The North Sea is a heavily used area, with wind parks, shipping routes, fishing, sand mining, and many future constructions planned. The Dutch Coastal Zone, the area of the North Sea containing the Dutch coast, is home to several natural areas, alongside extensive human-built flood defenses. Maintaining balance between these areas requires that we understand the complex dynamics of the system. This way, we can efficiently build what is required, while preserving the natural areas and their ecosystems. The Dutch Coastal Zone is about 70km wide, running parallel to the Dutch coast. It is relatively shallow and periodically stratified between the Rhine outflow at Rotterdam and IJmuiden. Its seabed mostly consists of sand (median grain size ~ 300 μm), with a fraction of fine sediment (median grain size < 63 μm) varying in space and time. The fine sediment fraction is important for the ecological functioning of the area, as turbidity strongly increases when it is suspended in the water column which hinders light penetration. Fine sediments are also responsible for the siltation of approach channels to important ports in the area. The yearly climate can be divided into a summer and a winter period, where the winter period has higher waves and wind speeds than the summer period. Previous research has shown that there is a higher concentration of fine sediment in the water column in winter, which is likely caused by the more frequent occurrence of storms and/or patterns in biological activity. However, the exact dynamics of fine sediments during these storms are still incompletely understood. It is also unclear how sediment stirred up by human interference behaves, and how it is transported through the system. This project sets out to analyze the behavior of fine sediments in the water column due to storms in the Dutch Coastal Zone, using a combination of field data and modeling. To interpret this data, we conceptualize the seabed using the two-layer model concept of van Kessel et al (2011). In this model, a thin top layer, consisting purely of fines, resides on top of a thicker lower layer, which is composed of a mixture of sand and fines. This expresses the heterogeneity in the vertical sediment distribution, caused by the mixture of sediment sizes. In this model, we assume that the top layer, called the ‘fluff layer’, is resuspended at every tide and deposited again during slack when conditions are calmest. The lower layer, called the ‘buffer layer’, is much more stable, eroding only during very energetic conditions such as storms. With existing knowledge of fine sediment dynamics, we created a theoretical model of the response of the seabed. Initially, both the fluff and buffer layer contain fine sediment. During the storm, the fluff and part of the buffer are eroded. When the sediment settles, the sand settles first followed by the fine sediment. The resulting bed is stratified by settling velocity, and the fines which were stored in the portion of the buffer layer that was eroded are now in the fluff layer. Gradually by some process, the portion of fines eroded from the buffer layer during the storm is re-entrained into the buffer layer. We assume that for each tidal cycle following the storm, the maximum suspended sediment measured is representative of what is available in the fluff layer. This suspended amount decreases over time, and through this value we can follow the recovery of the system. We quantify reaction to storms and subsequent recovery from our theoretical model using data gathered off the coast of Egmond aan Zee over 2 years in 10.5 m water depth, using a frame equipped with sensors in the bottom 2 m. Storms are defined in this project as energetic events (wave heights > 1 m at 1 km offshore), with clear peaks. We analyze individual events, and briefly touch on the effects of storms occurring in succession. We observe in the field data that individual storm events have corresponding peaks in suspended sediment near the bottom. Over several tidal cycles following the storm, the sediment concentrations gradually decrease until prestorm conditions are reached. This decrease per tide can be described with an exponential decay function, with a decay constant of around 0.1 per half tidal cycle, which appears to fluctuate seasonally by ± 0.03. A computational model, delwaq and Delft-3D, was used to determine the impact of advection on the reaction and recovery from the storm. We use a closed cell 1 dimensional model, with realistic boundary conditions, to determine the impact of advection which plays a role in the recovery of the storm. From the model we also determine that a varying floc size may be responsible for the reaction of SPM to storm conditions observed in the field data.

Sand ripples are small (10 cm height) bed forms which occur in the nearshore and surf zone under the impact of both waves and currents. They impact the roughness of the bed used in models of this zone, and contribute to sediment transport. Predictions of their geometry are important to properly model roughness, so correlations between the waves and ripples need to be established. In this project, data about the instantaneous bed level and wave conditions at two point locations in the surf zone was used to analyze the relationship between wave conditions and ripples on the bed. Values for significant wave height, peak period, skewness, ripple height and ripple period were found over bins of 15 minutes in length. It was found that there are relationships between ripple height and wave height, wave skewness and ripple height. These correlations are strongest for a zero shift in the signals, meaning the ripple height can adjust to the wave height and/or skewness within 15 minutes. These results can be used with further research, which includes local current measurements, to make more accurate estimates of the ripple height.