Coastal Erosion Control

Definitions

Coastal erosion is a natural or anthropogenic process in which sediment is worn away from the shoreline and seafloor due to natural and anthropogenic factors, such as storms, boat wakes, tidal currents, and rising sea levels.

Erosion control refers to erosion mitigation techniques based on soft and hard structural shoreline stabilization methods and nonstructural measures.

Hard structural stabilization refers to shoreline erosion control approaches based on the construction of man-made structures, such as seawalls, breakwaters, and groins.

Soft structural stabilization refers to shoreline erosion mitigation and control measures based on soft methods, namely, sand, pebble, or gravel fill, such as beach nourishment.

Nonstructural measures refer to any coastal erosion control strategy that does not involve man-made construction or other physical measures, but is based on good practices, policies, and education aimed at reducing anthropogenic and natural impacts, such as land use restriction and zoning.

Introduction

Estuaries are transitional zones where marine and riverine environments meet, i.e., where freshwater from a river mixes with saltwater from the sea. Here, many habitats, species, and ecological communities exist, and their ecosystem and naturalistic values are widely recognized. Along coasts, river banks, and nearshore profiles are governed by sediment transport equilibrium (i.e., erosion and deposition phenomena). Alteration of this equilibrium may result in shoreward recession, leading to land loss. Since estuarine areas are often used for tourism and recreational purposes, this leads to financial loss as well as ecological community and biodiversity impairment. State and local authorities thus aim to protect estuarine areas and to mitigate erosion. A review of the causes of coastal erosion and an evaluation of the erosion processes are provided below, together with an analysis of erosion control measures.

Causes of coastal erosion

There are many causes of coastal erosion processes attributable to natural and anthropogenic factors. A classification can be based on the temporal scale of such factors, distinguishing between short- and long-term events. Natural processes consist of short-term events that are generally the result of storms and river floods (i.e., high-energy content events), while long-term events relate to sea-level rise (Pranzini and Rossi, 1995; Khalil, 1997), tidal cycles, tectonic events, coastal subsidence (Khalil, 1997), climate change, river regimes, and discharge flux (Medina and Lopez, 1997).

In regard to anthropogenic factors, sediment loss is generally due to medium- and long-term events, such as decreasing sediment supply to coastal physiographic units (Simeoni et al., 1997; Eronat, 1999; Loizidou and Iacovou, 1999), deforestation in coastal and riverine watersheds (Eronat, 1999), non-sustainable man-made coastal structures and urban development (Fathallah and Gueddari, 2001; Rakha and Abul-Azm, 2001), flow regime and engineering structure changes, and riverbed sand and gravel extraction (PAP/RAC, 2000).

Erosion processes

Suspension and bed load transport can be distinguished (Fredsøe and Deigard, 1994), the latter mainly causing the loss of grain size material on the seabed and affecting long-term and short-term shoreline evolution. Such phenomena are primarily modeled by physical and mathematical and generally consider hydrodynamic and morphological factors. Hydrodynamic models are based on the classical equations of motion, with vertical averaged velocities (two-dimension models), and wave propagation, refraction, diffraction, and breaking. They evaluate velocity flow fields under generic forcing (Figure 1).

Coastal Erosion Control, Figure 1

Numerical results of hydrodynamic coastal field at Scalea beach, Italy.

Morphological models consider seabed characteristics and evaluate bed load and shoreline evolution, for both long-term and short-term conditions (Figure 2).

Coastal Erosion Control, Figure 2

Numerical results of bed load transport at Scalea beach, Italy.

Physical models are based on two-dimensional and three-dimensional laboratory scale similarity models that use channels or large basins (Figure 3) to study both bed load processes and evaluate the effectiveness of the structural design.

Coastal Erosion Control, Figure 3

Large basin simulation: example of an experimental investigation.

Finally, it should be pointed out that both physical and mathematical models require a detailed knowledge of seabed morphology, waves and currents (i.e., river or tidal currents or wave-generated currents), bathymetric surveys, and sediment characterization (pebble or sand beaches, sediment grain size, and erodibilty changes).

Coastal erosion risk mitigation

A coastal erosion control project should be developed with reference to coastal cells (Eurosion, 2004), which are lengths of coastlines in which a complete sediment balance can be identified. Spatial and temporal erosion phenomena scales should also be identified. Acute and structural erosion can be distinguished. Acute erosion is connected to waves, wind, and tidal action and typically covers temporal and spatial scales up to 1 month and from about 1 m to 100 km, respectively. On the contrary, structural erosion involves larger temporal and spatial scales, from about 1 month to 100 years and from 1 to 1,000 km, respectively (Safecoast, 2008).

