Modeling Form Roughness Induced by Tidal Sand Waves

Tide-dominated sandy shelf seas, such as the Dutch North Sea, are covered by sand waves. Yet, basin-scale hydrodynamic models do not include any sand wave information because their grid sizes are too coarse to resolve sand waves individually. We explore the possibility of parametrizing the effects of sand waves on the larger-scale tidal flow by means of a form roughness. Specifically, our aim is to see to what extent the flow over a sand wave field can be reproduced by that over a flat seabed with an increased effective roughness (accounting for both grain and form roughness). To do so, we use two process-based hydrodynamic models: a second order perturbation approach, and Delft3D. Both models demonstrate that the presence of sand waves causes amplitude decrease and phase shift of the tidal flow. We explore the dependencies of form roughness on different sand wave characteristics (wavelength, height and asymmetry). Shorter and higher sand waves cause a higher form roughness, while our analysis does not reveal any dependency on sand wave asymmetry. Notably, the consideration of a tidal flow, characterized by several tidal constituents, each represented by an amplitude and a phase, results in a more complex form roughness analysis than in a fluvial setting, where the flow is unidirectional and steady. We thus obtain an amplitude-based form roughness and a phase-based form roughness, each yielding a different value, yet displaying the same qualitative dependencies

Gradual Inlet Expansion and Barrier Drowning Under Most Sea Level Rise Scenarios

The expected increase in rates of sea level rise during the 21st century and beyond may cause barrier islands to drown. Barrier drowning occurs due to a sediment imbalance induced by sea level rise, causing inlets to open and expand. It is still unclear how fast barrier islands can drown. To gain insight into the morphodynamics of barrier systems subject to sea level rise, we here present results obtained with a novel barrier island exploratory model, BarrieR Inlet Environment-Drowning, that considers inlet expansion beyond equilibrium size. We quantify how much of a barrier island chain is drowned by calculating the fraction of its length that is below mean sea level due to sea level rise. Results show that barrier drowning is mostly sensitive to the wave height and the rate of sea level rise. In the model, it takes 100s of years for barrier islands to start drowning in response to high rates of sea level rise (more than 5 mm/yr, for a typical coastal environment). This lag in barrier response is caused by a gradual decrease in the sand volume of the barrier. Higher rates of sea level rise cause earlier and more severe barrier drowning. Modeled barrier systems that face higher waves undergo more frequent inlet closures that lower the rate of drowning, but they also have a deeper shoreface that increases the rate of drowning. In model simulations, the latter process dominates over the former when sea level rise rates exceed 5 mm/yr. Model results fairly agree with available field data

Nucleation of helium in liquid lithium at 843 K and high pressures

Fusion energy stands out as a promising alternative for a future decarbonised energy system. In order to be sustainable, future fusion nuclear reactors will have to produce their own tritium. In the so-called breeding blanket of a reactor, the neutron bombardment of lithium will produce the desired tritium, but also helium, which can trigger nucleation mechanisms owing to the very low solubility of helium in liquid metals. An understanding of the underlying microscopic processes is important for improving the efficiency, sustainability and reliability of the fusion energy conversion process. The spontaneous creation of helium droplets or bubbles in the liquid metal used as breeding material in some designs may be a serious issue for the performance of the breeding blankets. This phenomenon has yet to be fully studied and understood. This work aims to provide some insight on the behaviour of lithium and helium mixtures at experimentally corresponding operating conditions (843 K and pressures between 108 and 1010 Pa). We report a microscopic study of the thermodynamic, structural and dynamical properties of lithium–helium mixtures, as a first step to the simulation of the environment in a nuclear fusion power plant. We introduce a new microscopic model devised to describe the formation of helium droplets in the thermodynamic range considered. Our model predicts the formation of helium droplets at pressures around 109 Pa, with radii between 1 and 2 Å. The diffusion coefficient of lithium (2 Å2/ps) is in excellent agreement with reference experimental data, whereas the diffusion coefficient of helium is in the range of 1 Å2/ps and tends to decrease as pressure increases.Postprint (author's final draft

