wetlands provide a broad range of ecosystem services. Restored wetlands, especially in areas where the substrate and seed bank are intact, can regain the lost ecosystem services. The restoration may take from 1 to 5 years to reach ecosystem services comparable to a natural or reference system and some may never become fully functional (NRC, 2001). Managing the hydrological function (i.e., sediment elevation and drainage) and the wave climate (possibly with a nearshore structure such as a sill or breakwater) is essential in wetland creation. Once these habitat criteria have been met, marsh vegetation can be established in 2 to 3 years from planting, although sediment characteristics will take longer (perhaps decades) to achieve comparability with natural or reference systems. Relatively calm conditions are required for marshes to thrive. Therefore, marshes can benefit from breakwaters and sills. Quite often marshes are planted in combination with the installation of these structures to provide sediment stabilization. The success of such hybrid projects (structure-marsh hybrid) depends on the elevation of the marsh habitat because the plants cannot tolerate excessive periods of inundation. Over longer periods of time (possibly decades) bulkheads and seawalls have the potential to be detrimental to marshes, especially in areas with rapid relative sea-level rise. Bulkheads and seawalls are usually placed landward of the marsh preventing migration of the marsh shoreward as land becomes inundated, resulting in loss of the marsh as water levels rise or the marsh edge erodes. Bulkheads, revetments, groins, bridges, culverts, and diking, restrict tidal flow in marshes. Restricting flow leads to freshening of the marsh, increasing the likelihood for Phragmites invasion and loss of naturally occurring marsh grasses that support ecosystem functions. The impacts of groins and revetments on marshes have not yet been thoroughly studied and documented.
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Mitigating Shore Erosion Along Sheltered Coasts Mangroves For mangroves, establishment takes longer than for marshes (3 to 5 years, see Lewis et al., 2005). The ecosystem services provided by these restored or created habitats include all of those listed for natural systems, but it takes time for the systems to mature. Algal production dominates at first, followed by grasses. Fish are likely to begin to use these areas as refuges almost immediately and gradually for foraging. The benthic infauna are slower to stabilize because the sediment conditions may take several years to evolve. Birds and mammals, depending upon habitat requirements, may be present during restoration and creation or immediately thereafter. Just like marshes, mangroves can benefit from breakwaters and sills as they also need relatively calm conditions to thrive (Markley et al., 1992; Field, 1997; Snedaker and Biber, 1997; Milano, 1999). The impact of bulkheads, seawalls, groins and revetment on mangroves has not yet been documented. Macroalgae Breakwaters, sills, bulkheads, and seawalls are built with rocks, concrete or wood which are suitable substrates for macroalgae. As a result, the installation of these structures can lead to the growth of macroalgae on the subtidal portion of the structures if sufficient light is available. The algal diversity on these structures is usually lower than that found on rocky shores and new, invasive species can spread relatively quickly (see case study mentioned later in this chapter). Seagrasses Except for one study along a highly exposed area (2 meter waves) in Japan (Dan et al., 1998), the effect of breakwaters and sills on seagrasses has not yet been quantified. In this study, seagrasses grew in an area protected by a breakwater while they were absent in the adjacent unprotected area. The authors conclude that the breakwater is beneficial to the seagrasses along this wave-exposed shoreline. In the Chesapeake Bay at a relatively low energy environment (0.4 m storm waves), it appears that breakwaters can be detrimental to seagrasses over long periods of time (decades). Breakwaters tend to accumulate organic and fine particles shoreward of the structure, slowly making the area unsuitable for seagrass growth (Koch, 2005). Although no peer-reviewed studies were found in a literature survey, it appears that seawalls and bulkheads can be detrimental to seagrasses when trees are planted at the edge of the structure. Trees tend to shade seagrasses in the shallow waters adjacent to the structures. Also, as waters adjacent to revetments and seawalls become deeper, there may not be sufficient light to support seagrass growth, leading to the complete loss of this vegetation type. This remains to be quantified.
