COASTAL EROSION

By Niki EVELPIDOU, Anna KARKANI, Miltiadis POLIDOROU, Vasilis KOTINAS

Introduction

Coastal erosion constitutes a global issue, as 70% of the shorelines are retreating (Bird, 1985), while in Europe it is estimated that 15 km2 of shorelines are retreating annually. In Europe, more than 5 million people live in areas threatened by coastal erosion and coastal flooding. According to EUROSION 2004, in 2004 approximately 20.000 km of shorelines faced serious problems, with most of them being eroded (> 15.100 km).

In the last decades, sea level rise has become a subject of discussion not only in the scientific community but also in government agencies for decision making and problem solving. In addition, the continuous and sometimes arbitrary, residential coastal development, tourism development and destruction of coastal landforms along with the growing pressures from agriculture and industry require the study of sensitive coastal areas in order to find appropriate solutions for protection (Maglara, 2011).

The potential of predicting land loss rates and shoreline retreat is extremely important for designing future coastal zone management strategies and for assessing the future impact that may be brought about by extreme sudden or gradual physical processes (Maglara, 2011). Despite the fact that coastal erosion is a natural process, it is viewed as a natural disaster, because it causes problems on the coastal infrastructures (Doukakis, 2005).

For coastal management and the planning of land use, it is useful to separate the concepts of episodic and long-term erosion. Episodic erosion is caused during a storm and has serious effects on sandy shorelines in comparison to rocky ones. Long-term erosion, however, is owed mainly to sea level rise and results to the permanent retreat of the coast (Doukakis, 2005).

Coastal erosion (Fig. 1) is the result of a combination of factors that develop at different scales. Sea level rise, the intensity of storms and human interventions constitute the main parameters for coastal erosion (Table 1). For estimating the risk of sea level rise in a particular area, several factors are taken into account (Papanikolaou et al., 2011):

• The rate and extent of rise • The relationship between tectonics and eustasy in an area. Tectonically active

regions eliminate or reinforce sea level rise depending on block movements. • The relationship between sea level rise and changes in sediment supply: The

change may result from a climatic change (anthropogenic or natural) through which erosion rate will be affected as a consequence of precipitation variations and vegetation cover, or human interventions such as construction of dams, sand extraction, fires, etc.

Furthermore, other local factors affect coastal erosion (Doukakis, 2005; Papanikolaou et al., 2011), such as: coastal topography (protected shore or open), coastal geology (sandy or rocky), coastal morphology (inclination), climatic and wave regime

(longshore currents, wind, wave height), frequency and intensity of extreme weather and wave events, sediment supply of the area (coast near a river or not).

Table 1: Estimations erosion risk during sea level rise, and estimations of ecosystem

areas under threat in Greece (EUROSION 2000) Area

Shoreline length (km)

Shoreline length under erosion (km)

Area of coastal ecosystems in risk (km2)

North Aegean 1311 231 (17,6%) 349 South Aegean 3423 503 (14,7%) 929 Ionian 1056 260 (24,6%) 356 Crete 1148 756 (65,8%) 355 Total 6938 1750 (25,2%) 1989

Fig. 1: Coastal erosion in Marathon area (Attica). 2009: top image, 2010: image below

Types of coastal erosion

Mechanical erosion of waves The mechanical action of waves (Fig. 2) is the main erosion factor in coastal environments, through high energy waves or storm waves. In low energy conditions, the erosive action of waves is reduced; however, they still contribute significantly to the removal of weathered material. Apart from removing loose material through waves, two main results are the abrasion of rock surfaces and the pressure fluctuations induced on rocks by the waves.

Abrasion is owed to the erosive action of currents (of wave origin) and includes the rolling of rocks and sand along slightly inclined rocky surfaces and the ejection of coarse material on steep surfaces. The erosive action of water is greatly increased when transporting sand and pebbles. For example, Robinson (1977) found that the erosion rate on the base of cliffs was 15-20% greater in areas with sandy coasts, in comparison to those without one. Abrasion is greater when a fine layer of sediment is present (<0.1 m), while when a thick layer is present, the underlying rock surface is protected.