Depending on spatial scale, coastal erosion control measures can target (1) specific river bank sections and (2) larger areas or entire estuarine areas. Moreover, erosion control measures should consider the environment energy level, distinguishing between low-energy areas and ocean-facing beaches, the latter being higher-energy systems. This distinction is useful for focusing on the best erosion control project, which is strongly site specific. Coastal erosion control methods can be mainly classified as structural and nonstructural measures. Structural measures can then be further classified as “hard” and “soft” alternatives. Structural measures involve permanent concrete, rock, or wooden engineering structures, since nonstructural measures generally refer to best management practices or to actions not involving man-made structures.

The main hard erosion control measures are:

• Breakwaters

• Revetments, seawalls, and bulkheads

• Groins

These structures are designed to protect the areas behind them by “fixing” the shoreline or by modifying the flow-field circulation by “trapping” sediment.

The main soft erosion control measures are:

• Nourishment

• Rip-rap, gabions, and paved-lining revetments

• Marsh sills

• Planting vegetation

Rip-rap, gabions, paved-lining revetments, groins, marsh sills, and planting vegetation are generally effective for river bank protection, while in wave-exposed areas, coastal erosion control measures should be chosen depending on whether cross-shore or long-shore transport is dominant, with a preference for shorel-parallel structures (such as revetments, attached and detached breakwaters) or perpendicular structures (such as groins), respectively. Furthermore, good coastal planning and management practices that promote land use rehabilitation targets and eco-sustainable tourism are also suitable coastal erosion control measures. Lastly, the choice of erosion control method is strongly dependant on the environmental, social, and cultural characteristics of the areas concerned and on legislative, policy, and economic aspects. Thus, the combination of different methods is often an effective strategy.

Structural measures: hard stabilization

Hard stabilization methods are based on protective structures designed to stabilize the shore and to prevent waves and tides from reaching the area or to trap sediment. Such structures typically act by reducing incident wave energy and changing flow-field circulation. Stabilization effectiveness is strongly dependant on wave exposure, the latter being influenced by structure inclination and orientation (e.g., perpendicular or parallel to the coastline), permeability/impermeability, overtopping, etc. The main hard structures are described below.

Breakwaters

Breakwaters (Figure 4) are structures designed to protect shorelines from erosion by shielding waves (i.e., reducing incident wave energy) and changing littoral transport conditions. These structures can be directly connected to the shoreline or constructed shore-parallel, respectively, attached or detached. In the first case, breakwaters act as a revetment, protecting adjacent upland areas against scour induced by waves and currents, while detached breakwaters act by allowing sand accumulation from the original shoreline to the landward breakwater. Such sand accumulation is called a “tombolo” or “salient,” depending on whether or not the breakwater is reached by sand. Accretionary beach features are characterized by a dimensionless ratio X/B (Herbich, 1991), X representing the breakwater length and B the breakwater distance from the original shoreline. For X/B > 1, sediment deposition and accumulation behind breakwater forms until the shoreline is connected to the structure (permanent tombolo), while for X/B < 1, sediment forms from the shoreline in the lee of the structure, without reaching the breakwater (salient). Since sediment transport phenomena are in equilibrium, sand accumulation leads to the formation of an erosion zone. With reference to design and construction aspects, breakwaters can be classified as follows: (1) rock or concrete units with trapezoidal cross sections, (2) prefabricated triangular-shaped concrete units, and (3) sand-filled containers (caissons) with geotextile units.

Coastal Erosion Control, Figure 4

Detached breakwaters: definition sketch.

Breakwaters can be emergent and submerged, depending on crest position with respect to water level, namely, if the crest is positioned entirely below or above mean sea level (Figure 5). Both emergent and submerged breakwaters have a strong impact on sediment fluxes and on morphodynamic evolution, the latter depending on structure-induced waves, circulation fields, and wave overtopping. In order to avoid negative morphological effects, such as local scours, these aspects should be preliminarily and carefully analyzed. Lastly, it should be pointed out that during storm conditions, breakwater cannot stop or dissipate most of the waves, which result in low effectiveness, and thus other methods such as supplementary nourishment may be required.

Coastal Erosion Control, Figure 5

(a) Emergent; (b) submerged breakwaters.

Revetments, seawalls and bulkheads

Revetments, seawalls, and bulkheads are shore-parallel structures built adjacent to the shoreline. The main difference lies in their functional aspects (U.S. Army Corps of Engineers, 1984). In fact, revetments are stone or concrete structures built adjacent to the shoreline and are designed to protect the underlying soil from erosion. A larger stone layer is generally placed on the frontwater, with a smaller layer filter placed below it. The latter prevents underlying soil washing, while the main erosion protection is guaranteed by the upper stone layer.

