Coastal flood risk Denmark
The Danish coast
Denmark consists of the Jutland peninsula and more than 400 islands. The whole of the country is lowland. The surface was formed by Ice Age glaciers and glacial streams. The highest hill is approximately 170 metres above sea level. The coastline has a length of more than 7300 km. To protect low-lying land against flooding and storm surge, dikes or other permanent installations have been built along about 1800 km of coastline (1).
The Danish coastline partly comprises active coastal cliffs where the sea erodes material, and partly beach-ridge complexes, where the material is deposited in the lee of prevailing winds. About 80% of the population lives in urban areas connected to the coast. In recent years beach nourishment has increasingly been used to protect exposed stretches of coastline (1).
Sea level rise in the past
Global mean sea levels rose by around 17 cm over the twentieth-century, driven largely by the thermal expansion and melting glaciers, ice caps and ice sheets associated with anthropogenic global warming. On top of this global trend, there are significant regional differences in sea level change due to changes in ocean circulation and atmospheric pressure (2).
Because of natural variability, it is more difficult to detect a climate-change signal in the local sea level in Copenhagen than in global sea level. Over the last century, a linear trend of 0.44 mm per year (i.e. 4 cm per century) can be observed in Copenhagen water level data from the city Coastal Authority. The difference between this observed local trend and the global rise in sea level is due to local factors (changes in ocean and atmospheric circulation and local uplift) and to measurement and trend-extraction errors. The respective influence of these factors is still unclear (2).
The maximum observed rise is in southwestern Denmark, where the water level is rising by about 1 mm per year. In northern and eastern Denmark uplift of the land after the Ice Age is roughly in line with the rise in sea level (1).
Sea level rise in the future
A general rise in sea level by the year 2100 of 0.15–0.75 m is anticipated on the west coast and in Danish coastal waters. In extreme storm surge situations an increase in the maximum water level is expected of between 0.45–1.05 m on the west coast (3). For a high-end scenario of climate change (the so-called IPCC RCP8.5 scenario) a median sea level rise in Copenhagen has been projected of 68 cm and a 5−95% uncertainty range of 29−162 cm (28).
In Denmarks adaptation strategies a sea level rise of 0.1 - 0.5 m by 2050 and 0.2 - 1.4 m by 2100 is assumed. This is partly compensated for by a land rise of 0 - 0.1 m by 2050 and 0 - 0.2 m by 2100 30).
Storm surges in the past
The largest recorded event occurred in 1872; a strong storm (equivalent to a category 3 hurricane) tracked northwards across Europe and into the Baltic, causing a 3 m storm surge that led to severe flooding around the Danish Inner Waters (particularly in Lolland and Falster, the two islands south of Copenhagen). A detailed measure of water level in the Copenhagen harbour is not available as records start in 1890. The largest events in the record occurred in 1902 (154 cm above normal sea level) and 1921 (157 cm), but no information about flooded areas or damages are available (2).
The largest storm surges in recent history occurred during the winters of 2006 and 2007. On the 1st November 2006, for instance, an exceptionally strong storm developed south of Iceland and intensified past the west coast of Norway before travelling into the Baltic. Water was forced down from the Kattegat (to the north of Zealand and the Oresund) and from the Baltic. The ensuing build-up of water led to record water levels in the region, with the sea level in Odense (on the Island of Fyn to the west of Zealand) reaching two metres above normal (a more than 100-yr return-period event) and the level in the Copenhagen harbour reaching 131 cm. Also, on the 19th January 2007, the water level in the harbour was measured 142 cm above normal sea level. These events did not lead to significant damages in Copenhagen (2).
Extreme storm surges in Copenhagen are limited and cannot exceed 2 metres according to statistical analysis, making it very easy to protect the city with sea walls and dikes (2).