Weathering Coastal cliffs and intertidal coastal platforms are exposed to alternating wetting and drying from salt spray, wave swash, tides and rain. Subsequently, they constitute suitable environments for many physical and chemical processes of weathering. In protected areas and in erodible rocks, in particular, weathering is probably the main erosive mechanism. Wave action may be solely responsible for removing the weathered material.

In environments with storms and high energy waves, weathering may be important in some distance from the high tide level, where the waves have no access. Weathering is more efficient in the intertidal zone and above, as the mechanical action of waves.

Fig. 2: Wave impact on a coastal cliff (Cyprus)

Weathering processes are largely controlled by climatic factors (temperature, precipitation) and the rock type.

Physical weathering can derive from frost action and from alternating cycles of hydration – dehydration. The rock is fragmented through the development and subsequent widening of capillaries and larger cracks. If a frozen rock surface is submerged by a rising tide, the ice within the rock will melt. Therefore, the rock in the intertidal zone will undergo two cycles of freezing – melting during the day, when the tide is semidiurnal and temperatures are below zero, during the day. The intertidal zone of any rock surface, with semidiurnal tide, undergoes 700 cycles of hydration - dehydration per year. Frost action and the cycles of hydration – dehydration bring about significant weathering of the rock, especially if high saturation rates are obtained (Trenhaile and Mercan, 1984). These processes are important for material that absorb and maintain large water quantities, such as fine sedimentary rocks. The mechanical rock fragmentation is particularly important along rocky shores, in cold coastal regions, as the rocks are capable to retain high moisture levels and suffer multiple cycles of freezing – melting every year.

Chemical weathering is usually the result of various cooperating chemical reactions, including hydrolysis, oxidation, hydration and dissolution. The effectiveness of these processes is mainly determined by the water quantity available and the removal of insoluble products. If the weathered material remains in the system, chemical equilibrium may be achieved and prevent further weathering. It is difficult to rank the susceptibility of rocks to weathering, however, igneous rocks are considered less susceptible to chemical weathering in comparison to sedimentary. Chemical weathering is more important in warm and humid climates. Cold climates are characterized by slow rates of chemical weathering, although the lack of liquid water may be a more important factor rather than the low temperatures (Trenhaile, 1987).

Physical and chemical weathering often “cooperate” and are more effective when the rocky substrate undergoes often hydration – dehydration. Erosion due to weathering is particularly active around the margins of water basins in the supratidal or intertidal zone, where alternating hydration - dehydration occurs, as the water is replenished by spraying or tides. This type of weathering, known as water-layer leveling (Matsukura and Matsuoka, 1991), gradually removes the watersheds between rocky basins, enlarging them and eventually causing their merging. Water-layer leveling may play an important role in the lowering and alteration of coastal platforms in areas of high evaporation, but not as important in cold, wet climates.

Bio-erosion Bio-erosion is the removal of rock from organisms (Fig. 3). This process is more important in tropical regions, due to the abundance of marine biota and limestone substrates that are susceptible to biochemical and biophysical processes (Spencer, 1988). The crucial factor in the efficiency of bioerosion is the spatial distribution of marine organisms along the rock surface, which is largely controlled by the available moisture and consequently the characteristics of the tide levels and wave energy.

Mass movements The steep slopes of rocky shores testify that they are unstable and prone to mass movements. On the rocky shores many movement types may occur, depending mainly on rock properties (lithology and structure).

Rock falls (Fig. 4) and topplings are characteristic of hard rocks, particularly on the parts that rocks are well connected and the cliffs are undercut at their base by waves. They are also common on consolidated clay shores (tillites), although this material is not strictly characterized as “rocky”. Rock falls are more often during the winter months and this may be attributed to the frost action, the rain fall and increasing undercutting of the cliff base, due to storm waves.

