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This study explores the projected responses of tidal dynamics in the North Sea induced by the interplay between plausible projections of sea-level rise (SLR) and morphological changes in the Wadden Sea. This is done in order to gain insight into the casual relationships between physical drivers and hydro-morphodynamic processes. To achieve this goal, a hydronumerical model of the northwest European shelf seas (NWES) was set-up and validated. By implementing a plausible set of projections for global SLR (SLRRCP8.5 of 0.8 m and SLRhigh−end of 2.0 m) by the end of this century and beyond, the model was run to assess the responses of the regional tidal dynamics. In addition, for each considered SLR, various projections for cumulative rates of vertical accretion were applied to the intertidal flats in the Wadden Sea (ranging from 0 to 100% of projected SLR). Independent of the rate of vertical accretion, the spatial pattern of M2 amplitude changes remains relatively stable throughout most of the model domain for a SLR of 0.8 m. However, the model shows a substantial sensitivity toward the different rates of vertical accretion along the coasts of the Wadden Sea, but also in remote regions like the Skagerrak. If no vertical accretion is assumed in the intertidal flats of the Wadden Sea, the German Bight and the Danish west coast are subject to decreases in M2 amplitudes. In contrast, those regions experience increases in M2 amplitudes if the local intertidal flats are able to keep up with the projected SLR of 0.8 m. Between the different scenarios, the North Frisian Wadden Sea shows the largest differences in M2 amplitudes, locally varying by up to 14 cm. For a SLR of 2.0 m, the M2 amplitude changes are even more amplified. Again, the differences between the various rates of vertical accretion are largest in the North Frisian Wadden Sea (> 20 cm). The local distortion of the tidal wave is also significantly different between the scenarios. In the case of no vertical accretion, tidal asymmetry in the German estuaries increases, leading to a potentially enhanced sediment import. The presented results have strong implications for local coastal protection strategies and navigation in adjacent estuaries.

The integrity of a sea dike, especially its surface soil and biological revetment, is indispensable for coastal protection, as a dike breach would result in damages and economic losses. Estimates of the condition of a sea dike are typically established by on-site inspections and expert judgement at regular intervals. These status assessments of the protection level of the sea dike evaluate grass coverage and animal burrows, since structural inconsistencies deter the overall safety levels on coastal protection. In laboratory settings, erosion resistance of a sea dike is often determined by means of assessing critical shear stress induced by wave-run up and overtopping. Whereby the grain size distribution and soil aggregate formation on the one hand and the root penetration of the sample on the other are significant factors influencing critical shear stress and therefore erosion resistance. Drone-/UAV-based remote sensing can be used to easily determine the degree of coverage of the dike revetment via green value detection. Thermal spectroscopy is also already used in agriculture to detect the state of health of plants at an early stage, for example due to a shortage of water. In addition, plants can be classified using hyperspectral imaging data. We aim to derive transfer functions correlating ground truthing data, drawn from coastal real world- and a full scale laboratory dike, with plant species, its characteristic taxonomic traits and assessed top soil parameters. This approach bears the advantage of yielding an erosion-resistance estimate of the dike cover based on the plant classification using UAV-derived hyperspectral information. Furthermore, taxonomic species are sought to be paired with their respective, site specific, root architecture. Soil parameters such as nutrient availability and humidity will be observed and integrated into the approach, as they bear an impact on subterranean vegetation growth in that plants with lower nutrient availability develop a higher root network (high root length density [cm/cm³]). Finally, grazing livestock on the dike impacts the root system and soil structure as well and both aspects will be investigated comparing mowed against grazed areas as preliminary results show a dike cover void of grazing livestock exhibits a higher root shoot ratio than one with grazing. We hypothesize that classifying plants based on optical, hyperspectral UAV-derived data and the knowledge about the composition of the subsoil, the correlation of plant-specific root architecture and root growth with nutrient availability and agricultural maintenance could provide valuable information about erosion resistance of the dike cover to support dike inspection on an objective basis.

