Coastal erosion: European scale
Coastline length European countries
For a large number of European countries, the length of their coastline was inventorized (6):
|Country||Length (km)||Country||Length (km)||Country||Length (km)|
|Bosnia and Herzegovina||23||Italy||9,226||Slovenia||41|
Vulnerabilities - Coasts globally
Satellite imagery now provides a powerful alternative to derive reliable, global scale data on shoreline erosion and accretion. A global assessment, based on a fully automated analysis of 33 years of satellite images, covering the period 1984-2016, has been carried out recently (31). The researchers first detected sandy (and gravel) beaches worldwide. Next, they analysed more than 1.9 million images to quantify beach erosion rates. Over this period, 24% of the world’s sandy and gravel beaches have eroded more than 0.5 m/year, while 27% have accreted.
It is estimated that about 6,000 to 17,000 km2 of land will be lost during the 21st century due to enhanced coastal erosion associated with sea-level rise, in combination with other drivers. This could lead to a displacement of 1.6 to 5.3 million people and associated cumulative costs of 300 to 1000 billion USD (32).
Shoreline change along inlet‐interrupted coastlines
Future changes of sandy coastlines adjacent to tidal inlets mainly depend on the ratio of the effect of sea‐level rise and changes in fluvial sediment supply. This ratio governs the local sediment budget of the near-coastal zone, and hence the development of the position of the sandy shoreline. Projections of shoreline change have been made adjacent to 41 tidal inlets around the world (36). This was done by considering catchment‐estuary‐coastal systems in a holistic way in which the combined effect of sea-level rise and changes in future sediment supply determines the development of this system and, as a result, the position of the shoreline. Four scenarios of climate change were used for this (RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5).
According to these projections, the sandy shorelines adjacent to 90% of the 41 tidal inlets would retreat over the twenty-first century under all four scenarios of climate change. This retreat would be more than 100 m by the end of this century for 46% and 68% of the inlets under the low-end and high-end scenario of climate change, respectively (36).
Sandy coastlines worldwide under threat of erosion
Sandy beaches cover more than a third of the global coastline, a substantial proportion of which is already eroding (41). To predict future shoreline change, we need to know past rates of coastal change as well as the extent of future sea-level rise and storms. In a recent study, satellite data have been used to quantify trends of shoreline change in recent years. These trends were extrapolated into the future and combined with projections of the effects of future sea level rise and storm activity to derive projections of future shoreline change (40).
The dividing line between land and water along the world’s shorelines has been assessed from over 3 million satellite images covering the period 1984 to 2015. This was done for any given year in this period by finding the areas where water presence has never been detected on satellite images throughout the year. This permanently dry land defines the shoreline. From this information, past shoreline change was determined at a spacing of 250 m alongshore around the world’s entire coastline. These changes result from geological and hydrodynamic processes, including the impacts of human interventions on coastal sand budgets (40).
This satellite-based information of shoreline changes in previous decades provides the platform to predict future shoreline change due to sea-level rise and changes in storminess. Future projections of sea-level rise and storminess were based on a moderate (RCP 4.5) and high-end scenario (RCP 8.5) of climate change for two future time slices: 2050 and 2100. Sea-level rise was translated to future shoreline change based on scientific knowledge on the adaptation of shoreline profiles to changing sea levels (40).
The scientists conclude that shoreline loss will already be apparent by 2050, and that towards the end of the century, loss rates will increase further. Almost half of the world’s sandy beaches could be gone by the end of the century. A substantial proportion of the threatened sandy shorelines are in densely populated areas. The moderate scenario of climate change results in 17% less shoreline retreat compared with the high-end scenario in 2050, and 40% less retreat in 2100. This corresponds to a global average of around 42 m of preserved sandy beach width in the moderate scenario by the end of the century. Projected globally averaged shoreline retreat by 2100 is 86 m for the moderate and 128 m for the high-end scenario of climate change. This illustrates the importance of mitigating the emission of greenhouse gasses (40).
These future projections are combinations of shoreline changes because of geological and hydrodynamic processes, including impacts of human interventions, and the effects of sea-level rise and changes in storminess. The latter dominate future shoreline changes in about three quarters of the world’s coastline.