Storm surges in the future
In Denmarks adaptation strategies an increase in the set-up of severe storm surges of 0 - 0.1 m by 2050 and 0 - 0.3 m by 2100 is assumed due to higher wind velocities resulting in higher and longer waves. Peak storm surge levels may increase by up to 0.6 m by 2050 and up to 1.7 m by 2100 due to the combined effect of sea level rise and increasing surge set-up (30).
There is a tendency towards more frequent westerly winds and at the same time a shift of the storm tracks over the North Atlantic slightly eastward, leading to a small increase in storm activity over Denmark and the adjacent waters. On this basis, calculations with storm surge models show that the highest sea level in the more extreme cases could rise by 5-10% relative to today (about 0.3 m on the west coast) (1). A smaller increase of storm surge elevations for the continental North Sea coast of between 15 and almost 25 cm at the end of the 21st century has also been reported (16). In addition to this there is the global rise in sea level which the IPCC estimate at between 0.1-0.9 m over the level today (1).
New model studies on the effect of storminess changes on storm surge in Northern Europe showed statistically significant changes between 1961-1990 and 2071-2100 (based on four regionally downscaled GCMs, two runs with B2, one with A2, and one with an A1B emission scenario). Along the coast of the Netherlands, in the German Bay, along the west coast of Denmark, and for the northwest British Isles an 8 to 10% increase was found, mainly in the winter season (8). Within the German Bight a storm surge heigth increase of 20% between 1961-1990 and 2071-2100 has been projected (18).
From model studies based on two IPCC emission scenarios (SRES A1B and B1) for the period 1961–2100 (excluding sea level rise) it was concluded that, despite the remaining uncertainties toward the end of this century, extreme storm surge heights likely will show a small increase toward the coasts of the German Bight with stronger changes along the North Frisian Islands in case of anthropogenic climate change. This increase is superimposed by strong decadal variability. Human activities in the German Bight and along its coasts may be confronted with more frequent surge-induced impacts throughout the twenty-first century (20).
The 99 percentile of the annual 10 m height wind speed is a good measure of storminess because this percentile reaches about 18 m/s near the North Sea coasts (20). This is just above eight beaufort which is a storm benchmark. Linear trends in the results of these afore cited model studies are small compared to the decadal variations and range between 3 to 10 cm/century and between 0 and 14 cm/century for annual 99 percentile and maximum surge heights, respectively. Not all of these trends are statistically significant, however; all realizations show that larger and statistically significant changes are mainly limited to the south-eastern part of the North Sea. The more reliable changes of the ensemble mean show an increase of about 5% in the German Bight for the surge height. This agrees with the projected increase in frequency of stronger south-westerly and westerly winds which enhance the wind-setup toward the east (20).
It is not yet clear how climate change will influence the characteristics of extratropical cyclones. Climate physics tells us that a warming climate would have confounding effects on extratropical cyclones. However, while the body of evidence in this area gives a fairly clear picture that extratropical cyclones could become less frequent in both hemispheres, there could be a larger number of the most intense storms. The most robust result is that there will be a poleward shift in the position of the storm tracks, and therefore, some regions can expect to experience a lower frequency of storms, while others a higher frequency of storms (2). In Europe, studies project an increase in storm track density (the number of storms) over Northwestern Europe, in particular, the UK and Scandinavia. There is also evidence that the intensity of storms will increase over Europe (5,6).
Long-term records of sea levels around Europe already show signs of an increasing trend in the frequency of extreme sea levels (i.e. a reduced return-period for intense events) (2).
Global sea level rise
For the latest results: see Europe Coastal floods
For the latest results: see Europe Coastal floods
Extreme water levels - Global trends
More recent studies provide additional evidence that trends in extreme coastal high water across the globe reflect the increases in mean sea level (12), suggesting that mean sea level rise rather than changes in storminess are largely contributing to this increase (although data are sparse in many regions and this lowers the confidence in this assessment). It is therefore considered likely that sea level rise has led to a change in extreme coastal high water levels. It is likely that there has been an anthropogenic influence on increasing extreme coastal high water levels via mean sea level contributions. While changes in storminess may contribute to changes in sea level extremes, the limited geographical coverage of studies to date and the uncertainties associated with storminess changes overall mean that a general assessment of the effects of storminess changes on storm surge is not possible at this time.