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Mitigating Shore Erosion Along Sheltered Coasts Bluffs Ecosystem Services The ecology of marine bluffs (see Figure 4-3 and Chapter 1 for description) has been less thoroughly studied than other shorelines. As a result, ecosystem services have not been adequately documented, but are likely to include: Habitat Bluff habitat varies depending on differences in substrates, rainfall, wind exposure, and other physical factors. Bluffs, particularly forested bluffs, provide many unique and important habitat features. They tend to be covered in a variety of tree and groundcover species, with stable bluffs supporting old-growth forests and unstable bluffs being suitable for early successional species. These forest habitats provide cover for a variety of terrestrial organisms; in particular, they provide secure nesting sites and hunting perches for eagles, ospreys, herons, and a variety of other avifauna. Unvegetated and rocky bluffs can also provide habitat for cavity- and ledge-dwelling birds such as cliff swallows and peregrine falcons. An added benefit of some of these steep habitats is that they are relatively difficult to access, reducing human disturbance. These habitats are particularly significant FIGURE 4-3 Conceptual diagram of a bluff emphasizing its importance along sheltered coasts and outlining processes that occur on and adjacent to cliffs.
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Mitigating Shore Erosion Along Sheltered Coasts because many of the resident species are threatened or endangered. Large organic debris (e.g., fallen debris), tree leaves, and marine plant debris, tend to drift and accumulate along banks and bluffs. This substrate provides a complex habitat for insects, amphipods, and other organisms. Another potentially unique habitat associated with bluffs are groundwater seeps; while it is hypothesized that these seeps may host unique or rare species, they are relatively unstudied (Thom et al., 1994). Nutrients and Groundwater Dynamics Groundwater flow and associated nutrients, while poorly understood, may be an important service provided by bluffs. Bluff geomorphology will greatly affect the flow of groundwater to surrounding habitats such as beaches and marshes. This affects the delivery rate of nutrient inputs, such as nitrogen and phosphorus, by groundwater to these habitats (Thom et al., 1994). Shading An important but potentially overlooked service provided by vegetated bluffs is shading. Overhanging vegetation can shade beach or marsh substrates and nearshore waters, reducing and regulating temperatures. This can have a significant affect on species living in the shaded habitat. For example, shading may be crucial to juvenile salmonids in Puget Sound, which rely on the overhanging vegetation to maintain water temperatures, provide protective cover, and supply detritus and habitat for terrestrial prey items such as insects (Levings et al., 1991). Food Production Primary and secondary production on bluffs will depend on the type of habitat, but vegetated bluffs in particular provide food for a variety of organisms, including insects, small mammals, and raptors. Recreation Unvegetated bluffs can offer unimpeded views of the sea, making them appealing for housing development. Some bluffs may be attractive for recreation because they provide a means of access to the beach, while steeper bluffs may be attractive because they offer a physical challenge to climbers. The unique habitat provided by bluffs may draw bird-watchers and other naturalists.
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Mitigating Shore Erosion Along Sheltered Coasts Wave Attenuation Bluffs are very effective at wave attenuation although excessive wave energy leads to erosion at the toe of the bluff. Sediment Source Bluffs are a source of sediment for beaches, marshes, and mudflats; changing bluff erosion rates will also alter accretion rates in these depositional systems. Maintenance of Biodiversity As previously mentioned, bluffs can be important nesting and hunting habitat for threatened and endangered bird species such as bald eagles and peregrine falcons. Also, old-growth forests, groundwater seeps, and large organic debris drifts provide unique and often complex habitats that are important elements influencing diversity. Production of Raw Materials Bluffs may be mined/quarried for their raw material—sand, gravel, stone, and rock. Forested bluffs may also be a source of timber. The Impact of Shoreline Stabilizing Structures on Bluffs One of the major impacts of bluff stabilizing structures will be on the services provided by bluffs to downstream ecosystems, such as beaches and marshes. Bluffs are a source of groundwater, nutrients, sediments, and organic debris (i.e., leaves and trees). Structures may affect the natural groundwater regime and drainage of the bluff habitat, thus impacting physical processes such as slope stability. Modifications to the bluff morphology and vegetation associated with stabilizing structures will affect the species that rely on those habitats, particularly sensitive and essential nesting habitat for birds. Shoreline structures may also decrease overhanging trees and shrubs, thus decreasing the shading of substrates below the bluff and affecting species that rely on this shading. The integrity of sedimentary bluffs depends on the health of the adjacent beach and the capacity of the beach to attenuate waves. Therefore, any negative impact that shore-stabilizing structures impose on beaches (see section on the “impact of shoreline stabilizing structures on beaches and dunes”) will also affect bluffs.
