"uuid","repository link","title","author","contributor","publication year","abstract","subject topic","language","publication type","publisher","isbn","issn","patent","patent status","bibliographic note","access restriction","embargo date","faculty","department","research group","programme","project","coordinates"
"uuid:cc6848ff-ee93-4af8-a322-5b27d354da7f","http://resolver.tudelft.nl/uuid:cc6848ff-ee93-4af8-a322-5b27d354da7f","Lateral Flows and Sediment Dynamics in a Large Engineered Estuary","Zhou, Z. (TU Delft Coastal Engineering)","Wang, Zhengbing (promotor); Ding, Ping Xing (promotor); van Maren, D.S. (promotor); Delft University of Technology (degree granting institution)","2021","Estuaries are partially enclosed water bodies where river water mixes with sea water. Estuaries provide important ecological functions which are strongly regulated by estuarine hydrodynamics and sediment dynamics, and also by human interventions. Sustainable management of such systems therefore requires a thorough understanding of the interplay between hydrodynamics, sediment dynamics, and human interventions. However, estuaries are often complex systems influenced by river runoff and coastal hydrodynamics (tide, wind, and wave), which all interact with human interventions on various time and spatial scales. Our understanding of estuaries is still insufficient to understand the response of strongly engineered systems to both human interventions and to natural fluctuations. Many estuaries worldwide are strongly influenced by a wide range of human interventions, including engineering constructions, deepening, and land reclamations. An example of highly engineered estuaries is the Changjiang Estuary (CE), China. The upstream river discharge and sediment load is strongly influenced by the Three Gorges Dam (TGD), a multi-purpose dam in the Changjiang River aiming at optimizing flood control and irrigation, and generate hydropower. In the North Passage (NP), an outlet and the main navigation channel of the CE, the Deepwater Navigation Channel (DNC) has been constructed to improve channel navigability. The DNC project includes constructions of dikes and groynes, and regular dredging work. These various interventions strongly influence estuarine hydro- and sediment dynamics but take place concurrently, and therefore their individual impact is not straightforward to assess. A better understanding of the impact of these interventions requires systematic analysis of hydrodynamic and sediment transport processes in relation to the interventions. This dissertation aims to unravel the effect of groynes on lateral flows and sediment transport in a tidal channel-shoal system (i.e. the NP). Groyne fields provide buffer zones, with a salinity lagging behind that in the main navigation channel. The resulting lateral salinity gradients drive lateral density currents, which in turn modify longitudinal salinity gradients in the main channel. These salinity-driven currents also impact the lateral sediment exchange between the main channel and the groyne fields. The effects of groynes on lateral flows and lateral sediment exchange are analyzed using numerical simulations in combination with in-situ observations. Water-bed sediment exchange processes are investigated in more detail using measurements collected with two tripods deployed in the CE. Measured bed level changes are analyzed by semi-automatically fitting the Krone-Partheniades equations to the bed level data using observations of velocity and sediment concentration. This method provides continuous timeseries of sediment properties related to erosion and deposition. It is demonstrated that the erosion parameters are strongly fluctuating, and not constant as typically assumed in numerical models. Such a variability needs to be reflected in a model, either by time-varying parameters or including more detailed processes (for example, consolidation). This dissertation introduces a method to obtain a parameter space that includes the values and accuracies of all potential combinations of input parameters, which is important input for morphodynamic models. To further quantify effects of groynes on hydrodynamics and sediment dynamics, an idealized hydrodynamic model with a single channel with groynes is developed and analyzed. The idealized system has geometric features comparable to the NP, but is set up in such a way that the groyne field aspect ratios (the ratio of the distance between contiguous groynes to the length of groynes) can be systematically investigated. Model results reveal that groynes can influence channel hydrodynamics and local mixing conditions, which influence lateral flows and the longitudinal salt intrusion. Salt intrusion is highest for intermediate aspect ratios, but weaker for very wide or narrow groyne fields. These results highlight the complexity of the hydrodynamics in salt fresh-water transition zones, and specifically the role of human intervention thereon.","hydrodynamics; lateral flows; sediment dynamics; sediment transport; dikes and groynes; salt intrusion","en","doctoral thesis","","9789464235609","","","","","","","","","Coastal Engineering","","",""