According to the authors of this study, ‘the projected shoreline changes will substantially impact the shape of the world’s coastline. Many coastal systems have already lost their natural capacity to accommodate or recover from erosion, as the backshore is heavily occupied by human settlements, while dams and human development have depleted terrestrial sediment supply which would naturally replenish the shore with new material (42)’.
31% of the world’s sandy beaches are in low-elevation coastal zones with population density exceeding 500 people per km2. The results of this study indicate that about one-third of these low-elevation coastal zones will be seriously threatened by erosion by 2050, and more than half of them by 2100.
The results of this study are alarming. However, the authors emphasize that more sustainable management of coastal zones and river basins can prevent some of the erosion. They cite the Dutch coast as the most prominent example of this: effective site-specific coastal planning appears to be effective in stabilizing the Dutch coastline (43).
Vulnerabilities - European coasts
The results for Europe in the global summary presented above agree with the only pan-European assessment carried out in the past (the EUROSION study of 2004): 28% of Europe’s sandy and gravel shorelines are eroding (compared with 27% in the previous estimate) (31).
All European coastal states are to some extent affected by coastal erosion (4). About twenty thousand kilometres of coasts, corresponding to 20% face serious impacts in 2004. Most of the impact zones (15,100 km) are actively retreating, some of them in spite of coastal protection works (2,900 km). In addition, another 4,700 km have become artificially stabilised (1,5).
The risk of coastal flooding due to the undermining of coastal dunes and sea defences potentially affects several thousands of square kilometres and millions of people. Over the past 50 years, the population living in European coastal municipalities has more than doubled to reach 70 millions inhabitants in 2001 and the total value of economic assets located within 500 meters from the coastline has multiplied to an estimated 500-1000 billion Euros in 2000 (1).
The cost of coastal erosion (coastline protection against the risk of erosion and flooding) has been estimated to average 5,400 million euro per year between 1990 and 2020 (2).
Coastal erosion results in three different types of impacts (or risks):
- Loss of land with economical value
- Destruction of natural sea defences (usually a dune system) as a result of storm events, which may result in flooding of the hinterland.
- Undermining of artificial sea defences as a result of chronic sediment shortage
Coastal erosion is influenced by several human factors, including (1):
- Coastal engineering. The waterfronts of urban, tourism or industrial zones have usually been engineered by way of seawalls, dykes, breakwaters, jetties, or any hard and rock-armoured structures, which aims at protecting the construction or other assets landwards the coastline from the assault of the sea. Such structures modify wave and flow patterns in the near shore zone and therefore cause a redistribution of sediment. The net sediment volume in the coastal zone may not be strongly affected, but the sediment redistribution can induce erosion in some places and accretion in others.
- Land claim. Within tidal basins or bays (where land reclamation projects are most easily undertaken), land reclamation results in a reduction of the tidal volume and therefore a change in the ebb and flood currents transporting sediments. As a result, relatively stable coastal stretches may begin to erode.
- River basin regulation works. Damming has effectively sealed water catchments locking up millions of cubic metres of sediments per year. For some southern European rivers (e.g. Ebro, Douro, Urumea, Rhone), the annual volume of sediment discharge represents less than 10% of their level of 1950; for the Ebro this is even less than 5%. This results in a considerable sediment deficit at the river mouth, and subsequent erosion downstream as illustrated in Ebro delta, Playa Gross, Petite Camargue (Rhone delta) and Vagueira.
- Dredging. Dredging may affect coastal processes by removing from the foreshore materials (stones, pebbles) which protect the coast against erosion, and by contributing to the sediment deficit in the coastal sediment cell.
- Vegetation clearing. A significant number of cases have highlighted the positive role of vegetation to increase the resistance to erosion.
- Gas mining or water extraction. Gas mining or water extraction may induce land subsidence, causing sediment deficit and a retreat of the coastline.