Seawalls are primarily designed to protect the shore against wave action; bulkheads are retaining walls designed to provide protection in low-to-moderate wave energy environments. More specifically, seawalls are massive concrete or stone, vertical or sloped structures (Figure 6), with rubble, curved, or stepped face. A combination stepped-curved face may also be constructed. Revetment and seawall slopes should be no steeper than 1:3, and the length of the structure should be carefully selected to avoid erosion of the adjacent coastline.

Coastal Erosion Control, Figure 6

Seawall.

Bulkheads (Figure 7) are designed to protect adjacent upland areas and to retain the soil behind them. A sufficiently large embedded wall is required, and tie rods may be used to increase the stability of the structure. Toe protection is required for all these structures so as to prevent local scour.

Coastal Erosion Control, Figure 7

Wooden bulkheads (Rogers and Skrabal, 2001).

Groins

Groins are long, narrow coastal structures used both on open beaches and in estuaries, altering nearshore tidal flow patterns and deflecting currents. They are placed perpendicular or slightly perpendicular to the shoreline (Figure 8). Such structures are generally constructed in groups, so that compartments between adjacent groins can be identified, trapping sediments in each of them and extending coast longevity.

Coastal Erosion Control, Figure 8

Groins: definition sketch.

Groins are most effective when long-shore currents prevail in one direction.

Generally, two kinds of groins (Figure 9) can be distinguished (van Rijn, 2010; van Rijn, 2011):

Coastal Erosion Control, Figure 9

(a) Emergent; (b) submerged groins.

• Impermeable, high-crested structures, usually made of sheet piling or concrete structures. Crest levels are 1 m above MSL (mean sea level). A full long-shore current block is expected, so that the sand within each compartment is retained. A typical sawtooth shoreline profile should be created, thus increasing groin spacing.

• Permeable, low-crested structures, with crest level between MLW (mean low water) and MHW (mean high water). These structures act as flow resistance, reducing the littoral drift in the inner surf zone. A regular shoreline should be created.

Groins should only be constructed along coasts with recession rates in excess of 2 m/year. Their length should moreover be extended over the inner surf zone (Basco and Pope, 2004; Kana et al., 2004). High-crested, impermeable groin length (L) and spacing (S) typically varies between 50 and 100 m and between 1.5 and 3 times the length of the groin, respectively (van Rijn, 2011). Finally, groins induce local scour at the toe of the structures and thus require regular maintenance.

Structural measures: soft stabilization

Nourishment

Nourishment is a shoreline stabilization method using sand, pebbles, or gravel beach fill (Figure 10). A key parameter of successful nourishment design is the choice of fill material. In fact, sediment should have the following main characteristics (Department of Boating and Waterways and State Coastal Conservancy, 2002):

Coastal Erosion Control, Figure 10

Nourishment: (a) plane view; (b) cross section.

• No contamination

• Fine grain size fraction

• Grain size comparable to or larger than in situ material

With reference to uncontaminated sediments, the introduction of contaminated material to coastal systems not only compromises habitats and ecosystems but also creates financial loss in terms of tourism, these areas being often used for recreational purposes.

With regard to the choice of sediment grain size, sediment with comparable or larger than in situ material size characteristics is preferable. In fact, comparably, grain size material tends to have the same behavior as in situ material, while larger sediment generally results in a more stable solution. Vice versa, fine grain size usually results in less stable solutions and accelerated erosion.

Sediment grain size and contamination level are strongly dependant on the source of the nourishment material. The latter generally includes dredged sediment from harbor construction and maintenance, lagoon restoration, and offshore and inland (e.g., damming rivers dredging) sources. Fill material may be placed (1) on the dry beach (dune nourishment); (2) on the beach cross section, on the dry portion and near the waterline, and across the entire beach cross section (i.e., above and below water); and (3) offshore as a sand bar (National Research Council, 1995).

In dune nourishment configuration 1, fill material is placed high above the waterline. This configuration provides effective protection against storm waves, but no expansion in dry beach width and no increase in recreational coastal areas. In configuration 2, an immediate increase in beach width (i.e., recreational areas) is observed. Furthermore, once placed, fill material is redistributed offshore and alongshore below wave and current action until a stable configuration is achieved. If fill material is placed both above and below the waterline, an already stable configuration is attempted, and there is little offshore sand redistribution, which leads to minimal changes in dry beach width. Finally, in configuration 3, fill material is placed in the surf zone, and the sand gradually moves onshore below wave and current action, thus increasing the beach width.