"uuid:dde46a25-60c2-40f1-a7e0-5e879693fc90","http://resolver.tudelft.nl/uuid:dde46a25-60c2-40f1-a7e0-5e879693fc90","The Importance of Combined Tidal and Meteorological Forces for the Flow and Sediment Transport on Intertidal Shoals","de Vet, P.L.M. (TU Delft Environmental Fluid Mechanics; Deltares); van Prooijen, Bram (TU Delft Environmental Fluid Mechanics); Schrijvershof, R. A. (Deltares); van der Werf, J. J. (Deltares; University of Twente); Ysebaert, T. (Universiteit Utrecht; Wageningen University & Research); Schrijver, M. C. (Ministry of Infrastructure and the Environment); Wang, Zhengbing (TU Delft Coastal Engineering; Deltares)","","2018","Estuarine intertidal areas are shaped by combined astronomical and meteorological forces. This paper reveals the relative importance of tide, surge, wind, and waves for the flow and sediment transport on large intertidal shoals. Results of an intensive field campaign have been used to validate a numerical model of the Roggenplaat intertidal shoal in the Eastern Scheldt Estuary, the Netherlands, in order to identify and quantify the importance of each of the processes over time and space. We show that its main tidal creeks are not the cause for the dominant direction of the net flow on the shoal. The tidal flow over the shoal is steered by the water level differences between the surrounding channels. Also during wind events, the tidal flow (enhanced by surge) is dominant in the creeks. In contrast, wind speeds of order 40 times the typical tidal flow velocity are sufficient to completely alter the flow direction and magnitude on an intertidal shoal. This has significant consequences for the sediment transport patterns. Apart from this wind-driven flow dominance during these events, the wind also increases the bed shear stress by waves. For the largest intertidal part of the Roggenplaat, only ∼1–10% of the yearly transport results from the 50% least windy tides, even if the shoal is artificially lowered half the tidal range. This dominance of energetic meteorological conditions in the transports matches with field observations, in which the migration of the creeks and high parts of the shoal are in line with the predominant wind direction.","hydrodynamics; intertidal area; morphology; numerical model; sediment transport; wind","en","journal article","","","","","","","","","","","Environmental Fluid Mechanics","","",""
"uuid:51ee1355-eca1-46b1-8ec7-1164689e4593","http://resolver.tudelft.nl/uuid:51ee1355-eca1-46b1-8ec7-1164689e4593","How tides and waves enhance aeolian sediment transport at the sand motor mega-nourishment","Hoonhout, B.M. (TU Delft Coastal Engineering; Deltares); Luijendijk, Arjen (TU Delft Coastal Engineering; Deltares); Velhorst, R.L.C.; de Vries, S. (TU Delft Coastal Engineering); Roelvink, D. (IHE Delft Institute for Water Education)","Aagaard, T. (editor); Deigaard, R. (editor); Fuhrman, D. (editor)","2017","Expanding knowledge concerning the close entanglement between subtidal and subaerial processes in coastal environments initiated the development of the open-source Windsurf modeling framework that enables us to simulate
multi-fraction sediment transport due to subtidal and subaerial processes simultaneously. The Windsurf framework couples separate model cores for subtidal morphodynamics related to waves and currents and storms and aeolian
sediment transport. The Windsurf framework bridges three gaps in our ability to model long-term coastal morphodynamics: differences in time scales, land/water boundary and differences in meshes.
The Windsurf framework is applied to the Sand Motor mega-nourishment. The Sand Motor is virtually permanentlyexposed to tides, waves and wind and is consequently highly dynamic. In order to understand the complex
morphological behavior of the Sand Motor, it is vital to take both subtidal and subaerial processes into account. The ultimate aim of this study is to identify governing processes in aeolian sediment transport estimates in coastal environments and improve the accuracy of long-term coastal morphodynamic modeling.
