Coastal Erosion Management
A long time frame point of view
A stronger emphasis on the restoration and utilization of natural habitats for protection against erosion
Planned removal of anthropogenic structures to restore sediment supply
Factoring in both global and regional changes that will increase erosion problems (e.g., sea-level rise, tsunami potential, El Niño)
The spatial, temporal, environmental, physical, and social knowledge related to the area of interest forms the core of CEM to eliminate or at least minimize coastal erosion.
Current projections indicate that coastal erosion processes will reach unmanageable proportions due to increasing urbanization during the on-going warming climate and sea level rise. Coastal erosion processes and their management become much more critical as coastlines constitute some of the most densely populated and developed land zones in the world. That concentration of population isn’t likely to decline because the coastal zone is the ideal area for different productive activities (Gracia et al. 2018).
Coastal erosion is a normal process that has been active since land emerged from the sea. A suite of natural process and anthropogenic influences (most of the time interconnected) are involved in coastal erosion, which itself encompasses many different specific processes. Coastal erosion can occur as a slow pervasive process (i.e., due to ongoing subsidence) or rapid during extreme events (i.e., storm waves).
It is important to highlight that coastal erosion only becomes a problem when there is no room to accommodate the occurring changes (e.g., shoreline retreat). In this sense, coastal erosion is not an issue along areas where high, stable grounds back up the coast, but erosion is a severe problem for low elevated areas and uplands underlain by unstable materials, rural and urbanized. No matter the place, rates, and causes, coastal erosion is a process that currently raises a significant hazard to ecosystems and human activities and therefore must to be managed (Rangel-Buitrago et al. 2018).
World megacities (e.g., Jakarta, Manila) are located in coastal zones where the conjunction of geographic, economic, ecological, and historical conditions are attracting people, and stimulating different migration processes. Only during the last half century have world coastline populations recorded values close to 5% of the annual average for urban growth rate (UN-Habitat 2009). Under current and projected climate change scenarios, it has been estimated that along low coastlines, almost 35% of residences, if sited within 200 m from the sea, may be severely affected by erosion-related property losses over at least the next 50 years (UN-Habitat 2009).
All of the above make clear that the world needs sustainable development strategies accompanied by an optimal coastal erosion management. The coastal erosion management choice must be a function of a series of variables that include: erosion severity, causes, property rights, funding, legislation, and coastal use. In the same way, CEM must consist of all available knowledge, techniques, infrastructure, and institutional-political tools required to minimize or eliminate coastal erosion related impacts.
Coastal Erosion Management Approaches
The typology of coastal erosion management approaches can be summarized in five categories as follows.
The measures to be applied may be summarized in four specific structural solutions that can be developed stand-alone or in combination, depending on the particular conditions of the coast where they will be applied.
This approach has been used for CEM since ancient times. Structures are built of different kinds of materials (i.e., rocks, wood, concrete, steel), either parallel or perpendicular to the shore, to provide a barrier between sea and land that resists wave energy or to trap and accumulate sediment. Shore-hardening structures include:
Breakwaters: Structures placed onshore and offshore, generally constructed of hard materials, and designed to absorb wave energy before the waves reach the shoreline (Fig. 1a). These structures are necessary to protect harbor entrances but should have limited application for beach protection, because they disrupt lateral sand transport.
Groins: Structures that extend perpendicularly from the shoreline with the design purpose to trap and hold sand, thereby nourishing the beach compartments located between them. They cut the longshore transport of littoral drift.
Revetments and riprap: Shore-parallel, sloping features which break up or absorb the energy of waves but may let water and sediment pass through (Fig. 1b). Revetments are made of concrete or shaped blocks of stone laid on top of a layer of finer material. Riprap consists of layers of hardrock boulders with the largest, often weighing several tonnes, on the head. Riprap has the advantage of good permeability and looks more natural; however, these structures are in the sea-wall family with the same detrimental effects. Their permeability can contribute to piping which contributes to the wall’s sinking, requiring new material to be added.
Bulkheads and seawalls: Shore-parallel walls designed to hold bluffs and banks by completely separating land from water. Bulkheads act as retaining walls, keeping the earth or sand behind them from crumbling or slumping. Bulkheads are sometimes called “sand retention systems.” Seawalls are primarily used to resist wave action and fail for a variety of reasons including cutting off the sediment supply, steepening of the beach profile, and causing wave reflection and refraction that cause scour erosion in front of the wall, and increased erosion rates at the ends of the wall.
Gabions: These metal cages filled with rocks are stacked to form other structures (i.e., seawalls, revetments, gabions) and are one of the poorest choices for such construction on the coast.
Geotextiles: Permeable fabrics which can hold materials while water flows through. Geosynthetic tubes are large tubes consisting of a woven geotextile material filled with a slurry-mix (Fig. 1c). The mix usually includes dredged material (e.g., sand) from the nearby area but can also be a concrete mix or mortar. These tubes are placed in the same manner as seawalls or groins but may be buried as well.