In regard to environmental aspects, nourishment can have a strong impact on aquatic habitats on the seabed, near the shoreline. Thus, species ecosystem response tolerance and the burial adaptability thereof should also be considered. In all cases described above, because of the nourishment design characteristics, waves and currents gradually remove some of the sediment, and periodic maintenance is required.

Rip-rap, gabions, and paved-lining revetments

Shoreline revetments may be constructed using rip-rap revetments (Figure 11), gabions, and paved linings (Figure 12) that are wire cages filled with stones and placed as revetment along the shoreline and river banks, in vertical stacked or sloped configurations.

Coastal Erosion Control, Figure 11

Rip-rap revetments (Trowell, 2012).

Coastal Erosion Control, Figure 12

Gabions: definition sketch.

Marsh sills

Marsh sills are shore-parallel structures designed to protect planted wetland vegetation. An offshore wood or rock mound (sill) and an intertidal area are created between the sill and upland (Figure 13). Protection is achieved thanks to existing or planted vegetation in the intertidal zone, which dissipates wave energy, preventing it from reaching the upland (Rogers and Skrabal, 2001). An added value of this strategy is its ability to promote the creation of natural habitats.

Coastal Erosion Control, Figure 13

Marsh sills (Modified from Trowell, 2012).

Planting vegetation

Vegetation plays an important role in promoting shoreline stabilization, reducing wave and current energy, and trapping incoherent sediment in radical apparatuses. Thus, planting vegetation is considered a shoreline stabilization method, although its effectiveness is strongly site specific. In fact, planting vegetation generally allows good erosion control to be achieved in low-energy environments, such as in estuarine tidal zones, while on the contrary, in high-energy environment, vegetation seems ineffective. Finally, there should be a preference for the planting of native species.

Nonstructural measures

Policy and planning techniques

Policy and planning techniques for erosion control relate to strategies for coastal area use and anthropogenic pressure control, based on the introduction of sustainable coastal area management development logic. In fact, man’s use of coastal and estuarine areas for promoting economic activities generally requires intensive development and accelerated estuarine area modification processes, often leading to natural equilibrium alteration and increased vulnerability. Policy planning techniques involve a large number of factors, the majority of which are described herein.

Firstly, estuarine area management project measures are based on specific environmental policies, legislations (e.g., European Bird, 1979 and Habitats, 1992 Directives, Natura 2000 ecological networks of protected areas to name but a few), and project management development. The latter is generally based on “prevention” and “protection” measures, attending to primary coastal erosion risk management and consisting of both structural and nonstructural measures (Safecoast, 2008). Examples of “prevention” strategies are relocation, zoning, space allocation and reservation, coastal erosion risk education, and communication and raising awareness. Examples of “protection” strategies are based on building and maintaining structures for erosion control (such as breakwaters, nourishment and groins).

Good project management should be based on a precise site evaluation and decision logic systems, with focus on the following aspects (Safecoast, 2008):

• Physical and environmental characteristics

• Economic and ecological values, assessed by stakeholders and others, such as engineers, scientists, politicians, land use planners, and the public affected

• Historical and cultural background

• Policy measures and a general set of rules capable of identifying different scenarios (e.g., land use restriction)

• Coastal dynamics, effectiveness of erosion control measures, erosion phenomena, and spatial and temporal scales

On this basis, the resulting project will be the “optimum solution,” chosen in continuity with existing management policies, protection measures, and related operational procedures, such as institutional arrangement, operational responsibilities, and financing.

Conclusions

Natural and anthropogenic factors cause alterations in coastal sediment transport equilibrium, thus resulting in coastal erosion, with ecological and financial loss. Furthermore, cultural identity, resources, and recreational coastal land use are strongly compromised or lost.

Hard and soft control measures are effective tools for mitigating such phenomena. Hard stabilization methods in particular mainly consist of concrete and rock structures such as groins, revetments, breakwaters, etc., while soft methods consist of shoreline protection measures based on beach nourishment and the planting of vegetation, thus allowing recreational tourist areas to be developed. Good practices in the planning and management of coastal areas are also effective coastal erosion control measures and important targets for many states. The choice of stabilization methods should be the “optimal” method among possible solutions and should be selected starting from a hydrodynamic, environmental, social, and cultural site characterization. Finally, the combined use of different stabilization methods enables considerable coastal erosion control objectives to be achieved.

Cross-references

• Bulkheads

• Revetments

• Sediment Grain Size

• Shore Protection