Fig. 3: Bioerosion on limestone coasts (Theologos coastal zone, central Greece)

Landslides are large failures that occur when the compressive strength of the rock is exceeded by the overlying load. The failure starts when the strain at some part of the rock mass exceeds its strength, so that the cohesion at that part is zeroed. Landslides are usually distinguished in translational, rotational and mudslides.

All types of landslides driven by three main factors: 1) the addition of material, possibly from mass movement at the upper part of the slope that causes an increase of the overlying load, 2) the increase of slope inclination due to the base undercutting, from marine processes and 3) the reduction of the compressive strength of the rock due to the increase of the moisture content or weathering. Landslides are often activated by changes in moisture or increased wave action. For this reason they are most common in winter than in summer.

Mass movements associated with movements of high moisture material are known as flows. Depending on sediment size, they are distinguished in debris flows (coarse grained) and mudflows (fine grained). Flows take place under conditions of heavy rainfall and the presence of groundwater.

Fig. 4: Rocks falls (Theologos coastal zone, central Greece)

Main parameters defining coastal erosion

Climate The climatic conditions in the coastal zone are a significant factor for the intensity of coastal erosion. The climatic regime defines the weather conditions of an area and therefore the physical phenomena of the coastal zone, such as waves, underwater currents, coastal currents and storm surges. The wind regime is the most important climatic variable for coastal erosion processes.

The wind defines the wind waves and the resulting coastal currents. The stronger the wind in the coastal zone, the larger is the wave height and therefore the erosive action. The influence of wind on the waves is owed to the drift of the surface layer of sea water through friction. Gradually, this movement of water molecules is extended to the deeper layers, resulting in the development of wind waves. The latter ones, as they move towards the shore, impact on the coast with great velocity, causing intense erosion on existing cliffs with undercutting, or by sweeping coastal sediments offshore.

Storm conditions also bring about intense erosion on coastal environments. The resulting storm surges are much larger in height than the average wave height of an area, as on the latter one an additional height is added, owed to the disturbance of the sea mass because of the storm and the related barometric low.

Wave regime Wave action is a defining parameter in the configuration of coastal geomorphology. Marine waves are periodic mechanical oscillations of water molecules, by which energy is transferred (Karymbalis, 2010). A large part of this energy is consumed by the breaking of waves on the coast. Waves have a double part, as they not only undercut coastal cliffs, but they also transport the weathering products that are accumulated at their base.

The wave regime is directly related to the wind conditions in a coastal area. The more intense the wind the more erosive is their action, as they break on the coast.

Coastal currents regime Coastal currents transport loose sediments of the coastal zone, which derive either from fluvial processes inland and are deposited on the coast or from the weathering of coastal cliffs. In this way, they are a significant factor in the sediment balance in the coastal zone and therefore in coastal geomorphology.

Coastal wind currents owe their genesis to the waves that approach the shore and are responsible for moving large volumes of sediment along the shoreline.

The continuous arrival and breaking of waves on the shore causes the accumulation of water in the surf zone, resulting to the development of currents, either parallel to the shoreline (longshore currents) or directed to the open sea (rip currents) (Karymbalis, 2010).

The type of coastal currents in an area depends on the angle of wave incidence on the coast, the morphological characteristics of the shore as well as on the submarine geomorphology. If the waves approach the shoreline perpendicularly or almost perpendicularly, then a closed circulation regime is formed by the action of longshore currents and rip currents. In case the waves reach the shore obliquely, then only longshore currents are formed, whose action is limited in front of the breaker zone. Their characteristics depend on the incidence angle of the waves on the shore and their velocity varies between a few centimeters per second and 1 meter per second.

