The Dutch coast

The Dutch coastline including all estuaries has a length of about 1000 km. The coastal zone can be divided into three regions with different characteristics (1):

• the southwest region with a large number of (previous) tidal inlets and islands
• the central connected coast
• the northern region with the Wadden Sea coast and its islands

The central connected coast is about 350 km long of which 75% consists of dune areas of varying widths, ranging from less than 100 meters up to a width of several kilometers.

Almost 50% of the European population lives in the 50 km coastal strip. 85% of the Dutch and Belgian coast and 50% of the German coast has less than 5 meter elevation (10).

The rest of the flood prone area of the Netherlands can be divided into (1):

• the IJsselmeer lake district
• the upper river courses of Rhine and Meuse
• the lower (tidal) courses of Rhine and Meuse.

Historically, flood risk management in the Netherlands was based on building dikes high and strong enough to prevent floods that had occurred in the past from happening again. This strategy of embankments carried on for 1000 years and thus resulted in a country that came to rely completely on its flood defence system (1).

It is not without reason that the Dutch flood defence system has the highest safety standards worldwide. Almost two thirds of the Netherlands is prone to flooding from the sea or from the rivers Rhine and Meuse (although only a part of this may be flooded in one event). 70% of the Dutch Gross National Product is earned below sea level. The embankments protect 9 million people (1).

Flood prone areas in The Netherlands

The table below shows the percentage of the Netherlands, its urban area and its population that is located in dike rings (embanked areas) and in flood-prone areas (river area + coastal zone; from (37)). Note that not all of the embanked areas are actually flood prone: there are higher grounds within the dike rings. Hence, 'only' about half of the urban area and population within these dike rings are in the flood-prone zone.

Dike rings Flood-prone zone Total area 55% 34% Total urban area 62% 31% Total population 67% 35%

Sea level rise in the Netherlands

Past

Since 1900 sea level rise of the North Sea near the Dutch coast has been 19 cm, which is comparable with the global average (4). Over the last decades, sea level rise near the Dutch coast has increased to 3 mm per year, an increase by 50% compared with the average rate of sea level rise over the 20th century (59). In addition, there is subsidence of the Dutch soil up to 8 mm/year, depending on the location in The Netherlands (52).

Future

According to the most recent scenarios of the Royal Netherlands Meteorological Institute, sea level on the Southern North Sea will be 25 to 80 cm higher in 2071-2100 (averaged year 2085) than in 1981-2010. For 2100 an upper level of sea level rise is projected of 100 cm (46). In addition, the subsidence of the Dutch soil, due to drainage of peatlands and areas reclaimed from the sea, expansion of built-up areas and the infrastructural network on soft soil, and salt mining and gas extraction, will continue up to several mm's and locally even cm's per year, depending on the location in The Netherlands (58).

Greenland and Antarctica tend to lose more ice than was presented in the last report of the IPCC (AR4). Observations in the period from 2002 to 2009 show that the mass loss of both ice sheets has accelerated over time, implying that the ice sheets contributions to sea level rise also becomes larger over time. For 2100, the high-end projection for global mean sea level rise is higher (0.55 -1.15 metres) than global estimates, as reported by the IPCC AR4 (0.25-0.76 metres), compared to 1990 levels. This implies a rise along the Dutch coast of 0.40 to 1.05 metres, by 2100 (excluding land subsidence) (23).

Global sea level rise

Observations

For the latest results: see Europe Coastal floods

Projections

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 (31), 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 (32). 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 (33).

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 (54). 

Storm surge

According to the Royal Netherlands Meteorological Institute, changes in the wind climate in the 21st century will be small with respect to natural variability (4). For Dutch policy on flood protection it is considered unlikely that the storm regime along the Dutch North Sea coast and the associated maximum storm surges will change significantly in the 21st century (6,55,56). For the Dutch coast no statistically significant change in the 10,000-year return values of surge heights was projected for the 21st century because projected wind speed changes were not associated with the surge-generating northerlies but rather non-surge generating south-westerlies (26).

