The bedload layer in a 1D sand-gravel morphodynamic model

The effective thickness of the bedload layer over a river bed can be schematised as the total volume of bedload sediment that is in transport, divided by the surface. In practice, this layer is considered to have a constant thickness over time. Main question of this thesis is whether this assumption is valid. By analyzing the mass balance, it is possible to gain an approximation of the reality. By means of numerical approximations, it is possible to build a morphological model which imitates these equations. This imitation makes it possible to analyze the effect of specific parameters on the sediment transport and the bed surface elevation. In this way, the effect of neglecting the derivative over time of the effective thickness of the bedload layer can be investigated. The model of a river section of the Rhine between Emmerich am Rhein and Lobith results in a maximum influence of the derivative of the effective thickness of the bedload layer on the morphodynamic changes as the result of one flood event that is smaller than 4%. A parameter study makes it possible to test the obtained result on sensitivity. Because the morphodynamic model is based on a lot of input parameters, a well founded choice between the large variety of parameters has to be made. A rough sensitivity test shows that the at maximum 4% influence on the derivative is subject to a possible variation of approximately a factor 2, depending on the variation in the input parameters.Hydraulic EngineeringCivil Engineering and Geoscience

Modelling sediment transport and morphology during overwash and breaching events

Currently, morphodynamic models as XBeach show substantial overestimations of the erosion rates during breaching and overwash events at barrier islands. The presently used limitations on the Shields parameter and the sediment concentration do hinder erosion, but have undesirable side effects, e.g. the breaching process is suppressed. By implementing additional physics, e.g. the erosion hindering effect of dilatancy and a proper bed slope effect, substantial improvements are achieved for idealised cases. However, two hurricane case studies showed that these model improvements do not hinder erosion sufficiently to achieve reasonable results. A proper description of bed roughness, which is preferably depth dependent and accounts for vegetation, together with calibration of the wave skewness and asymmetry is found to be very important. If this knowledge is applied on a newly introduced case study of Fire Island (hurricane Sandy, 2012), both breaching and overwash are modelled much more in line with reality. However, the complexity of having various morphodynamic processes within one model domain makes calibration a challenging task, requiring a more advanced bed roughness formulation.NUS-TUD Double MSc Degree ProgrammeHydraulic EngineeringCivil Engineering and Geoscience

Intertidal Flats in Engineered Estuaries: On the Hydrodynamics, Morphodynamics, and Implications for Ecology and System Management

Intertidal flats — regions of estuaries that emerge every tide from the water — form unique ecosystems. Benthic communities living in the bed are a valuable food source for wading birds. Salt marshes present on these flats further enhance the biodiversity. Through the damping of waves, intertidal flats also contribute to the safety of the hinterland against flooding. In engineered estuaries, human interventions such as storm surge barriers, navigation channels, dams, and levees affect these ecologically valuable intertidal flats and may even threaten their existence. Therefore, these systems should be managed with care, requiring a thorough understanding of the mechanisms shaping intertidal flats. This dissertation aims to identify and quantify the natural and anthropogenic processes driving hydrodynamics and morphodynamics of intertidal flats, and to reveal the implications for ecology and system management. The Eastern Scheldt and Western Scheldt estuaries (the Netherlands) were selected for this study. These were chosen because of the extensive datasets measured in both estuaries and the different types of human interventions affecting these systems. In the Eastern Scheldt, a storm surge barrier closes during storm conditions and reduces tidal flow velocities inside the estuary at normal conditions. Tidal velocities are also reduced by dams in the branches of this estuary. In the Western Scheldt, sediment is being relocated from too shallow parts of the navigation channel to other parts of the estuary, enabling navigation to economically important harbors. In this dissertation it is shown that it is the aggregated system of natural forces and human interventions that drives the eco-morphological evolution of intertidal flats in estuaries. Intertidal flats respond to local as well as to system-wide changes in sediment availability and hydrodynamics due to human interventions. Even under major human interventions, the natural forces remain relevant. Due to many spatial and temporal scales involved in the eco-morphological response of intertidal flats to changing natural and anthropogenic forces, estuaries require adaptive management strategies.Coastal Engineerin

The differences in morphological development between the intertidal flats of the Eastern and Western Scheldt

Human interventions have a large impact on estuarine morphology. The intertidal flats in the Eastern Scheldt and Western Scheldt estuaries (The Netherlands) have faced substantial morphological changes over the past decades. These changes are thought to be caused by human interventions, such as the construction of the storm surge barrier in the mouth of the Eastern Scheldt, and the deepening of the navigation channels of the Western Scheldt. This paper analyses several datasets and numerical simulations of hydrodynamics, providing an overview of the various morphological characteristics of the intertidal flats in the two estuaries over time and space. Apart from the volume, area and average height of these areas, also the integral steepness of each flat is quantified based on its full geometry. The analyses focus on the intertidal flats surrounded by water, which allows for a robust comparison between the different flats. The intertidal flats in the Western Scheldt appear to be substantially steeper compared to those in the Eastern Scheldt. The data indicates that a larger average height of a flat is related to a larger steepness. Despite variations in the evolution of the different flats, distinct characteristics of both estuaries are observed. An opposed trend is identified over time: the flats in the Western Scheldt have mainly increased in height, whereas the flats in the Eastern Scheldt have lowered after the completion of the storm surge barrier. This opposing development is associated with differences in tidal flow velocities in the estuaries, which are the result of human interventions.Environmental Fluid MechanicsCoastal Engineerin

Where and why do creeks evolve on fringing and bare tidal flats?

Although tidal flats appear homogeneous from a distance, morphological variations are found on various spatial scales. These are driven by physical and/or biological processes. In this paper, we consider the creeks that are present on fringing tidal flats and which are orientated approximately perpendicular to the main channel. To explain why these creeks occur, we analysed high-resolution aerial pictures and yearly measured LiDAR data of the Ems-Dollard and Western Scheldt estuaries, located in the Netherlands. We selected nine bare fringing tidal flats, with and without creeks in both estuaries. Subsequently, we related the flat shape to the creek occurrence by evaluating cross-sections of tidal flats from the two estuaries. Finally, we studied how the flat shape affects the cross-shore flow velocity with a 1D numerical model to link creek occurrence to tidal flow. The results show highest ebb velocities, the largest velocity gradients and the largest erosion potential at the transition area between the lower and the upper flat. The milder the slope of the upper flat and the shorter the transition zone, the larger the flow velocities. Based on the data analysis and numerical model outcomes, we conclude that the conditions are favourable for creeks on convex-up intertidal flats with a sharp transition between the upper part and lower part of the flat and that they are predominantly found in this transition zone. We finally argue that these tidal creeks are not only a consequence of the tidal flat profile, but also affect the (equilibrium) profile of the tidal flat.Coastal EngineeringEnvironmental Fluid Mechanic

Winds of opportunity: The effects of wind on intertidal flat accretion

Intertidal ecosystems are threatened by sea level rise and anthropogenic pressures. Understanding the processes controlling the morphodynamic developments of tidal flats is crucial for sustainable management of these systems. Analysis of three extensive fieldwork campaigns carried out on two adjacent mudflats fringing the Dutch Western Wadden Sea (from 2016 to 2018) provides important new insights into the conditions controlling a permanent increase of tidal flat elevation (‘accretion’), in which the wind and consolidation processes play a pivotal role. Sediment temporarily settles (‘deposition’) on the flats during a period of high suspended sediment availability and water level setup (often following a storm). A tidal flat accretes when a new layer of sediment over-consolidates: a state in which the bed strength is much larger than it would attain during inundated conditions, due to high stresses experienced during prolonged drying. This happens when a phase of sediment deposition is followed by a sufficiently long period with a low ambient water table (phreatic level) and aerial exposure. The chronological order of sediment deposition and over-consolidation provides a window of opportunity for tidal flat accretion. Such a window of opportunity depends on the hydrodynamic forcing (tides, waves, wind), on the consolidation state of the bed, and on sediment availability. Wind plays a crucial role in creating the conditions for tidal flat accretion because the wind direction influences the duration of a low water table and aerial exposure and therefore (over-)consolidation rates, which we refer to as the ‘winds of opportunity’. An abundance of sediment may even limit tidal flat accretion, because large deposition rates substantially increase consolidation timescales.Environmental Fluid MechanicsCoastal Engineerin