Prevention of physical damage to property as a result of waves and flooding
Prevention of loss of [economic] production and income
Prevention of land loss through erosion
Prevention of loss of infrastructure (mainly focused on recreational activities)
Accelerated bottom erosion in front of the hard structures and downdrift scouring (beach loss)
Disturbance of sediment supply and beach reduction (beach and habitat loss)
Restricted public access
Potential risks for bathers (e.g., generation of rip currents, boating hazards, bottom holes)
Loss of aesthetic visual effects on the seaside landscape
Perhaps the principal problem with the use of hard structures is that once built, they have fixed the location of the coastline in its position at the time of construction. This is highly problematic because coasts are dynamic landforms which respond to several processes such as rising sea levels and wave climate. Management should allow the shoreline to migrate, both landward or seaward; once “held in place,” the system is in disequilibrium.
Beach nourishment or artificial sand supply: The artificial addition of sediment of suitable quality to a beach area that has a sediment deficit (Fig. 1d). This may be achieved through a direct placement of sands on the beach, through pumping or trickle charging (placing sediments at a single point). Sediment placement can be on the emerged part of the foreshore (“beach nourishment”) or under the water line (“underwater nourishment”). Often construction of artificial dunes accompanies such beach nourishment projects. This approach is most common, but it is expensive, destructive of habitat, requires frequent maintenance, may reduce aesthetics, and can actually increase the rate of beach erosion.
Beach drainage: Beach drainage is achieved by a system of perforated pipes under the beach to pump out water, lowering the water table. This allows water to percolate into the beach during backwash, reducing the seaward movement of sediment. The few documented examples were generally not successful.
Beach scraping: Artificial reprofiling of the beach when sediment losses are not severe enough to warrant the importation of large volumes of sediment. Reprofiling is achieved using existing beach sediment. This is a very short-term approach to combating erosion as the sand is lost in the next big storm.
Sand by-passing: Reactivation of sediment transport processes by pumping sediments that accumulated up-drift of coastal infrastructure and injecting them down-drift to maintain beaches. Although successful, this approach is rarely adopted because of the high costs and needed maintenance of the pumping station system. A variant of sand by-passing is to place sediment dredged from navigational channels onto adjacent beaches to reactivate sediment supply and transport.
Construction of stable bays: By increasing the length of the coastline, wave energy per unit length of the coast is diluted. While some coastline segments are protected, erosion continues between these hard points leading to the formation of embayments.
Cliff drainage: In order to reduce cliff failure by slumping and land sliding, the pore pressure in the rock or sediment is reduced by piping water out of the cliff or bluff, therefore preventing accumulation of water at weak rock boundaries.
Cliff profiling: Slumping and landsliding of cliff and bluff faces can be reduced by lowering the slope angle to increase cliff stability. The angle at which a cliff becomes stable is a function of rock type, geologic structure, and water content.
Rock pinning: Prevention of slippage in seaward dipping rocks can be achieved by bolting layers together to increase cohesion and stability. This does not prevent wave attack at the cliff base but reduces the threat of mass movement and thus reduces net erosion rates.
All of these soft protection techniques need ongoing and constant monitoring, maintenance, and engineering (Edge et al. 2003). Hence, high costs and capability needs should be considered when such approaches are considered. The implementation of soft defenses represents a significate shift from an action-reaction basis (erosion problem = structure) to the adoption of a more proactive approach.
Land Reclamation/Land Claim
The primary goal of this approach is simple: create new land from areas that were previously below high-tide level. This approach is considered an “aggressive” way to protect the coast to such an extent that it also may be called “advance the line,” “attack the line,” “land reclamation,” or “reclamation fill.” Land fill and reclamation is practiced with the sole objective to gain land area to be used for economic purposes, and its use is particularly prevalent in large urbanized coastal areas where land values are elevated (i.e., Hong Kong).
Most of the time, the land claim approach is employed in deltas or estuaries due to the shelter afforded to economic developments, such as ports, as well as the availability of large areas of flat land, that is accessible from both sea and land (French 1997). Generally, this approach is completed with the enclosing and protection of shore or nearshore areas (e.g., diking/polders), as well as sand filling practices, often using the same techniques used in beach nourishment.
The main advantage of the use of land claim approach is the gain of additional coastal land that can be used for diverse purposes (i.e., urbanization) and its implementation requires the use of mature soil that is optimal for agricultural production. This approach provides an attractive and valuable source of land in places where increases in the coastal population are projected.
