Mega nourishments are a novel approach to stimulating coastal safety and resilience. Mega nourishments are intended to spread along the coast on a decadal time scale by natural sediment transport processes with a minimum of intrusion into the natural coastal system. The supratidal morphodynamic behaviour of mega nourishments is not well understood due to complexities introduced by limitations in sediment availability to aeolian sediment transport. Consequently, the effectiveness of mega nourishments to stimulate coastal safety and to influence coastal landscape and habitat development remains unknown. In this paper we present a detailed 4-year hindcast of the morphological development of the Sand Motor mega nourishment in The Netherlands. We use the aeolian sediment transport and availability model AEOLIS that focuses specifically on the simulation of spatiotemporal variations in sediment availability. The model includes the recurrence relation between sediment availability and aeolian sediment transport through self-grading and beach armoring. We show that the model is able to reproduce multi-annual aeolian sediment transport rates in the Sand Motor domain in the four years after its construction. The RMSE is 3⋅10⁴m³ (7% of the total sediment accumulation) and R² is 0.93 when comparing timeseries of total sediment accumulation in the dunes, dune lake and lagoon. The combination of spatial and temporal variations in aeolian sediment availability, due to the combined influence of soil moisture, sediment sorting and beach armoring, is essential for an accurate estimate of the total sedimentation volume. The simulated feedback between aeolian sediment availability and transport is required for accurately describing compartmentalization of the beach and locating the aeolian sediment source areas in the Sand Motor domain.

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Shortly after the Sand Motor was constructed, strong winds suspended large clouds of fine sediment in the air. These local sand clouds could reach up to 100 m high and travel several kilometers inland into the urban areas of Kijkduin. About half a year later, the nuisance disappeared as suddenly as it had appeared. What happened to this extreme aeolian activity, unprecedented along the Dutch coast?@en

Coastal landscape change represents aggregated sediment transport gradients from spatially and temporally variable marine and aeolian forces. Numerous tools exist that independently simulate subaqueous and subaerial coastal profile change in response to these physical forces on a range of time scales. In this capacity, coastal foredunes have been treated primarily as wind-driven features. However, there are several marine controls on coastal foredune growth, such as sediment supply and moisture effects on aeolian processes. To improve understanding of interactions across the land-sea interface, here the development of the new Windsurf-coupled numerical modeling framework is presented. Windsurf couples standalone subaqueous and subaerial coastal change models to simulate the co-evolution of the coastal zone in response to both marine and aeolian processes. Windsurf is applied to a progradational, dissipative coastal system inWashington, USA, demonstrating the ability of the model framework to simulate sediment exchanges between the nearshore, beach, and dune for a one-year period. Windsurf simulations generally reproduce observed cycles of seasonal beach progradation and retreat, as well as dune growth, with reasonable skill. Exploratory model simulations are used to further explore the implications of environmental forcing variability on annual-scale coastal profile evolution. The findings of this work support the hypothesis that there are both direct and indirect oceanographic and meteorological controls on coastal foredune progradation, with this new modeling tool providing a new means of exploring complex morphodynamic feedback mechanisms.@en

In order to simulate coastal dune dynamics and to be able to explain morphological features at the dry beach, it is important to include limitations in sediment supply as well as feedback between sediment transport processes and morphological changes. For this purpose we extend the process based model for supply limited sediment transport at the coastal zone AeoLiS (Hoonhout & de Vries, 2016) with a spatially varying wind field The original intention of the AeoLiS transport model was to be able to simulate the influx of Aeolian sand into the dune area. It is shown that the limitations of sediment supply on the beach (e.g. surface moisture, armouring) are more important than the Aeolian transport capacity. At present, intending to use AeoLiS as a hindcast tool for dune morphology, an adequate prediction of the wind field is needed. Even more so, the feedback mechanism between wind field perturbation and topographical changes is crucial. We chose to extend the model with the shear stress perturbation theory in the boundary layer by Weng et al. (1991). Here, we validate the model formulation on the growing rate and migration velocity of a 1D (linear dunes) or 2D (Barchan dunes) initial heap of sand. Migration velocity is known to scale with wind speed and dune height. The growing rate is known to depend on the level of saturation of Aeolian transport upwind of the profile and the initial shape of the profile. Given the framework of AeoLiS as a model for supply-limited transport, we are able to relate the saturation level to coastal indicators such as beach width. Results indeed show growing or shrinking dunes, that migrate downwind. Furthermore, it is numerically confirmed that larger beach widths give rise to faster growing dunes. @en