On the basis of studies of observed trends in extreme coastal high water levels it is very likely that mean sea level rise will contribute to upward trends in the future.
Extreme waves - Future trends along the Western European coast
Recent regional studies provide evidence for positive projected future trends in significant wave height and extreme waves along the western European coast (13). However, considerable variation in projections can arise from the different climate models and scenarios used to force wave models, which lowers the confidence in the projections (14).
The large natural variability has a greater impact on the local North Sea wind field than potential anthropogenic-induced trends. For the North Sea region reliable predictions concerning strongly wind- influenced characteristics such as local sea level, storm surges, surface waves and thermocline depth are still impossible (31).
Vulnerabilities - Coastal flood probability
Around 2% of the population of Copenhagen lives below an elevation of 1 m, 4% lives below an elevation of 2 m and 13% lives below an elevation of 5 m above sea level. The most exposed areas of the city lie on the island of Amager and along the coastline of the larger island of Zealand (particularly to the south). The most densely populated area of Copenhagen, to the west of the centre, is at relatively low vulnerability, as it lies away from the coast at an elevation of at least 5 m above sea level, and in some parts more than 10 m (2,15).
Copenhagen is very well protected against storm surges. In the Copenhagen city centre and in the harbour, quays are at more than 2 m above current sea level. Considering the maximum possible storm surge in the current climate is estimated at 2 m, this protection level suggests that the historical centre – where population density is very high – is not at risk of coastal floods today. Considering this protection level, it seems that Copenhagen is very well defended, and in some places even over-protected (e.g., western part of Amager island). As a consequence, even a large amount of sea level rise could be managed by the current protection system (2,15).
While it is impossible to approximate the scale of the change of return-periods, recent evidence suggests that the return-period for a given amplitude of storm surge will reduce in the future due to increased storminess around Northwestern Europe (2).
Vulnerabilities - Potential coastal damage
A statistical analysis has been carried out of past storm surges in Copenhagen, matched to a geographical-information analysis of the population and asset exposure in the city, for various sea levels and storm surge characteristics. From this analysis it was concluded that Copenhagen is not highly vulnerable to coastal flooding (2,15).
In the absence of protection, however, the total losses (direct and indirect) caused by the current 120-yr storm surge event, at 150 cm above normal sea level, would reach EUR 3 billion. In the absence of protection, moreover, future sea level rise would significantly increase flood risks beyond this level. For instance, with 25 cm of mean sea level rise (SLR) total losses caused by a future 120-yr event would rise from EUR 3 billion to EUR 4 billion, to EUR 5 billion with 50 cm of mean SLR, and to almost EUR 8 billion with 100 cm of SLR (2,15).
As a result of rising mean sea levels, the exposures associated with a 120-yr storm surge could more than double by the end of the century, in an unchanged city of Copenhagen. By 2030, the exposure from such an event is estimated to rise to around 33,000 – 35,200 people. By the end of the century, the exposure is estimated to reach around 44,400 – 71,200 people (2).
New waterfront construction, port-related operations and sanding up of harbour entrances will pose special problems. Cities located at river mouths at the bottom of fjords may face a very complex set of problems, since they can be under pressure from higher sea levels, increased precipitation and runoff, as well as changes in groundwater levels (3).
Vulnerabilities - Coastal flood protection
The level of flood protection in Copenhagen is based on a water level of 150 cm above the mean sea level, which is associated to a return-period of 120 years (2).
The level of protection will have to be upgraded in case of sea level rise: even with only a 25 cm sea level rise, the city protection level would decline from 1-in-120 years to 1-in-10 years (2).