Although the direct gain of land is beneficial, the land claim can generate negative impacts. This approach requires either the enclosure of intertidal habitats by hard defenses or their elevation to be increased above sea level to prevent inundations, thus producing a direct loss of coastal ecosystems. In the same way, hard defenses used to claim land causes erosion and scour of the coast and can hinder and even prevent natural ecosystem adjustments in response to changing factors such as sea level rise. Another disadvantage is related to acidification and pollution of the coastal zone. Acidification is linked to the action of bacteria in new sediments which create sulfuric acid when exposed to air, while pollutants can be introduced through the use of dredged sediments for raising land elevation. The above can be a big issue if the claimed land is to be used for agriculture or when coastal waters are essential for development of fishing activities.
Surge Barriers and Closure Dams
The constructions of structures to prevent extreme water levels penetrating a specific area are large-scale coastal protection projects able to defend rivers, estuaries, and tidal inlets. Closure dams are fixed structures that permanently close off a delta or estuary while surge barriers are gates or movable/fixed obstacles that can be closed to prevent flooding when an extreme water level is expected. Both cases are commonly applied at narrow tidal inlets, where the length of the structure is not required to be so high, and where defenses behind the barrier can be reduced both in height or width.
Closure dams and surge barriers may be beneficial for economic purposes. After construction, both structures can be used to develop trade activities. In the same way, they can provide a flexible approach to maintain most of the natural dynamics while still offering reliable flood protection. In particular cases, these structures allow the additional benefit of enhancing a system’s natural capacity to clean itself. This can be achieved by independently opening and closing selected barriers, depending on water movement. Closing barriers improve the ability to drive water out of the system, therefore increasing its mobility and dispersing pollutants.
The main disadvantage of implementation lies in its high costs. To implement such protection, high economic investment is required during the construction, and continuous maintenance needs are high. In the same way, surge barriers and closure dams can generate floods on the landward side of the barrier when water levels are high and, in the case of movable barriers, if the defense remains closed for an extended period. When a movable barrier is used an extra investment in flood warning systems is required to provide information about closure times. This approach can change the chemical, physical, and biological properties of the coastal system altering the inflow and outflow of water. This may generate alterations to water salinity, temperature, suspended matter, and nutrients affecting ecosystems.
Technologies that allow physical changes to accommodate increased coastal erosion such as, flood proofing, restoration, and floating agricultural systems.
Information systems to enhance understanding of coastal erosion risks in order to develop appropriate responses to eliminate or at least, minimize impacts. These include early warning systems, indicators, and mapping/zoning.
Accommodation provides opportunities for inundated land to be used for new purposes and some compensatory economic benefits could be derived from this action. However, considerable costs may be involved in the planning and restructuration of land uses. The necessary expenditure may place significant stress on national budgets, especially in developing countries.
The social and cultural implications of the implementation of the accommodation approach can be significant. A shift in the economic activity of an area can change lifestyles of the coastal inhabitants. Also, this approach may lead to living conditions being less desirable, for example, if properties are subject to occasional flooding, or if problems with sewage disposal occur. Public safety and health may thus be adversely affected by this option.
By answering large-scale coastal erosion challenges through accommodation, the full range of coastal erosion collateral effects can be addressed within a longer-term perspective. This may also reduce the need for specific direct coastal protection measures. However, accommodation must be implemented proactively as it requires advanced planning and the acceptance that some coastal zone values could be lost (IPCC CZMS 1990).
Adequate elevation of the building above the expected flood level plus the addition of wave height (i.e., building raised on stilts so flood water and erosive storm waves and/or surge can pass under the structure without inducing scour).
Keep ground-level under building open or use breakaway walls to allow passage of flood waters without causing significant structural damage or erosional scour.
Adequately anchor structures against flood flows so they are not swept away to become battering rams against other structures.
Locate utilities and septic systems below ground at depths not likely to be reached if erosional scour does occur.
Floating Agricultural Systems
Floating agricultural systems are an adaptation approach to cope with increased or prolonged flooding and erosion. In a more specific way, such systems utilize waterlogged areas over long time periods to produce new soil, food, and at the same time avoid damage from flooding and erosion.
This approach uses a substrate of rotting vegetation that acts as compost for soil creation and crop growth in waterlogged areas. The vegetation floats on the surface of the water, thus creating new spaces for agricultural purposes. Floating agricultural systems mitigate flooding erosion because plant roots create new soil and alter soil properties (i.e., aggregate stability, improve hydraulic function, and shear strength). In this way, cultivatable area can be increased, and communities can become more self-sufficient and at the same time control coastal erosion.
Floating agricultural systems are a relatively labor-intensive approach, so they can provide employment opportunities within communities. Such systems may also aid in food security by increasing the land output while supporting poor and landless people. The main disadvantages of this approach are there are very limited areas where such physical systems can be developed (e.g., shoal areas with low wave and current energy), and such systems are limited by climate conditions.
Forecasting and Early Warning Systems
Long-term weather forecasting provides some guidance in focusing on where coastal erosion events may occur in the future. This allows both managers and the public to be warned so that optimal decisions can be taken to reduce the adverse effects. As such, the primary goal of forecasting and early warning systems is to reduce the exposure.