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Mitigating Shore Erosion Along Sheltered Coasts ECOSYSTEM SERVICES PROVIDED BY TECHNIQUES TO MITIGATE SHORELINE EROSION Summary Shoreline stabilizing structures also provide some ecosystem services. Although usually different and of a lower quality than those of the natural environment, the ecosystem services provided by shoreline stabilizing structures should be considered in the design and implementation of projects to protect eroding areas. The challenge for shoreline planners and permit agencies is evaluating the postconstruction services relative to those in place prior to erosion mitigation. In some instances, specific services provided by the mitigation structure or approach are desirable to specific interest groups, e.g., habitat for specific recreational fishing species, making the consideration of the change in services more complex. A summary of ecosystem services provided by the major types of shoreline protection structures is presented below and presented in Table 4-3. Bulkhead The location of the bulkhead is essential in determining its effect on its environment. Spalding and Jackson (2001) show that bulkheads located high in the intertidal did not affect the beach habitat while a bulkhead in the subtidal promoted the loss of sediment and associated meiofauna. When partially submersed, bulkheads could serve as a substrate to molluscs, algae, and associated organisms. Therefore, partially submersed bulkheads appear to only add a minor ecosystem service (if any at all) to an area. The losses of ecosystem services overweigh the gains. Revetment Positive and negative effects have been reported for revetments (Quigley and Harper, 2004). Negative effects were observed when revetments led to the loss of natural vegetation which was shown to always be superior in providing ecosystem services than the engineered structure. Positive effects were observed when revetments were installed in combination with other environmental management practices, such as storm water management, in highly degraded areas. The stones used in revetments become colonized by micro and macroalgae which provides food for organisms. Additionally, the rocks serve as habitat for filter feeders, and these can improve water clarity by removing particulates, including microalgae (Newell and Ott, 1999). Bryophytes and associated meiofauna on riprap have also been found to increase spatial diversity (Linhart et al., 2002). Sculpins were more abundant at revetments than sandy or cobble beaches (Toft et al., 2004). In summary, revetments may provide some ecosystem services (mainly habitat) particularly in areas undergoing extensive erosion. However,
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Mitigating Shore Erosion Along Sheltered Coasts TABLE 4-3 Summary of Ecosystem Services Provided by Natural Coastal Ecosystems as Well as by Commonly Used Techniques to Mitigate Shoreline Erosion NOTES: The darker symbol (●) represents the highest degree of contribution for the ecosystem service listed while the lighter symbol (○) represents a low contribution. The symbol represents an intermediate contribution and a dash (–) suggests that the ecosystem service in not relevant or nonexistent. Please note that the ecosystem services assigned to techniques to mitigate shoreline erosion are best estimates. Extensive research is still needed to determine the ecosystem services provided by these techniques.
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Mitigating Shore Erosion Along Sheltered Coasts even in these degraded environments, some ecosystem services will be lost with the installation of a revetment. Breakwater and Sill Coastal vegetation that thrives in sheltered environments can benefit from breakwaters, at least in the short term. Kelp (Macrocystis pyrifera), marsh (Spartina alterniflora), and seagrasses (Zostera marina) have been shown to benefit from breakwaters along high energy shorelines (Rice et al., 1989; Allen et al., 1990; Rennie, 1990). Some caution is needed though, as it appears that the accumulation of fine and organic particles shoreward of breakwaters may lead to the loss of seagrasses in the long term (Koch, unpublished data). Breakwater units, if built of stone, have also been shown to provide hard substrate that is beneficial to algae, barnacles, and oysters, and creates a foraging area for fish (USACE et al., 1990). In one estimate, the primary production of algae colonizing the hard substrates of a marina (breakwaters, pilings) apparently compensated for the loss of primary production due to deepening of the nearshore zone during construction (Iannuzzi et al., 1996). The benefit of breakwaters can be enhanced when these are covered with oysters which provide the ecosystem service of water filtration and improvement of water quality (Newell and Koch, 2004). It has been proposed that breakwaters can provide a variety of benefits, particularly to fish communities. However, the evidence for these benefits is mixed. Some studies suggest that breakwaters can serve as a habitat for fishes (Stephens et al., 1994) and often have a higher fish species richness than natural reefs (Lincoln Smith et al., 1994). As a result, breakwaters