Coastal flood risk Germany
The German coast
Germany’s coast extends over 3700 km on both the North (1600 km) and Baltic Seas (2100 km). In political and administrative terms, five states (out of 16 states making up the Federal Republic of Germany) border these coasts: Lower Saxony, Bremen, and Hamburg belong to the North Sea region; Mecklenburg-Vorpommern belongs to the Baltic Sea region; and Schleswig-Holstein shares coasts with both seas. Along the coast large low-lying areas are already threatened by recurring storm flood events and erosion. Accelerated sea-level rise therefore exacerbates a high-risk situation (1).
Two-thirds of the 3,700 km coastline are eroding. The German coastline is mainly shallow, i.e., marsh, dune coast, or beach wall, while only approximately 11% of the coast (420 km) is steep. On the Baltic, more than half of the coastline belongs to the so-called Bodden Coast (Bodden are shallow bays and inlets cut off from the open Baltic Sea by islands, peninsulas, and narrow spits) (1).
Sea level rise in Germany in the past
Along the North Sea coast, a long-term (so-called secular) rise of 20–25 cm/100 year has been recorded on a regional scale (4). Along the Baltic coast, the average secular rise has been slower (i.e., average 15 cm/100 year), as suggested by the records from several tide gauges (5). These values include regional effects of slow isostatic subsidence (following the last glaciation of northern Europe) of approximately 5 cm/100 year, which add to the climate-related effects of sea-level rise (1).
More recently, absolute mean sea level rise in the southern German Bight in the last 100 years was reported to be around 11 - 17 cm; for this period, a land subsidence of around 4 to 16 cm was quantified, being spread very unevenly from one place to another (30). For this area, no significant acceleration of sea level rise can be demonstrated yet (31).
Sea level rise in Germany in the future
According to IPCC estimates, global sea level rise will accelerate significantly in upcoming decades, increasing to three to four times the current rate by 2100. The mid IPCC estimate for sea level rise during this period amounts to 49 cm (6). However, in shallow seas like the North and Baltic Seas, sea level rise caused by thermal expansion alone is thought to be proportionally higher. Consequently, combined with the above mentioned geological effects, a 60 cm rise of mean water level is assumed to be more plausible for this area (1).
The recorded trend of sea level rise in the southern North Sea appears to be accompanied by a simultaneous increase of the tidal range by 0.15 cm/year (7). Therefore, in the German Bight, the change in mean high water (MHW) levels is foreseen to be greater than mean water level change. From the comparison of all hydrographical parameters, the current assumption of the coastal authorities in Schleswig-Holstein is that MHW might rise by 0.65 cm/year during the 21st century (1).
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 (23), 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 (24). 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 (25).
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 (40).
Storm surges in the past
Along the German North Sea coast and the Baltic Sea coast storm surge levels are changing, but this is not due to changes in storm activity (41).
According to recent tide gauge observations along the North Sea coast, extreme water levels have reached greater heights during the last four decades than before the so-called ‘‘flood of the century’’ that occurred in February 1962 (large portions of Hamburg City and the neighbouring North Sea coastal lowlands were flooded). The storm flood levels for both 1976 and 1981 were up to 50 cm higher than those in the 1962 event. Six storm surges higher than the 1962 level have also been recorded at the tide gauge station in Hamburg; four since 1990 (1). However, to date, there is no clear evidence suggesting a significant long-term change of storm activity in the German coastal regions (42). Observed changes in storm surge climate are largely related to the local mean sea level rise (43). The increase of extreme sea levels along the German North Sea coast is due to the rising mean sea level, and possibly partly due to changes in the tidal regime, but changes in the wind climate produced no noticeable long-term trend (41). Also for the Baltic Sea, studies showed no significant increases in extreme Baltic Sea levels so far (44).
In the Ems estuary near the border to The Netherlands, the storm surge of January 1994 was the highest ever recorded (8). A significant increase in the frequency of (moderate) storm floods can be shown statistically for the North Sea and the Baltic (9). Significant trends are not currently available for strong and extreme storm floods, partly because of the lack of longer data series (10).
Similarly, along the Baltic coast the long-term records show a significant increase of storm surges. At Travemuende, which has had surge records since 1830, there is an increasing trend of storm surges through the 20th century (1).
Storm surges in the future
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 (19,26). Within the German Bight a storm surge heigth increase of 20% between 1961-1990 and 2071-2100 has been projected (28).