Forecasting systems help anticipate potential events before they occur. They may determine the hazard and risk. The forecasts increase lead-time for the people to prepare and evacuate and help provide a short time for minimal responses to minimize potential erosion damage. The lead-time can range from hours up to a few days, depending on local hydrological and topographical conditions (susceptibility). The forecasting systems estimate expected erosion using data inputs from simulation tools and models (e.g., storm surge models).
Assessments and knowledge of coastal erosion-related process in the area
Local hazard monitoring (forecasts) and warning service
Erosion risk dissemination and communication service
Community response capabilities
Forecasting and early warning systems go together. Once erosion is forecasted, early warning systems provide the crucial step to communicate the threat. Due to the current climate change scenarios, both are of vital importance and help to manage and minimize the catastrophic effects of coastal erosion. It is important to highlight that forecasting and early warning systems are not a stand-alone response to minimization of the impacts of coastal erosion. Both must be coupled with emergency planning measures and should also contain an awareness raising element.
As stressed above, forecasting and early warning systems are not sufficient on their own to reduce coastal erosion risk. People’s reactions to warnings – their attitude and the nature of their response – have a significant bearing upon the effectiveness of this approach. These systems are only useful when all inhabitants of a coastal area know what the forecasting and system of warning means, what the stages of warning are, and what to do when the warnings are given (Tompkins 2005). The accuracy is a vital point in the implementation of this approach. System inaccuracies might lead to complacency if previous forecasting and warnings were unfounded. Also worthy of note, in the USA, weather reports often state “beach erosion is expected” which usually means the back beach area will be eroded (i.e., bluff, dunes, buildings’ footings) and storm surge and overwash are likely (the beach will still be there after the storm – the houses, maybe not).
Geoindicators: Describe the physical/morphological features of coastal systems and can be grouped into geology (expressing processes and materials – their relative resistance to erosion) and landforms (expressing the coastal type).
Driver-based indicators: Used to assess coastal erosion vulnerability, namely, in large-scale applications involving semi-quantitative coastal indexes (e.g., scaled in classes from 1 to 5 or from very low to very high). These indicators include the parameters driving coastal erosion (i.e., wave characteristics, sea level, wind).
Process-based indicators: These reflect the interaction between coastal erosion driving mechanisms and the coastal morphology. These indicators are obtained through formulations or models that combine the driving variables (e.g., wave height, sea level) and the morphological parameters (e.g., cliff height, lithology). The resulting indicator has a physical meaning, directly related to the coastal erosion hazard.
Indicators are used to evaluate erosion susceptibility of coastal areas through a relatively fast and low-cost approach. And indicators are of practical use in coastal zone planning and management, because they allow a different kind of monitoring in coastal erosion management. However, indicators are not sufficient on their own to reduce coastal erosion but are tools and the selection of the appropriate indicator and associated thresholds is fundamental to obtain optimal coastal erosion hazard estimation.
Ideally, mapping should be the first step in CEM, because this is the broadest approach to define those coastal areas which are at risk of erosion under specific conditions (Fig. 2c). Coastal zone mapping usually consists of a range of map types from traditional (e.g., topography, land forms) to specific hazards (e.g., FEMA flood maps, storm surge maps, erosion rate maps). Computer modelling combined with other map data bases allows construction of hazard-specific maps (e.g., Puerto Rico’s Tsunami Zone maps). With computers, combining various types of coastal hazard maps provide for hybrid maps to express general coastal vulnerability, usually expressed in terms of a graded coastal vulnerability index (CVI). Coastal erosion is always an aspect of this type of mapping, and all of these map types can be applied to CEM. FEMA flood maps have designated “wave velocity zones” which can be taken as erosion zones, especially for coasts of erodible material. Storm surge maps similarly may indicate the reach of erosive waves and overwash/backwash currents. Projected sea-level rise inundation maps show where new vulnerable shorelines will be. In the Pacific, modelling of El Niño flood areas can be mapped. And old maps and air photos provide data bases for mapping historic erosion rates and changes in those rates. The applied goal of all of these maps is risk reduction, but they also support CEM in working to reduce the impact of coastal erosion. Mapping shows the expected extent of the erosion process in a given location, based on various scenarios.
Mapping is usually the basis for zoning, the common management approach for appropriate land use planning, especially in erosion-prone areas. Coastal zoning is the division of areas into zones that can be assigned different purposes and user restrictions, based on the knowledge of the coastal erosion hazard and other restrictions. Coastal zoning allows multiple users to benefit from a coastal area under a broader sustainable management strategy. Coastal zoning schemes can constitute the regulatory and planning framework for CEM and other coastal management issues. As noted by Bapulu and Sinha (2005), mapping creates easily read, rapidly accessible data sets that facilitate the identification and zonation of areas at risk of erosion and also help prioritize mitigation and response efforts.