could contribute to the fish larval pool (Stephens and Pondella, 2002). In contrast, another recent study suggested that shoreline hardening will have a negative impact on certain fish species and their nearshore habitat (Seitz, R.D., et al., 2005). Breakwaters have also been shown to have lower observed total diversity (including all plants and animals) than that of rocky shores (Moschellaa et al., 2005). While it is not valid to compare breakwaters to rocky or sandy beach habitat, clearly there will be a trade-off when a natural habitat is replaced by a man-made structure. When an eroding bank, narrow beach, or nearshore is changed to a stable bank, marsh or stone breakwater or sill, there is a trade-off that affects habitats. The encroachment should only be as much as the level of protection required. The stable marsh fringe and stone provide a myriad of desirable habitats, but at the expense of a narrow band of benthic communities, requiring an evaluation of the trade-offs by state regulatory agencies similar to those conducted for proposals to add or trap sand. Impacts to adjacent, unprotected coasts must also be considered. In summary, breakwaters may provide some ecosystem services related to fish, oyster, and plant habitat, and may contribute to primary production although to a limited degree. Breakwaters may also increase spatial biodiversity although this ecosystem service may not always be welcomed (i.e., introduction of new
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Mitigating Shore Erosion Along Sheltered Coasts species to a system) but, when compared to natural systems such as rocky shores, biodiversity decreases. Beach Nourishment The most detrimental direct effect of beach nourishment is the burial of shallow reefs and invertebrates reducing food availability for birds, fishes, and crabs (Peterson and Bishop, 2005). There is basically a replacement of habitat from nearshore benthic community to an intertidal and supratidal beach and dune. This is not always viewed as positive and some states are reluctant of allow this option even though it is a nonstructural alternative. The benefits of beach nourishment have not yet been appropriately quantified (Peterson and Bishop, 2005). The addition of vegetation in the form of dune grass plantings is recommended not only as added level of protection but an important habitat component. Rarely considered but also of potential importance are habitat changes in the borrow area for the beach fill (Nordstrom, 2005) where deep holes are created in the nearshore and benthic habitats are dramatically changed, at least immediately after dredging. CUMULATIVE AND SECONDARY IMPACTS OF TECHNIQUES TO MITIGATE SHORELINE EROSION As in many instances of mitigation (NRC, 2001), the cumulative effects of multiple shoreline structures or protection measures are rarely addressed. The scale of cumulative erosion control measures in some areas is massive. One of the best-documented examples is Mobile Bay, where by 1997, 30 percent of the bay’s shoreline was armored (Douglass and Pickel, 1999). This has led to the loss of 4-8 hectare (approx. 10-20 acres) of intertidal area and 6-13 kilometers (approx. 4-8 miles) of shoreline. The loss of intertidal habitat as a result of extensive placement of vertical bulkheads certainly limits recreational opportunities for local residents and reduces the availabilities of shallow intertidal habitat for nekton, although any population level effects of this loss of habitat are difficult to quantify. Similar assessments in Puget Sound (Gabriel and Terich, 2005) showed that shore protection structures have proliferated over the last 40 years with currently over 80 percent of properties protected from erosion in some areas. It is the goal of this chapter to provide factual information, not to evaluate the negative or positive impacts of the changes.The placement of mitigation structures in separate places along a shoreline can result in disruption of linkages between environments. This is of ecological concern as it can potentially interfere both with the coupling of systems in terms of energy and nutrient exchange, and as habitats of common use by organisms are disconnected. The importance of linkages between subtidal and intertidal marsh habitats for nekton has been well established (Weisberg and Lotrich, 1982). However, the effects of cumulative changes in shoreline character on system level provision of ecosystem services,
