Vulnerabilities - 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 (3):

• 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 (1,3).

Erosion of the coastal zone

Coastal erosion was estimated to occur over 134 km (2), spread along half of the Dutch coast (4). Large sections of the Dutch coast have been eroding for centuries, at some locations resulting in a retreat of 5 km in four centuries. In the past, only an ad-hoc policy against coastal erosion was followed: measures were only taken when the safety of polder land was at stake or when special values in the dune area, e.g. drinking water areas, nature reserves, camping places, were threatened (4).

Discussion on a new policy for coastal defence of dune coasts started in the 1980's (5). In 1990 Parliament decided to adopt a new policy called “Dynamic Preservation of the coast line” in order to stop further retreat of the coast, meaning that the entire coastline will be maintained at its 1990 position. Further erosion will be counteracted by sand nourishments. Sand nourishment has been a common measure to combat coastal erosion in the Netherlands since the end of the 1970's. When a nourishment project is carried out, sand excavated from the bottom of the North Sea (outside the -20 m depth contour), is added to the near shore zone (4).

Initially, sand nourishments involved 6 M m3 sand per year. Following ongoing research and especially taking into account the sustainability of the deeper shore, it was concluded that a higher amount is needed. From 2001 onwards, the Netherlands has raised the total nourishment volume in its coastal system from an average of 6 to an average of 12 M m3 per year (4).

Vulnerabilities - Erosion of the dunes

Large parts of the Netherlands are below sea level, protected against floods from the North Sea by a coastal flood defence system consisting of dunes, dams and storm-surge barriers. Over the last decades these flood defences were designed and maintained to be high and strong enough to withstand storm surge levels that may occur with a probability of 1/10,000 per year. The extremely high safety standards of the Dutch flood defence system are unique worldwide. Investments are needed to maintain this high safety level, under current conditions near the coast (and on the rivers and lakes) and with respect to the projected impacts of climate change. Scientific studies inform policy makers on possible impacts of climate change on sea level rise, and wave and storm surge conditions, and assist flood defence managers to take appropriate measures. Part of this research focuses on the dunes of the central connected coast that protects cities like Amsterdam, Rotterdam and The Hague.

Sea level rise will affect dune erosion by allowing waves to attack the dunes at a higher level. Sea level is projected to rise 0.15 to 0.8 m towards the end of this century (13). Extreme waves and storm surge, however, are not expected to change under a changing climate, not even for conditions with a probability of 1/10,000 per year (14). There are, however, indications that the corresponding wave direction could shift (15), because of a poleward shift of the storm track (16).

The effect of climate change on dune erosion along the Dutch coast was studied under wave and storm surge conditions at the 1/10,000-year event (12). This was done for two locations. Currently the Dutch coast is eroding. The study shows that the volume of eroded dune sand increases linearly with sea level rise by little over 20 % per meter sea level rise. This increase is primarily due to the larger water level in front of the dune.

Changes in the offshore angle of wave incidence also affect the volume of eroded dune sand. Although the height and energy of the extreme waves along the Dutch coast are not expected to change, the corresponding wave direction may shift to more westerly directions (14). A shift by 30° could have the same eroding effect as 0.4 metre sea level rise. This increase is related to strong alongshore currents, generated as a result of the obliquity of the waves, that enhance stirring and hence offshore transport. Climate change-induced change in angle of wave incidence cannot be ignored in dune erosion studies. The authors conclude that coasts exposed to extreme wave conditions from different directions should assess changes in the wave direction under climate change conditions, as this impact can be as large as, or in the same order of magnitude as sea level rise (12).

The current Dutch policy towards sea level rise is to add the same volume of sand to the profile of the near-coastal zone as the water volume increases as a result of sea level rise. According to the authors this approach is conservative: a focus on dune strengthening in case of large sea level rise may be more effective. The current level of flood protection could be reached in the future with less additional sediment if the sand is used to increase the volume of the dunes (12).

Dune erosion events alternate with prolonged periods of dune accretion through aeolian processes. Climate change may affect these processes too. Changes in wind patterns and beach width can influence yearly aeolian supply to the foredunes. More dune erosion could lead to more blowouts and, as a corridor for aeolian transport, stimulate the vertical growth of the more landward dunes. More erosion may, accordingly, not necessarily be bad for the beach dune system as a whole (12).