There are, however, several publications in the scientific literature that point at a possible storm surge increase:

• Climate simulations indicate some further increase in wind speeds and storm intensity in the north-eastern Atlantic during at least the early part of the 21st century (2010 to 2030) (7), with a shift of storm centre maxima closer to European coasts (8).
• Ensemble modelling of storm surges and tidal levels in shelf seas, particularly for the Baltic and southern North Sea, indicate fewer but more extreme surge events under some SRES (IPCC) emissions scenarios (9).
• Extreme wind speeds increase between 45°N and 55°N, except over and south of the Alps, and become more northwesterly than currently… leading to more North Sea storms and a corresponding increase in storm surges along coastal regions of Holland, Germany and Denmark, in particular (11,34).
• 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 (27). Within the German Bight a storm surge heigth increase of 20% between 1961-1990 and 2071-2100 has been projected (39).
• 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 (41). According to these model studies, linear trends in the 99 percentile of the annual 10 m height wind speed and the maximum surge heights range between 3 to 10 cm/century and 0 to 14 cm/century, 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 (41).

Coastal flood probability - Safety standards

The dunes, dikes, dams and storm surge barriers have to meet safety standards set by law. This law, the 1996 Flood Protection Act, is of relatively recent date but the standards have already been recommended and accepted since 1960 for the coastal zone and since 1977 and 1993 for the flood prone areas near the rivers Rhine and Meuse, respectively (2).

The recommendations of the safety standards for the coastal zone followed after the 1953 storm surge induced flooding and killed over 1800 people in the southwest of the Netherlands. The standards demand for minimum height and strength of the flood defences surrounding a given area, thus protecting this area from flooding from the sea, the main rivers and large lakes. Such an enclosed area protected by one set of dikes is called a dike ring. The flood prone part of the Netherlands consists of 53 dike rings (and a number of small embankments along the Meuse) (2).

The dikes for which the legal standards hold, are called primary dikes. These standards do not hold for smaller (secondary) dikes, generally bordering smaller water bodies. The dikes are managed by so-called ‘water boards’, democratic bodies solely responsible for water management in certain territories (2).

The safety standards reflect the probability of occurrence (return period) of the highest water levels that need to be safely contained by the flood defences. For rivers, for example, the safety standard is 1/1250 per year: this means that high river water levels, occurring with a chance of 6% per human lifetime, must be safely contained. At higher water levels, the hinterland may be flooded (2).

For the dike rings of the Netherlands 5 different standards hold. The highest standards (the lowest flood probability) hold for the coastal zone: a safety standard of 1/10,000 per year for most of the provinces of North and South Holland, and a safety standard of 1/4,000 per year for the rest of the coastal zone. The difference in the height of the safety standard for the coastal zone reflects differences in population density and economic value (2).

Actual flood probability

The safety standards indicate a minimum level of safety. A safety standard of 1/10,000 per year means that the coastal flood defence must be high and strong enough to withstand storm surges that have a likelihood of occurrence of 1/10,000 per year. The actual coastal flood probability is even (much) lower. The actual flood probability cannot be quantified exactly because it depends on many factors, such as the strength of dikes and the likelihood of storm surges, that cannot be quantified exactly. According to estimates, actual flood probability of the low-lying, densely populated area in the western part of the country, with the major cities of Amsterdam, Rotterdam and the Hague, may be less than 1/100,000 per year (19).

At present about a third of all flood defences (including those of the coast, the rivers and the large lakes) do not comply with the current standards. For about half of these defenses improvements are being implemented; the improvements of the other half of the defences that failed the assessment still have to be planned (35).

Uncertainties in flood probability

Several problems arise when translating the ‘accepted risk’ of a 1/10,000 per year coastal flood risk into the sea level being exceeded (on average) only once in 10,000 years (13):

• as the observational records of tidal stations are only 100 years in length, the surge level with an average return period of 10,000 years requires an extrapolation of two orders of magnitude. It is unclear how reliable the estimate from such an extrapolation is;
• various probability functions can be fitted to the observational records of extreme surges, leading to different results in the 10,000-year return levels;
• extrapolation from observational records does not contain information about surges in a greenhouse gas-induced changing climate;
• a second population of rare but intense storms, originating from a different kind of meteorological system, would result in higher return values than estimated from standard extreme-value analysis of the available short records.