One of the drawbacks of past mapping is that the conclusions for some hazards, such as coastal erosion, flood levels, and storm surge levels, are based on past records (historic) or models which also use past information (e.g., wave data, storm frequency) and do not have mechanisms for including the changes that are known to be occurring (e.g., sea-level rise, subsidence rates, impacts of land-use changes). Some newer mapping techniques are now including these variables and generating maps that are forward looking, rather than expecting conditions to remain as they were in the past (e.g., AMBUR, Jackson et al. 2012).
Climate change must be a primary consideration in future mapping. The effects of climate change are far more reaching than just the sea-level rise, and many of the coming changes will create new interactions with the dynamic nature of the coast. For example, changes in extreme waves, occurring as a result of climate change, will generate changes in the areas susceptible to erosion. So traditional maps will require periodic updates to reflect the changing risk of erosion and also to update the existing zonation. Maps should have many final users including developers to determine areas at risk of erosion and by insurers to assess premiums in areas where property is at risk. Maps must be integrated with other procedures, such as emergency response planning and town planning before the full benefits can be realized.
Use of Ecosystems
Develop or maintain ecosystems quality
Ensure the best way for delivery of different ecosystem services to human well-being
CEM based on habitat diversification can be applied worldwide, particularly in areas that have sufficient space between existing urbanization and the coastline to allow for ecosystem development (for example, in accommodation strategies). Because ecosystems have the natural ability to reduce extreme wave effects (Shepard et al. 2011), their growth can keep pace with sea-level rise through sediment accretion if available (Kirwan et al. 2010).
The use of this approach gives multiple benefits in comparison with common conventional methods of shore hardening. These benefits include natural habitat conservation, the creation of recreational spaces, carbon sequestration, improvement of water quality, production of fisheries, and wave attenuation. In fact, many existing coastal ecosystems already provide some degree of protection with no installation cost.
Despite the many advantages, it is essential to highlight limitations in the implementation of ecosystem-based coastal erosion management. Ecosystems demand space to flourish and sometimes require more space than conventional protection approaches. Flexibility in planning is required as ecosystems usually have a recovery capacity after losses (e.g., storm damage), but this recovery is not immediate. In some cases, recovery can be prolonged and can even become lost due to external factors. In the same way, ecosystem development and functionality will depend on the coastal setting, hydrodynamics, human factors, and habitat dimensions, together with the severity of coastal erosion. Some common protective ecosystems with respect to CEM are as follows.
These ecosystems have many functions in the coastal erosion management process, but the most important are sediment generation and energy dissipation (Fig. 3a). The organisms that make up reefs have skeletons composed of calcium carbonate which allows them to build the rigid reef framework and also to produce sediment when broken up by wave activity or bioerosion. So reefs serve a double function in building the wave-resistant ecosystem and generating sediments. Fragmentation and erosion processes (including bioerosion) transform reef structures into biogenic, calcareous sands, able to feed the coast, and maintain its stability. Reef organisms can also contribute to sedimentation through allogenic processes related to mucous secretions. Coral secretions trap suspended particles in the water column, forming aggregates that sink rapidly to the bottom. Secretions by corals can make it the dominant form of suspended organic matter within and around coral reefs (Marshall 1968).
Their shape and structure allow coral reefs to act as barriers that dissipate wave energy, providing a natural submerged breakwater. Their geometry (porosity, surface, tortuosity, roughness, and the overall void matrix), water depth above the reef system (depth of flow), and length in the direction of wave propagation are vital points in the wave energy dissipation process (Gracia et al. 2018).
Corals collaterally generate environmental and socioeconomic benefits. These ecosystems create high-value biodiversity hotspots that also filter waters, positively affecting overall water quality. From a socioeconomic point of view, reefs provide an essential source of food and income for local livelihoods (i.e., fishing and tourist activities) and contain marine organisms with chemical compounds that have a high potential for medical use.
The main disadvantage lies in that protection activities can be complex technologically and politically – mainly when stress sources are found, and significant economic development activities demand to be foregone in order to protect the reefs. Unfortunately, global warming and acidification of sea water are drivers of reef degradation which cannot be managed and eliminated on a local level.
Wetlands (Mangroves and Salt Marshes)
These are land areas saturated with water, seasonally or permanently, such that they take on the characteristics of a distinct ecosystem (Fig. 3b, c). The prime factor that distinguishes wetlands is the characteristic vegetation of aquatic plants adapted to the unique hydric soil conditions. The major coastal wetlands, mangroves, and salt marshes, are termed “the genuine ecosystem engineers” because of the conjunction of their intrinsic properties (width, structure, tree size), their link with other ecosystems, their function as marine nursery grounds, and their ability to contribute to sediment production and reduction of coastal erosion.
