1,721,047 research outputs found
Biological influences on morphology and bed composition of an intertidal flat
Get PDFBiological activity is known to influence sediment strength at bed-water interfaces. However, its precise effect on geomorphology and on bed composition is not known. This paper proposes a parameterization of sediment destabilizing and stabilizing organisms on three parameters that describe the erosion and mixing processes of the sediment bed, namely the critical bed shear stress, the erosion coefficient and the bioturbation coefficient. This parameterization is included in a 3D sand-mud morphodynamic model to form the sand-mud-bio model. The performance of the sand-mud-bio model is demonstrated by testing it on the Paulinapolder intertidal flat in the Western Scheldt estuary of The Netherlands. Model results show that biological influences on sediment strength result in significant morphological change and bed composition variations. Destabilizing organisms always cause a significant decrease in mud content in the bed and an increase of erosion. On the other hand, stabilizing organisms can, but do not necessarily, cause an increase of mud content and additional sedimentation
Hydrodynamics and morphodynamics of a seasonally forced tidal inlet system
No full textTidal inlets along the central coast of Vietnam are highly dynamic and variable under the influence of seasonal climatic regime. The strong episodic processes of river flow and wave action cause the inlet morphologies to be changed frequently and unpredictable. Despite the interests and research on tidal inlets have been growing in recent decades, the mechanisms behind morphodynamics and behaviours of tidal inlets located in tropical monsoon areas with strong episodic river flow influences are still poorly understood. Effective engineering solutions for the inlets could not be found without understandings of the physical processes underlying the morphodynamics and behaviours of the inlets. This research work focuses on the hydrodynamics and morphodynamics of a tidal inlet system located on a microtidal wave dominated coast in the region. Further insights into the morphodynamics and behaviour of the inlet system has been gained in the present study based on the analyses observed data and numerical model simulation to address the role and the influence of each physical process on the morphology and the behaviour of the inlets. Although the study is focused on a specific tidal inlet system, the principles, approaches and some results of the study may be applied to, or at least, give a hint for other inlets in the regionHydraulic EngineeringCivil Engineering and Geoscience
Eastern Scheldt Inlet Morphodynamics
No full textIn the south-western part of the Netherlands, the system of estuaries, tidal basins and islands called the Delta, has been shaped and studied by humans for centuries. By far the largest event that determined its current configuration was the storm surge that occurred in 1953. This giant flooding gave rise to one of the largest engineering programs in the world: the Delta Plan. The aim of this plan was to ensure safety against flooding, while at the same time allow for other utilisations of the Delta. This system of dams, barriers and sluices has had, and is still having a strong effect on the morphology of the Delta coast and basins, especially the Eastern Scheldt tidal basin. The objective of this research is to gain understanding of the mechanisms that govern the exchange of sediment between the Eastern Scheldt basin and its ebb-tidal delta, and the effects of human interventions on these mechanisms. In order to gain better understanding of the processes determining the morphology of the Eastern Scheldt inlet, analysis of bathymetric and hydraulic data is combined with process-based numerical modelling. From this study on the Eastern Scheldt and its surroundings, it has become clear that the Eastern Scheldt is a basin that has been shaped strongly by a multitude of human interventions throughout the past few centuries. It will also take in the order of centuries before the morphological effects of these interventions will have levelled out.Hydraulic EngineeringCivil Engineering and Geoscience
Tidal and sediment dynamics in a fine-grained coastal region: A case study of the Jiangsu coast
No full textThe Jiangsu coast is located in eastern China bordering the South Yellow Sea. It is strongly affected by the semi-diurnal tides. Both the tidal range and the tidal current vary greatly in space due to local tidal wave systems and morphologies. The radial tidal current pattern identified at the central coast is suggested to play a primary role in the evolution of a large-scale radial-shape sand ridge system. Another feature of the Jiangsu coast is the diversity of the bottom sediments with pronounced silt content. Inspired by the characteristics of both the hydrodynamics and sediment dynamics throughout the Jiangsu coast, this thesis focuses on advancing our understanding of the coastal tidal dynamics and the resulting sediment transport. Regarding the radial tidal current pattern at the central Jiangsu coast, there have been plenty of studies exploring relevant formation mechanisms. A generally accepted inference is that the radial tidal current pattern is a consequence of the interaction between the northern rotating tidal wave system and the southern progressive tidal wave. In this study, we examine the emergence of the radial tidal current in a schematized semi-enclosed tidal basin by introducing the tidal Current Amphidromic Point (CAP) and the tidal current inclination angles. After comprehensive numerical experiments, we find that the overall basin scale and the cross-basin phase difference play roles in the emergence