Only a crude estimate of the 10,000-year surge level can be made from a single record with a length of the order of 100 years…. 10,000-year wind speed from 100-year records always has to be interpreted as a lower limit (13).

Changes in flood damage in 20th century

During the 20th century the amount of urban area in the flood-prone part of the Dutch delta (river area + coastal zone) has increased about six-fold. This increase in urban area in the flood-prone zone has led to an exponential increase in potential flood damage during the 20th century: 16 times the damage of 1900 by 2000. However, GDP increased more than potential flood damage, and the capacity to deal with catastrophic flood losses has actually almost doubled in 2000 with respect to 1900 (37).

Potential coastal damage - Flood scenarios for the current situation

The extent of the potential damage due to a coastal flood depends on the scenario of the flood that unfolds. Clearly, a local flood because of one dike breach causes (far) less damage than several breaches along a large part of the coastal zone. Dutch authorities use several scenarios as a basis for their flood protection and contingency planning policies, and for their strategy to adapt to the consequences of climate change.

The most extreme scenarios have been drawn up for contingency planning (1). These scenarios reflect the idea of ‘think the unthinkable’. For large-scale floods in the Netherlands this idea has been given an upper limit called ‘worst credible floods’: an upper limit for floods that are still considered realistic or credible by experts. Two scenarios for a ‘worst credible’ coastal flood have been drawn up: a flood of the southwestern and central coastline, and a flood of the northern (Wadden Sea) coastline.

The scenario for the southwestern and central coastline results in a flooded area of 4300 km2 , of which 50% gets flooded already in the first 8 hours, and 80% after one day. Not the entire flood prone area gets flooded: roughly half of the largest dike rings remains dry. In approximately 50% of the flooded area the water depth is less than 2 metres. A potential damage of up to 121 billion Euros and up to 10,000 casualties have been estimated for this scenario (1).

The Wadden Sea coast scenario results in a flooded area of 4560 km2 but the estimated number of casualties (some 3,000) and potential damage (40 billion Euros) is far less since this part of the Netherlands is less densely populated. The flooding proceeds at a slower rate than the scenario for the south-western and central coastline: 50% of the 4560 km2 gets flooded 12 hours after the breaches, 73% after one day. Again, not the entire flood prone area gets flooded: higher ground and objects stop the flood in parts of the area. In approximately 70% of the flooded area the water depth is less than 2 metres (1).

The order of magnitude of the estimates of the potential damage caused by a ‘worst credible flood’ are in line with similar estimates that were made as part of an evaluation of flood risk policy in the Netherlands (19).

Potential flood damage in 2040 - Second Sustainability Outlook

In the Second Sustainability Outlook, the Netherlands Environmental Assessment Agency (PBL) presents scenarios of the Netherlands in 2040, including options to adapt to the consequences of climate change.

In this outlook, the sea level rise and associated reduction in options for guaranteeing the free discharge from the rivers are considered to be determining factors for the long-term future of the Netherlands. The need for protection is further intensified because the majority of new urban development is in the flood-risk parts of the Netherlands. This substantially increases the potential for economic damage in the event of flooding in the period to 2040 (17).

The consequence of opting for further investment in the Randstad (the area with cities like Amsterdam and Rotterdam) and the low-lying Netherlands is that the economic vulnerability of the Netherlands will continue to grow. By 2040 the potential economic damage will have increased by 100 to 250%, owing to the appreciation in value of existing buildings and infrastructure and the construction of new housing. Depending on the economy, demography and the housing market, the new built-up area will contain 20 to 30% of the total potential damage.

For economic reasons, a rise in the safety levels of some dike rings is justified. In the 2020-2040 period, this will require an additional investment of 0.8 billion Euros, compared with 1.5 billion Euros to maintain current safety levels in the same period (18).