Migration of nearshore sandbars results from nonlinear interacting hydro- and morphodynamic processes in the surf zone of wave dominated sandy shorelines. To gain new insights in the largely varying temporal effects of shoreface nourishments within these areas, the principle component analysis is applied to sixteen data sets with i) natural sandbar migration on different timescales and ii) interfering shoreface nourishments. The statistical methodology is able to separate the non-stationary effects of shoreface nourishments from the natural stationary migration of the sandbars. Depending on other long-term morphodynamic changes and nourishment interactions in the data, the duration of these interferences (the nourishment lifetimes) can be quantified. Relating these nourishment lifetimes ( L n ) with respect to the bar cycle return periods ( T r ) of these areas (e.g. the relative nourishment lifetime) of twenty-one shoreface nourishments with several design parameters, and parameters of the sandbar system in which they were placed, remained inconclusive. A negative exponential fit with the bar cycle return period itself was the most significant ( r 2 = 0.41). Assuming a linear system of the parameters increases the predictive capability ( r 2 = 0.67). The most influential parameters in this relation are the nourishment concentration (+), the nourishment depth (+), the concentration of the migrating sandbars (−) and the bar cycle return period (−), thereby indicating the prominent roles of both nourishment design and natural sandbar system in the inter-site variability of shoreface nourishment lifetimes.

Traditionally, it has often been assumed that the flow conditions in a moonpool are only moderately altered when an object is introduced therein. Moreover, the hydrodynamic forces acting on the object has typically been estimated by Morison’s equation for small volume structures, using the fluid kinematics of the empty moonpool as a basis and applying correction factors for the confined flow conditions, as for an object in a tube or a channel. To investigate the validity of the traditional approach, an experimental study on the forces acting on objects in a moonpool was performed at NTNU/MARINTEK in Trondheim, Norway in 2013. The experiments were done using a simplified 2-dimensional moonpool model which was given a forced heave motion. Two objects, both with square cross sections but of different sizes, were put inside the moonpool one at the time. The resulting wave elevations inside the moonpool and the forces acting on the objects were recorded and analyzed. To get a deeper understanding of the flow characteristics in the moonpool, PIV measurements were used to obtain the fluid velocity fields. The experiments revealed that even moderately sized objects (relative to the size of the moonpool) change the fluid motions in the moonpool to a large extent; the overall wave elevation amplitude is strongly reduced and the resonance period is altered. A consequence of this is that there is a large discrepancy between the hydrodynamic forces acting on the objects measured in the experiments and the forces calculated using the traditional approach. The PIV results showed the formation of vortices at the inlet of the moonpool and at the edges of the objects, which is the main source of non-linear damping of the wave elevation inside a moonpool.

The decades of collected monitoring data of coastal profiles in combination with the decades of experience with sand nourishments in the Wadden Sea, forms an invaluable basis to study the inter-site efficiency of sand nourishment design. However, a systematic data-driven study of this type needs to be applicable for the inter-site varying (i) nourishment design strategies, (ii) coastal monitoring data sets and (iii) natural morphodynamics of the shorelines, respectively. This study introduces a four-step method able to systematically classify the influence of individual nourishment design parameters on the nourishment lifetime (i.e. the period of influence on the natural dynamics of a coastal profile). With the non-linear and adaptive principle component analysis (PCA) method, nourishment lifetimes of beach- and shoreface nourishments are extracted from data sets that describe different natural morphodynamics. Based on an application of the method to a limited number of nourishments placed in two coastal areas in the Netherlands (Ameland) and Germany (Sylt), increasing nourishment concentration, alongshore nourishment length and absolute nourishment peak elevation seem to increase the lifetime of beach- and shoreface nourishments. Nourishment lifetimes at profile more downstream seem to decrease for beach nourishments, but increase for shoreface nourishments. The method supports inclusion of additional coastal profiles and parameters related to the nourishment design, natural morphodynamics of the coastal profile and hydrodynamic forcing, to quantify nourishment design influences on nourishment lifetimes at different locations.