Sea-level change in the Dutch Wadden Sea

Rising sea levels due to climate change can have severe consequences for coastal populations and ecosystems all around the world. Understanding and projecting sea-level rise is especially important for low-lying countries such as the Netherlands. It is of specific interest for vulnerable ecological and morphodynamic regions, such as the Wadden Sea UNESCO World Heritage region. Here we provide an overview of sea-level projections for the 21st century for the Wadden Sea region and a condensed review of the scientific data, understanding and uncertainties underpinning the projections. The sea-level projections are formulated in the framework of the geological history of the Wadden Sea region and are based on the regional sea-level projections published in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). These IPCC AR5 projections are compared against updates derived from more recent literature and evaluated for the Wadden Sea region. The projections are further put into perspective by including interannual variability based on long-term tide-gauge records from observing stations at Den Helder and Delfzijl. We consider three climate scenarios, following the Representative Concentration Pathways (RCPs), as defined in IPCC AR5: the RCP2.6 scenario assumes that greenhouse gas (GHG) emissions decline after 2020; the RCP4.5 scenario assumes that GHG emissions peak at 2040 and decline thereafter; and the RCP8.5 scenario represents a continued rise of GHG emissions throughout the 21st century. For RCP8.5, we also evaluate several scenarios from recent literature where the mass loss in Antarctica accelerates at rates exceeding those presented in IPCC AR5. For the Dutch Wadden Sea, the IPCC AR5-based projected sea-level rise is 0.07±0.06m for the RCP4.5 scenario for the period 2018–30 (uncertainties representing 5–95%), with the RCP2.6 and RCP8.5 scenarios projecting 0.01m less and more, respectively. The projected rates of sea-level change in 2030 range between 2.6mma−1 for the 5th percentile of the RCP2.6 scenario to 9.1mma−1 for the 95th percentile of the RCP8.5 scenario. For the period 2018–50, the differences between the scenarios increase, with projected changes of 0.16±0.12m for RCP2.6, 0.19±0.11m for RCP4.5 and 0.23±0.12m for RCP8.5. The accompanying rates of change range between 2.3 and 12.4mma−1 in 2050. The differences between the scenarios amplify for the 2018–2100 period, with projected total changes of 0.41±0.25m for RCP2.6, 0.52±0.27m for RCP4.5 and 0.76±0.36m for RCP8.5. The projections for the RCP8.5 scenario are larger than the high-end projections presented in the 2008 Delta Commission Report (0.74m for 1990–2100) when the differences in time period are considered. The sea-level change rates range from 2.2 to 18.3mma−1 for the year 2100. We also assess the effect of accelerated ice mass loss on the sea-level projections under the RCP8.5 scenario, as recent literature suggests that there may be a larger contribution from Antarctica than presented in IPCC AR5 (potentially exceeding 1m in 2100). Changes in episodic extreme events, such as storm surges, and periodic (tidal) contributions on (sub-)daily timescales, have not been included in these sea-level projections. However, the potential impacts of these processes on sea-level change rates have been assessed in the report