Direct anthropogenic effects on effective sea-level rise (ESLR)
From an assessment of contemporary effective sea-level rise (ESLR) for a sample of 40 deltas distributed worldwide it was concluded that direct anthropogenic effects determine ESLR in the majority of deltas studied, with a relatively less important role for eustatic sea-level rise. According to this study, serious challenges to human occupancy of deltaic regions worldwide are conveyed by other factors than the climate change sea-level rise (3). The IPCC states that the primary drivers of widespread observed coastal erosion are human drivers other than climate change so that there is very low confidence in the detection of impacts related to climate change (7).
For any delta, ESLR is a net rate, defined by the combination of eustatic sea-level rise, the natural gross rate of fluvial sediment deposition and subsidence, and accelerated subsidence due to groundwater and hydrocarbon extraction. The deltas in this study represent all major climate zones, levels of population density, and degrees of economic development. The study includes the European deltas of Danube, Ebro, Po, Rhine, and Rhone. Collectively, the sampled deltas serve as the endpoint for river basins draining 30% of the Earth's landmass, and 42% of global terrestrial runoff. Nearly 300 million people inhabit these deltas. For the contemporary baseline, ESLR estimates range from 0.5 to 12.5 mm per year (3).
Decreased accretion of fluvial sediment resulting from upstream siltation of artificial impoundments and consumptive losses of runoff from irrigation are the primary determinants of ESLR in nearly 70% of the deltas. Approximately 20% of the deltas show accelerated subsidence, while only 12% show eustatic sea-level rise as the predominant effect. Extrapolating contemporary rates of ESLR through 2050 reveals that 8.7 million people and 28,000 km2 of deltaic area in the sample set of deltas could suffer from enhanced inundation and increased coastal erosion (3).
Vulnerabilities - Europe's sandy beaches and sea level rise
As a result of sea level rise, the shorelines of sandy beaches in Europe may ‘potentially’ retreat by tens to a few hundred metres between now and 2100, scientists conclude (35). The results of their study are summarized below.
A focus on sea level rise only
The potential impact of sea level rise on shoreline retreat depends on the extent of sandy beaches along the European coast, the slope of the beach profile, and the scenario of climate change. For all of these three variables the scientists had two datasets. They combined these data into eight assessments and calculated potential shoreline retreat of sandy beaches in Europe by 2100, relative to the baseline year 2010.
Several natural processes and human influences also determine shoreline change, including natural gradients in alongshore sediment transport, the effects of port construction, changes in fluvial sediment supply, sand nourishments, and land reclamations. These were not taken into account. They focused on sea level rise only.
Tens to a few hundred metres retreat
Under the high-end scenario of sea level rise, the potential shoreline retreat, relative to the baseline year 2010, is projected to be about 18 - 54 m by 2050, and 51 - 242 m by 2100. This would result in a potential total coastal land loss of 473 - 1,410 km2 by 2050, and 1,334 - 6,316 km2 by 2100. These ranges are the 5 - 95% confidence interval of the results of the calculations. Under the moderate scenario of climate change, these numbers are almost 35% (2050) and 55% (2100) lower.
The median value of potential shoreline retreat by 2100 is about 54 m to 97 m under a moderate (RCP 4.5) to high-end (RCP 8.5) scenario of climate change, respectively. This translates to a potential coastal land loss of about 1,400 km2 to 2,500 km2 along the European coastal zone.
Does sea level rise dominate over other influences?
Is sea level rise the dominant mechanism that determines future shoreline retreat? The scientist made an estimate of the potential impacts of all the other natural processes and human influences that also determine shoreline change. They did so by quantifying historical change rates of shoreline position and extrapolating them into the future.
They concluded that under a moderate scenario of sea level rise the combined impact of all these other processes and influences is bigger than the impact of sea level rise in 2050; the impacts are the same in 2100. Under a high-end scenario of sea level rise, the impact of sea level rise on shoreline retreat dominates over the impact of the other processes and influences both in 2050 and 2100.
Not all sandy coastlines will retreat
Not all coastlines will be eroding this century. About 14% of the European coastlines have historically advancing trends larger than that of the potential future retreat even under the high-end scenario of climate change. For these coastlines, overall net changes on coastal land area are a land gain if the accreting historic trends continue.