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Mitigating Shore Erosion Along Sheltered Coasts such as nursery habitat for nekton, is difficult to assess as the specific value of the habitats is itself poorly defined (Beck et al., 2003). A recent review by Airoldi et al. (2005), showed that the proliferation of shore protecting structures can have critical effects on regional species diversity. A large number of nearby structures can disrupt natural barriers (e.g., extensive sand beaches serve as a barrier for the dispersion of organisms associated with rocky shores) enhancing the dispersal of species characteristic of rocky shores in regions that were naturally poorly connected. This can also lead to dispersal routes for invasive species as seen in northeastern Italy where the invasive macroalga Codium fragile subsp. tomentosoides rapidly expanded along the 300 km (approx. 190 miles) of protected shorelines. In summary, a landscape approach that includes population dynamics needs to be considered when permitting shore protecting structures. The best examples on how to manage natural resources at a regional scale can be found in the design of marine protected areas (see Kinlan and Gaines, 2003; Lubchenco et al., 2003), which also needs to consider habitat fragmentation, loss of habitat connectivity, and dispersal of species at the landscape level (Airoldi et al., 2005). CASE STUDIES Ecological Impacts of Low-Crested Breakwaters in Europe Extensive work on the ecological consequences of building low-crested structures (LCS; i.e., sills) has been recently completed in Europe (see Moschellaa et al., 2005). Six structures exposed to a variety of hydrodynamic conditions were studied in detail. Surprisingly, despite geographical, hydrodynamic, and engineering differences, some clear patterns emerged. Ecological changes induced by the LCS were mainly a result of a modification of hydrodynamic conditions and sediment composition landward of the structures. The presence of sills increased the overall species diversity of infauna mainly due to a larger number of species found in the sheltered area (Martin et al., 2005). The introduction of new species is seen as a negative transformation of the environment by the authors (Martin et al., 2005). The sills also attracted fish species typical of rocky shores, but these remained in the juvenile stages. As a result, intense fisheries activities around the structures were absent. Additionally, the installation of sills or breakwaters killed the benthic organisms in the footprint of the structure. The accumulation of fine and organic rich sediments landward of the LCS, especially those along relatively sheltered coastlines, resulted in a shift in the composition of the infauna (Martin et al., 2005). In some instances, the same processes that led to the accumulation of fine and organic particles also led to the excessive accumulation of macroalgae such as Ulva lactuca creating unpleasant conditions (rotting algae) for recreational activities. In order to minimize the negative impacts associated with low-crested breakwaters (altered hydrodynamics and deposition of fine and organic sediment),
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Mitigating Shore Erosion Along Sheltered Coasts Martin et al. (2005) suggest that the structure be built: (1) as far away from shore as possible, (2) as porous as possible, (3) with as much overtopping (i.e., water flowing over the structure) as possible, (4) with maximum gap size and number, (5) without beach nourishment, (6) without lateral groins, and (7) be avoided if at all possible, in areas dominated by fine sediments. The manuscript concludes that “the number of LCS should be reduced to the minimum necessary to protect the coast, avoiding large-scale effects of habitat loss, fragmentation and community changes.” These ecological considerations have been incorporated into a model to design more environmentally friendly structures to protect shorelines from erosion (see Zanuttigh et al., 2005). Mills Island, Chincoteague Bay, Maryland In some locations, shoreline erosion creates habitat for ecologically important species. Shoreline stabilization in these areas would lead to loss of habitats and ecosystem services. This is the case at Mills Island, located on the western shore of Chincoteague Bay, MD, an area of high relative sea-level rise. This marsh island is eroding at a rate of half a meter (approx. 2 ft) yr−1 exposing compacted peat in the subtidal areas. This sediment is not suitable to seagrass growth, a plant type that serves as habitat for a variety of commercially and ecologically important species. As a result, marsh erosion is also leading to seagrass loss with the exception of areas where dunes located within the marsh island are also eroding (Wicks, 2005). Where the dunes are eroding, a layer of sand is deposited on top of the compacted marsh peat making it a suitable seagrass substrate. The construction of any structure to reduce or stop shoreline erosion will lead to deepening of the offshore area and will no longer allow sand to cover the unsuitable compacted marsh peat, leading to the loss of seagrass habitat (Wicks, 2005). Therefore, the benefits of shoreline erosion (e.g., sediment supply) need to be considered when developing a regional shoreline protection plan. FINDINGS A general lack of information exists about the ecosystem services provided by structures to mitigate shoreline erosion. Techniques to mitigate shoreline erosion lead to the loss of some ecosystem services. The loss of ecosystem services associated to the mitigation of eroding shorelines can be localized when only a few structures exist within a system but can alter the whole area (even where such structures are absent) when a certain critical percentage of shoreline modification is exceeded. Techniques to mitigate shoreline erosion may contribute some ecosystem services, but not the range of services provided by natural systems.
Representative terms from entire chapter: