The coastline is pro-actively maintained in the Netherlands to prevent negative effects of erosion. Since the 1990’s significant nourishment efforts have been carried out for this purpose. Here, the beach topography of a relatively undisturbed stretch of coast, with the exception of nourishment efforts, was investigated to understand the effect of three decades of coastal preservation. The evolution of the beach volume, coastline position, and coastline orientation were compared before and after the implementation of the coastal preservation policy. Structural erosion of the coastline was put to a halt thanks to regular nourishments. On average, the coastline built out, which was proven necessary to maintain the most erosive parts of the coast. Especially adjacent to breakwaters/groynes and nourishments hotspots an expansion of the coastline can be observed. It can thus be concluded that coastline maintenance is achieved with regular nourishments and that sediment accumulates in the beach zone at certain locations along-shore.

This work investigates the impact of sea level rise (SLR) of up to 3 m on flood protection and freshwater availability in the Netherlands. We applied an exploratory modeling approach to consider the large degree of uncertainty associated with SLR. The results show the current degree of flood protection can be technically and financially maintained for up to three meters of SLR. A primary finding of this work is that a similar degree of safety against floods can be maintained. There are, however, several challenges: First, maintaining this degree of safety against floods requires considerable spatial allocations to maintain and upgrade flood defenses, often in populated areas with limited space. Second, the supply of sand for coastal nourishments will be challenging due to other functions in the North Sea (wind energy, shipping) and explosive remnants of war. Third, an acceleration in the rate of SLR may impact the overall feasibility of maintaining flood defenses. Maintaining the freshwater strategy will be challenging due to SLR-induced salt intrusion, which aggravates climate impacts including droughts. Continued flushing of salinized areas of regional water systems and polders with fresh river water will increasingly compete with other demands. Our analysis highlights the vulnerabilities of the flood protection and freshwater strategies and gives input to follow-up analyses on societal impact and perspectives of actions for adaptation.

Shoreface nourishments have become a popular management option to mitigate coastal retreat for sites with abundant sand supplies. With shoreface nourishments, relatively large volumes of off-site sand are placed under water in typical water depths of 4–10 m. This part of the nearshore zone has a high bed level variability and contains a myriad of (rhythmic) morphological features. As a result, understanding and forecasting shoreface nourishment morphodynamics and impacts is challenging. Significant progress on this topic is needed in due time, especially in light of emerging climate-change effects. This review paper presents an overview of field, laboratory and numerical modeling studies on shoreface nourishment morphodynamics. We have identified 4 key knowledge gaps. First, the spreading of nourished sand through the coastal zone is poorly understood, and has not been quantified. Second, it is unclear how design variables such as size, placement location and grain-size affect the lifetime, spreading and impact of shoreface nourishments. Third, the cumulative effect of repeated shoreface nourishments (scale: 1–10 km, 1–10 years) on the coastal system as a whole (100+ km, 50+ years) is largely unknown. Fourth, numerical models cannot reliably predict the complete morphological development and impact of shoreface nourishments. To tackle these knowledge gaps we propose a research agenda to ensure the generation and valuation of scientifically robust and societally relevant knowledge.

Coastal erosion threatens flood safety and other uses of beaches and dunes globally. In the Netherlands a coastline maintenance policy was implemented in the 1990's to address the negative effects of erosion, with sand nourishments as the primary means. In this study, the cumulative effects of these nourishments are evaluated against the strategic goal of sustainable preservation of the uses and values of the coast. This research aims to inform national and international policy makers, practitioners, and scientist about the possible long-term effects of coastal management with structural sand nourishments. Coastal indicators were analysed to quantify the morphological evolution of the coast before and since coastline maintenance. It is observed that regular nourishments serve to halt structural coastline retreat. The coastline built out, on average, which was necessary to achieve maintenance of the most erosive areas. Additionally, strong dune growth is observed since the start of coastline maintenance, thanks to wind-driven transport of nourished sand and more dynamic dune management. Nourishments thus contribute positively to flood safety, although flood safety is not an automatic benefit of coastline maintenance. Space for recreation and nature is maintained or improved: the dry beach width was unaffected, and dune areas have grown. Further, it is reported that the impact of nourishments on the coastal ecosystem is local and temporary, leading to the inference that uses and values of the coast are being maintained sustainably through regular nourishments. Overall, the approach of coastline maintenance with regular pro-active nourishments has thus proven to be successful.

