Uncertain Accelerated Sea-Level Rise, Potential Consequences, and Adaptive Strategies in The Netherlands
2. The Netherlands and the Delta Programme
3. Materials and Methods
4.1. The Potential Impact of Sea-Level Rise on the Dutch Delta
4.2. Strategies to Cope with Sea-Level Rise
- Retreat, which involves reducing the exposure to coastal hazards by moving people, assets, and activities landward to higher ground. This choice can be motivated by excessive economic or environmental impacts of protection or long-term adaptation needs;
- Accommodate, which implies that people continue to use the land at risk but do not attempt to prevent the land from being flooded. This option includes creating emergency flood shelters, elevating buildings on piles or mounds, converting agriculture to fish farming, or growing flood or salt tolerant crops;
- Protect, which involves hard structures such as seawalls and dikes, as well as nature-based solutions such as dunes and vegetation, to protect the land from the sea so that existing land uses can continue. Protection can be provided by strengthening the existing coastline or by advancing a higher new coastline.
- Close them off from the sea completely by dams, navigation locks, and sluices. This ‘protect-closed’ strategy protects the inland area against SLR and the related salinization. However, handling river discharge requires enormous pumping stations, with a capacity of several thousand m3/s, in combination with large areas for temporary flood storage.
- Maintain the open connection with the sea. In this ‘protect-open’ strategy, rising sea level extends its influence upstream along the rivers, flooding unembanked (harbor) areas and requiring extensive dike improvement programmes to maintain flood protection standards. In addition, a large inland area may be affected by salinization of surface waters.
- Until 2050, SLR projections for different emission scenarios vary only little. Therefore, a continuation of the present protect strategy seems logical as a first step. A complicated societal debate is avoided, choices can be postponed until more knowledge about SLR becomes available, and present land use can be continued by increasing the present technical measures in flood protection and water management, presumably up to a sea-level rise of 2 m and a related rate of SLR. The increasing volumes involved in beach nourishments (up to 60 million m3/year) can be dredged on the Dutch continental shelf. This strategy largely depends on scaling up present sand extraction and beach nourishments operations of generally 1–5 million m3 per project. An interesting pilot is the so-called ‘Sand motor’, a 1.28 km2 beach nourishment of 21.5 million m3, costing about EUR 70 million. After 10 years of monitoring, it was concluded that the ‘Sand motor’ acted as a ‘feeder beach’ from which the sand is transported to adjacent dunes and beaches by natural processes such as wind, tidal currents, and waves . It also illustrated the ‘benefits of scale’ effect, since such larger volumes enable the use of more efficient large vessels and nourishment techniques, and reduces the costs of material mobilization and demobilization costs for ground works. As a consequence, the usual beach nourishment costs were almost halved.
- In addition to these kind of pilots, low-regret measures can be taken, such as spatial reservations for sand extraction in the North Sea, to avoid conflicts with other marine users, including wind farm developers. Nevertheless, a large loss of intertidal areas in the Wadden Sea cannot be prevented if SLR rates consistently exceed 6 mm/year (western Wadden Sea) or 10 mm/year (Eastern Wadden sea). Nor can the salt intrusion be fully avoided. In this way, the protection pathway is continued, with the risk of large transfer cost for future generations, if a shift to other strategies is needed.
- With rising sea levels, the first dilemma that may emerge is whether to permanently close the estuaries or to keep them open to facilitate free river discharge. To keep both options open in the coming decades, it is necessary to avoid new developments in unembanked areas (or at least make them flood-proof), and reserve space for future dike improvements. A first inventory shows that future dike raising requires 12–17 m additional horizontal space per meter raised. This requires valuable space, especially in urban areas such as Rotterdam, which can only be used for temporary activities. Furthermore, in the closed option, drainage of river discharge requires large pumping capacity, up to several thousand m3/s or even more. This pumping requirements can be reduced when flood waters can be temporarily stored. This storage capacity can be found in present water systems, or in creating new systems in the coastal zone, attached to the river mouths. Model simulations can help find the optimum combination of dike raising, storage volumes, and pumping capacity.
- Regarding agriculture and horticulture, a gradual transition to salt (and drought)-tolerant crops is low-regret, since salinization of the coastal area is expected in both the retreat and protection strategies.