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 (29).
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 (29). 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 (29).
Vulnerabilities - Coastal flood probability
The ‘‘baseline scenario’’ of the common methodology developed by the IPCC Coastal Zone Management Subgroup assumes a sea-level rise of 1 m by the year 2100 (11). When applying this scenario to the storm flood frequency distribution, the recurrence of extreme (i.e., hazardous) water levels shows a significant reduction of return periods. For example, at Cuxhaven, the 1-in-100-year flood event today is reduced to a 5-year flood event (1).
At Travemuende along the Baltic coast, the 1 m sea level rise scenario would lead to an even higher increase in the frequency of storm surges, as the absence of tides generally leads to a gentler storm flood frequency curve. Therefore, maximum flood levels, showing a frequency of 1 in 250 years in the past (as estimated from morphological-geological investigations) would be reduced to a 1 in 2–10 year period (1).
Vulnerabilities - Potential coastal damage
From a socio-economic perspective it is essential to delineate the coastal zone threatened by impacts of sea level rise and storm flood events as precisely as possible. For the North Sea coast, which has a meso-tidal regime (tidal range of 1.5–4 m), the landward boundary was taken at the 10 m contour line; for the microtidal environment of the Baltic Sea (tidal range 0.1–0.2 m) historic storm surge levels are significantly lower and, thus, the 5 m contour line was considered to appropriately delineate the flood prone area (1).
The total size of this area is more than 15,000 km2, the largest portion of which lies on the North Sea coast. However, this only represents 4.2% of the country’s total land area. The surface area below the 5 m contour line represents 3.8% of the German territory (1).
The low-lying coastal region is densely populated and intensely used. As many as 3.2 million people live within this coastal strip, concentrated mainly in a number of large coastal towns. The four biggest of these are the port cities of Hamburg (1.6 million inhabitants, of which 180,000 are in the risk area), Bremen (630,000), Kiel (245,000), and Rostock (180,000). Moreover, there are about 10 seaboard towns with between 50,000 and 120,000 inhabitants, most of them with historic city centers, e.g., Luebeck, Flensburg, Wismar, Stralsund, Greifswald (Baltic area), Cuxhaven, Wilhelmshaven, and Emden (North Sea area) (1).
Vulnerabilities - Coastal flood protection
Few sections of the Baltic coast, mostly the densely populated areas, are protected by dikes. Along the cliffed coast and around the inlets, there are no protective (hard) structures. In total, only 560 km, or 27%, of this coast is protected by dikes. This is in strong contrast to the North Sea coast, where 1340 km, or 85%, of the coast is dike-protected. For the entire German coastline, 1900 km, or 52%, is protected by dikes, dunes, or other constructions (1).
Hamburg lies about 140 km upstream of the North Sea and is connected by the Elbe estuary. Hamburg has often been subject to storm surges with significant damages. The history of storm surges in Hamburg, as documented since 1750, had three phases: the frequent damage-period prior to 1850, the calm period between 1855 and 1962, and a period of elevated but well-managed storm surge levels since 1962 (12).
The big flood of 1962 caused severe damage all along the German North Sea coast. Many dikes in Hamburg broke and more than 300 lives were lost in Hamburg (12).
Before the 1962 flood, the height of the flood defense system of Hamburg was NN +5,70 m (NN = the German Ordnance Datum). The highest flood of 3 January 1976 was NN +6,45 m. Since then, the flood defense system near Hamburg was strongly improved and now consists, a.o., of 77.5 km of dikes, 22.5 km of floodwalls, 6 flood barriers and 6 sluices (2,3). The flood defense protects an area of 270 km2 (1/3 of the area of Hamburg), over 180,000 citizens and over 10 billion Euros (3). After the 1962 flood the dikes were raised to NN +7,20 m, and after the flood of 1976 the dikes were raised again to a level between NN +8.00m and NN +9.30m (12).
The increase of the storm surge height in Hamburg is mainly due to modifications of the river Elbe: the improvement of the coastal defense and the dredging of the shipping channel (12). The intensification of the North Atlantic Oscillation during the period between 1960 and 1995 may have contributed a minor increase in level (14).