of the radial tidal current. The radial tidal current only has an opportunity to emerge in a basin where the basin length (L) is larger than width (B) (i.e. L/B>1), a lateral depth difference exists or the offshore incoming tidal wave has an oblique angle. The Yellow Sea is featured by these aforementioned prerequisites favouring the emergence of the radial tidal current. Furthermore, we discover that the radial tidal current is related to the cross-basin CAP distribution pattern. When the radial tidal current emerges, the focal point is the CAP related to the velocity vectors rotating anti-cyclonically in the Northern Hemisphere. The CAP distribution deserves more attention for the identification of the radial tidal current pattern. To understand the sediment dynamics in a silt-enriched environment in more detail, we have carried out a series of flume experiments under various wave and current conditions with field-collected silt-sand mixtures. According to the experiments, we find that the silt fraction has different features originating from both the sand fraction and the clay fraction. A high concentration layer is observed near the bottom together with ripples under pure wave conditions. Sediment concentrations inside the high concentration layer are quasi-stationary with the bulk Richardson number approaching a constant value. The thickness of the high concentration layer can be scaled with approximately two times the damped wave boundary layer thickness. Thus, the wave motion induced turbulence is considered to be the main reason generating the high concentration layer. Moreover, suspensions inside the high concentration layer have a certain amount of sand content, which is different from the fluid mud in the cohesive muddy bed. For the vertical concentration profile, the silt fraction is also distributed differently from the sand fraction, since the silt concentration decreases logarithmically within high concentration layer, while it is homogeneously distributed outside the high concentration layer. Considering the specific features of the silt fraction, we recalibrated the formulations of van Rijn (2007a, b) based on our experiments and further developed a multi-fraction sediment transport model to predict the vertical concentration profile for silt and sand classes, and then tested the existing sediment formulations. The results show a promising agreement with the measurements, for both wave-only and wave-with-current conditions. Finally, the Jiangsu Regional Model is set up utilizing the aforementioned findings on tides and sediments. The Jiangsu Regional Model is used to examine whether our existing knowledge can be integrated for a relatively long-term (i.e. time scale of years) predictions on the sediment transport and the morphological changes of the Jiangsu coast. To this end, we first reasonably construct the bed composition throughout the model domain. Subsequently, the model is calibrated and validated against two independent measurements on water level, flow velocity and the sediment concentration. The results indicate that the present model can produce good results. The simulated annual-averaged SSCs depict a high value in the coastal region between the Old Yellow River Delta and the northern Radial Sand Ridge Field. The simulated morphological changes show a spatially distributed alternating-erosion-sedimentation pattern in the Old Yellow River Delta rather than pure erosion. Over the Radial Sand Ridge Field, the ridges are continuously growing and the adjacent tidal channels are deepening. The simulated annual-averaged tide-induced sediment budget shows that the northern (i.e. the Old Yellow River Delta) and southern (i.e. the southern Radial Sand Ridge Field) Jiangsu coast are under erosion, while the central coast (i.e. the northern and central Radial Sand Ridge Field) is still in progradation. Furthermore, the simulated sediment bed in the Old Yellow River Delta shows a gradually coarsening trend while an overall fining trend is pronounced in the northern Radial Sand Ridge Field. All these long-term results are in good agreement with observation-based estimations. The present modelling framework indeed has the ability for simulating sediment transport and morphological changes over a relatively long time span (i.e. time scale of years). This thesis addresses series of findings on the radial tidal current pattern, characteristics of the silt-dominated sediments as well as the sediment transport and morphological changes along the Jiangsu coast. The proposed modelling approaches can serve as a basis and provide information on large-scale hydrodynamics and sediment dynamics for the management and planning of the Jiangsu coast. Future studies may be focused on (1) detailed investigation on the influencing factors on the emergence of the radial tidal current by the CAP system distribution; (2) the physics of the layered-bed system (i.e. the hard layer under ripples) for silt dominated mixtures; (3) improving the computational efficiency of the Jiangsu Regional Model for longer time scale (i.e. tens of years).Hydraulic EngineeringCivil Engineering and Geoscience
Progradation and erosion of a fine-grained tidally dominated delta: A case study of the Jiangsu coast, China