Rip currents are responsible for transporting sediments from the shore to the open sea. These are strong currents, of small amplitude, and they initiate at the breaker zone. Their characteristics depend on the sea level rise at the breaker zone, due to the accumulation of water mass in front of this zone. Their length may reach 60-750 meters, while their velocity ranges between 20cm/sec and larger than 2 m/sec.

Furthermore, in the breaker zone, currents from wind waves and tides also contribute to the longshore coastal transport of sediments; however, their contribution is less significant in relation to the currents that are developed by waves approaching the shores (Davidson-Arnott, 2010).

Lithology of coastal rocks Rocky coasts constitute eroded coasts and it is particularly important to identify the factors affecting their retreat rate. The main parameter controlling erosion is the rock hardness, namely its resistance to subaerial and marine erosion. The rock’s resistance to erosion is largely defined by its lithology. Sunamura (1992) correlated erosion rates to lithology as follows:

• < 0.001 m per year for granitic rocks • 0.001 – 0.01 m per year for limestones • 0.01 – 0.1 m per year for flysch and shales • 0.1 – 1 m per year for chalk and Tertiary sedimentary rocks • 1 -10 m per year for Quaternary deposits • >10 m per year for loose volcanic deposits

Other factors are equally important. For example, tectonically vulnerable areas because of intense faulting and thrusting may provide sites of accelerated erosion. Exposure to wave activity is also significant, not only because waves actively erode the cliffs but because they also remove material from their base. The presence of a sandy beach in front of a coastal cliff reduces its erosion rate, by protecting its base

(Fig. 5). Furthermore, the height of the cliff is considered also significant; the lower cliffs are eroded quicker than the higher ones, since less material needs to be removed for the cliff to retreat. Human activities can also greatly affect the erosion rate of coastal cliffs. Coastal constructions, in particular, may lead to the disturbance of coastal currents and the relocation of the protective beach in front of the cliff, leading to accelerating erosion. After all, the construction of buildings, such as houses, at the cliff tops increase the existing weight, making them prone to failure.

The evolution and retreat of coastal cliffs are occasional processes, with most morphological changes and erosion taking place during event with heavy rainfall and waves. Therefore, the interpretation of short-term erosion rates should be made with caution. The erosion rates of cliffs are often particular for specific regions and they are hard to predict, if not using past records of sufficient duration.

In coastal zones, with large-scale and wide beaches and no rocky cliffs, the erosion of sediments is more extensive and depends of the granulometry. In coarse-grained coasts, erosion is less significant in comparison to fine-grained coasts, as they have more resistance to wave energy (better penetration of waves and therefore energy absorption). Wave penetration is promoted by the existence of voids between grains. The opposite occurs in the case of fine-grained coasts. Here, erosion is very significant, because grain compaction is greater, and as a result, the penetration of waves within is very limited.

Fig. 5: Coastal cliff with the presence of a sandy beach (central Greece)

Global sea level rise During the last century, global temperatures have increased by 0.3 – 0.6°C and the eustatic sea level has risen by about 0.2 m. Although sea level rise is a normal phenomenon during an interglacial period, it appears that this rise is amplified by the anthropogenic global warming. The continuous global warming results to the continuous sea level rise, due to the thermal expansion of ocean water in combination with ice sheet melting. Global warming is attributed to the increasing atmospheric concentrations of greenhouse gasses (mainly CO2), due to fossil fuel combustion. Since humanity is unlikely to stop burning fossil fuels in the near future, gas concentrations of the greenhouse effect in the atmosphere are expected to continue to increase over the next century. Climate models have estimated that global temperatures may rise by 2°C by 2100, with the sea level rising by 0.11 – 0.77 m, during the same period (Church et al., 2001).

The ongoing and expected rise of eustatic sea level is causing a great concern in coastal countries. Sea level rise can cause coastal erosion, flooding of wetlands and salt contamination of coastal aquifers. Human activities on the coastal zone will be greatly disturbed, traditional means of transport will be affected and farmlands will become unsuitable.