Land subsidence and uplift paint different pictures locally
The study highlights the potential impact of sea level rise on the retreat of Europe’s sandy coastlines. The word ‘potential’ is important here. The scientists assumed a sufficiently wide erodible beach for the shoreline to erode. At some locations the erodible beach width could be smaller than the projected retreat, however, leading to an overestimation of coastal land loss therein.
The scientists also did not include the effect of land subsidence and (postglacial) land uplift on shoreline retreat. In areas with significant subsidence, such as the Po Delta in Italy, relative sea level rise can be larger than the values used in this study and shoreline retreat may be underestimated. In areas with significant land uplift, such as the Baltic Sea region, relative sea level is dropping which contributes to land gain in these areas.
Vulnerabilities - Wetlands globally
Globally, between 20 - 90% of existing coastal wetland area is projected to be lost by 2100 (33), depending on different scenarios of climate change. These projected changes vary regionally and between different types of wetlands. High risk of total local loss is projected under the high-end scenario of climate change (RCP8.5) by 2100, especially if landward migration and sediment supply is constrained by human modification of shorelines and river flows (34).
Vulnerabilities - European wetlands
Salt marsh vulnerabilities under sea level rise
According to widespread perceptions, future sea level rise would result in large losses of salt marshes: regional and global assessments predict that sea level rise alone will lead to a 20–50% loss of marshland by the end of the current century (14). This may be highly overestimated. Many marshes will survive in place for the majority of emission and sea level scenarios considered by the IPCC, and the most rapid scenarios of sea level rise will not exceed thresholds for marsh survival for several decades, scientists argue (15).
According to them sea level rise over the next few decades is not an immediate, catastrophic threat to many marshes: marshes will survive in place under relatively fast rates of sea level rise (>10 mm per year) where sediment delivery to the coast is not restricted by dams (16). They state that previous studies underestimate marsh resilience by not fully accounting for feedbacks that lead to increasing accretion rates with sea level rise or the potential for marshes to migrate inland.
Dynamic feedbacks: A crucial process that should be included in models of marsh response to sea level rise is the dynamic feedbacks between tidal inundation and increased vertical accretion of mineral and organic sediments. In general, increased tidal inundation promotes more frequent and longer episodes of mineral sediment settling on the marsh platform, enhanced vegetation growth and faster rates of organic matter accumulation (17) than is generally assumed. In fact, models that simulate this dynamic feedback indicate that marshes generally survive relative sea level rates rates of up to 10–50 mm per year. The sea level rise salt marshes can handle largely depends on the suspended sediment concentration in the water that floods the marsh system, and on the local tidal range. Where suspended sediment concentrations are larger than 30 mg/l and tidal range exceeds 1 m, the models predict that marshes can adapt to fast relative sea level rise rates of several centimetres per year (18).
Inland migration: A primary mechanism for marsh survival is transgression into adjacent uplands. Marsh migration is already occurring in low-lying areas, where saline intrusion driven by sea level rise triggers forest dieback and causes agricultural losses (19). Transgression of marshes into adjacent uplands may allow marshes to survive, or even expand, in response to future sea level rise. This is not possible, however, where artificial structures border the marshes, which is the case for almost all marsh areas in northwest European estuaries (20); in these cases, erosion of marshes from the ocean side and hardened shorelines at the mainland side result in ‘coastal squeeze’, with salt marshes and coastal ecosystems confined to a shrinking area and prevented from migrating into adjacent uplands (20,21). An example area the salt marshes in the UK and the Netherlands: these contracted in size over the past decades because transgression limited by dykes could not compensate for sustained lateral retreat of up to several metres per year (22).
Vulnerabilities - Global wetlands
The global extent of and change in tidal flats has been mapped over the course of 33 years (1984-2016). The results show that tidal flats, defined as sand, rock or mud flats that undergo regular tidal inundation, occupy at least 127,921 km, and that about 16% of these tidal flats were lost between 1984 and 2016. Extensive degradation from coastal development, reduced sediment delivery from major rivers, sinking of riverine deltas, increased coastal erosion and sea-level risesignal a continuing negative trajectory for tidal flat ecosystems around the world (30).