Mega nourishments are intended to enhance growth and resilience of coastal dunes on medium to long time scales by stimulation of natural sediment transport processes. The growth and resilience of coastal dunes largely depends on the presence of a continuous supply of aeolian sediment. A recent example of a mega nourishment is the 21 Mm³ mega nourishment known as the Sand Motor. The Sand Motor is intended to nourish the entire Holland coast over a period of two decades. Four years of bi-monthly topographic measurements of the Sand Motor domain provide an opportunity to analyze spatiotemporal variations in aeolian sediment supply using an aeolian sediment budget analysis. It appears that more than 58% of all aeolian sediment deposits originate from the low-lying beaches that are regularly reworked by waves. Aeolian sediment supply from higher beaches diminished after half a year after construction of the Sand Motor, likely due to the formation of deflation lag deposits that constitute a beach armor layer. The compartmentalization of the Sand Motor in armored and unarmored surfaces suggests that the construction height is an important design criterion that influences the lifetime and region of influence for any mega nourishment.

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This thesis explores the nature of aeolian sediment availability and its influence on aeolian sediment transport. The aim is to improve large scale and long term aeolian sediment transport estimates in (nourished) coastal environments. The generally poor performance of aeolian sediment transport models with respect to measurements in coastal environments is often accredited to limitations in sediment availability. Sediment availability can be limited by particular properties of the bed surface. For example, if the beach is moist or covered with non-erodible elements, like shells. If sediment availability is limited, the aeolian sediment transport rate is governed by the sediment availability rather than the wind transport capacity. Aeolian sediment availability is rather intangible as sediment availability is not only affected by aeolian processes, but also by marine and meteorological processes that act on a variety of spatial and temporal scales. The Sand Motor 21 Mm3 mega nourishment is used to quantify the spatiotemporal variations in aeolian sediment availability and its effect on aeolian sediment transport. The Sand Motor was constructed in 2011 along the Dutch coast. Aeolian sediment accumulation in the Sand Motor region is low compared to the wind transport capacity, while the Sand Motor itself is virtually permanently exposed to wind and accommodates large fetches. Aeolian sediment availability is therefore likely to dominate aeolian sediment accumulation. Multi-annual bi-monthly measurements of the Sand Motor's topography are used for a large scale aeolian sediment budget analysis. The analysis revealed that aeolian sediment supply from the dry beach area, that is almost permanently exposed to wind, diminished a half year after construction of the Sand Motor. The reduction in aeolian sediment supply is likely due to the development of a beach armor layer. In the subsequent years, two-third of the aeolian sediment deposits originate from the low-lying beaches that are frequently flooded and therefore often moist. The importance of the low-lying beaches in the Sand Motor region is tested during a six-week field campaign. Gradients in aeolian sediment transport are measured during the field campaign as to localize aeolian sediment source and sink areas. A consistent supply from the intertidal beach area was measured that was temporarily deposited at the higher dry beach. The temporary deposits were transported further during high water, when sediment supply from the intertidal beach ceased, resulting in a continuous sediment supply to the dunes. The temporary deposition of sediment at the dry beach was likely promoted by the presence of a berm that affects the local wind shear. Moreover, the berm edge coincided with the onset of the beach armor layer that might have further promoted deposition of sediment. The measurements on spatiotemporal variations in aeolian sediment availability and supply inspired an attempt to capture the characteristics of aeolian sediment availability in coastal environments in a comprehensive model approach. The resulting model simulates spatiotemporal variations in bed surface properties and their combined influence on aeolian sediment availability and transport. The implementation of multi-fraction aeolian sediment transport in the model introduces the recurrence relation between aeolian sediment availability and transport through self-grading of sediment. The model was applied in a four-year hindcast of the Sand Motor mega nourishment as first field validation. The model reproduces the multi-annual aeolian sediment erosion and deposition volumes, and the relative importance of the intertidal beach area as source of aeolian sediment well. Seasonal variations in aeolian sediment transport are incidentally missed by the model. The model accuracy is reflected in a R2 value of 0.93 when comparing time series of measured and modeled total aeolian sediment transport volumes in the four years since construction of the Sand Motor. The results suggest that indeed significant limitations in sediment availability, due to soil moisture content and beach armoring, govern aeolian sediment transport in the Sand Motor region. A comparison with a simulation without limitation in sediment availability suggests that aeolian sediment availability in the Sand Motor region is limited to about 25% of the wind transport capacity. Moreover, both spatial and temporal variations in aeolian sediment availability as well as the recurrence relation between aeolian sediment availability and transport are essential to accurate long term and large scale aeolian sediment transport estimates. @en

