Coastal aeolian sediment transport is influenced by supply-limiting factors such as sediment sorting by grain size. Sorting processes can cause a coarsening of the bed surface and influence the formation of aeolian ripples. However, it is not fully understood what influence sorting processes have on the magnitude of this transport. In this study, sorting processes of different grain sizes when interacting with wind and their influence on the magnitude of aeolian transport are investigated by performing a tracer study and developing a conceptual model. Sand grains were painted in different colors according to particle size and placed on Noordwijk beach, Netherlands. Several experiments were conducted with varying wind speeds. Surface sampling and cameras tracked the sand color movement on the bed surface, and wind speed and direction were measured. The tracer experiments show that for moderate wind conditions, ripples developed. Once the ripples have formed, the supply of finer tracer grains in downwind direction decreased over time, while the supply of coarser grains remained constant. A strong linear relationship between ripple speed and wind speed was found. For higher wind velocities, no ripples or differences in transport of grain size fractions were observed. Alternating phases of erosion and deposition of upwind sand were observed which could not be related to local variations in wind speed. Based on these results and literature, a conceptual model for an Active Bed Surface Layer (ABSL) with two regimes corresponding to moderate (I), and high (II) wind speeds was developed. The concept enables to estimate the magnitude of aeolian sediment transport as a function of wind speed, bed characteristics, and upwind sediment supply. For Regime I, a linear relationship between sediment transport and wind speed is suggested and for Regime II a third power relationship in combination with a process-based model accounting for supply limitations is suggested. ... Read more

‘The Dutch are not going to lose against the sea’, is a commonly accepted quote of the citizens of The Netherlands. Having developed a country below sea level, Dutch people are aware of the challenges of the sea, but are committed in an endless fight against it. Nowadays, with global warming increasing, the challenge for them is even bigger. Sea level rises threateningly, and the estimations about the future conditions are characterized by deep uncertainty. For that reason, the Dutch government identified 13 weak links in the coastal defence system, one of which is the area of interest for this research, the Scheveningen district and its surroundings, belonging to the Municipality of The Hague. Apart from its vulnerable coastal defence position, the area is densely populated and the most popular beach destination of the country. The aim of this research is to provide water safety solutions in the long term (until 2100), while taking into account the stakeholders demands. Secondly, compare the designs and identify specific aspects in which decision makers in the Municipality of The Hague would be forced to make compromises in order to implement a final solution for the project. Thirdly, investigate on additional measures that can complement and optimize the water safety design. For that reason, a background research was conducted through literature reviewing, interviewing experts and stakeholders, in order to collect information about possible sea level rise scenarios, existing boundaries of the system and stakeholders perspectives. The water safety issue was tackled with nature-based solutions following the principles of Integrated Coastal Zone Management by Building with Nature a framework that delivers solutions for sustainable infrastructure. Two solutions packages were obtained. The first one uses a Preserve strategy, attempting to maintain the current coastline position with soft (sandy) interventions where possible, keeping construction costs relatively low. The second package uses an Advance strategy, extending land in the seaward direction, and creating a large space for the development of natural habitats and human activities. To evaluate and compare the resulting designs, a Multi Criteria Analysis was conducted according to five criteria: Recreation and tourism, Social values, Ecology, Economy and Finance, Sustainability, Design. The goal of this part of the analysis is not the selection of a winner option but the assistance to the decision making process by providing strengths and weaknesses of each option, as well as a comparison between the two in terms of the above mentioned criteria. From this point on, this research identified a total of eight aspects where the decision maker, the Municipality of The Hague, would be forced to make some sort of compromises between different interests, in order to implement the project. The evaluation process resulted in a slight preference for the Advance strategy as the most integrated solution package. The optimization of this design was realised and four additional measures were proposed, leading to a more holistic proposal with more chances to bring acceptance among all stakeholders involved. ... Read more