At the Sand Motor beach armoring occurs on the dry beach. In contrast to the dry beach, no armor layer can be established in the intertidal zone due to periodic flooding. Consequently, during low tide non-armored intertidal beaches are susceptible for wind erosion and, although moist, may provide a larger aeolian sediment supply than the vast dry beach areas. Hence, subtidal processes significantly influence the subaerial morphology and both need to be accounted for to understand the long-term aeolian morphodynamic behavior of the Sand Motor.","hydrodynamics; sediment transport; morphodynamics; dunes and ecomorphology; numerical modelling; coasts and climate","en","conference paper","","","","","","","","","","","Coastal Engineering","","",""
"uuid:66be9199-2065-407e-abbf-f0d4c10cf4fb","http://resolver.tudelft.nl/uuid:66be9199-2065-407e-abbf-f0d4c10cf4fb","Hydrodynamic Drivers of Sediment Transport Across a Fringing Reef","Bodde, W.P.; Pomeroy, A.W.M.; Van Dongeren, A.R.; Lowe, R.; van Thiel de Vries, J.S.M.","","2014","Coral reefs are highly valuable ecosystems, which are under an increasing number of environmental pressures. Sedimentation and sediment transport patterns are among key physical drivers of coral reefs, so it is important to improve our understanding of these poorly studied dynamics on reefs. To this purpose, flume experiments were performed on a scaled fringing reef in the laboratory facilities of Deltares in Delft. The objective was to improve the understanding of hydrodynamic and sediment transport processes across fringing reefs. The water depth and bed roughness were shown to have influence on many processes such as short wave breaking, infragravity (IG) wave generation, IG wave transformation, reef flat seiching, wave-induced setup and wave reflection. The measurements showed that long waves dominate over the short waves at the back of the reef flat. The flow velocities in the rough cases were lower than those in the smooth cases as a result of the bed friction. Analysis of the third order velocity moment and the bed level changes indicates that both the short and the long waves play a role, but that the long waves appear to be the dominant factor in sediment transport and bed profile development - especially close to the beach. Bed roughness affected the shape of a swash bar, which was more pronounced for the smooth than for the rough cases. This demonstrates that the dominance of long waves in a fringing reef lagoon results in different sediment dynamics than, for example, on a regular sandy beach.","fringing reef; sediment transport; infragravity waves; flume experiment; hydrodynamics; morphodynamics; seiching; bed friction; wave setup; ICCE 2014","en","conference paper","Coastal Engineering Research Council","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:f112b25d-bfc6-4771-8201-2ad5860cf373","http://resolver.tudelft.nl/uuid:f112b25d-bfc6-4771-8201-2ad5860cf373","Macrophytes in estuarine gradients: Flow through flexible vegetation","Dijkstra, J.T.","Stive, M.J.F. (promotor); Uijttewaal, W.S.J. (promotor)","2012","Aquatic plants –or macrophytes- are an important part of coastal, estuarine and freshwater ecosystems worldwide, both from an ecological and an engineering viewpoint. Their meadows provide a wide range of ecosystem services: forming a physical protection of the shoreline, enhancing water quality and harbouring many other organisms. Unfortunately, these vegetations such as salt marshes, seagrasses or mangroves have been on the decline as a result of anthropogenic pressure and climate change, despite costly conservation and restoration efforts. The low success rate of these efforts might partially be due to a lack of understanding of the complex bio-physical interactions between plant properties, plant growth, hydro- and morphodynamics and water quality. The capability of plants to alter their abiotic environment via these interactions is referred to as ‘ecosystem engineering’. Many experimental studies, both in the field and in laboratory flumes, have been performed to unravel these interactions. Since such experiments are always hampered by practical limitations such as flume dimensions, available time, or uncontrolled conditions, this knowledge cannot always be generically applied. Therefore, the primary objective of this study is to develop a generically applicable model for feedbacks between flexible macrophytes and their physical environment. To warrant this general applicability under the various circumstances occurring in estuaries, the model development follows a process based approach; a data-orientated approach is merely applicable to known conditions. Modelling starts out on the scale of one plant to finish at the scale of a meadow. The focus is on seagrass, as seagrasses are well studied, highly flexible, have a relatively simple shape and are among the most productive as well as threatened ecosystems. The first step was to create the numerical model called ‘Dynveg’, by combining a novel dynamic plant bending model based on a Lagrangian force balance to an existing 1DV k-? turbulence model (Chapter 2). The plant bending model is based on measurable biomechanical properties of plants: length, width, thickness, volumetric density and the elasticity modulus. Because very flexible plants can assume a position almost parallel to the flow direction, friction too needed to be incorporated rather than pressure drag alone. Flume measurements on strips of eelgrass-like proportions provided the actual values for drag- and friction coefficients, as well as validation data for predicted strip positions and forces. The effect of multiple plants on hydrodynamics was incorporated by assuming that all plants in a meadow do the same, and by defining two turbulence length scales: One for internally generated turbulence, related to the wakes behind individual stems, and one for larger eddies created in the shear layer above, penetrating the canopy depending on