Potential flood damage in 2100

Future projections of socio-economic changes, based on scenarios for 2100 constructed in line with scenarios available for 2040 (38), show a further increase of urban area in the flood-prone zone in 2100 by +30% (scenario low economic growth) to 125% (scenario high economic growth) with respect to 2000 (river area + coastal zone) (37). It is projected that in both scenarios these developments take place in relatively unsafe locations with potential inundation depths often exceeding 2.5 m. These projections only refer to socio-economic changes: the impact of flood protection works and changing hydraulic conditions (due to e.g. climate change) are not included.

These projections indicate that potential damages will continue to increase during the 21st century, two- to three-fold and tenfold by 2100, for the scenarios of low and high economic growth respectively. The capacity to cope with these increased flood damages, due to changes in GDP, probably will not change much (37).

Coastal flood protection

Safety against flooding from the sea can be ensured with current, available methods, even in the worst case scenario of 1.5 meters sea level rise per century (6).

Coastal flood risk

Dams, dikes and coastal defences in the Netherlands have never been stronger: the probability of encountering floods from rivers or from the sea has substantially declined since the last flooding in the south-western part of the Netherlands in 1953. However, the risks of casualties and of economic damage from flooding have become much greater since this event. This paradoxical situation is the result of a growing discrepancy between the existent set of design standards for the height and strength of dams, dikes and coastal defences set around 1960, and a steady social and economic development since that time (2).

The average yearly economic expansion since 1960 has been twice as high as expected at that time and the population at risk in the Netherlands has more than doubled. In the period between 1960 and the present, the design standards have not been corrected for the increased economic value and population (2).

Compared with other risks, the societal risk of flooding (the probability of large numbers of casualties) in the Netherlands appears to be several orders of magnitude larger than the sum of the societal risk of other known external hazards (e.g. industrial hazards and plane crashes). A further increase in flood risk is expected owing to further economic and social development (2).

A part of the flood defences do not yet comply with the legal standards (6). It is expected that all flood defences will comply by 2015. A cost-benefit analysis has been carried out to estimate whether these defences, once they comply with the standards in 2015, offer a sufficiently high level of flood protection with respect to the economic value and the number of people behind the dikes. The results indicate that this is indeed the case for the northwestern and northern part of the country. For a large part of the southwestern coastal area, however, a higher safety standard is recommended (for instance 1/20.000 per year for the Rotterdam area instead of 1/10,000 per year). Also for the river area higher safety standards are recommended (22). The costs of improving the flood defences to make them comply with these recommended higher safety standards have been estimated as 1.7–7 billion Euros (22).

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 (36). The results show:

1. 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.
2. 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.
3. 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 policy

Current water policy already takes account of the possible consequences of climate change. This involves not only technical measures, but also landscape and land use measures to ensure a more secure and robust water system (20).

Most policy documents and projects look forward 50 to 100 years. For the coast, a low scenario (20 cm/century) is used for decisions with a short-time design life (5 years), a middle scenario (60 cm/century) for decisions with a longer-time design life (50–100 years), and a high scenario (85 cm/century) in the case of reserving land for measures in the very long term (time horizon of 200 years) (21).

Adaptation strategies - Delta Programme

In the Netherlands a Delta Programme has been initiated aimed at improving the flood defence system, such that the dikes comply with the current safety standards, and preparing an adaptation strategy for the future (up to 2100). The Dutch pro-active approach of looking ahead to the end of this century and making sure that current measures fit in a long-term strategy of climate adaptation is unique. This allows the Dutch to combine flood safety and water security with other interests, such as economy, spatial planning and nature, while being flexible enough to adjust the adaptation strategy to changing views on the impact of climate change. The Delta Programme is steered by the Delta Commissioner (3,42).

The Delta Programme focuses on three main topics: flood safety (o.a. by assessing the possibilities of so-called Delta Dikes), fresh water security (by means of more self sufficiency in the regions, and by optimizing the fresh water distribution in the main and regional water systems), and new urban development and restructuring (based on the National Adaptation Strategy) (3).

The adaptation strategy in the Delta Programme is based on the 2006 climate change scenarios of the Royal Netherlands Meteorological Institute (4). An essential component of the strategy is the adaptation tipping points approach (5). Adaptation tipping points are points where the magnitude of change due to climate change or sea level rise is such that the current strategy will no longer be able to meet the objectives. This gives information on whether or when a water management strategy may fail and alternative strategies are needed. Adaptation strategies should be both robust and flexible. An example of a robust strategy is increasing the river discharge capacity by making more room for the river. An example of a flexible strategy is the use of sand nourishments for the improvement of the coastal flood defence (3).