Wetlands can significantly reduce the energy of any fluid that is moving through them. The energy power lost when wind and waves pass through roots and branches can range from 15% to 65%, minimizing seabed scour and further sediment movement (Spalding et al. 2014). At the same time, their structure can reduce winds across the surface of the water, reducing the reformation or propagation of waves. In the 2004 Indian Ocean tsunami, some areas fronted by mangrove stands suffered less damage than unvegetated shores. Vegetation structure is essential in determining possible protection during extreme erosive wave events and tsunamis. Of course, this protection capacity also depends on the size and forward speed of the storm, tsunami characteristics, and coast setting. However, it has been determined that wetlands are more efficient at reducing surge levels if the event passes over relatively quickly.
Mangrove and marsh-grass roots help generate and bind new sediments and soils. The above-ground roots slow down the water-flow process, stimulate sediment deposition, and reduce erosion. Sedimentation rates correlate with root density and between 70% and 80% of suspended sediment brought in from coastal waters may be trapped in wetlands (Furukawa et al. 1997).
Wetlands also provide habitat for a diverse range of plant and animal species (breeding and nurseries for fish, birds, shellfish, and mammals) and can release organic matter to the ocean (e.g., mangroves) providing support for marine life. From an economic point of view, they often serve as an essential source of food and resources for livelihoods (e.g., wood, fishing, tourism) for local communities.
Seagrasses can significantly influence the hydrodynamic environment by reducing flow velocity, dissipating wave energy, and stabilizing sediment. When a wave reaches the ecosystem, a negligible energy reflection and weak wave attenuation by friction are produced. This situation is opposite of what happens with most hard structures, where the same process gives rise to higher energy reflection and wave attenuation by breaking and friction with further loss of sediment. Additionally, meadows can stabilize and maintain sediments in shallow areas. Due to their ability to dampen waves and currents, seagrass canopies can increase sediment deposition, decrease resuspension, and even directly intercept suspended sediment. Also, substances secreted by epiphytes can bind sediment particles to seagrass leaves. In the same way, they can influence the original seafloor bathymetry through the accretion of rhizomes and roots in the sediments, thus exerting new forces over hydrodynamics and sedimentation.
The efficiency of this ecosystem is primarily and firmly based on the density, standing biomass, plant stiffness and incident energy flux (Gracia et al. 2018). Optimal conditions for enhancing coastal erosion defense provided by seagrasses can be reached at shallow waters and low wave energy environments. Like reefs, seagrass ecosystems can create new high-value biodiversity hotspots and also can filter waters, positively affecting overall water quality. Seagrass meadows support commercial fisheries (offering nursery functions for commercial species).
Artificial plantings of mangroves and seagrasses to reestablish existing ecosystems or encourage new growth should be a consideration for any CEM plan. Removal of either of these systems usually leads to an increase in the shoreline rate of erosion. A primary problem in implementing their use is that some coastlines have high levels of recreational or development activities that disturb these ecosystems.
Shellfish Beds, Banks, and Hardgrounds
A shellfish community or single shellfish species can create shell banks or beds on the sea floor that produce sediment as well as offering a wave-energy dissipating effect. Like reef skeletal material, most shells are calcium carbonate and produce sediment when fragmented. Some, like serpulid worms (polychaetes), are encrusters and bind sediment or may build small reef-like structures. If organisms have a cement binding, they produce a hardground that is resistant to erosion. These ecosystems are topographically rough, with fractal complexity capable of reducing wave energy and erosion (Commito and Rusignuolo 2000). Their structures can act as barriers that generate dams, to hold pools of water, and increase immersion time above the shoreward bank margin, facilitating sediment deposition. Extensive shellfish banks and beds can minimize the impacts of direct water flow, extreme waves, storm surges, and can stabilize the shoreline. Under low-to-moderate flow action, increased deposition on beds causes sediment to build up to form banks that can be higher than the ambient substrate.
Can provide habitat for other species by creating a hard substrate with high surface complexity, acting as attachment sites for sessile organisms and refuges for mobile microorganisms, supporting high levels of species diversity.
Can accrete dead shell material such that the bank grows in size and mass over time (except where restricted by tidal exposure or when harvested) with decay occurring at varying rates.
Can provide food for other organisms, either when consumed directly or through the species assemblages they support.
Can filter the water and reduce turbidity by extracting phytoplankton and organic and inorganic particles from the water column.
Carbonate shells accumulate carbon in the calcium carbonate of their shells that help reduce the concentration of greenhouse gasses.
Natural dunes offer both a protective barrier during storms and an obvious sand supply for natural nourishment of adjacent beaches (Fig. 3d). Some management schemes include bulldozing sand dikes or ridges under the guise of “dunes,” but true dunes are a complex ecosystem that relies heavily on vegetation and a soil biota. Dune vegetation stimulates dune growth by trapping and stabilizing wind-moving sand. Small plants located on the face of eroded dunes can enhance the natural development above the limit of direct wind or wave attack. These coastal ecosystems need conservation in any CEM plan. Dunes are extremely sensitive to foot traffic and should be protected by such means as constructed walk overs, or limited access paths.