No full textA large proportion of the world population lives in the coastal zone, which is facing a variety of natural and anthropogenic impacts on its evolution. A better understanding of coastal evolution can be obtained when it is based on an accurate knowledge of coastal dynamics. This thesis aims to study the long-term (hundreds of years) coastal evolution of the Jiangsu coast, China, under both natural and anthropogenic impacts. The recent (hundreds of years) evolution of the Jiangsu coast is closely related to the shifting course of the Yellow River. The coastline of Jiangsu experienced a rapid progradation stage (1128~1855), when the Yellow River discharged into the Yellow Sea, and a following modification stage (after 1855), when the Yellow River abandoned its piror course. Simultaneously, the sedimentary environment of the Jiangsu coast changed from sandy to silty. In contrast, the tide, which is a dominant driving force for the sediment transport and morphological changes along the Jiangsu coast, has been stable for at least thousands of years. As one of the fundamental factors controlling the coastal evolution, local hydrodynamics has at the outset drawn our attention. A large tidal wave model containing the Bohai Sea, the Yellow Sea and the East China Sea is established to investigate the near-field hydrodynamic conditions of the Jiangsu coast. For instance, existing knowledge always suggested that the geographical position of the Shandong Peninsula is crucial for the local tidal wave system off the Jiangsu coast. However, this is a hypothesis without any verification. Moreover, the role of the local bathymetry on the formation of the radial tidal current off the Jiangsu coast is debatable. These two factors of influence are assessed based on the tidal wave system (Chapter 2). The results demonstrate that the Shandong Peninsula plays a secondary role, rather than a crucial role on the Jiangsu local tidal wave system. The radial tidal current is independent of the geographic position of Shandong Peninsula and local ridge-channel morphologies. Besides the existence of the special tidal wave system, another characteristic of the Jiangsu coast is the existence of the silt-dominated sedimentary environment. To gain insight into the characteristics of silt-dominant sediment, flume experiments with various wave and current conditions have been conducted with two sediment samples from the field (Chapter 3). A high concentration layer is observed near the bottom together with ripples under wave-only conditions. Moreover, a significant vertical sediment sorting phenomenon has been found near-bottom for both sediment samples. As a widely used instrument for measuring suspended sediment concentration in both field and laboratory conditions, the Optical Backscatter Sensor (OBS), has been introducted, requiring additional caution due to its sensitivity to sediment grain size. As observed in the flume experiments, vertical sorting leads to vertical grain size differences in a water column. Thus, traditional approaches converting OBS signals based on one fixed calibration curve (i.e. against the bottom sediment) may lead to large deviations over silt-sand mixed suspensions. To extend the application of OBS for fine mixed sediment, we take the grain size effect into account and propose a new approach (Chapter 4). The new approach introduces an accompanying sediment sample besides the original bed samples for calibrating the OBS instrument. Besides, a multi-fraction sediment model is developed to predict the vertical distribution of sediment grain sizes. The reliability of this approach has been validated by the flume experiments. Regarding the aforementioned coastal evolution of the Jiangsu coast since 1128, previous studies using a geological approach failed to reach a satisfactory agreement. To investigate the long-term evolution of the Jiangsu coast, a large-scale morphodynamic model is established (Chapter 5, 6). Due to the scarcity of historical data, we have simplified several data, such as the bathymetric data and the hydrodynamic conditions (e.g. river discharge). Historical maps, records and geological measurements have been utilized for determining model settings and validating the model results. We first focus on the formation and the development of the deltaic system (Chapter 5). In spite of the simplifications on model setting, the model shows a good capability to reproduce the development of the AYD. Sensitivity analyses of uncertainties (e.g. sediment discharge, accommodation space) on the performance of long-term morphodynamic model are further conducted. Subsequently, we focus on the modification stage of the AYD, when the Yellow River shifted its course after 1855. In this stage, the fluvial impact on the coastal evolution disappeared, while the impact of wave and human activity rose. Therefore, we take tides, waves and artificial revetments (sea dikes) into account in the model to understand the ongoing coastal behavior (Chapter 6). Based on modelling results, the Yellow River has been proven as one of the sediment sources for the RSRs, and the relevant contribution is neither restricted to the northern RSRs nor to a certain period (even at present). Moreover, the results show that the sea dikes play a significant role on the evolution of the nearshore regions, and that wind waves play a remarkable role on the erosion of offshore shoals. In the perspective of time and hydrodyanmic forces, tide is the most dominant force governing the overall evolution of the Jiangsu coast. This thesis addresses the long-term morphological evolution of the Jiangsu coast through a process-based morphodynamic modelling approach. The corresponding insights and findings improve our understanding on the long-term evolution of the Jiangsu coast.Hydraulic EngineeringCivil Engineering and Geoscience
Millennial to annual volume changes in the Humber Estuary
No full textAnalysis of stratigraphic records and historic charts has allowed comparison of Holocene
and historical rates of accommodation space change in the Humber Basin. Apparent rate
differences are explained using a morphological model and an analytical approach to
separate out the effect of differing forcing signals. This understanding is then used to