Future sediment transport from the North Sea coasts to the Dutch Wadden Sea for various future sea level scenarios has been studied because it influences the future sand nourishment demand for the maintenance of the coastline and because it determines bio-geomorphological development of the Wadden Sea. The present study focuses on two questions which have not yet been considered in the previous modelling studies using ASMITA: How will the transport develop around drowning of the intertidal flats in the Wadden Sea? How will tidal range change influence the future sediment exchange? By using SLR scenarios with faster acceleration and running the simulations for longer periods of time some inlets exhibited drowning, i.e., where the tidal flat volume vanishes. When drowning occurs, the sediment import rate approaches a maximum or a minimum, depending on the initial morphological state of the tidal inlet system. This maximum or minimum rate for a certain tidal inlet system depends on the SLR scenario. Theoretical analysis as well as modelling results show that tidal range change will influence the sediment import to the Wadden Sea. A tidal range increase will cause a decrease of the sediment demand in the Wadden Sea resulting into less sediment import to the Wadden Sea. It is thus important to study the tidal range development in the Wadden Sea by considering the interaction between SLR, tidal range change and morphological development in the system. It is further concluded that the empirical relation used in the previous studies is not representative of conditions in a tidal basin with fixed basin area, even though this relation has been derived from field observations in many tidal inlet systems worldwide. The equilibrium channel volume should be proportional to the tidal prism instead of to its 1.5^th power.

Globally coasts are under pressure owing to stressors such as human use and climate change. From the 1970s onwards, Integrated Coastal Management gradually emerged as a strategic approach that strives for sustainable integration of the sometimes conflicting interests of human use, natural values and protection against flooding. Developing and implementing effective integrated (coastal) policy requires meaningful interaction between policymakers and scientists. However, bridging the gap between science and policy remains problematic. Developing, implementing and conducting research specifically for policy is an ongoing challenge for both coastal managers and scientists. Therefore, the central question in my research is: How can we connect science and policy in support of sustainable coastal management, not at a global scale, but regionally in the Netherlands?

This research has two parts. The first part examines how the policy-oriented Coastal Genesis 2 research programme has influenced Dutch coastal policy and management. Specifically, it analyses how the programme has contributed to bridging the gap between science and policy. The (revised) conceptual model of the long-term sediment budget of the Dutch coast plays a central role in the analysis. The second part of the research addresses one of the most important uncertainties in this conceptual model, namely the long-term morphological development of the Dutch Wadden Sea.

The long-term sediment demand of the Dutch coast is integral to the current Dutch Coastal Flood and Erosion Risk Management policy. The Coastal Genesis 2 research programme was initiated to address the sustainability of this policy under sea level rise by focusing on key uncertainties in the conceptual model of the sediment demand of the Dutch coast. The substantive scientific contributions of the Coastal Genesis 2 research programme are analysed in this paper by applying an output-outcome-impact framework. The direct outputs of the programme are categorised in terms of the knowledge types of a 5-element framework, namely measurement data, simulation models, system understanding, conceptual models, and policy and practice. The research outcomes arise from the interactions of these knowledge types. Our analysis of these outcomes highlights that synthesising new scientific insights into shared conceptual models is critical to achieving impact in policy and practice. In the Dutch situation, a new shared conceptual model of the long-term sediment demand enabled the development of four potential nourishment strategies aiming to meet the strategic goals of the Coastal Flood and Erosion Risk Management policy on a timescale up to 20 years. In 2021, the Minister of Infrastructure and Water Management officially articulated her intention to adopt the advised nourishment strategy from 2024 onwards. This represents a lasting impact of the Coastal Genesis 2 research programme in policy and practice. Further, the insight regarding the pivotal role of shared conceptual models as intermediary between science, policy and practice may prove useful in the design of future research programmes aiming to influence policy.

