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).

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).

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).

• 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).

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).

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).

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).

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).

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).

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)

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).