The Delta Programme discriminates between 6 regions, each having their own vulnerabilities with respect to climate change, and thus asking for tailor-made adaptation measures:

1. Rhine estuary – Drechtsteden: flood protection of the Rhine – Meuse - Delta
2. Southwestern Delta: climate change impacts on flood safety, fresh water availability, nature and regional economic development from 2050 onwards
3. IJsselmeer Region: long-term water level management for flood safety (free discharge of lake water on the Wadden Sea under sea level rise) and fresh water security (a larger reservoir for dry summers)
4. Rivers: implementation measures for increasing discharge capacity Rhine and Meuse (short-term); securing flood safety along with addressing fresh water supply, shipping, nature and regional development (long-term)
5. The Coast: focus on a sustainable flood protection strategy and options for coastal expansion
6. Wadden Region: focus on several issues, a.o. integrated coastal and island management, innovation in dike construction, sediment budgets, and climate proofing areas outside the dikes

Delta Decisions in 2015

In 2015 the Delta Programme will result in Delta Decisions for flood safety and securing fresh water reserves for 2050 with an outlook towards 2100. Five Delta Decisions will be taken, covering Flood risk management, Freshwater strategy, Spatial adaptation, the Rhine-Meuse delta and Water level management IJsselmeer Region. These decisions determine and are directional for work to be carried out as from 2015. Central in 2013 will be the development of potential strategies (DP2013). These will be detailed in preferential strategies in DP2014 and as proposals for Delta Decisions in DP2015 (25).

Proposals for the following Delta Decisions are foreseen (25):

1. Delta Decision Flood risk management. Updating flood protection standards and development of area-based strategies for flood protection The area-based strategies provide insights into promising combinations of measures (delta) dykes, river-widening and/or spatial development measures including natural safety measures (“Building with nature”), adaptive construction and organization], insights into financial requirements, chances for spatial adjustment, social base, planning and feasibility of implementation.
2. Delta Decision Freshwater strategy. Strategy for sustainable and economically effective freshwater supplies in the Netherlands. This strategy provides insights into supply of and demand for freshwater and water security; makes statements on potential for water savings, optimal water distribution and future service levels in relation to functions and their impact on these functions; and clarifies the division of responsibilities between government, market and user.
3. Delta Decision Spatial adaptation. National policy framework new urban developments and restructuring and recommendations around flooding and heat stress. The proposal for this Delta Decision yields a strategy on means and conditions for robust development in built-up areas in the Netherlands. The policy will at least cover the topics of the built-up area in- and outside the dykes as well as in, on and around water defence systems and reserved areas from the angle of flood risk management and pluvial flooding.
4. Delta Decision Rhine-Meuse delta. Strategy for flood protection in this crucial transitional delta area, together with solutions for freshwater supplies. The Rhine-Meuse delta is the location of the major rivers, Rhine Estuary-Drechtsteden and the southwest delta. This is a key transitional area in the Dutch delta. River and sea come together here, and there is a wide range of interests requiring protection – both in terms of population and economic activity. The proposal for the Delta Decision comprises one or more strategies to ensure flood protection and sustainable freshwater supplies up to 2050 followed by a forward view to 2100.
5. Delta Decision Water level management Ijsselmeer Region. Strategy for water reserves in this lake in view of freshwater supplies and flood risk management. The proposal for the Delta Decision IJsselmeer area comprises a strategy for water level management in the IJsselmeer area for the period 2015-2050 with a forward view to 2100.