MODELLING DUNE EROSION, OVERWASH AND BREACHING AT FIRE ISLAND (NY) DURING HURRICANE SANDY

In 2012, Hurricane Sandy caused a breach at Fire Island (NY, USA), near Pelican Island. This paper aims at modelling dune erosion, overwash and breaching processes that occured during the hurricane event at this stretch of coast with the numerical model XBeach. By using the default settings, the erosion rates are substantially overestimated, which was also concluded in several previous case studies. If the discretization of bed roughness along with wave skewness and asymmetry are improved in the model, XBeach is capable of simulating the various morphological changes within the chosen model domain

Sediment Disposals in Estuarine Channels Alter the Eco-Morphology of Intertidal Flats

Dredging of navigation channels in estuaries affects estuarine morphology and ecosystems. In the Western Scheldt, a two-channel estuary in the Netherlands, the navigation channel is deepened and the sediment is relocated to other parts of the estuary. We analyzed the response of an intertidal flat to sediment disposals in its adjacent channel. Decades of high-frequency monitoring data from the intertidal flat show a shift from erosion toward accretion and reveal a sequence of cascading eco-morphological consequences. We document significant morphological changes not only at the disposal sites, but also at the nearby intertidal flats. Disposals influence channel bank migration, driving changes in the evolution of the intertidal flat hydrodynamics, morphology, and grain sizes. The analyzed disposals related to an expansion of the channel bank, an increase in bed level of the intertidal flat, a decrease in flow velocities on this higher elevated flat, and locally a decrease in grain sizes. These changes in turn affect intertidal flat benthic communities (increased in quantity in this case) and the evolution of the adjacent salt marsh (retreated less or even expanded in this case). The shifts in evolution may occur years after dredged disposal begins, especially in zones of the flats farther away from the disposal locations. The consequences of sediment disposals that we identify stress the urgency of managing such interventions with integrated strategies on a system scale.Coastal EngineeringEnvironmental Fluid Mechanic

The impact of wind on flow and sediment transport over intertidal flats

Sediment transport over intertidal flats is driven by a combination of waves, tides, and wind-driven flow. In this study we aimed at identifying and quantifying the interactions between these processes. A five week long dataset consisting of flow velocities, waves, water depths, suspended sediment concentrations, and bed level changes was collected at two locations across a tidal flat in the Wadden Sea (The Netherlands). A momentum balance was evaluated, based on field data, for windy and non-windy conditions. The results show that wind speed and direction have large impacts on the net flow, and that even moderate wind can reverse the tidal flow. A simple analytical tide–wind interaction model shows that the wind-induced reversal can be predicted as a function of tidal flow amplitude and wind forcing. Asymmetries in sediment transport are not only related to the tide–wind interaction, but also to the intratidal asymmetries in sediment concentration. These asymmetries are influenced by wind-induced circulation interacting with the large scale topography. An analysis of the shear stresses induced by waves and currents revealed the relative contributions of local processes (resuspension) and large-scale processes (advection) at different tidal flat elevations.Environmental Fluid MechanicsCoastal Engineerin

Physical Processes Driving the Morphological Evolution of the Roggenplaat Tidal Flat

The flow velocities in tidal channels are already rather complex by the presence of various tidal components, wind driven flow and estuarine circulations. An extra level of complexity is introduced when the flow on top of an intertidalflat is considered (Le Hir, 2000). This research aims at understanding the complex flow patterns on top of a large-scale intertidal flat and on assessing the morphological consequences. The focus of this study is on the Roggenplaat, which is with an intertidal area of 14.6 km2 the largest intertidal flatfully surrounded by channels of the Eastern Scheldt (The Netherlands, see Figure 1). The flat is subject to a mean tidal range of 2.6 m and is characterized by a typical sediment grain size of 0.25 mm. Two large tidal creeks in the Northwestare the remainder of the merging of separate flats 80-150 years ago. Since the late 1980s, the flats in the Eastern Scheldt have been eroding severely because of the construction of a storm surge barrier and various compartment dams (Louters, 1998). A nourishment of 1.65 million m3 is planned on this flat for 2017, to compensate for its lowering. This study combines the results of an Acoustic Doppler Current Profiler (ADCP) measurement campaign with the results of a numerical model. Apart from validation material for the numerical model, the ADCP data is also analysed individually. The focus of this study is on the present-day hydrodynamics and morphodynamics of the Roggenplaat, which is essential knowledge for the design of appropriate nourishment strategies. Furthermore, physical insights achieved in this study are relevant for the understanding of other large-scale intertidal flats around the world.Environmental Fluid MechanicsCoastal Engineerin