Fig. 6: Erosion of coastal cliff (coastal zone of Theologos, central Greece)

In order design the future uses of the coastal zone and to mitigate the problems caused by sea level changes, it should be possible to predict the response of the coastal environment to the rising sea level. In reality, coastal environments respond to sea level rise by redistributing coastal sediments. Usually sea level rise results to coastal erosion and shoreline retreat. However, it is also possible that the shoreline position may not change or move towards the sea. A crucial factor is sediment budget. A coastal area with positive sediment budget may expand instead of be eroded, under sea level rise conditions. Likewise, coastal erosion due to sea level rise will be intensified in case of negative sediment budget.

As a consequence, the impacts of sea level rise on the coastal environments are complicated and not easy to assess. Coastal response depends largely on location and the predictions of coastal response to rising sea levels require careful analysis of the local coastal morphodynamics and sediment budget. Even if a coastal system is well understood, predictions may not be reliable due to feedback mechanisms and thresholds in the systems and because of uncertainties related to the future environmental conditions (rate of sea level rise and climate change). Furthermore, the geomorphological response to increasing sea levels will reflect other aspects of climate change, such as storminess periods and direction of waves. Particularly important for sediment budget in the nearshore zone is the possibility of changes in the incident waves’ regime, due to climate change, which can alter the rate and direction of littoral drift.

Man-made interventions

A large variety of activities are hosted in the coastal zone, which support the economy and serve many needs. At the same time many environmental problems occur, which eventually lead to the deterioration of citizens’ lives.

Deltaic coastal areas are constantly evolving as a result of the interaction of terrestrial (river discharge, coastal morphology) and marine processes (waves, coastal currents, tides). However, the construction of dams in the last decades has brought about a drastic reduction of the sediment volumes reaching the estuaries globally, at percentages ranging from 10 – 90% (Cheng, 1980; Poulos, 1999; Syvitski et al., 2005; Gupta et al., 2012).

Although the contemporary sea level rise contributes to the loss of many deltas globally, the degradation of wetlands is related to manmade interventions, which include sand extraction, coastal constructions and dams.

Dams The construction of a dam constitutes a significant human intervention to the balance of an area. The usefulness or not of these projects and their overall design has been a major subject of discussion in recent years. Dams are constructions designed to store and divert water, change the natural physical distribution and synchronization of river flows, in order to serve human needs.

Fig. 7: Coastal erosion destroying part of the road (Atalandi coastal zone, central Greece)

The purpose of the first dams was primarily to provide flood protection and water storage for irrigation and water supply purposes. Later they were used for hydroelectric power, for aquaculture, tourism and recreation.

The need for better planning of dams and taking measures quickly emerged in order to minimize the environmental and other impacts and losses. Nowadays, it is generally accepted that cultural, social, humanitarian and environmental values should be taken into account in the planning, design and construction of large constructions and their evaluation should go beyond the inclusion of traditional economic values.

The environmental impacts of each dam depend on various factors, such as its structure and functioning, the local hydrology, the river processes, the sediment volume, the geomorphological constraints, the climate and the basic properties of the local biota (Kondolf, 1997; Friedl & Wuest, 2002; McGinnis et al., 2006).

Some of the environmental changes are mentioned next. The functioning of a dam alters the supply of the river downstream with significant effects on local hydrology, downstream water and sediment transport. In river deltas it is possible to cause a shoreline shift of about 5 – 10 m per year, resulting to erosion and alteration of the local ecosystem. The fauna downstream is negatively affected, due to the changes in river flow as well as possible changes in water temperature, salinity and oxygen. The microclimate of the area may also be disturbed due to the changes in the water regime, causing changes in moisture content, air temperature, air movements, etc. and changes in the local topography. At the same time, the development of a reservoir can cover places of historical interest, geological or aesthetic value (Chu and Zhai, 2006; Kondolf, 1997).