With respect to flood safety a number of interim conclusions has been drawn in 2012, among others (25):

• Flood risk management programme. The basis for the flood risk management programme, to be presented in 2014, includes multi-layer flood risk management. Primarily, this deals with solutions aimed at flood prevention, but also possible supporting measures from the spatial development and disaster management angles.
• Multi-functional flood defence systems. Multifunctional deployment of flood defence systems is – linked to the integral character of the Delta Programme – an interesting option. This is certainly the case for urban areas where space is in short supply, and near ‘delta dykes’.
• Natural flood risk management measures. Natural flood risk management measures can be cost-effective and deliver added value. Where possible they will be developed as alternatives within the Delta Programme.
• The Maeslant barrier in the Nieuwe Waterweg plays an important role in protecting the west of the country from the sea and this will continue for the next several decades. Possible improvement to the safety of this flood defence system will be reviewed.

In addition it has been concluded that (35):

• Southwest: For the Oosterschelde tidal basin and the Westerschelde estuary, optimisation of the current safety strategy appears to suffice for tasking until 2100, combined with innovative concepts for dykes and adjustments to the Oosterschelde storm-surge barrier when sea level rise exceeds 50 cm.
• Central coast: Safety along the coast can be largely kept at the required standard by means of sand replenishments over the next few decades. However, the volume of the replenishments must be sufficient for the coastal foundation zone to meet the standard in light of the measured rise in sea level.
• Northern (Wadden Sea) coast: The current safety strategy will be sufficient for the next few decades. It can be optimised by combining dykes with salt marshes, or by making dykes overtopping-resistant and organising the hinterland in such a way that damage caused by overtopping is minimised. Sand replenishments along the North Sea coast may be optimised by assessing them in conjunction with sediment transport to the dunes on the islands and via the outer deltas to the sea. Using innovative dykes may enable combinations with other functions. Thorough preparation for disasters and disaster management may reduce the risk of casualties further. The moment at which the Wadden Sea can no longer keep pace with the rising sea level and the tidal flats ‘drown’ can be deferred by intervening in the system (e.g. large-scale replenishments in the outer delta or in the Wadden Sea itself ).

Adaptive deltamanagement

The Delta Programme works along the lines of the so-called ‘adaptive delta management’ approach. This approach offers chances to switch to other strategies or to carry out measures in such a way as to enable later expansion or adjustment.  It is about doing what is necessary, neither too much nor too little, without ruling out future options (25,42,60).

As sea levels rise faster and higher, sand nourishment volumes to maintain the Dutch coast may need to be up to 20 times larger than to date in 2100, storm surge barriers will need to close at increasing frequency until closed permanently, and intensified saltwater intrusion will reduce freshwater availability while the demand is rising (57).

Adaptation strategies - Future flood protection

The Netherlands is opting for sand replenishment as a way of enabling the coastal foundation zone to grow concurrently with the rise in sea levels. Where possible, this is to take place by distributing and transferring sand naturally along the coast. In addition, the Cabinet is opting for a cohesive approach to area development that allows for a balanced development of nature, economy and accessibility in the existing coastal areas (6).

Along the Holland coast an experiment is being carried out with a concentrated mega-nourishment with 20 million m3 of sand (the so-called Delfland Sand Engine). The present coastal maintenance practice of small-scale nourishments is climate-robust for existing beaches and dunes, as it is flexible and adaptable. Mega-nourishments are expected to mitigate some of the negative impacts of small-scale nourishments and create additional wildlife habitats and opportunities for recreation and economic activities. The mega-nourishment experiment is considered a logical next step in the existing coastal maintenance strategy with shore nourishment in the Netherlands (43).

It has been estimated that adaptation strategies could reduce the damages of climate change in the Netherlands by a significant amount and at a relatively low cost: optimal investment in protective infrastructures would reduce the damages of climate change to the Netherlands from 39.9 billion Euros to 1.1 billion Euros in the 21st century, at a cost of 1.5 billion Euros (12).

Salt marshes in front of sea dikes 

Salt marshes in front of sea dikes may reduce the heights of waves attacking sea defences, even under extreme conditions. This has been assessed for the Wadden Sea dikes in the North of the Netherlands. If these salt marshes could keep pace with sea level rise they may result in a reduced dike reinforcement task (53). 

Adaptation strategies - Withdrawal to higher grounds?