Vulnerabilities - Coastal flood risk
Raising flood defences by the amplitude of sea level rise maintains the flood probability, but increases flood risks (because the extent of possible damage increases). To maintain flood risks, therefore, it is necessary to heighten flood defences by more than sea level rise (2).
Economic impacts of sea level rise for Europe
The direct and indirect costs of sea level rise for Europe have been modelled for a range of sea level rise scenarios for the 2020s and 2080s (17). The results show:
- First, sea-level rise has negative economic effects but these effects are not particularly dramatic. In absolute terms, optimal coastal defence can be extremely costly. However, on an annual basis, and compared to national GDP, these costs are quite small. On a relative basis, the highest value is represented by the 0.2% of GDP in Estonia in 2085.
- Second, the impact of sea-level rise is not confined to the coastal zone and sea-level rise indeed affects landlocked countries as well. Because of international trade, countries that have relatively small direct impacts of sea-level rise, and even landlocked countries such as Austria, gain in competitiveness.
- Third, adaptation is crucial to keep the negative impacts of sea-level rise at an acceptable level. This may well imply that some European countries will need to adopt a coastal zone management policy that is more integrated and more forward looking than is currently the case.
Adaptation strategies - Planning
The Danish government considers planning legislation an important means of reducing the negative socio-economic consequences of climate change. Regulations for the coastal zone already restrict new construction areas on open coasts. Each coastal town must develop an adaptation plan taking into account climate change impacts in the coastal zone. The coastal adaptation plans focus on shoreline management (29).
Adaptation strategies - Infrastructure
Protection of the hinterland against flooding at the Danish North Sea coast will continue to be achieved by keeping the protection line in its current position, with the exception of those areas where coastal retreat is regarded as acceptable and no human interference preventing it is deemed necessary. At the Danish Wadden Sea coast existing dykes are strengthened to meet prevailing safety levels including the anticipated safety margins for climate change effects (30).
Also in the case of reinforcing dikes/dunes or adapting harbour installations and ferry berths, which are relatively simple constructions, it will be possible for individual owners to adapt to ongoing climate changes. With respect to new construction or renovation of dikes, coastal protection or harbor installations, it is important to consider how many years of climate change should be included in the basic design, since these installations have a lifetime of 50 –100 years, and the climate is expected to change dramatically in that period (3).
It is also important to consider whether it is possible to accept the reduced safety of dikes and other high water protection resulting from climate changes or indeed whether to give up dikes or coastal protection altogether and return to a more natural coastline with more frequent flooding and natural erosion (3).
Past experience demonstrates that the retrofit of coastal defence structures is a lengthy process requiring forward thinking and planning. For example, there was a 30 year lag between the decision to build the Thames barrier and its actual implementation. It is necessary to start thinking about long-term adaptation in coastal cities today, even if the risks of climate change are not imminent (2).
As extreme sea level events (referred to as “storm surge events”) are not particularly high in Copenhagen, the city is relatively easy to protect with dykes and sea walls and the residual risk is low. Because flood exposure will increase regardless of the protection level, the consequence of protection failure will increase with sea level rise. Faultless maintenance will, therefore, become even more crucial than today. Also it requires “soft” adaptation measures, such as emergency plans and warning systems, to prepare for the possibility of failure (2).
Adaptation strategies - Risk communication and management plans
There should be measures to ensure that updated information on climate developments and relevant risk analyses – for example, illustrated on maps – reaches the municipalities at all times for use in their planning. Information should also be made available for citizens and businesses (3).
The EU Directive on the Assessment and Management of Flood Risks, which took effect in 2007, stipulates that this type of information must be provided. Member States must by the end of 2011 indicate those areas thought to be at risk of flooding. The designation will be based on an interim estimate of flood risks, including as a result of climate change. For every designated area, maps must be made by the end of 2013 showing the flooding extent and the potential number of inhabitants that would be affected, potential environmental and economic damage, etc. On the basis of these maps Member States must draft by the end of 2015 risk management plans for flooding and set adequate targets for managing flood risks (3).