Expanding knowledge concerning the close entanglement between subtidal and subaerial processes in coastal environments initiated the development of the open-source Windsurf modeling framework that enables us to simulate multi-fraction sediment transport due to subtidal and subaerial processes simultaneously. The Windsurf framework couples separate model cores for subtidal morphodynamics related to waves and currents and storms and aeolian sediment transport. The Windsurf framework bridges three gaps in our ability to model long-term coastal morphodynamics: differences in time scales, land/water boundary and differences in meshes. The Windsurf framework is applied to the Sand Motor mega-nourishment. The Sand Motor is virtually permanentlyexposed to tides, waves and wind and is consequently highly dynamic. In order to understand the complex morphological behavior of the Sand Motor, it is vital to take both subtidal and subaerial processes into account. The ultimate aim of this study is to identify governing processes in aeolian sediment transport estimates in coastal environments and improve the accuracy of long-term coastal morphodynamic modeling. At the Sand Motor beach armoring occurs on the dry beach. In contrast to the dry beach, no armor layer can be established in the intertidal zone due to periodic flooding. Consequently, during low tide non-armored intertidal beaches are susceptible for wind erosion and, although moist, may provide a larger aeolian sediment supply than the vast dry beach areas. Hence, subtidal processes significantly influence the subaerial morphology and both need to be accounted for to understand the long-term aeolian morphodynamic behavior of the Sand Motor.@en

This paper shows the first results of measured spatial variability of beach erosion due to aeolian processes during the recently conducted SEDEX2 field experiment at Long Beach, Washington, U.S.A.. Beach erosion and sedimentation were derived using series of detailed terrestrial LIDAR measurements of beach morphology during three low tide periods. Results show significant measured sedimentation and erosion up to 10-20 mm/hour during moderate wind conditions. Spatial variability in bed level changes were found which appeared to be related to the wind orientation and varying bed level characteristics. Around the high waterline, erosion is found during onshore winds whereas sedimentation is observed on the upper beach. The terrestrial lidar data also resolves the migration of bed forms migrating on the upper beach demonstrating its utility of for a range of aeolian sediment transport applications.@en

Spatial variations in aeolian sediment transport were measured at the Sand Motor mega nourishment in The Netherlands during a six week field campaign in the fall of 2014. A consistent significant increase in sediment transport in downwind direction (positive gradient) was measured over the intertidal beach area, indicating that the intertidal beach is a primary source of aeolian sediment, despite the high soil moisture contents. A small positive increase in transport in downwind direction was measured over the dry beach, indicating that local aeolian sediment supply was hampered. A consistent decrease in sediment transport in downwind direction (negative gradient) was measured at the transition between intertidal and dry beach, indicating local deposition of sediment. The negative gradients coincide with the berm edge and the onset of a shell pavement. Therefore deposition might be promoted by morphological feedback between a berm and the wind and the entrapment of sediment in the beach armor layer. The local sediment deposits cause the sediment supply to the dunes to be continued even during high water, resulting in a phased process. The influence of the beach armor layer reduces during storm events as the armor layer itself is being mobilized.