Dune vegetation can be transplanted to encourage the growth of new foredunes along the toe of existing dunes, as long as these species are tolerant to occasional seawater inundation. Planting vegetation from seed can be undertaken but will not usually be successful in the dynamic foredune environment. Transplanted grasses also need fertilization to encourage the associated soil biota (e.g., Vesicular-Arbuscular Mycorrhizal Fungi).
Vegetation sowing and transplanting per se will not construct new dunes or completely prevent erosion. Plants encourage natural recovery by creating a reservoir of sand within the dunes that better enable their ability to withstand erosion. Fencing, thatching, and beach recycling are often necessary to help in sand accretion. Additionally, these works can provide extra protection from waves and will reduce damage due to trampling. Once vegetation is well established, dunes may become self-sustaining, although any erosion damage will need to be quickly repaired.
The use of dune vegetation can decrease adverse effects on landscapes (even artificial dunes). In the same way, developing a dune system can restore a degree of the natural character to places that once had natural dune complexes before development and provide a valuable coastal habitat for animals. From an economic point of view, dunes help to reach multiple management objectives, such as public access to recreational resources and hazard mitigation.
Transplanting and management of appropriate dune vegetation will have no damaging impact on the natural environment of the receiving area but can be harmful to the borrow area, particularly when sand is bulldozed off of the beach. Over-harvesting of transplants from any region can give rise to increased erosion in specific points. This may be most significant for salt-tolerant sand-couch vegetation, as the borrow area will necessarily be a foredune susceptible to wave over-washing and wind erosion. Also, the use of dune vegetation in coastal erosion management typically requires dune fencing or thatching to achieve success. The construction of fences or thatching will disrupt public use of the coast, so provision must be made for controlled access.
This approach works well in large areas where retreat strategies allow coastal ecosystems (e.g., wetlands) to adjust to increased levels of the sea through a slow landward migration. On the other hand, in small areas (e.g., islands), retreat does not offer a suitable alternative. There may be little or no land for resettlement and also many ecosystems, heritage, and cultural assets that will be affected or could be lost entirely.
Managed/planned retreat raises significant transboundary implications. The institutional capability to conduct a retreat on a temporary or permanent basis must be established. Currently, managed/planned retreat is supported under practices and legal tools that allow moving property and people out of coastal zones affected by erosion.
Managed realignment is the process of moving back existing defenses in a coastline allowing the reestablishment or creation of a new intertidal habitat area through its natural regenerative capacity or reclaimed land (Fig. 4). This intertidal habitat will be highly effective at attenuating wave energy and also will reduce offshore sediment transport and therefore erosion. Intertidal habitats also can form dense root mats which increase the stability of intertidal sediments, helping to mitigate coastal erosion (USACE 1989).
This approach involves well thought out plans for the retreat of communities and ecosystems with a landward shift of the ecosystems. It is appropriate in areas where low-cost agricultural land holds sway, where there are few structures or directly affected stakeholders, and costs of compensating owners are minimized.
Removal: Defenses are removed allowing the coastline to respond more naturally to hydrodynamic process.
Breaching: Selected existing defenses are removed to allow tides and waves into the previously protected land.
Realignment: Change in the position of the line of defense to favor the development of a more dynamic coastline.
Controlled tidal restoration: Tidal flow into the embanked area is controlled while defenses are maintained.
The main disadvantage of the managed realignment is the loss of private properties and commercial income. This approach is not politically and economically viable in urban areas or where agricultural land is of high quality.
- Setbacks – hold the line: A coastal setback can be defined as the distance to keep structures out of coastal erosion or at least at a reasonable distance from a hazardous process. The idea of a coastal setback is simple: to allow room for the high water mark to naturally move inland throughout all the service life of the property. This approach enables erosion to continue along specific sections of the coast, while further development is not authorized or at least restricted. Setbacks can be divided into two types:
Fixed: Limits growth for a set distance landward from a specific reference feature.
Rolling: The regulatory line shifts landward according to topography or shoreline movement (dynamic or natural processes are used to lay out the setback line).
The adoption of arbitrary distances which do not indeed represent the threat from erosion can generate problems; in that sense, setbacks should be established based on real information (i.e., shoreline evolution or extreme water levels).
Rolling easements: This legally enforceable expectation is that the shore or human access along the shore can migrate inland instead of being squeezed between eroding coasts and a fixed property line or physical structure (Titus 2011). In a more specific way, it is a legal instrument that allows publically owned land, and land use restrictions, to migrate inland as shorelines retreat.
Rolling easements, like coastal setbacks, are measured against a coastal benchmark (e.g., high water line, dune crest or vegetated area). Because these benchmarks are expected to move dynamically with changing coastal conditions, the easement is said to “roll.”
- A rolling easement can work in two ways:
As a regulation that prohibits shore protection.
As a property right to ensure that an area moves inland with the natural retreat of the shore.