According to Dutch engineers it will be possible to maintain the current high level of flood protection for centuries to come, even under far higher sea level rise scenarios than are considered likely based on the current knowledge of climate change (14). It may be questioned, however, whether in the long-term a gradual relocation of companies, plants, and infrastructure may take place to the higher eastern part of the country should sea level rise much faster than currently anticipated. This could, for instance, be triggered in response to a (near) flood, and result in domino effects to other companies (14).

Scientists relate a possible scenario of withdrawal to higher ground to extreme high sea level rise of 5-6 meter within centuries, which is again related to the unknown but probably small probability that the West-Antarctic Ice Sheet (WAIS) will collapse because of anthropogenic climate change. The WAIS comprises about 10% by volume of the entire Antarctic ice sheet, and in volume is equivalent to a 5-6 meter rise in sea level (15).

Although people often speak about a ‘‘WAIS collapse’’, meaning the loss of most or all of the WAIS land based ice, this process would in fact be slower than collapse might imply. It is difficult to postulate significant loss occurring in less than a few hundred years. It has been concluded that the scientific opinion allows for a 5% probability of the WAIS causing a sea level rise of at least 10 mm/yr (or 1 m/century) within 200 years. In terms of total rise due to the WAIS contribution, scientists estimated a 5% probability of a 0.5 meter rise by 2100, about 2.3 meter by 2500, and about 3.2 meter rise by 4000. Hence, none of these estimates equate to a total WAIS collapse (16).

Given the current status of scientific knowledge, the total collapse of the WAIS in relatively short timescales of 100–200 years cannot presently be stated as completely impossible. This is supported by discussions with several glaciologists who all thought such rapid collapse highly unlikely, but felt the system was not well enough understood to totally discount such a rapid change (14).

Adaptation strategies - Second Sustainability Outlook

In the Second Sustainability Outlook, the Netherlands Environmental Assessment Agency (PBL) concluded that the Netherlands will probably remain climate-proof and protected against sea level rise for some centuries to come, and that structural spatial measures, such as a shift in investment to the upland areas of the Netherlands or to a much wider coastal zone, are not urgently required (17).

The diminishing probability of gravity (free) discharge from the rivers is a determining factor in the long-term sustainability of the Netherlands. Should sea levels rise by about two meters, other structural solutions may have to be found for dealing with peak discharges from the rivers Rhine and Meuse. A rise in sea level as high as this, at the upper end of estimates by the Royal Netherlands Meteorological Institute, could occur within two or three centuries. The heavily populated lower reaches of the major rivers, which include the cities of Rotterdam and Dordrecht, are especially vulnerable (17).

In the southwest delta region, the river areas and the IJsselmeer area, land should be reserved to keep longer-term options open for changing the discharge regimes and water storage capacity of the rivers and lakes. These designated inundation areas also make the Netherlands more resilient to any unexpected increases in the rate of sea level rise this century. Areas for extra water storage are most needed in the low-lying Netherlands, the deeper areas of some of the land reclaimed from lakes being the prime candidates. These areas are most suitable, because this would also help counter salt-water intrusion, combating desiccation in surrounding nature conservation areas and would be of additional benefit to recreation and green residential areas (17).

A separate salt-water drainage system could support the development of water based recreational facilities. It is assumed that any new urban areas will be designed with extra space for water storage, given the limited options and high cost of later modification (sewerage, water storage space). This is also an important ingredient in the restructuring of existing urban areas (17).

Urban planning and infrastructure developments have long-term effects that will have consequences for several generations. Decisions taken in the coming decades, therefore, will partly determine the scope for future solutions with regard to adaptation to climate change (17).

The safety and long-term future of the Netherlands can, in principle, be controlled by (17):

• maintaining or enhancing the level of protection through engineering measures, such as strengthening coastal defences and dikes;
• reducing the effects of flooding by adapted building methods, compartmentalisation, awareness raising, risk and crisis communication and emergency evacuation plans;
• steering spatial development to minimise damage and casualties in the event of flooding and keeping options open for future spatial planning measures.