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This study presents some recent developments in coastal morphological modeling focusing on flexible meshes, flexible coupling between models operating at different time scales, and a recently developed morphodynamic model for the intertidal and dry beach. This integrated modeling approach is applied to the Sand Engine mega nourishment in The Netherlands to illustrate the added-values of this integrated approach. A seamlessly coupled modeling system for Delft3D and AeoLiS has been developed and applied to compute the first years of evolution of the Sand Engine, both for the subaqueous and subaerial areas. The subaqueous bed level changes have been computed with the new Flexible Mesh version of Delft3D, resulting in comparable accuracy levels as to the standard Delft3D version. The integrated morphodynamic prediction of both subaqueous and subaerial reveals a qualitative behavior which is very similar to observations. Model results confirm that after the first year after construction the sand supply for aeolian transports is predominantly from the intertidal area. The AeoLiS model results indicate a significant intertidal erosion volume of about 230,000 m3 over the five year period, which is a not to be neglected volume, especially in multiyear or decadal predictions. Interestingly, the model results show that the spit, developed by the wave-related processes, is also subject to aeolian transports acting on the emerged spit during lower tides. The seamlessly coupled models are now able to combine the dry beach behaviour with subaqueous morphodynamic evolution, which is important in medium-term to decadal morphodynamic predictions but also relevant for designing such sandy solutions incorporating lakes, lagoons, and relief.@en

Aeolian sediment transport is influenced by a variety of bed surface properties, like moisture, shells, vegetation, and nonerodible elements. The bed surface properties influence aeolian sediment transport by changing the sediment transport capacity and/or the sediment availability. The effect of bed surface properties on the transport capacity and sediment availability is typically incorporated through the velocity threshold. This approach appears to be a critical limitation in existing aeolian sediment transport models for simulation of real-world cases with spatiotemporal variations in bed surface properties. This paper presents a new model approach for multifraction aeolian sediment transport in which sediment availability is simulated rather than parameterized through the velocity threshold. The model can cope with arbitrary spatiotemporal configurations of bed surface properties that either limit or enhance the sediment availability or sediment transport capacity. The performance of the model is illustrated using four prototype cases, the simulation of two wind tunnel experiments from literature and a sensitivity analysis of newly introduced parameters. @en

The Sand Motor is an artificial sandy peninsula extruding from the Dutch coast about 1 kilometer into the North Sea (Stive et al., 2013). It is virtually permanently exposed to tides, waves and wind and is consequently highly dynamic. In order to understand the complex morphological behavior of the Sand Motor, it is vital to take both subtidal and subaerial processes into account. About 70% of the Sand Motor area is located above 2m+MSL and is therefore uniquely shaped by subaerial processes. These dry areas are hardly eroded due to the presence of a coarse sand armor layer that was naturally established over time. However, significant aeolian transport is observed originating from the intertidal beaches surrounding the Sand Motor. Due to periodic flooding no armor layer can be established in the intertidal zone. Consequently, subtidal processes significantly influence the subaerial morphology. An international collaboration initiated the development of the open-source Windsurf modeling framework that enables us to simulate multi-fraction sediment transport due to subtidal and subaerial processes simultaneously. The Windsurf framework couples separate model cores for subtidal (XBeach; Roelvink, 2006) and subaerial morphodynamics (Coastal Dune Model; Duran and Moore, 2013) and multi-fraction aeolian sediment supply (AeoLiS; based on work by de Vries, 2015). Preliminary model results from a one-year one-dimensional simulation show a concentration of aeolian sediment supply from the intertidal beach area during calm conditions and an elevated aeolian activity shortly after a strong wind or surge event. The period of elevated transport may last for over a week resulting in the immediate initiation of recovery after a surge. Here we will present an application of the Windsurf modeling framework on the Sand Motor and a detailed description on how the interaction between subtidal and subaerial processes explain its complex morphological development. @en

