Proceedings of the XXVIIIth TELEMAC User Conference 18-19 October 2022

Hydrodynamic

Modelling currents and sediment transport phenomena in shelf seas and estuaries

Numerical models are an important tool for describing and understanding hydrodynamic and transport processes in the marine and estuarine environment. The success of these models is due to the fact that they are based on physical laws which describe in detail the processes involved. Two aspects of modelling, which are closely related, are highlighted. The first one focusses on the numerical aspects, while the second deals with validating the models through intercomparison with other models or analytical solutions and through field measurements. A2.50 vertical plane ice-ocean model and a 3D barotropic ocean model has been developed which simulates the wind induced currents in high latitude seas. The models solve the hydrodynamic and ice dynamic equations using finte different techniques. Because the domain of integration is very often smaller than the ocean basin non physical boundaries have to be introduced. Numerically this means that an open boundary condition has to be implemented. The Orlanski and the Camerlengo-O'Brien open boundary conditions have been introduced in an existing 2D depth averaged hydrodynamic model and have been validated by applying the model to some well defined test cases. It was found that the Camerlengo and O'Brien condition gave the best results. The Camerlengo-O'Brien open boundary condition has been used to simulate the flow behind a backward facing step in a long channel with the downstream boundaries open in order to investigate the quality of the results as a function of the advection scheme (1st order upstream and 3rd order QUICK).The 2.5D ocean model was validated by comparing the results to those of an other model and to an analytical solution. The 2.5D ice-ocean model was forced by a katabatic wind. It is shown that coastal polynyas can be formed by theses strong off coastal winds. The differences found between both model results have been summarized and explained. The 2.5D model has been applied to an (idealized) ocean covered by an ice layer and deal with the behaviour of an ocean in the vicinity of an ice edge and a continental slope.The second part is devoted to the study of mud transport in a part of the Scheldt estuary using field measurements and numerical models. The observations show the great variations in mud concentration found in the Scheldt estuary. The mud transport is simulated using a 2D and a 3D model. The 2D transport model is a Lagrangian model. The model has been validated by comparing the model results to the data obtained from a laboratory model. This experiment has shown that the model is able to reproduce qualitatively and quantitatively well the observations, but that the predictive capability of the model suffers from a good description and integration of the various physical processes which governs the mud transport. The 2D mud transport model has then been applied to a part of the Scheldt estuary. The results of the model have been compared to the results of two other mud transport models.The 3D mud transport model and several observations have been used to the study the mud transport in an access channel. It is shown that the high mud deposition in the access channels is mainly due to the density gradients existing between the river and the access channel and in the vertical plane

Modelling scour around submerged objects with TELEMAC3D - GAIA

Hydrodynamic

Online proceedings of the papers submitted to the 2020 TELEMAC-MASCARET User Conference October 2020

Hydrodynamic

Hypsometric and geometric controls on hydrodynamics, tidal asymmetry, and sediment connectivity in shallow estuarine systems.