Although the regulatory approach is the more common way to prevent shore protection, the nonregulatory approach may sometimes work better. Private land trusts, government agencies, and (for some plans) even private citizens can buy (or secure donations of) rolling easements from property owners. An owner who has voluntarily engaged in the creation of the rolling easement is more likely to perceive the arrangement as fair than a landowner subjected to government regulation.
No-build/No dwelling zones: These are areas, defined under laws and policies, that are not recommended, or are forbidden for human habitation and infrastructure development because of a high degree of coastal erosion susceptibility. The basis of this approach lies in the conservation of an eroding shoreline which is more than just a buffer zone and usually is a state policy to uphold the people’s constitutional right to life and property.
Construction disincentives: Legal strategies to encourage people not to build in high-risk zones or areas of critical habitat. This approach is based on the premise that construction over regions with high erosion rates cannot be covered for insurance or any federal assistance (i.e., small business loans and funding to rebuild infrastructure).
Land acquisition is the process wherein coastal lands are acquired by the government to protect them from future coastal erosion related problems. This acquisition can be made through negotiated agreements where owners voluntarily sell the property, or, in extreme cases using compulsory purchase.
In some nations, the acquisition approach is developed through a legal figure called “coastal public reserves.” It consists in the designation of a strip of land measured from the high water mark and running along the shoreline as land owned by the state. This approach provides benefits concerning conservation, providing public access to the shore, contributing to recreational and tourism needs, preserving aesthetics, and protecting habitat.
Active: Undertaken by moving a building back either before it is threatened, or, if threatened, before it is damaged.
Passive: Rebuilding a destroyed structure in another area, away from the shore, and out of the coastal hazard zone.
Long-term: Implies a broader strategy through community zoning or land use plans that identify a frontal zone of buildings likely to be affected by known erosion rates.
Although relocation may represent the most effective long-term adaptation strategy for some coastal communities, this option is still considered outside the range of acceptable possibilities due to political, institutional, sociocultural, and economic considerations. Also, the “where” and “when” to relocate has not always been clear or adequately enforced.
As a “do nothing” approach in which buildings are regarded as having a fixed lifespan, and when their time comes to fall into the sea, no attempt is made to protect them.
As an emergency program that encourages relocation of buildings before a hazard event to avoid economic loss.
As a law that bans post-erosion reconstruction or by requiring relocation landward of the revised post-erosion setback control line.
As denial of insurance and subsidy programs in order to discourage rebuilding actions after erosion.
Identify the rationale behind the decision.
Use demonstration sites to present the feasibility of accommodating retreat.
Assess the geomorphological/ecological changes and indicate the advantages of allowing them to occur.
In the twenty-first century, the wise implementation of coastal erosion management is facing a challenging set of global changes: an increasing population in the coastal zone, the parallel increase in property development, the on-going sea-level rise, increasing the reach of erosional processes, potential increasing storminess (wave energy, storm, surge, rainfall runoff), and the unsustainability of traditional antierosion projects (e.g., increasing costs for hard and soft approaches, and decreasing sand supplies for nourishment). Past coastal erosion management has not been particularly successful, primarily because social-political systems are more driven by the profit motive than by utilizing the knowledge base and tools to avoid and/or mitigate erosion. The time has arrived to do more than just implementing one or a combination of the previously presented approaches. Improving CEM requires policies and implementation processes involving more in-depth knowledge of the processes implicated in coastal erosion and optimal strategies and operative management frameworks. The basis should be based on historical and scientific knowledge in order to come to solutions that fit CEM frameworks and should be carried out following the best available techniques. Clearly, some traditional strategies must be abandoned and current underutilized approaches given more consideration in addressing erosion problems (e.g., use of ecosystems, managed retreat).
CEM is not about just solving a problem by the use of one of the five approaches presented in this entry but rather about future livability with the coastal environment. CEM must be the critical point, together with the ongoing process, that demands a constant identification of risks and opportunities, implementation of coastal erosion reduction measures, and especially the review of strategy effectiveness. The optimal performance of any strategy must be carefully monitored and assessed and results feedback through the entire cycle to improve use and future interventions.
Strategic planning for CEM relies on understanding the processes responsible for shaping coastal morphology. This process not only needs to include the local or regional procedures, but also the inclusion of the processes that lead to the formation (even the physical and chemical weathering) and the physical sediment supply that are of importance for the coast. Likewise, such strategies must be developed within policy frameworks that set clear objectives, and in an institutional environment where stakeholders have different defined roles.
CEM can be a slow process, so expected results are likely to be met in the medium- to long-term time frame. In this sense being proactive is the crucial issue. Flexible strategies instead of reactive measures should be adopted to strengthen CEM and thus improve the coastal environmental quality. A considerable level of cooperation is required from all stakeholders to take actions using CEM. This will help foster and close existing links between CEM, disaster risk reduction, and climate change adaptation, as well as between science and policy.
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