There is a need for ongoing adaptation of rescue and storm surge preparedness as well as information on conditions significant to planning coastal constructions in future risk areas. Generally speaking, it is individual land owners' own choice to protect themselves from flooding and erosion. Therefore, there are no general laws or regulations stipulating protection, or to what degree owners must protect themselves (3).
Disaster prevention, preparedness, response and recovery should become even more of a priority for Member States (7).
Adaptation strategies - The costs of adaptation
Both the risk of sea-level rise and the costs of adaptation to sea-level rise in the European Union have been estimated for 2100 compared with 2000 (19). Model calculations have been made based on the IPCC SRES A2 and B1 scenarios. In these projections both flooding due to sea-level rise near the coast and the backwater effect of sea level rise on the rivers have been included. Salinity intrusion into coastal aquifers has not been included, only salt water intrusion into the rivers. Changes in storm frequency and intensity have not been considered; the present storm surge characteristics are simply displaced upwards with the rising sea level following 20th century observations. The assessment is based on national estimates of GDP.
The projections show that without adaptation (no further raising of the dikes and no beach nourishments), the number of people affected annually by coastal flooding would be 20 (B1 scenario) to 70 (A2 scenario) times higher in 2100 than in 2000. This is about 0.05 - 0.13% of the population of the 27 EU countries in 2010 (19).
Without adaptation, damage costs would increase roughly by a factor of 5 during the century under both scenarios, up to US$ 17×109 in 2100. Total damage costs would amount to roughly 0.04% of GDP of the 27 EU countries in 2100 under both scenarios. Damage costs relative to national GDP would be highest in the Netherlands (0.3% in 2100 under A2). For all other countries relative damage costs do not exceed 0.1% of GDP under both scenarios (19).
Adaptation (raising dikes and beach nourishments in response to sea level rise) would strongly reduce the number of people flooded by factors of 110 to 288 and total damage costs by factors of 7 to 9. In 2100 adaptation costs are projected to be US$ 3.5×109 under A2 and 2.6×109 under B1. Relative to GDP, annual adaptation costs constitute 0.005 % of GDP under B1 and 0.009% under A2 in 2100. Adaptation costs relative to GDP are highest for Estonia (0.16% under A2) and Ireland (0.05% under A2). These results suggest that adaptation measures to sea-level rise are beneficial and affordable, and will be widely applied throughout the European Union (19).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Denmark.
- Danish Ministry of the Environment (2005)
- Hallegatte et al. (2008)
- Danish Government (2008)
- Binnerup (2000), in: Fenger (2000)
- Fenger et al. (2008)
- Fischer-Bruns et al. (2005)
- Commission of the European Communities: Green paper (2007)
- Debernard and Roed (2008), in: IPCC (2012)
- Bindoff et al. (2007), in: IPCC (2012)
- Church and White (2011), in: IPCC (2012)
- Velicogna (2009); Rignot et al. (2011); Sørensen et al. (2011), all in: IPCC (2012)
- Marcos et al. (2009); Haigh et al. (2010); Menendez and Woodworth (2010), all in: IPCC (2012)
- Debernard and Roed (2008); Grabemann and Weisse (2008), both in: IPCC (2012)
- IPCC (2012)
- Hallegatte et al. (2011)
- Woth (2005)
- Bosello et al. (2012)
- Woth et al. (2006)
- Hinkel et al. (2010)
- Gaslikova et al. (2013)
- Cazenave et al. (2014)
- IPCC (2014)
- Watson et al. (2015)
- Yi et al. (2015)
- Church et al. (2013), in: Watson et al. (2015)
- Shepherd et al. (2012), in: Watson et al. (2015)
- Church et al. (2013), in: Watson et al. (2015)
- Grinsted et al. (2015), in: Arnbjerg-Nielsen et al. (2015)
- Dronkers and Stojanovic (2016)
- Niemeyer et al. (2016)
- Schrum et al. (2016)