The sediment exchange between the Dutch Wadden Sea and the North Sea coastal zone is of key importance to Dutch coastal management. Net sediment import from the coastal zone to the Wadden Sea results in coastal erosion which needs to be compensated through nourishments. At the same time net sediment import is the source of sediment for the intertidal flats in the Wadden Sea to adapt to sea level rise (SLR). Understanding the current and future sediment exchange is therefore essential for sustainable coastal management. Insights in the sediment exchange directly influence the coastal nourishment strategies applied to the Dutch coasts. Projections of the future sediment exchange between the Dutch Wadden Sea and the North Sea are established using the aggregated morphodynamic model ASMITA for five sea level rise scenarios, viz. the present rate of 2 mm/yr and accelerated rates of 4, 6, 8 and 17 mm/yr in 2100. The differences in the projected import rates between the five sea level rise scenarios until 2100 are not as large as the differences in sea level rise rates may suggest. For the Eastern part of the Dutch Wadden Sea, where the morphology is near its dynamic equilibrium, the projected import rate in 2100 varies with a factor 3 (300%), for sea level rise rates from 2 to 17 mm/yr (factor 8.5, 850%). In the western part of the Dutch Wadden Sea, where the morphology is still far from equilibrium due to the closure of the Zuiderzee, the projected import rate in 2100 varies a factor 1.45 (145%) for these sea level rise rates. For the total Dutch Wadden Sea this is a factor 1.7 (170%). The projected increase of the import rate until 2100 with respect to the present situation (2020) is up to a factor 1.45 (145%) for the highest sea level rise scenario, which is significant but not substantial.

Coastal areas world wide are highly dynamic areas, subject to continuous deformation processes. Both natural and anthropogenic processes constantly cause changes at various spatial scales. Sandy beaches in the Netherlands fall under a regulation, according to which moving sand is permitted, if the volume change remains below a certain threshold. The threshold holds for volume changes within a cross section of 1 m width of the beach. The enforcement of this rule is currently labor intensive, because monitoring generally happens only on a yearly basis, or incidental and non-quantitative. Improved observation capabilities with remote sensing are advancing the supporting technology for this kind of regulations. Permanent laser scanning is a potential tool for monitoring and quantifying volume changes of a section of the beach. We develop and implement methodology to extract time series of volume change with respect to a reference date of 01-01-2020 covering January 2020 until the end of April 2020. The method is applied on point cloud data from a permanent laser scanner on the coast of Noordwijk, The Netherlands. We analyse the time series for incidents, where the threshold in volume change is passed, and find all shortest intervals during which the threshold is passed. Then we analyse potential underlying cause in order to support not only enforcement, but also evaluation of the current regulation. This will ultimately help to work towards a better understanding of the influence of small scale human activities on coastal development.

The Dutch coast is one of the most heavily nourished coasts globally. An average of 12 mln. m3 is annually added to the coastline of only 432 km for dynamic coastline conservation. This study provides an overview of the operational aspects of the more than 300 nourishments for coastline maintenance that have been performed since the 1990s and discusses the evolution of the nourishment approach and lessons learned with regard to the nourishment design. The first nourishments were beach and dune nourishments to repair local beach and dune erosion. In the 1990s the nourishment efforts increased when nourishing the coastline was set in policy as the formal strategy to dynamically preserve the coastline. Simultaneously shoreface nourishments emerged, which aim to feed the coast gradually over a longer period than beach nourishments. In 2001 the volume of sand used for nourishments increased from 6.4 to 12 mln. m3 per year, to enable the coastal zone to stay in equilibrium with sea level rise. Channel wall nourishments were introduced around that time because they can slow down the landward migration of tidal channels and can accommodate large volumes of sediment. Nowadays, underwater nourishments are preferred because of the lower costs associated, but the decision for a beach, shoreface, or channel wall nourishment also depends on the morphology, the local setting, and the purpose of the nourishment. All nourishments combined have succeeded in conserving the coastline at its desired position over the past 30 years.