Impact of mean sea-level rise on the long-term evolution of a mega-nourishment

Mean sea-level rise (MSLR) will induce shoreline recession, increasing the stress on coastal systems of high socio-economic and environmental values. Localized mega-nourishments are meant to alleviate erosion problems by diffusing alongshore over decades and thus feeding adjacent beaches. The 21-st century morphological evolution of the Delfland coast, where the Sand Engine mega-nourishment was built in 2011, was simulated with the Q2Dmorfo model to assess the Sand Engine capacity to protect the area against the effects of MSLR. The calibrated and validated model was forced with historical wave and sea-level data and MSLR projections until 2100 corresponding to different Representative Concentration Pathways (RCP2.6, RCP4.5 and RCP8.5). Results show that the Sand Engine diffusive trend will continue in forthcoming decades, with the feeding effect to adjacent beaches being less noticeable from 2050 onward. Superimposed to this alongshore diffusion, MSLR causes the shoreline to recede because of both passive-flooding and a net offshore sediment transport produced by wave reshaping and gravity. The existing feeding asymmetry enforces more sediment transport to the NE than to the SW, causing the former to remain stable whilst the SW shoreline retreats significantly, especially from 2050 onward. Sediment from the Sand Engine does not reach the beaches located more than 6 km to the SW, with a strong shoreline and profile recession in that area, as well as dune erosion. The uncertainties in the results are dominated by those related to the free model parameters up to 2050 whilst uncertainties in MSLR projections prevail from 2050 to 2100

Efectes de les variacions del nivell del mar en la dinàmica a llarg termini del Zandmotor

The Dutch coast is profoundly threatened by sand erosion and sea level rise due to climate change consequences. In order to keep the country save, the Dutch government is highly implicated in the renourishment of beaches and dunes, and the construction of groynes and dykes, so that the shoreline is maintained at its 1990 position. In this framework, a sand mega-nourishment was constructed in the Delfland coast in 2011, the Zandmotor (ZM). With this work we aim at studying the long-term evolution of the ZM, under the effects of wave-driven sediment transport but also taking into account the consequences of sea level variations, including the daily variability associated with astronomical, barometric and wind stress tides, and the slow rise due to global warming. The fact that the ZM is the first of its kind in the world makes it necessary to accomplish this study by means of computational simulations. Simulations have been done using the Q2D-morfo model, starting at 2012, one year after construction, up to 2100, in order to properly assess the effects of climate change. The model has been forced with wave and sea level series constructed from historical data and including mean sea level rise (MSLR) under different IPCC scenarios of global warming. Before performing the long-term simulations, the model has been calibrated with measured bathymetries of the first 3 years. Results show an adverse evolution of the SW coast (located at 6 – 10 km from the centre of the ZM) compared to the NE one. The SW coastline suffers an important retreat under MSLR situations. Yet, the NE coast is maintained at the same position from 2015. This situation yields a feeding asymmetry, being the feeding towards the NE coast 25% larger than in the SW coast by 2100. Further, as a consequence of this behaviour, the dunes located at the SW coast are in great danger. Simulations indicate that by 2100 the coastline will almost have reached the dune base at high tide and, under a extreme inundation event, the dune base would already be reached by 2050. Also, the effects of daily sea surface level variability have been assessed, although MSLR acts on the coastline more significantly. Particularly, the effects of MSLR on the dry beach area have been assessed yielding a loss of between 25% to 44% of initial area, increasing as MSLR scenarios are more pessimistic. These effects are also visible on the mean lifetime (time necessary for the amplitude to decrease by a factor e) of the ZM resulting in a period from 30 to 26 years. Hence, the most important recommendations of this study are that a new mega-nourishment should be constructed in this area and, in the case the same shape is chosen, it should be placed at the south-west from the original position of the ZM. Finally, in future studies about the effect of MSLR in this area it is important to account for the erosion of the coast, as done in the present study, given that it accounts for 50% of the dry beach area loss (the other 50% being due to inundation)

Calculating sand wave-induced form roughness coefficients for a section of the Netherlands Continental Shelf.

Coastal environments exhibit various bedforms, with sand waves being of particular interest due to their influence on flow, sediment transport, and morphology (van der Meijden et al., 2023). While numerous models simulate tidal flow, none explicitly includes form roughness induced by these sand wave fields. For instance, the roughness coefficient in the Dutch Continental Shelf Model (DCSM) is calibrated solely to match observed water levels. Portos-Amill et al.'s (submitted) process-based modelling approach addresses this gap by quantifying form roughness induced by sand waves, based on water depth, tidal flow amplitude, grain size, and sand wave characteristics. This provides four criteria to compute form roughness value; either amplitude-based or phase-based, to match the M2 depth-averaged flow or sea surface elevation. The resulting roughness may vary depending on the criterion chosen, highlighting the complex interactions between sand waves and tidal currents