Physics of coral reef systems in a shallow tidal embayment

Ongoing deforestation in the tropics involves higher river discharges and an increase of runoff, which has consequences to coastal ecosystems. The dispersal of fluvial sediment and freshwater by marine processes affects the environmental determinants of coral reefs near the coast, which include temperature, salinity, light and nutrient concentration. The central objective of this thesis is to identify the physical mechanisms that govern (1) the regional variability of vertical mixing, salinity, turbidity, and the supply of terrestrial sediment in coastal reef environments and (2) the local sedimentary processes at reef slopes, determining turbidity and sedimentation. The object of study was the Bay of Banten, which represents a shallow coastal embayment where coral reefs are subject to relatively turbid marine conditions. Field data were gathered within the framework of the Teluk Banten Research Programme, which is an Indonesian-Dutch research programme focussing on the social-economy, biology, geology and physical oceanography of to the Bay of Banten, Indonesia. The Teluk Banten Research Program is part of the Global Change Program of the Royal Netherlands Academy of Sciences (KNAW), and coordinated by the Foundation for the Advancement of Tropical Research (WOTRO). The study revealed that the key issues determining the delicate subsistence of shallow-water reefs in the Bay of Banten include mixing in the freshwater source region, the seasonal coupling between coastal processes and local resuspension of sediment. The topography-controlled mixing of small river discharges with the ambient marine waters inhibits the direct exposure of coral reefs to river-derived sediment suspensions, high nutrient loads and low salinity. The reefs near the coast benefit from relatively strong currents in front of an abandoned delta, which hydrodynamically acts like a coastal headland. Due to a seasonal coupling between river discharge, wind-driven throughflow and wave attack on the eroding delta, coral reefs in the Bay of Banten are protected from prolonged inundation by high-turbidity water masses that originate from the eroding delta. Local currents that induce resuspension of sediment at the reef slopes are the main control on turbidity variation in the waters that surround the coral reefs. Sediment that is already available may be resuspended and deposited repeatedly, which limits the role of regional advection of sediment. Backscatter data from an Acoustic Doppler Current Profiler (ADCP) have provided a valuable measure of suspended sediment concentration, revealing the spatial structure of sediment clouds connected to the reefs. The analysis of tides in the Bay of Banten gave cause for a general study on the asymmetry of sediment transport in mixed diurnal ? semidiurnal tidal regimes. In these regimes, tidal asymmetry is not only caused by nonlinear tidal interaction, but is also generated by astronomical tides in absence of shallow-water effects. In particular, the K1, O-1 and M2 constituents give an asymmetrical periodic flow pattern, with the largest peak velocities persistently in the same direction. A theoretical explanation for this phenomenon has been presented, and an inventory has been made of other, subordinate constituents that may contribute to repetitive asymmetric flow patterns. Analytical expressions have been derived which quantify the residual transport of sediment due to the K1, O-1 and M2 tides, as a function of the phases and amplitudes of these constituents, and the time lag between variation in suspended sediment transport and flow velocity

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

THE USE OF MARINE RADAR FOR INTERTIDAL AREA SURVEY AND MONITORING COASTAL MORPHOLOGICAL CHANGE

Surveying and monitoring the dynamic morphology of intertidal areas is a logistically challenging and expensive task, due to their large area and complications associated with access. This thesis describes a contribution to the nearshore survey industry; an innovative methodology is developed and subsequently applied to marine radar image data in order to map topography within the intertidal area. This new method of intertidal topographical mapping has a reasonable spatial resolution (5 m) and operates over a large radial range (~4 km) with the required temporal resolution to observe both event-based and long-term morphological change (currently bi-weekly surveys). This study uses nearly three years of radar image data collected during 2006-2009 from an installation on Hilbre Island at the mouth of the Dee estuary, northwest UK. The development of the novel 'radar waterline method' builds on previous waterline techniques and improves upon them by moving the analysis from the spatial to the temporal domain, making the analysis extremely robust and more resilient to poor quality image data. Results from radar topographical surveys are compared to those of a LiDAR survey during October 2006. The differences compare favourably across large areas of the intertidal zone, within the first kilometre 97% of radar-derived elevations lie within 1 m of LiDAR estimations. Concentrations of poor estimations are seen in areas that are shown to be shadowed from the radar antenna or suffering from pooling water during the ebb tide. The full three-year dataset is used to analyse changing intertidal morphology over that time period using radar-derived surveys generated every two weeks. These surveys are used to perform an analysis of changing sediment volume and mean elevation, giving an indication of beach 'health' and revealing a seasonal trend of erosion and accretion at several sites across the Dee estuary. The ability of the developed technique to resolve morphological changes resulting from storm events is demonstrated and a quantification of that impact is provided. The application of the technique to long-range (7.5 km) marine radar data is demonstrated in an attempt to test the spatial and operational limitations of this new method. The development of a mobile radar survey platform, the Rapidar allows remote areas to be surveyed and provides a platform for potential integration with other survey instruments. A description of the potential application to coastal management and monitoring is presented. Areas of further work intended to improve vertical elevation accuracy and robustness are proposed. This contribution provides a useful tool for coastal scientists, engineers and decision-makers interested in the management of coastal areas that will form part of integrated coastal management and monitoring operations. This method presents several key advantages over traditional survey techniques including; the large area of operation and temporal resolution of repeat surveys, it is limited primarily by topographical shadowing and low wind conditions limiting data collection