Vulnerabilities - The future of the Wadden Sea

The Wadden Sea is a coastal region that spans a distance of nearly 500 km along the North Sea coast of the Netherlands, Germany and Denmark. It consists of a chain of barrier islands that shelter an area of extensive intertidal flats and salt marshes, dissected by tidal channels and creeks. No rivers debouch into the basins of this coastal zone. All sediment that settles into the channels and onto the intertidal flats is imported from the North Sea. The accretion of the intertidal flats under sea-level rise, therefore, also depends on the import from the North Sea. If this import is insufficient, the intertidal flats may not keep up with sea-level rise at some point in the future, and drown. The ecological value of the Wadden Sea is high. Especially the intertidal flats support a wide range of wildlife. If these flats would drown under sea-level rise, a most valuable nature reserve would be lost.

With the current rate of sea level rise and the current replenishment policy, sufficient sediment is transported to the Wadden Sea to allow the area to grow apace with the sea level (9,17). In the future, particularly in the case of accelerated sea level rise and soil subsidence, the supply of sufficient sediment may become more problematic, as the outer deltas will then shrink in size (9).

The impact of accelerated sea-level rise and soil subsidence

For the Wadden Sea’s future it is the relative sea-level rise that is important: the combination of absolute sea-level rise and the subsidence of the intertidal flats. A projection of possible future developments of this area is presented in a recent study that includes state of the art understanding of sea-level rise, soil subsidence, and sediment flows (17). The study focuses on the years 2030, 2050 and 2100.

Projected mean sea-level rise between now (2018) and 2100 is 0.41 m, 0.52 m and 0.76 m, under a low, intermediate and high-end scenario of climate change (the so-called RCP2.6, RCP4.5 and RCP8.5 scenarios), respectively. The subsidence results from the extraction of gas and salt in the area, and from natural processes (compaction of sediments, postglacial adjustment of the earth’s crust). The estimated effect of gas and salt extraction on subsidence in future decades varies locally, from zero to 1.6 mm per year. Natural subsidence is less than 1 mm per year (17).

Under the low-end scenario of climate change, projected sea-level rise will not exceed the critical rate for drowning of the intertidal flats this century. Under the intermediate scenario of climate change this critical rate will be exceeded in one part of the Wadden Sea in 2030, whilst under the high-end scenario this rate will be exceeded in more parts from 2030 onwards. However, even under this high-end scenario the intertidal flats will not disappear on short notice. Drowning of the intertidal flats results from a gradual erosion of the flats. This process will take several centuries. Thus, for the near future (up to 2030), the effect of sea-level rise on the Wadden Sea will be hardly noticeable. Over the long term, up to 2100, the projected changes largely depend on the climate scenarios. There will be hardly any effect under the low-end scenario, whereas effects will be noticeable already in 2050 under the high-end scenario (17).

Recently, an assessment has been carried out of possible upper limits of sea-level rise, based on recent studies (18) that indicate that ice melt of Antarctica proceeds much faster than has been assumed so far (19). The assessment indicates that sea-level rise may be up to 2-3 metres in 2100. This would agree with a rate of sea-level rise of 14 mm per year in 2050, and possibly increasing up to 60 mm per year in 2100. Under these extreme scenarios, the Wadden Sea will be practically drowned before 2100 (17).

Other studies show similar results. Model results indicate that the extensive tidal flats in systems such as the Dutch Wadden Sea may slowly diminish or even disappear entirely, which may have such massive socioeconomic impacts that the continued existence of some local communities may become untenable in the long-term (7). These results are based on a modelling approach over a 110-year study period with three different scenarios for Relative Sea Level Rise (RSLR): (a) No RSLR, (b) IPCC lower sea level rise (SLR) projection (0.2 m SLR by 2100 compared to 1990) and  land subsidence, and (c) IPCC higher SLR projection (0.7 m SLR by 2100 compared to 1990) and land subsidence.

These model results indicate that under the No RSLR condition, the tidal flats continued to develop while under the high RSLR scenario the tidal flats eventually drowned, implying that under this condition the system may degenerate into a tidal lagoon. The tidal flats were more or less stable under the low RSLR scenario implying that this may be the critical RSLR condition for the maintenance of the system.