The interface between the land and sea is complex. During high energy conditions large waves and elevated water levels can cause severe beach and dune erosion, whereas recovery takes place during extended periods of calm. Recent advances in numerical modeling have improved our ability to accurately model both storm-induced coastal hazards (e.g., XBeach; Roelvink et al., 2009), aeolian sediment transport in supply limited conditions (de Vries et. al. 2014), and dune eco-morphodynamics (e.g., Coastal Dune Model; Duran and Moore, 2013). However, process based numerical models incorporating the co-evolution of the coastal zone due to both subaqueous and subaerial processes to our knowledge, do not exist; hindering accurate forecasts of seasonal- to decadal-scale coastal evolution. Addressing this community need, a recent international collaboration has initiated the development of the open-source coupled numerical model Windsurf. Under the Windsurf framework, the coupled system resolves the most important subtidal and supratidal physical and ecological processes during both calm accretive conditions and high energy erosive periods. Here we present the coupled model’s ability to simulate short-term beach and dune building processes on daily to seasonal time scales. The welding of intertidal sandbars to the shoreline has long been recognized as an important mechanism for beach and dune building (e.g., Houser, 2009) as beaches are often supply limited. Field experiments on the Oregon coast indicate that discrete sandbar welding events can deliver as much as 20 m3/m of sand from the nearshore to the backshore via the intertidal, resulting in shoreline progradation, backshore aggradation, and dune growth. Here we compare these field observations to model simulations demonstrating Windsurf’s ability to simulate beach-dune exchanges in supply limited scenarios. @en

Coastal dunes are generally dynamic due to a combination of marine and aeolian sediment transport processes. The growth of coastal dunes is generally governed by wind driven sediment transport. Quantifying and predicting aeolian sediment transport processes is a scientific and practical challenge. This is caused by the uncertainties in the relative importance of the transport capacity of the wind and the availability of sediment (or sediment supply). Especially sediment supply has recently been hypothesized to be of governing importance but no quantitative knowledge is available yet. The intertidal zone adds further complexity since sediment supply is likely influenced by alternating marine and aeolian processes on the tide timescale. However, while sediment availability is likely to vary along the coastal zone, no measurements of sediment supply and availability has been successful in the past. In this study we use detailed measurements of wind driven erosion and sedimentation in the intertidal and supra-tidal coastal zone to quantify sediment supply for aeolian sediment transport and associated dune growth. During the 6 weeks SEDEX2 field campaign, a RIEGL 200VZ laser scanner is used to collect high resolution topographic data with 15-minute intervals during several tidal cycles. The data reveals the small but significant erosion in the intertidal zone due to wind driven processes within a tidal cycle for the first time. At the same time the small but significant sedimentation at the dry beach and dunes is measured on the tidal timescale. These data are essential in understanding sediment exchange between marine and aeolian zones and the growth of coastal dunes.@en

Duran and Moore (2013) recently extended the model of Hermann et al. (2008) to create an aeolian eco-morphodynamic model (the Coastal Dune Model, CDM) to simulate the formation of coastal foredunes. De Vries et al. (2014) initiated development of a model to simulate the influence of supply-limiting factors on aeolian transport (now the AeoLiS model). Roelvink et al. (2009) developed a modelling approach to dune erosion, overwashing and breaching (XBeach), which connects the upper shoreface with dune systems during storms. Despite the importance of interactions that occur across the spatial domains and range of conditions represented by these process-based models, CDM and AeoLiS treat storm erosion in a schematized way and XBeach does not address inter-storm development of topography. Thus, we are collaborating to develop WindSurf, consisting of: (1) XBeach (2) CDM and (3) AeoLiS. In addressing both subaqueous and subaerial sediment transport and erosion during storms as well as inter-storm evolution of subaerial topography, the resulting coupled model will allow, for the first time, process-based simulation of event- and decadal-scale co-evolution of nearshore, beach and dune systems. @en

Supply-limiting factors, like moisture content and sediment armoring, influence coastal aeolian sediment transport and subsequently dune evolution significantly. We organized a 6-week field experiment on the influence of spatiotemporal variations in supply on coastal aeolian sediment transport at the Sand Motor, The Netherlands. Due to the presence of a strongly curved coastline and complex intertidal bathymetries, a large spatial variation in supply is to be expected at the Sand Motor, which makes the area particularly suitable for a field experiment on this subject. Preliminary results show that not the largest surface area of sand, nor the biggest fetch or the most severe storm result in significant aeolian sediment transport events, but persistent moderate winds over large intertidal beaches are the key to coastal aeolian transport and subsequently dune evolution.@en