The Wadden Sea is a unique intertidal wetland area, forming an important hub for migratory water birds. A feared effect of accelerated sea-level rise (SLR) is the gradual loss or even disappearance of the ecologically valuable intertidal flats. To date, the effect of SLR on the time-evolution of the intertidal areas in the Dutch Wadden Sea has not been studied. To explore the sensitivity of the intertidal flats to SLR and the spatial differentiation of the response, simulations are carried out with the reduced-complexity model ASMITA for four sea level rise scenarios: one with a stable rate of 2 mm/yr (current rate), and three with accelerated sea level rise rates to respectively 4, 6 and 8 mm/yr. In addition, a scenario with a linearly increasing rate to 17 mm/yr in 2100 has been added to get an impression of what may happen under more extreme SLR-rates. The results show that the intertidal flats in the larger basins are most vulnerable to drowning. Due to differences in tidal flat geometry, the intertidal flats in the smaller basins mainly reduce in average height, while the intertidal flats in the larger basins mainly reduce in surface area. Within the basins, largest losses are expected to occur just off the land reclamation works and along the western part of each tidal watershed. The intertidal flats are sensitive to the rate of SLR. With doubling the rate of SLR, losses nearly double as well. Complete drowning is not predicted for any of the considered scenarios, but for the larger basins volume losses of nearly 50% by 2100 are predicted for the highest considered scenario. This will transform these basins into more lagoon-like basins, which is expected to have major consequences for the ecology.

The development of the Coastal Genesis 2 research programme and its role in contributing to Dutch coastal policy are described in the paper. The organisation of policy development related to coastal flood risk and erosion in The Netherlands is addressed, highlighting the division of responsibilities between the policy and operational directorates of the Ministry of Infrastructure and Water Management. A conceptual model of the long term sediment budget of the Dutch coast that underpins the current Coastal Flood and Erosion Risk Management policy is detailed. The role of the operational directorate Rijkswaterstaat in coordinating a ‘Research for Policy’ cycle as a means of generating new insights on the coastal system and ensuring their subsequent inclusion in a new/revised conceptual model, is highlighted. By detailing the new conceptual model of the long term sediment budget, the paper demonstrates how key uncertainties related to this model guided the determination of the research agenda for Coastal Genesis 2. The paper concludes by reflecting briefly on the outcomes of the research programme and the role of the ‘Research for Policy’ cycle in ensuring the sustainable future of the Dutch coast.

A man-made dune-beach-spit system at the south-east side of the island of Texel (Prins Hendrik site) has been built in 2018–2019 to strengthen the traditional dike. The core of the dune-beach-spit system consists of medium fine sand with a d₅₀ of 0.25–0.3 mm. The beach is covered with an armour (protection) layer of coarse materials with relatively large gravel and shell fractions to reduce wind erosion and thus maintenance costs. In the design phase of the project the aeolian sand transport model of Bagnold was used to estimate the long-term erosion losses of sand at the new dune-beach system. This transport model was validated in the design phase by using detailed sand transport and bed roughness measurements at a nearby site called The Hors. This site is a wide natural beach plain of sand (d₅₀ = 0.23 mm), where 147 high-quality datasets have been collected using a wind mast equipped with 5 cup anemometers and various sand traps. It is shown that the measured sand transport rates at the Hors can be reasonably well represented by the modified Bagnold-equation for dry sand. After completion of the new dune-beach system, a field experiment was performed at the Prins Hendrik site to verify the sediment transport predictions and erosion loss of sand. Data from two permanent wind masts and one short, mobile wind mast were used to derive the effective roughness of (stationary) bed forms. Sand transport rates were measured at various locations using a new trap sampler. The measured sediment transport in the armoured beach zone can be reasonably well represented by the Bagnold-equation using a multi-fraction approach with hiding-exposure coefficient. The predicted transport rates have been used to estimate the annual loss of sand from the Prins-Hendrik site.