Some characteristic examples include the following:

• Dam in California: reduction of sediment volume by 23%, of the order of 2.300.000 m3 (Slagel & Griggs, 2008)

• China: the large reservoirs developed at Huanghq river, have reduced the volume sediment reaching its delta from 1.1 billion metric tons to 0.4 billion metric tons (Li et al., 2004; IPCC, 2007)

• Danube, Ganges-Brahmaputra, Indus, Mahanadi, Mangoku, McKenzie, Mississippi, Niger, Nile, Shatt el Arab, Volga, Huanghe, Yukon and Zambezi: through the analysis of satellite images it was found that a total loss of 15.845 km2 of delta wetlands took place during 14 years (Coleman et al., 2005). At each delta, the loss of land showed different percentages, and human activities represented more than half.

• Greece: After the construction of the dams of Ladona (1955) and Floka (1962), it was found that a significant retreat of more than 400 m occurred at the delta of Alfeios river (Ghionis et al., 2013), with a rate of 8.3 – 26.6 m per year. At the deltas of the rivers Axios and Aliakmonas, the volume of sediments were drastically reduced by 2.5x106 m3 per year, after the

construction of reservoirs and hydroelectric dams after the ‘70s (Kapsimalis et al. 2005).

Sand extraction Extensive sand extraction has played a crucial role, in the Greek coastlines (Fig. 8). This phenomenon has its roots in the 1950s, when the reconstruction of the country began after the destructions of the Second World War and the civil war. The sand of the Greek coasts has very good physical properties for building constructions, consisting of fine-grained material. This method was a source of easy and quick profit for many contractors. This fact combined with the effort to find resources from the local government, led to the extraction and transport of large quantities of sand from several coasts. Additionally, illegal sand extraction was taking place from isolated beaches in order to avoid the high cost of marine sand.

Sand extraction was declared illegal in 1986 by the Law Act 1650. Today this law is enforced to a few coasts, because usually ignorance is the main reason of environmental disasters on the coasts (Spirou, 2010).

Fig. 8: A small lake has developed due to extensive sand extraction and reduced sediment

input. The coastal zone of Marathon nearby is actively eroded.

Coastal construction projects Construction projects on the natural shoreline usually result to the development of a new artificial coastline, with different geomorphological and sedimentological features.

These shorelines are eroded by the waves, reducing the water clarity of the surrounding marine area and altering the granulometric composition of the seabed sediments, resulting to the degradation of marine flora and fauna. This type of pollution is known as mechanical pollution.

The infillings that usually accompany harbor works, such as piers and breakwaters, in addition to aesthetic pollution, often cause changes in coastal currents. As a result, the sediment budget of the shoreline is altered causing areas of erosion and areas of deposition. It is not uncommon, that sometimes even harbor works are covered by sediments, as a result of this balance alteration. Consequently, the design and implementation of a technical project on the coastline requires a multidisciplinary approach, with the collaboration of engineers and oceanographers in order to maximize project efficiency and minimize environmental impacts (Panagiotidis and Hadjibiros, 2004).

Touristic facilities An important factor for the degradation of the coastal zone is the uncontrolled/unregulated constructions (buildings) (Fig. 9), mainly for touristic purposes. At the same time, working in the tourism sector resulted in the abandonment of agriculture and as a consequence the soil was hardened and became more difficult to be transported to the sea. As a result, coasts are no longer supplied from the land, and coastal erosion is active. This phenomenon is further intensified due to climate change and the intensity of storms and waves.

Fig. 9: Uncontrolled constructions on the coastal zone. On the top photo, the river mouth

of Oinois River can be seen. In front of the photo below, there used to be a road.