A strategy of differentiated safety standards, compartmentalisation and overflow dykes can be pursued in order to considerably limit the risks, particularly the casualty risks, and to make the river system more robust. This will require an investment of over 3 billion Euros in 2020 to 2040, but would reduce the potential annual damage by 35% and – depending on the engineering options – reduce the casualty risk by possibly as much as 70% (17)

Adaptation strategies - Contingency planning

Disaster prevention, preparedness, response and recovery should become even more of a priority for Member States (24).

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 (40). 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 (40).

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 (40).

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 (40).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for the Netherlands.

1. ten Brinke et al.  (2010)
2. ten Brinke et al.  (2008a)
3. Ministry of Transport, Public Works and Water Management, Ministry of Agriculture, Nature and Food Quality, andMinistry of Housing, Spatial Planning and the Environment (2010)
4. Platform Communication on Climate Change (2006)
5. Kwadijk et al. (2010)
6. Ministry of Housing, Spatial Planning and the Environment (2009)
7. Meier et al. (2004); Räisänen et al. (2004), both in:Alcamo et al. (2007)
8. Knippertz et al. (2000); Leckebusch and Ulbrich (2004); Lozano et al. (2004), all in: Alcamo et al. (2007)
9. Hulme et al. (2002); Meier et al. (2004), both in: Alcamo et al. (2007)
10. Commission of the European Communities: Green paper
11. Beniston et al. (2007)
12. Bosello et al. (2007), in: Carraro and Sgobbi (2008)
13. Van den Brink et al. (2005)
14. Tol et al. (2006)
15. Lythe et al. (2001); Vaughan and Spouge (2002); Oppenheimer and Alley (2004), all in: Tol et al. (2006)
16. Vaughan and Spouge (2002), in: Tol et al. (2006)
17. Netherlands Environmental Assessment Agency (PBL) (2010)
18. Klijn et al. (2007), in: Netherlands Environmental Assessment Agency (PBL) (2010)
19. RIVM (2004)
20. Ministry of Transport, Public Works and Water Management (2000a)
21. Ministry of Transport, Public Works and Water Management (2000b)
22. Ministry of Transport, Public Works and Water Management (2008)
23. Katsman et al. (2011)
24. Commission of the European Communities: Green paper (2007)
25. Ministry of Infrastructure and the Environment, and Ministry of Economic Affairs, Agriculture and Innovation (2011)
26. Sterl et al. (2009), in: IPCC (2012)
27. Debernard and Roed (2008), in: IPCC (2012)
28. Bindoff et al. (2007), in: IPCC (2012)
29. Church and White (2011), in: IPCC (2012)
30. Velicogna (2009); Rignot et al. (2011); Sørensen et al. (2011), all in: IPCC (2012)
31. Marcos et al. (2009); Haigh et al. (2010); Menendez and Woodworth (2010), all in: IPCC (2012)
32. Debernard and Roed (2008); Grabemann and Weisse (2008), both in: IPCC (2012)
33. IPCC (2012)
34. Woth (2005)
35. Ministry of Infrastructure and the Environment, and Ministry of Economic Affairs, Agriculture and Innovation (2012)
36. Bosello et al. (2012)
37. De Moel et al. (2011)
38. CPB et al. (2006), in: De Moel et al. (2011)
39. Woth et al. (2006)
40. Hinkel et al. (2010)
41. Gaslikova et al. (2013)
42. Zevenbergen et al. (2013)
43. Van Slobbe et al. (2013)
44. Cazenave et al. (2014)
45. IPCC (2014)
46. Royal Netherlands Meteorological Institute (KNMI) (2014)
47. Watson et al. (2015)
48. Yi et al. (2015)
49. Church et al. (2013), in: Watson et al. (2015)
50. Shepherd et al. (2012), in: Watson et al. (2015)
51. Church et al. (2013), in: Watson et al. (2015)
52. Hoogland et al. (2012)
53. Van Loon-Steensma (2015)
54. Schrum et al. (2016)
55. Vousdoukas et al. (2016)
56. Howard et al. (2014); Sterl et al. (2009), both in: Vousdoukas et al. (2016)
57. Haasnoot et al. (2020)
58. Stouthamer et al. (2020)
59. Website Dutch Met Office www.knmi.nl (download 22 Jan. 2022)
60. Van Alphen et al. (2022)