construct a simple behavioural model of how estuary volumes change over time that is
consistent over millennial to annual time-scales. Given the desire to promote the
sustainable management of estuaries, the approach provides a means to identify the
natural variability that should be anticipated. Distinguishing such signals from
influences such as sea-level rise is also important for the proper attribution of impacts
due to climate change
Morphodynamics of Ameland Inlet: Medium-term Delft3D Modelling
No full textThe Dutch Wadden Sea is consisting of 7 tidal inlet systems separated by sandy barrier islands. The general morphodynamics and hydrodynamics of tidal inlets are relatively well understood, but the detailed interactions remain a prominent research interest. In this study, process-based computer models are employed as a tool to investigate the medium term mechanisms of the Ameland Inlet. Ameland is selected as this inlet is fairly undisturbed from human intervention and the relative large morphodynamic changes are well-documented. The aim of this study is to improve existing Delft3D models of Ameland Inlet. The model used is based on Fockert (2008) and uses a ‘Mormerge’ approach to derive morphodynamic predictions over a 12 year time frame. The model-data comparison focusses on two key aspects. Firstly, channel stability is analyzed based on the measured bathymetry datasets from 1926 to 2011. This analysis shows that the morphodynamic changes over the last 22 years (1989-2011) are representative for the present-day developments. Over this time-frame, the main channel is relatively stable and experienced a lateral shift up to half channel width (about 500m) and a depth variation in the range of plus and minus 1 m. The volume changes are analyzed over a more recent bathymetry (1999-2011). During this timeframe the basin increased in volume (9.6 Million m3) and the ebb-tidal delta (including coast) lost 29.7 Million m3 of sediment. Through a rigorous sensitivity analysis the effects of the most important morphodynamic parameters: the morphological acceleration factor, bed-slope effects, sediment composition are investigated. The use of spatial varying bed sediment grain size, and the Van Rijn bed-roughness predictor for flow, is essential in controlling the depth (stabilizing) the main inlet channel. An important finding of this study was the effect of the island sediment transports on the inlet development. A sensitivity analysis of the SedTHr, minimum water depth for sediment computations, indicates that erosion of the island tips plays an important role in the formation of secondary inlet channels. Allowing more or less island tip erosion, results in the more or less pronounced formation of a secondary inlet channel. The size of this secondary channel starts to dominate the morphodynamic change of the inlet over longer time frames. The sensitivity of long-term morphodynamic change to the setting of SedThr indicates that the interaction of the adjacent barrier island with the inlet is very important. This interaction is poorly understood and it is recommended to study this in further detail. In addition, improvements in the model are made in the schematization of the model boundary conditions. A new morphological tide has been derived based on the principle of Latteux (1995). Sediment transport through selected (key) transects along the main channel are analyzed. Based on this analysis, seven morphological tides can be selected that show (approximately) equal skill in reproducing the sediment transport based on the full spring-neap cycle. As an additional step, the sediment transport through cross sections between two barriers and correlation factor between the sediment transport of spring-neap tide cycle and morphological tide are evaluated to derive the single, most optimum morphological tide. The final model is able to predict the erosion of the Bornrif (nearshore Ameland island), the sedimentation in the eastern ebb-tidal delta and along the Ameland coastline. But the magnitudes of them are 5 times lower than the measurement. For other areas, the model could not show identical morphological behavior with the measured data. The main problem is the tide overestimate the sediment transport from basin to offshore, lead to the sedimentation of the ebb-tidal delta.Hydraulic EngineeringHydraulic EngineeringCivil Engineering and Geoscience
The morphological behavior of shoal connecting channels
No full textA shoal connecting channel connects two main channels over a shoal area. Shoal connecting channels are dynamic in their behavior and create areas alternating in height. By doing so the connecting channels make the area ecologically more valuable. Over time the dynamics of the shoal connecting channels in the Westerschelde have decreased. The cause for this decrease is not sufficiently understood. This present study provides an analysis on the morphological changes and hydraulic processes that affect the connecting channels. The following research questions will be answered: Which hydraulic processes influence the orientation of the shoal connecting channel? What is the influence of a shoal connecting channel on the water level difference between main channels? And what is the influence of morphological changes of the main channels on the connecting channels?Civil Engineering and GeosciencesHydraulic Engineerin
Mathematical modelling of morphological changes and hyperconcentrated floods in the Yellow River