Modeling coastal erosion

Coastal Vulnerability Index – CVI One of the problems of the coastal areas is to define the erosion risk and the future sea level rise. The intensity and extent of coastal erosion is expected to increase due to the sea level rise, but the erosion trend of is not easy to predict because of the interaction of various factors, such as the sediment budget and coastal hydrodynamics. Research has shown that the global sea level has risen by about 18 centimeters in the last century. One of the effects of rising sea level is coastal erosion. In order to address this problem U.S. Geological Survey developed a method for identifying the erosion risk on those coasts that may be more vulnerable to future sea level rise. This methodology is known as Coastal Vulnerability Index – CVI, and aims to identify vulnerable and susceptible areas.

Coastal Vulnerability Index is an objective, simple, dynamic and easy to use mathematical model of risk determination in coastal zones in relation to possible future sea level changes. For the quantification of this methodology, six variables are taken into account (Equation 1).

(a) Morphology of the coastal zone

(b) Historical horizontal shoreline changes

(c) Coastal slope

(d) Tidal range

(e) Mean wave height

(f) Relative sea level rise

CVI = �(a ∗ b ∗ c ∗ d ∗ e ∗ f)

6

Equation 1: Coastal Vulnerability Index

A relatively simple, vulnerability classification system (Table 2) allows the six variables to be quantified and in this way to numerically express the relative vulnerability of the coast to natural changes due to future sea level rise. This method produces the figures/numbers and cannot be completely equated with natural results, however they are an indicator. Each of the variables receives a vulnerability value from 1 (very low) to 5 (very high), in order to determine the vulnerability and susceptibility of a region to natural change.

Table 2: Classification of variables for CVI (Equation 1) based on five categories of vulnerability, according to Pendleton et al. (2004)

Variables Very low Low Middle High Very high

(1) (2) (3) (4) (5)

Morphology Rocky

shores,

high

cliffs

Cliffs of

average

height

Low cliffs,

alluvial

plains

Shores with

pebbles,

lagoons

Barrier

islands,

deltas,

sandy

shores

Shoreline change (m/a) >2.0 1.0 – 2.0 -1.0 – 1.0 -2.0 - -1.0 <-2.0

Coastal slope (%) >1.20 1.20 – 0.90 0.90 – 0.60 0.60 – 0.30 <0.30

Sea level change (mm/a) <1,8 1.8 – 2.5 2.5 – 3.0 3.0 – 3.4 >3.4

Wave height (m) <0.55 0.55 – 0.85 0.85 – 1.05 1.05 – 1.25 >1.25

Tidal range (m) >6.0 4.0 – 6.0 2.0 – 4.0 1.0 – 2.0 <1.0

These variables can be divided into two groups: a) Geological variables b) Physical variables. Geological variables include geomorphology, historical horizontal shoreline and the coastal slope; these represent the relative resistance to erosion.

Physical variables include the average significant wave height, the tidal range, and the relative rise in sea level. The aforementioned variables contribute to the risk and their impact in the development of the phenomenon can last from hours to centuries. Regarding the morphology of each region, geology and coastal landforms are the variables to be taken into consideration.

The CVI was first applied along the US coasts, using field data, topographic and geoenvironmental information in combination with Geographic Information Systems (GIS). This approach is a relatively simple and reliable way to quantify the vulnerability of the coastline. Occasionally, a modified CVI has been proposed and used. Some examples are listed in the following table (Table 3):

Table 3: Modifications of CVI

Geometric average CVI =(a ∗ b ∗ c ∗ d ∗ e ∗ f)

6

Modified Geometric average CVI =

a ∗ b ∗ c ∗ �12 ∗ d� ∗ (1

2 ∗ (e + f)4

Average sum of squares CVI =

a2 + b2 + c2 + d + e2 + f2

6

Modified Geometric average CVI =a ∗ b ∗ c ∗ d ∗ e ∗ f

52

Sum of multiplications CVI = 4 ∗ a + 4 ∗ b + 4 ∗ c + 2 ∗ d + 2 ∗ (e + f)

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Other Sources

• Study of coastal erosion from wave action (South Corinth Gulf, Greece), https://www.youtube.com/watch?v=nkMhPYiF2no