Estuaries and tidal basins are highly dynamic coastal systems that serve as a transition zone between the river and the ocean. The morphological evolution of these diverse environments is modulated by non-linear feedbacks between tides, meteorological forcing, and sediment transport processes. This thesis focuses on the fundamental links between these physical processes, and the geomorphologic characteristics of shallow estuarine systems, specifically: (i) how shallow basin geometries and hypsometries affect hydrodynamics and tidal asymmetry, (ii) how wind-induced currents modify velocity asymmetry in shallow basins, and (iii) defining the relationships between geometry, hypsometry, and sediment connectivity inside shallow estuarine systems. Linking geomorphological characteristics and tidal processes in shallow tidal basins The links between tidal basin geometry and hypsometry, bed shear stress patterns, tidal velocity- and slack water asymmetry, and hypsometric profile shapes were explored for six shallow microtidal basins of Tauranga Harbour, New Zealand. Model results, obtained from a depth-averaged numerical model developed in Delft3D for the full estuarine system, indicated that tidal distortion increases with distance from basin entrance. A simple ratio between tidal basin width and entrance width was defined to describe the planform shape of the basin. This metric, termed the ‘basin dilation factor’ indicates whether a basin can be designated as a divergent or convergent geometry. Shallow basins with a constricted geometry and relatively deep entrance channels were found to be associated with small bed shear stress values and high rates of flood-directed tidal velocity asymmetry in the sheltered basin centres. These results suggest substantial potential for sediment deposition of larger particles. Moreover, slack water asymmetry within these basins was weakly ebb-directed, indicating a small potential for export of fine sediments. These divergent, depositional basins were found to be characterized by convex hypsometric profiles with elevated intertidal regions. Conversely, unconstricted, convergent basins were associated with larger bed shear stress values and more ebb-directed tidal velocity asymmetry within basin centres. Consequently, there was limited potential for overall sediment deposition inside these basins. The slack water asymmetry was weakly flood-dominant, suggesting limited potential for fine sediment input. The comparatively high-energy conditions within these exposed tidal basins were associated with a less convex hypsometric intertidal profile. This study highlights the impacts of specific geomorphologic basin characteristics on tidal processes in shallow estuarine systems. The ability to predict the links between tidal asymmetry and morphological changes in tide-dominated systems is beneficial for coastal management, as the morphological evolution of estuarine systems affects coastal ecosystem functioning, port and estuary navigability, and potential for coastal protection. Understanding the effects of wind-driven currents on velocity asymmetry in shallow tidal basins Numerical modelling experiments were conducted for a series of idealized basins in which planform shape and bathymetry were varied. The model results were used to examine how wind-generated currents modulate horizontal velocity asymmetry patterns in shallow tidal basins. This study revealed that wind-driven currents primarily influence mean and peak flow velocities inside the basins, with a limited effect on tidal harmonics. Faster wind speeds led to more extreme horizontal velocity asymmetry (larger velocity asymmetry values), without substantially modifying overall spatial patterns of velocity asymmetry. The velocity asymmetry was found to be strongly depth-dependent, with changes to asymmetry patterns being most evident for wind speeds of 6 m/s and greater, and for wind directions parallel to the main axes of the tidal channels in the basins. Shallow intertidal regions inside the basins were characterized by a downwind-directed increase in velocity asymmetry, whereas deeper subtidal channels experienced asymmetry changes in the opposite direction. Wind event duration and timing were also found to influence the velocity asymmetry patterns. The differences between the relative size of the peak flood- and ebb directed currents were most evident for wind events with a duration of 6 hours or less that coincide with flooding tides. The results of this study highlight that hydrodynamics, sediment transport, and morphological evolution in shallow estuaries are modulated by tidal processes as well as meteorological forcing. Since anthropogenically induced climate change is expected to increase the intensity of extreme meteorological events, the ability to predict future pathways of morphological change in shallow estuarine systems, based on specific meteorological conditions as well as well-defined local tidal regimes, is vital for the management of these dynamic systems. An examination of sediment connectivity in a shallow estuarine system The sediment connectivity framework was used to examine links between hydrodynamics, sediment transport pathways, and local hypsometry inside a shallow estuarine system (Tauranga Harbour, New Zealand). The estuary was divided into twenty geomorphic cells, representing tidal channels, intertidal flats, and shallow sub-basins. Depth-averaged numerical modelling simulations were carried out to quantify tide-driven sediment connectivity between the cells for five sediment grainsize classes. Connectivity matrices were developed for the different grainsize classes, based on modelled sediment mass loads. Sediment connectivity inside the estuary was found to be modulated by tidal energy, estuarine morphology (depth), sub-basin hypsometry and geometry (planform shape), and sediment characteristics. The connectivity matrices, combined with metrics such as link density and cell strength, illustrated that sediment mass loads, and hence connectivity, were largest in the high-energy environments of the deep tidal channels located in the main estuary. In the more sheltered upper estuary, and inside the shallow sub-basins, connectivity was reduced. For fine sediments ( 275 μm) was found to be ~20%, with transport pathways primarily confined to the deeper regions of the estuary. An in-depth analysis of sediment transport pathways between the shallow sub-basins emphasized that flood-dominant, divergent basins with a convex-shaped hypsometric profile mainly function as sediment sinks, whereas ebb-dominant convergent basins act as sediment sources. This thesis highlights the substantial dependence of tidal asymmetry, morphology, sediment transport and connectivity on hypsometry, geometry, and grainsize characteristics inside shallow estuaries. Additionally, the effects of wind-driven currents on the non-linear physical processes inside these highly dynamic environments are described. Overall, this work provides a novel elucidation of some of the relationships between geometric parameters and forcing mechanisms applicable to many shallow coastal systems

Numerische Modellirung der Küstenprozesse in der Dithmarscher Bucht unter Einbeziehung von Naturdaten

