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

Sediment disposal data Western Scheldt

Dataset supporting the publication ‘Sediment Disposals in Estuarine Channels Alter the Eco-Morphology of Intertidal Flats’. It contains all bathymetric, grain size, and macrobenthos data analysed in this publication

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

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.</p

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.</p

Data underlying the publication: Variations in Storm-Induced Bed Level Dynamics Across Intertidal Flats

Dataset supporting the publication "Variations in Storm-Induced Bed Level Dynamics Across Intertidal Flats". It contains data on bed level, morphodynamics, hydrodynamics, and macrobenthos

Adaptation timescales of estuarine systems to human interventions

Many estuaries and tidal basins are strongly influenced by various human interventions (land reclamations, infrastructure development, channel deepening, dredging and disposal of sediments). Such interventions lead to a range of hydrodynamic and morphological responses (a changing channel depth, tidal amplitude and/or suspended sediment concentration). The response time of a system to interventions is determined by the processes driving this change, the size of the system, and the magnitude of the intervention. A quantitative understanding of the response time to an intervention therefore provides important insight into the processes driving the response. In this paper we develop and apply a methodology to estimate the response timescales of human interventions using available morphological and hydraulic data. Fitting an exponential decay function to data with sufficient temporal resolution yields an adaptation timescale (and equilibrium value) of the tidal range and deposited sediment volumes. The method has been applied in the Dutch Wadden Sea, where two large basins were reclaimed and where long-term and detailed bathymetric maps are available. Exponential fitting the morphological data revealed that closure of a very large part of a tidal basin in the Wadden Sea initially led to internal redistribution and import of coarse and fine sediments, and was followed by a phase of extensive redistribution while only fine-grained sediments are imported. Closure of a smaller part of a smaller basin led to shorter response timescales, and these response timescales are also more sensitive to rising mean sea levels or high waters. The method has also been applied to tidal water level observations in the Scheldt and Ems estuaries. Exponential fits to tidal data reveal that adaptation timescales are shortest at the landward limit of dredging. The adaptation time increases in the landward direction because of retrogressive erosion (Scheldt) or lowering of the hydraulic roughness (Ems). The seaward increase in adaptation time is related to the seaward widening of both systems