Uncertainties in the physical mechanisms governing tidal inlet response to RSLR are high, however; previous findings indicate that the tidal flats can keep up with much higher RSLR rates, up to 8 mm/year (20) or even up to 10.5 mm/year) (8).

Adaptation strategies

Currently, depending on the market situation, some 12 million m3 of sand is replenished annually (21) and an increase to 20 million m3 is needed for the entire coastal base to grow apace with the current rise in sea levels. With a steady rise in sea level, the volume of sand needed to maintain the coastal base also increases apace. According to initial estimates this can vary, rising to an annual average of 30 million m3 in 2050, rising to as high as 65 million m3 annually in 2100 (6).

The current replenishment volumes along the coast are not sufficient to also have the Wadden Sea fully grow apace with the sea level. As yet it is uncertain where the sand shortfalls will eventually create safety problems in the Wadden Sea area. Changes in the estuary could mean increasing high water levels in the Eems-Dollard. Given these factors it is likely that safety tasking will increase in the long run (6).

Delfland Sand Engine, from reactive to proactive coastal protection

The provinces of North and South Holland on the North Sea coast of the Netherlands are protected by a 120-km-long sandy shore, the Holland coast. Before 1990, the coastal protection was managed with a ‘hard’ engineering strategy (dams, reinforcements and acceptance of beach erosion). After 1990, the policy changed and nowadays the coast is maintained by nourishing it with sand mined offshore. In 2000, after measurements showed a steepening of the lower shoreface (6–8 m below mean sea level), it was decided to extend the sand nourishment strategy to deeper water. The new maintenance zone, called the ‘coastal foundation’, extends down to the -20-m deep contour. Since 2000, the average volume of sand used to nourish the entire Dutch coast has amounted to 12 million m3 per year (10).

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. The Delfland Sand Engine was completed mid-2011. This mega-nourishment was expected to reduce construction costs, create a more natural coastal profile in the long run and provide a number of ecological and recreational benefits. In the long run, the body of sand will be redistributed over shoreface, beach and dunes, thus enriching the entire coastal cell between Rotterdam and Scheveningen.

The environmental impact assessment (11) described the pilot project as follows: ‘A sand engine is a large sand nourishment just off the coast. The sand is distributed by waves, currents and wind in such a way that the coast continues to grow naturally. This creates a buffer of sand on the coastal foundation against sea level rise, thus guaranteeing the safety of the coast in the longer term. It also creates extra space for nature and recreation. The scale of the sand engine is larger than ever seen before. That is why the construction of the sand engine off the Delfland coast has the character of a pilot, meant to gather knowledge on how to build with nature for climate adaptation’.

The shape of the Sand Engine will change over time. Morphological model projections indicate that the sand will be gradually redistributed over the beaches, dunes and foreshore over several decades, at a rate of some hundreds of metres per year.

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. Van der Meulen et al. (2013)
2. www.eurosion.org, in: Van der Meulen et al. (2013)
3. ten Brinke et al.  (2010)
4. de Ronde et al. (2003)
5. Hillen et al. (1995), in: de Ronde et al. (2003)
6. Ministry of Infrastructure and the Environment, and Ministry of Economic Affairs, Agriculture and Innovation (2011)
7. Dissanayake et al. (2012)
8. Van Goor et al. (2003), in: Dissanayake et al. (2012)
9. Ministry of Infrastructure and the Environment, and Ministry of Economic Affairs, Agriculture and Innovation (2012)
10. Rijkswaterstaat (2011), in: Van Slobbe et al. (2013)
11. Fiselier (2010), in: Van Slobbe et al. (2013)
12. De Winter and Ruessink (2017)
13. De Vries et al. (2014), in: De Winter and Ruessink (2017)
14. Sterl et al. (2009); De Winter et al. (2012), both in: De Winter and Ruessink (2017)
15. De Winter et al. (2012, 2013), in: De Winter and Ruessink (2017)
16. Bengtsson et al. (2006); Harvey et al. (2012); Chang et al. (2012), all in: De Winter and Ruessink (2017)
17. Van der Spek (2018)
18. Oppenheimer and Alley (2016); Le Bars (2017)
19. Haasnoot et al. (2018)
20. Van Dobben et al. (2022)
21. Brand et al. (2022)