Based on North Sea storm surge scenarios (including sea level rise), it is projected that water level extremes, in terms of the mean maximum water level in a storm season, would rise by 15 cm ± 5 cm in Cuxhaven at the mouth of the river and 20 cm ± 5 cm in Hamburg until around 2030, relative to 1980-90 levels. Such an increase does not cause significant concern among coastal engineers. However, at a later time in the 21st century, say 2085 representing the last three decades of the century, the increase may amount of about 50 cm ± 15 cm in Cuxhaven and 60 cm ± 20 cm. Such an increase would need adaptations in both places, Cuxhaven and Hamburg (15).
These projections have been calculated under the assumption that the river topography will remain unchanged in the future. In fact, in seems unlikely that this assumption will be fulfilled. Because of disadvantageous patterns of sediment transport, plans are now made to slow the hydrodynamic regime in the Elbe estuary. When the tidal regime in the Elbe is slowed down, then not only ecology and sediment transport is affected but also the movement of water, including tides and storm surges. Therefore part of the earlier increase in storm surge heights in Hamburg may possibly be taken back, so that the perspectives for future storm surges may be smaller than what was envisaged under unchanged topographic conditions (12).
Vulnerabilities - Coastal flood risk
Bremen, with 92% of its area situated in the low-lying Wesermarsh, is conspicuously the coastal state most exposed to sea level rise as it is followed by the state of Schleswig-Holstein, which has 33% of its state area exposed. Other states have less area at risk: Hamburg (30%), Lower Saxony (20%), and Mecklenburg-Vorpommern (9.5%) (1).
The picture for population affected in each state is similar to that of area at risk: i.e., Bremen is again the most affected, with 92% of all inhabitants at risk according to this scenario, followed by Schleswig-Holstein (23%), Niedersachsen (19%), Mecklenburg-Vorpommern (17%), and Hamburg (11%). This low percentage for Hamburg is due to the fact that much of Hamburg’s territory lies on higher ground, e.g., on terminal moraines. However, virtually all of Hamburg harbour, by far the most important port of Germany, is within the potential flood zone (1).
Nationally, fewer than 30,000 people are at risk based on the current probability of flooding, which includes a moderate historical sea level rise of 15–25 cm. The probability of future flooding, if sea level rises by 1 m, is much greater and will grow by a factor of at least 10 in both coastal regions. Consequently, the currently valid numbers for people and capital values at risk would, without further coastal protection measures, increase tenfold by the year 2100. Therefore, 309,000 people and capital values of more than 300 billion US$ (based on 1995 values) are faced with an annual flood risk in the German coastal zone (1).
For the city of Bremen, one of the largest ports of Germany, a total damage of about 18.5 billion euros (6.5 billion euros direct damage + 12 billion euros macroeconomic effects such as the loss of labor) has been estimated for an extreme flood under climate change (assumption: 55 cm sea level rise) (18).
Further aspects of vulnerability of the coastal system in the region include ● the reduction of sandy beaches and of dune ecosystems by erosion ● the problem of higher water levels for the terrestrial drainage of flat areas ● increased intrusion of saltwater into groundwater and soil caused by higher mean sea level ● permanent submergence (i.e., loss) of beach zones and coastal wetlands (1).
When speaking about future risks, one has to take into account also changing patterns of vulnerability of coastal populations. It seems that the vulnerability of the population has increased in the recent past. The effective coastal defense has created a perception of absolute security, even if scientists have demonstrated that a slight modification of past storms (in terms of path and speed) could cause significantly exaggerated high storm surges. The vulnerability increases also because of the influx of people not originating from the coastal zone, who simply are not aware of the severity of the risk (12).
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 (27). 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 - Current flood protection
Along the East and North Frisian islands, only the densely populated areas are protected by dikes. Additionally, almost all tide-influenced tributaries of the Ems, Weser, Elbe, and Eider have been protected by storm surge barriers since the 1950s. However, the slow subsidence of the older marsh areas (i.e., the first ones to be protected by dikes) near the edge of the upland area creates a particularly difficult situation for coastal protection and terrestrial drainage. Furthermore, it has been observed that the heavy weight of dikes causes them to subside, while at the same time the calculated water level is rising (1).
In the past, coastal engineering design had not taken these problems fully into consideration because acceleration of sea level rise due to climate change had not been seriously considered by coastal authorities. As a result, coastal protection standards in some areas are now insufficient for a drastic sea level rise scenario, requiring new evaluation of the required dike heights and the resulting costs. Taking a regional perspective, the dikes are not in an adequate condition in some areas of Niedersachsen (mainly in the district of Weser-Ems). Even without the extreme 1 m sea level rise scenario, reenforcement of the dikes is needed there in the next 20 years and beyond (1).