the space between the stems. Dynveg compared favourably with the measurements of hydrodynamic characteristics in mimicked eelgrass by Nepf & Vivoni [2000]. Next, Dynveg was combined with the large-scale hydro- and morphodynamic model Delft3D to simulate two-dimensional spatial processes in and around meadows of flexible macrophytes (Chapter 3). The leading principle for this integration is the conditional similarity between flow characteristics in flexible vegetation and those in rigid vegetation: If the rigid vegetation has i) the same height as the deflected vegetation, ii) its plant volume redistributed over the vertical accordingly and iii) a drag coefficient representative of the streamlined shape, the flow is practically analogous for a range of plant properties and hydrodynamic conditions. This modelling method was validated by comparing model results with flume experiments on two seagrass species, showing good agreement for canopy height, flow velocity profile and flow adaptation length. A field measurement campaign in a French macrotidal bay bordered by an eelgrass meadow provided validation data for application to real meadows (Chapter 5). Along with a detailed bathymetry survey by jetski, time-series of flow velocity and sediment dynamics inside a meadow and over a bare adjacent area were measured over two tidal periods. The applied sediment transport formula [van Rijn, 1993] deals with vegetation effects on sediment pick-up and transport via the effects of plants on hydrodynamics. Vegetation-specific interactions such as particle trapping by blades or flow intensification directly around shoots were not taken into account. Nevertheless, the three-dimensional numerical model was able to reproduce the main features of the observations, indicating that the processes of vegetation bending in non-stationary flow and sediment transport through vegetated areas are incorporated correctly. Thus, the objective of making a model for feedbacks between flexible macrophytes and their physical environment has been met. The model can be applied as a tool in conservation and restoration studies or in long-term biogeomorphological feedback studies. Recommended extensions are the incorporation of plant-wave interactions, more intricate plant morphologies and a vegetation-specific transport formula. The second objective of this thesis was to use the developed model(s) as a tool to learn more about biophysical interactions under different conditions. In Chapter 4, Dynveg and the two-dimensional model were used to assess the ecosystem engineering capacities of three plant species that partly co-occur in temperate intertidal areas: the stiff Spartina anglica, the short flexible seagrass Zostera noltii and the tall flexible seagrass Zostera marina. The flow velocity inside the canopy, the canopy flux and the bed shear stress were used as proxies for the species’ ability to respectively absorb hydrodynamic energy, the supply of nutrients or sediment and the ability to prevent erosion. This analysis showed that a species’ eco-engineering capacities depend on its spatial density, its size, its structural rigidity and its buoyancy, but also on environmental conditions. Therefore, biomass, leaf area index or other lumped parameters that neglect structural properties are no good generic indicators of ecosystem engineering capacities. Rigid plants have more potential to trap sediment due to a higher canopy flux than flexible plants. This canopy flux showed to be inversely related to spatial density along the entire natural range. For flexible plants, the canopy flux is only related to density in relatively sparse meadows; in denser meadows the canopy flux is constant with increasing density. Flexible plants are better at preventing erosion because they are more efficient in reducing bed shear stresses than rigid plants. For very thin plants, buoyancy is the most important determinant of position in given flow conditions. For intermediate flexible plants, the structural rigidity is the most influential parameter, whereas for (nearly) rigid plants, the spatial density is dominant. In Chapter 6, the three-dimensional model of the macrotidal bay was used to study the effects of different types of macrophytes on (residual) sediment transport and light availability. The effects of the real, relatively sparse eelgrass meadow were compared to those of a meadow with rigid plants of the same spatial density, with a dense eelgrass meadow, and with a bare bed. Though the differences between these four vegetation scenarios were small –only a few percent- the consequences on long timescales can be considerable. In deep water, sparse flexible vegetation kept more sediment inside the bay than rigid or denser plants. When vegetation only occupies a small part of the water column, plants prevent erosion rather than promote deposition and they have more effect on bed-load transport than on the transport of suspended sediment. Stiff and denser plants affect the bed-load more than sparse flexible vegetation, thereby blocking the transport from outside to inside. The presence of dense or stiff macrophytes increased the light availability at the bed over a tidal cycle up to 7% with respect to a bare bed. The increase of light availability was less pronounced for the relatively open eelgrass meadow: up to 3%. Overall, this study has resulted in a widely applicable model for the interactions between flexible aquatic plants, flow and sediment transport and in more insight in some of these interactions. Other researchers are encouraged to use this tool complementary to fieldwork and laboratory experiments, and to extend it with other functionalities, e.g. for wave attenuation or vegetation development.","macrophyte; flexible vegetation; estuary; hydrodynamics; turbulence; ecology; sediment transport; light climate; seagrass","en","doctoral thesis","","","","","","","","2012-03-06","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""