No full textHyperconcentrated flow/flood is a water-driven sediment transport phenomenon, which is characterized by high sediment concentrations between normal sediment-laden flow and debris/mud flow. In a hyperconcentrated flow, strong interactions exist between water flow, sediment, and river bed, which may not only change the flow rheological properties, but also affect the characteristics of sediment transport and bed deformation. In the lower reach of the Yellow River (China), hyperconcentrated flows/floods are frequently observed (with sediment concentrations > 200 kg/m3) and primarily Newtonian, turbulent flows (van Maren et al., 2009a). This reach is characterized by two special, and possibly unique, hydrodynamic and sediment transport phenomena that are associated with hyperconcentrated flow: a downstream increasing peak discharge (at a rate far exceeding the contribution from tributaries) during hyperconcentrated floods, and a downstream decreasing runoff due to water diversions. Assuming spatially continuous diversions along a constant-width channel, previous studies suggest a longitudinally convex bed at the equilibrium state. However, the validity of a convex bed profile, for discrete diversions in natural channels of longitudinally varying width, remains to be justified. Also, such equilibrium analysis does not reveal the morphological time scale (MTS) associated with water diversions. Moreover, though many explanations have previously been proposed for the peak discharge increase, they have focused on only one possible mechanism (e.g., bed roughness change, bed erosion, floodplain influences) and no consensus has been achieved. The underlying physics still remain largely unknown. Research efforts are therefore needed to further investigate these two issues, which comprise the main work of the present PhD research and this thesis. For the study of the downstream peak discharge increase phenomenon, mathematical modelling is the main research method, together with the field data analysis. High-resolution morphodynamic modelling of complex fluvial processes, such as in a hyperconcentrated flood, has so far been limited by model accuracy or computational efficiency. In order to account for the strong interactions during the hyperconcentrated flood and to acquire accurate and efficient solutions in the field scale, a fully coupled morphodynamic model has first been developed using the finite volume method for structured grids. Physically, this model is based on the concept of non-capacity sediment transport, and it incorporates the effects of sediment density and bed deformation on the flow (both in mass and momentum), as well as the influences of turbulence and sediment diffusions. Numerically, this model combines the high accuracy of high-order upwind schemes and the efficiency of centered schemes by the extension of a recent upwind-biased centered (UFORCE) scheme (Stecca et al., 2010) originally developed for clear flow and scalar transport over a fixed bed, to sediment-laden flows over an erodible bed. For stability, a two-stage splitting approach together with a second order Runge-Kutta method is used for the source terms. Moreover, the full set of governing equations is solved at one time to obtain synchronous solutions in mathematics. The model is verified in a number of dam-break tests, covering a wide range of complex (sediment-laden) flows. It is demonstrated to accurately simulate shock waves and reflection waves, as well as rapid bed deformations at high sediment transport rates. Using this model, the relative role of bed roughness change and bed erosion on the downstream peak discharge increase is then investigated in schematized 1-D channels for two hyperconcentrated floods. The results reveal that although erosion effects may contribute to the downstream discharge increase (especially in case of extreme erosion), for most cases the increase is mainly due to a reduction in bed roughness during peak discharge conditions. Additionally, based on the concept of channel storage reduction, the effects of decreasing bed roughness and (very strong) bed erosion can be integrated in the explanation of the peak discharge increase. Later, this model is also applied to reveal the floodplain influences on the peak discharge increase in schematized 2-D channel-floodplain reaches. The results indicate that the cross-sectional changes of channel erosion and floodplain deposition during hyperconcentrated floods are often limited and that it is difficult to drive a peak discharge increase in the downstream direction. For the study of the water diversion impact, a general theoretical framework is proposed to predict the equilibrium state of the fluvial system, which is applicable to both continuous and discrete water diversions in a longitudinally width-varying channel. Numerical experiments by the SOBEK-RE software (version 2.52.005, Delft Hydraulics, 2005) complement the MTS studies for water diversions. The effects of diversion intensity, diversion placement (discrete and continuous) and diversion schemes (pure water and water-sediment mixture) are also systematically studied. The present work confirms the previous findings that water diversions lead to a decrease of the equilibrium depth with respect to natural conditions and a convex bed in a constant-width channel. Moreover, it reveals that in a widening channel a convex bed also develops under conditions of water diversions, while convex, concave or quasi-linear beds may occur in a narrowing channel. Non-monotonic beds may develop in a strongly narrowing channel, depending on the diversion schemes. On a large spatial scale, diversion placement is less important for the equilibrium development. The MTS for water diversions and natural development are very similar and large, indicating considerable influences of water diversions on river morphology. The present thesis advances our understanding of the long-term impact of water diversions on the evolution of a river.Hydraulic EngineeringCivil Engineering and Geoscience
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