Along the Baltic coast, little more than a quarter of the total coast is protected from flooding by dikes, revetments, and other protection systems. Many existing dikes do not fulfill the requirements for the calculated water level determined during the most catastrophic storm flood of 1872, and even without considering an increasing sea level, reenforcement of the dikes is needed (1).
Consequently, based on a 1 m sea level rise scenario, the current (annual) coastal protection costs of the five coastal states is generally 200 million Euros/year. This sum does not yet include the so-called ‘‘soft coastal protection’’ (i.e., beach nourishment, dune protection, etc.). Along the Baltic coast a 30–50% increase of the erosion on beaches and steep coasts has been observed as a long-term medium value. In the 1 m rise scenario, total protection costs will need to include measures that will address this problem at an estimated cost of 25–50 million Euros/year (1).
In the long run, spatially differentiated adaptation strategies will be more appropriate, if not unavoidable. These measures could probably ensure that dramatic changes, in particular large-scale losses of wetland habitats, might be avoided and that the existing littoral ecosystem values and functions could be sustained. First steps in this direction have been initialised in two states (Schleswig-Holstein and Mecklenburg-Vorpommern), which address the issues of possible retreat and natural coastal system adjustments in their protection master plans as possible long-term options (1).
Adaptation strategies - Future flood protection
Four federal states are responsible for adaptation strategies along the German North Sea coast: the Free and Hanseatic City of Hamburg and of Bremen, Lower Saxony, and Schleswig Holstein. All four states account for future climate change effects in the evaluation of design water levels by adding a general provision margin of 50 cm for 100 years, equivalent to a sea level rise of about 40-45 cm per 100 years. In addition, in Bremen, Hamburg and Lower Saxony, solid structures are constructed so as to accommodate an increase in water level beyond the anticipated safety margin up to 75 cm (Bremen), 80 cm (Hamburg) or 50 cm (Lower Saxony) (39).
Hamburg generally employs the strategy of keeping the protection line in its current position. But very recently, some new infrastructure like large public buildings has been erected on dwelling mounds to prevent them flooding if dyke sections fail during a storm surge. The strategy in Schleswig Holstein for dykes at the mainland North Sea coast and on the North Frisian Islands is similar: in the current protection line dykes will be repeatedly strengthened relative to safety levels and safety margins. Bremen will keep the protection line in its current position. For its mainland coast and along the tidal estuaries Ems-Dollard, Weser and Elbe, Lower Saxony will do the same (39).
Adaptation strategies - Drainage
If the sea level on the North Sea were to rise significantly, efforts and cost requirements for the protection of the terrestrial drainage would be higher. Most floodgates currently allow the natural drainage of inland waters at low tide cycles. This would have to be changed to pumping drainage stations (as used widely in the Netherlands) in order to pump the water out continuously (1).
Continuous pumping is the only way to avoid saltwater intrusion into the soil and groundwater and to protect agricultural utilisation. Expert opinion estimates that in the assumed extreme scenario, the cost of drainage measures in the three North Sea states might correspond to those of the dike construction costs and might possibly be higher (1).
Adaptation strategies - Contingency planning
Disaster prevention, preparedness, response and recovery should become even more of a priority for Member States (16).
Flood proofing Hamburg
The city of Hamburg developed one of the most innovative designs for flood control of a city. The HafenCity, an adaptation strategy followed by the municipality of Hamburg as a more robust development of flood proofing new urban areas, is located on the waterside of the main dike line, and is thus within the flooding area of the Elbe estuary. The existing elevations of the HafenCity are between 4.4 m and 7.2 m above sea level and not adequately protected against flood. Therefore, almost every public road and bridge and all buildings will be elevated to the minimum height of 7.5 m above sea level. The buildings’ foundations will serve as ground floor garages, which can be flooded in severe cases. Roads and paths are constructed above the flood line to ensure unrestricted access for the fire and emergency services in the event of an extreme storm tide. Total costs are estimated to reach about 600 million euro’s (17).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Germany.
- Sterr (2008)
- Gönnert and Triebner (2004)
- Freie und Hansestadt Hamburg (2007)
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- Marcos et al. (2009); Haigh et al. (2010); Menendez and Woodworth (2010), all in: IPCC (2012)
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- IPCC (2012)
- Woth (2005)
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