Climate change, and especially the associated acceleration of sea-level rise, forms a serious threat to the Wadden Sea. The Wadden Sea contains the world’s largest coherent intertidal flat area and it is known that these flats can drown when the rate of sea-level rise exceeds a critical limit. As a result, the intertidal flats would then be permanently inundated, seriously affecting the ecological functioning of the system. The determination of this critical limit and the modelling of the transient process of how a tidal basin responds to accelerated sea-level rise is of critical importance. In this contribution we revisit the modelling of the response of the Wadden Sea tidal basins to sea-level rise using a basin scale morphological model (aggregated scale morphological interaction between tidal basin and adjacent coast, ASMITA). Analysis using this aggregated scale model shows that the critical rate of sea-level rise is not merely influenced by the morphological equilibrium and the morphological time scale, but also depends on the grain size distribution of sediment in the tidal inlet system. As sea-level rises, there is a lag in the morphological response, which means that the basin will be deeper than the systems morphological equilibrium. However, so long as the rate of sea-level rise is constant and below a critical limit, this offset becomes constant and a dynamic equilibrium is established. This equilibrium deviation as well as the time needed to achieve the dynamic equilibrium increase non-linearly with increasing rates of sea-level rise. As a result, the response of a tidal basin to relatively fast sea-level rise is similar, no matter if the sea-level rise rate is just below, equal or above the critical limit. A tidal basin will experience a long process of ‘drowning’ when sea-level rise rate exceeds about 80% of the critical limit. The insights from the present study can be used to improve morphodynamic modelling of tidal basin response to accelerating sea-level rise and are useful for sustainable management of tidal inlet systems.

The Wadden Sea is a unique coastal wetland containing an uninterrupted stretch of tidal flats that span a distance of nearly 500 km along the North Sea coast from the Netherlands to Denmark. The development of this system is under pressure of climate change and especially the associated acceleration in sea-level rise (SLR). Sustainable management of the system to ensure safety against flooding of the hinterland, to protect the environmental value and to optimise the economic activities in the area requires predictions of the future morphological development.
The Dutch Wadden Sea has been accreting by importing sediment from the ebb-tidal deltas and the North Sea coasts of the barrier islands. The average accretion rate since 1926 has been higher than that of the local relative SLR. The large sediment imports are predominantly caused by the damming of the Zuiderzee and Lauwerszee rather than due to response to this rise in sea level. The intertidal flats in all tidal basins increased in height to compensate for SLR.
The barrier islands, the ebb-tidal deltas and the tidal basins that comprise tidal channels and flats together form a sediment-sharing system. The residual sediment transport between a tidal basin and its ebb-tidal delta through the tidal inlet is influenced by different processes and mechanisms. In the Dutch Wadden Sea, residual flow, tidal asymmetry and dispersion are dominant. The interaction between tidal channels and tidal flats is governed by both tides and waves. The height of the tidal flats is the result of the balance between sand supply by the tide and resuspension by waves.
At present, long-term modelling for evaluating the effects of accelerated SLR mainly relies on aggregated models. These models are used to evaluate the maximum rates of sediment import into the tidal basins in the Dutch Wadden Sea. These maximum rates are compared to the combined scenarios of SLR and extraction-induced subsidence, in order to explore the future state of the Dutch Wadden Sea.
For the near future, up to 2030, the effect of accelerated SLR will be limited and hardly noticeable. Over the long term, by the year 2100, the effect depends on the SLR scenarios. According to the low-end scenario, there will be hardly any effect due to SLR until 2100, whereas according to the high-end scenario the effect will be noticeable already in 2050.