Previous large-scale assessments on the response of coastal wetlands to sea-level rise may be too dramatic. They have failed to properly consider two feedback mechanisms that stimulate the growth of coastal wetlands, scientists state (27). First, coastal wetlands more easily build up vertically by sediment accretion under increasing inundation heights and frequencies. Because of this, coastal wetlands may even benefit from accelerating sea-level rise (28). Second, the suggestion in previous studies that coastal flood protection structures, coastal roads and railway lines, settlements, and impervious land surfaces are barriers to inland wetland migration (29)is too pessimistic. There is more accommodation space for new wetlands to develop inland, and these new wetlands may compensate for the loss of existing wetlands.
These scientists carried out a new assessment of global-scale changes in coastal wetland areas by 2100, including current knowledge on vertical wetland accretion and inland wetland migration. They used a low-, intermediate and high-end scenario of climate change, corresponding to 29, 50 and 110 cm of sea-level rise by 2100, respectively.
They concluded that, in the absence of further accommodation space in addition to current levels, the loss of global coastal wetland area until 2100 will range between 0 and 30%. In fact, global wetland gains of up to 60% of the current area are possible, if more than 37% (their upper estimate for current accommodation space) of coastal wetlands have sufficient accommodation space, and sediment supply remains at present levels. The resilience of global wetlands is primarily driven by the availability of accommodation space, they conclude. This is strongly influenced by the building of anthropogenic infrastructure in the coastal zone and such infrastructure is expected to change over the twenty-first century. Thus, large-scale loss of coastal wetlands can be avoided, if sufficient additional accommodation space can be created through careful nature-based adaptation solutions to coastal management.
Tipping point of delta survival globally
11,000 year ago global warming induced a rapid rise in sea level. Sea level rise slowed down around 7500 to 7000 years ago. Geological research has revealed that this slow down coincided with the beginning of the formation of a large part of the world’s deltas. Radiocarbon dating of sediments of 36 deltas showed that the formation of 33 of them began between 9000 and 7000 years ago. Scientists concluded that “the concurrence of a slowing sea level rise and delta formation is close enough to suggest a strong causal significance”. In fact, the scientists who carried out this research concluded that, based on their data, the world’s deltas formed when sea level rise slowed to between 5 and 10 mm per year (23).
Apparently this led to a dynamic balance – or tipping point – between sediment supply, erosion, and sea level rise, thus creating the circumstances that favoured coastal progradation (24). Bearing this in mind, the scientists wondered whether this geological information about sea level rise slow down and delta formation may be interpreted the other way around: does it inform us about accelerating sea level rise this century being a tipping point of delta collapse? What if the past is a key to the future?
If most modern deltas initiated when sea level rise fell below a critical value or tipping point, then the reverse is likely true, they argued. The world’s marine deltas, broadly speaking, will begin to collapse when the forecasted sea level rise exceeds this tipping point. They presented evidence that this may occur when sea level rise reaches between 5 and 10 mm per year (25). Currently, sea level rises globally at an average rate of 3.4 mm per year. The IPCC projected that sea level rise would ‘‘likely’’ be 8 to 16 mm per year by the end of this century. In fact, this may be an underestimation since these numbers do not include the latest insights in potential rapid ice mass loss from the Antarctic ice sheet. A recent update projects an intermediate, high, and extreme sea level rise of 10, 20, and 25 mm per year, respectively, by 2050, and 15, 35, and 44 mm per year, respectively, by the end of the century (26).
The combination of their geologic data on delta formation in the past and projected sea level rise in the next decades led them to conclude that the tipping point between modern delta resilience and collapse will likely occur in the next 50 years as sea level rise reaches between 5 and 10 mm per year. If they are right, the impact will be dramatic. Changes to the existing coastal geomorphology will have regional, national, and international repercussions, occur nearly concurrently, and will compromise existing trade networks, settlements, and ecosystems (23).
According to a more recent study, global delta land area will decrease if relative sea-level rise exceeds about 5.5 mm per year (37). The study shows that, from 1985 to 2015, delta land gain has slowed down mainly because of sea-level rise. This decrease is expected to result in a net land loss once the rate of sea-level rise has reached 5.5 mm per year. By the end of the century, sea-level rise will be the dominant driver that determines the future of our deltas. According to this study, sea-level rise under a high-end scenario of climate change (RCP8.5) will lead to the disappearance of about 5% of total global delta area by the end of this century. In addition, waves and tides more and more leave their mark on the shape of deltas at the expense of the influence of the river (37).