- With continued SLR and increasing technical efforts, a fundamental choice between ‘advance’ or ‘retreat’ becomes inevitable. The ‘advance’ strategy can be seen as a next step in the present protective strategy, heavily relying on technical measures such as flood defenses and pumps. Therefore, some experts advocate to start with this strategy already, and avoid short-term expenses on maintenance and on upgrading the present flood and water management infrastructure. The retreat strategy marks a significant ‘change of mind’, which may be reached after a long societal debate, or become reality when natural disaster creates a ‘fait accompli’.
4.3. The First Step
- research aimed to reduce the uncertainty regarding SLR. This requires international cooperation on polar research, but also on monitoring SLR in coastal seas;
- develop a method to signal a potential acceleration of SLR timely and with sufficient confidence. This involves statistical research on SLR data and projections;
- establish the potential effects on the flood risk and water management of the Dutch delta and related infrastructure and establish the lifetime of the present policies. Detailed model computations for different SLR scenarios must be performed to predict future coastal erosion, necessary beach nourishment volumes, hydraulic loads and required flood defense strengthening, storage and pumping capacities, and increase of fresh water supply. Special attention is paid to the discharge of river water with the rising sea-level , which is a major challenge for many deltas worldwide;
- explore long-term options which might become necessary when present policies become short. In this situation, the present territory and land use of the Netherlands might be affected by regular or permanent flooding and salt intrusion. In cooperation with other authorities and NGOs, it must be explored whether present investment agendas on, e.g., housing, infrastructure, and sustainable energy may occupy land that might be necessary for future strategies to deal with SLR, and how these agendas can be aligned to create synergy with these future strategies;
- prepare the implementation of the necessary policies and measures by:
- collecting expertise and knowledge on large-scale long-term societal transitions;
- starting pilots to gain experience with the possibilities to scale up present techniques and methods (e.g., building with nature);
- propose spatial reservations for future sand extraction, flood protection, water discharge, storage, and supply; and
- introducing the adaptive approach in the design of projects that are planned for the coming years in housing, infrastructure, and replacement of present (aging) infrastructure.
5. Discussion and Conclusions
- research into the behavior of Antarctic ice, combined with the development of a method to signal SLR acceleration in a timely way with sufficient confidence;
- research into the consequences of accelerated SLR on present water management infrastructure such as flood defenses, storm surge barriers, locks and pumping stations, and fresh water intake locations. How will SLR affect their lifetime, maintenance, replacement, or reconstruction? Consequently, what will be the effect on the land use that this affected infrastructure serves? Which strategic choices in water management and land use strategies become inevitable? E.g., is the present type of fresh-water-based agriculture in the coastal zone sustainable with increasing salinization due to SLR? Or do we continue with new urban development in low-lying coastal areas which require increasing efforts to maintain dry?;
- the exploration of long-term strategies (e.g., ‘protect’, ‘advance’, ‘accommodate’, and ‘retreat’) that might become necessary when present policies start to fail. Vice versa, identify necessary short-term measures to keep these future strategies open and avoid regret investments. An important no-regret measure is the spatial reservation for future sand extraction (for beach nourishments) and for future expansion of flood defenses, water discharge, and water storage.
- flexible measures, which are important to bridge the coming decades of uncertainty in SLR. Nature-based solutions, such as beach nourishments or stimulating natural sedimentation on flood defense forelands, are relatively cheap and more flexible to adapt to changing conditions than concrete structures. In addition, they have beneficial side effects on, e.g., nature restoration and recreation. Pilots may be helpful to improve the effectiveness of these measures and establish their ‘scaling up’ potential, as illustrated by the Sand motor mega beach nourishment .
- adaptive design. When decisions have to be made regarding the (re)construction of long-lifetime robust infrastructure, an adaptive design should be promoted, to make future expansion possible without excessive costs.
Institutional Review Board Statement
Informed Consent Statement
Conflicts of Interest
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van Alphen, J.; Haasnoot, M.; Diermanse, F. Uncertain Accelerated Sea-Level Rise, Potential Consequences, and Adaptive Strategies in The Netherlands. Water 2022, 14, 1527. https://doi.org/10.3390/w14101527
van Alphen J, Haasnoot M, Diermanse F. Uncertain Accelerated Sea-Level Rise, Potential Consequences, and Adaptive Strategies in The Netherlands. Water. 2022; 14(10):1527. https://doi.org/10.3390/w14101527Chicago/Turabian Style
van Alphen, Jos, Marjolijn Haasnoot, and Ferdinand Diermanse. 2022. "Uncertain Accelerated Sea-Level Rise, Potential Consequences, and Adaptive Strategies in The Netherlands" Water 14, no. 10: 1527. https://doi.org/10.3390/w14101527