The main purpose of this dissertation is to investigate the applicability of numerical models in the study of the sediment transport in a tidal channel of the Dithmarschen Bight in the North Sea. The field data collected at different stages of the neap-spring tidal cycle was used for the evaluation of hydrodynamic and sediment dynamic of Delft3D package (Delft Hydraulics, The Netherlands) numerical model. The package was employed in both two and three dimensional. The investigation was conducted in two stages. At first the performance of the two dimensional model (2DH) was appraised against the field data. The model was calibrated and validated at this stage. Afterwards, the assessment and comparison of the three dimensional model results (3D) with the field data and also with the two dimensional model results was investigated. It was found that both the 2DH and 3D models are capable of predicting hydrodynamics of the area in good agreement with the field data. It was also observed that the effect of waves on water levels and current velocities was not significant under moderate conditions. The comparison of the modelled and measured suspended sediment concentration showed some disagreements especially during ebb current, due to some model shortcomings and to some measuring approximations and methods. The lack of precise techniques for deriving or calculating settling velocity and critical bed shear stresses were found to be the main sources for the model shortcoming. Two sources of error were also suggested relevant to the limitations of the measuring coverage and devices. The first was insufficient field measurements of grain size distributions, specifically on the tidal flat area; the second was the use of transmissometer device for deriving suspended sediment concentration at shallow water regions. Furthermore the difference between results, i.e. the current velocity and the suspended sediment concentration from the two and the three dimensional simulations are not significant as a result of well-mixed nature of the area. Thus, the use of the 3D model may not be justified with the currently available data for the area of investigation. However, the 3D model made it possible to find some weak points and deficiencies in the overall model. The three dimensional model has also provided very useful information about the flow and sediment dynamics of the area and the type of data required for further studies. Finally it was concluded that at this specific area of investigation to develop a model capable of predicting both hydrodynamic and sediment dynamics in agreement with those in the field, we do need to improve both measuring techniques and also model capability in the tidal flat area.Das Hauptaugenmerk dieser Arbeit liegt bei der Evaluierung der Anwendbarkeit von numerischen Modellen zur Untersuchung des Sedimenttransports im Prielsytem der Dithmarscher Bucht in der Nordsee. Naturdaten, gemessen während verschiedener Phasen im Tidenzyklus, wurden zum Aufbau und zur Auswertung der Modelle des Delft3D-Pakets (Delft Hydraulics, Niederlande) genutzt. Es wurde sowohl ein 2-, als auch ein 3-dimensionales Model erstellt. Die Untersuchung wurde in zwei Schritten ausgeführt. Zunächst wurde das 2-dimensionale Modell (2DH) anhand der Messdaten evaluiert. In diesem Schritt wurde das Modell kalibriert und validiert. Darauf aufbauend wurde das 3-dimensionale Modell (3D) erstellt und mit Hilfe der Naturmessungen und der Ergebnisse des 2DH-Modells bewertet. Die Untersuchung ergab, dass beide Modelle in der Lage sind, die Hydrodynamik des Interessengebietes in guter Übereinstimmung mit den Messwerten wiederzugeben. Weiterhin wurde festgestellt, dass der Einfluss von Wellen auf die Wasserstände und Fließgeschwindigkeiten unter moderaten Bedingungen nicht signifikant ist. Der Vergleich von modelliertem und gemessenem Sedimenttransport in der Wassersäule ergab einige Unterschiede, speziell während der Ebbphase. Dies liegt an einigen Beschränkungen des Modells, der Messgenauigkeit und den Messmethoden. Der Mangel an präzisen Techniken zur Bestimmung oder Berechnung von der Fallgeschwindigkeit und der Sohlschubspannung wurden als Hauptgründe für die Beschränkung des Modells identifiziert. Zwei Fehlerquellen lagen bei der Dichte der Feldmessungen und bei der Wahl der Messgeräte. Der erste betriff die ungenügende Messdichte bezüglich der Korngrößenverteilung der Wattenflächen, der zweite die Verwendung eines Transmissometers zur Messung der Sedimentkonzentration im Flachwasser. Beim Vergleich der Ergebnisse des 2- und 3-dimensionalen Modells, vor allem bei Fließgeschwindigkeit und Sedimenttransport, waren nur geringe Unterschiede erkennbar. Hier bringt die Verwendung eines 3D-Modells unter den gegebenen Umständen, ohne genügende Messwerte, keine signifikanten Vorteile. Gleichwohl wurden durch die Verwendung des 3D-Modells Schwachstellen und Mängel des allgemeinen Modells offengelegt. Das 3D-Modell trug ferner zum besseren Verständnis der Hydrologie und des Sedimenttransports im Untersuchungsgebiet bei. Ebenso wurde ein Überblick geschaffen über die Art von Messungen, welche für eine weitere Entwicklung des 3DModells erforderlich sind. Als abschließende Erkenntnis wurde festgestellt, dass sowohl die Messtechniken als auch die Modellkapazität verbessert werden muss, um ein gut funktionierendes Hydrodynamik- und Sedimenttransportmodell für das vorliegende Interessengebiet zu erstellen

Socio-economic Impacts—Fisheries

Fishers and scientists have known for over 100 years that the status of fish stocks can be greatly influenced by prevailing climatic conditions. Based on historical sea surface temperature data, the North Sea has been identified as one of 20 ‘hot spots’ of climate change globally and projections for the next 100 years suggest that the region will continue to warm. The consequences of this rapid temperature rise are already being seen in shifts in species distribution and variability in stock recruitment. This chapter reviews current evidence for climate change effects on fisheries in the North Sea—one of the most important fishing grounds in the world—as well as available projections for North Sea fisheries in the future. Discussion focuses on biological, operational and wider market concerns, as well as on possible economic consequences. It is clear that fish communities and the fisheries that target them will be very different in 50 or 100 years’ time and that management and governance will need to adapt accordingly