Adaptation strategies - Four key recommendations
Four key recommendations have been proposed to make coastal erosion problems and risks in Europe manageable (1):
- Increase coastal resilience by restoring the sediment balance and providing space for coastal processes. A more strategic and proactive approach to coastal erosion is needed for the sustainable development of vulnerable coastal zones and the conservation of coastal biodiversity. In light of climate change it is recommended that coastal resilience is enhanced by: (a) restoring the sediment balance; (b) allocating space necessary to accommodate natural erosion and coastal sediment processes and (c) the designation of strategic sediment reservoirs (supplies of sediment of ‘appropriate’ characteristics that are available for replenishment of the coastal zone, either temporarily (to compensate for losses due to extreme storms) or in the long term (at least 100 years)).
- Internalise coastal erosion cost and risk in planning and investment decisions. Public responsibility for coastal erosion risk should be limited and an appropriate part of the risk should be transferred to direct beneficiaries and investors. Risks should be monitored and mapped, evaluated and incorporated into planning and investment policies. Current practices observed in Europe reveal that the tax payer – through expenditures executed by public authorities - supports the major part of the costs associated with coastal erosion risk. Almost no cases are found were the parties responsible for coastal erosion or the owners of assets at risk paid the bill. The contribution of private funding for coastal erosion management in European member states probably does not reach 10% of the public expenditure (except for Denmark: a contribution from private owners up to 50% of the overall cost of coastal defence).
- Make responses to coastal erosion accountable. Coastal erosion management should move away from piecemeal solutions to a planned approach based upon accountability principles, by optimising investment costs against values at risk, increasing social acceptability of actions and keeping options open for the future.
- Strengthen the knowledge base of coastal erosion management and planning. Over the past hundred years the limited knowledge of coastal sediment transport processes at the local authority level has often resulted in inappropriate measures of coastal erosion mitigation. In many cases, measures may have solved coastal erosion locally but have exacerbated coastal erosion problems at other locations – up to tens of kilometres away – or have generated other environmental problems.
Adaptation strategies - IPCC classification
The IPCC classification of coastal adaptation strategies consisting of retreat, accommodation and protection is now widely used and applied in both developed and developing countries (8):
- Protection aims at advancing or holding existing defense lines by means of different options such as: land claim, beach and dune nourishment, the construction of artificial dunes, hard structures such as seawalls, sea dikes and storm surge barriers or removing invasive and restoring native species.
- Accommodation is achieved by increasing flexibility, flood proofing, flood-resistant agriculture, flood hazard mapping, the implementation of flood warning systems or replacing armored with living shorelines.
- Retreat options include allowing wetlands to migrate inland, shoreline setbacks and managed realignment by, for example, breaching coastal defenses allowing the creation of an intertidal habitat.
Ecosystem-based adaptation is increasingly attracting attention (9). Adaptation measures based on the protection and restoration of relevant coastal natural systems such as mangroves (10), oyster reefs (11) and salt marshes (12) are seen as no-or low-regret options irrespective of the future of climate change (13).
Adaptation strategies - Traditional versus nature-based solutions
Traditional coastal protection includes hard structures, such as seawalls, breakwaters, groynes and dikes. These structures are non-adaptive. They must be upgraded, repaired, and rebuilt in response to a changing climate. Therefore, they are expensive to build. Besides, they have environmental and social costs: coastal habitats, such as dunes, beaches, mangroves, and saltmarshes, are replaced with artificial ones, and human access to natural shorelines is disturbed (38).
Nature-based coastal defences include soft and hybrid methods. Soft methods incorporate the restored habitat only, such as dune planting. Hybrid methods use a combination of hard structures integrated with the natural habitat, such as rock sills in front of saltmarsh or mangroves. Nature-based coastal defences can self-repair after storm events and adapt to changes in climate, reducing maintenance costs compared to traditional defence structures that will need to be re-designed (38). Oyster reefs, for example, have been shown to have vertical reef growth at a rate that matches sea level rise (39).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.
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