81 research outputs found
Proefeiland IJburg: Bepaling theoretisch zandtraject tijdens sproeiproces
No full textOm aan de blijvende vraag naar woningen te kunnen voldoen is de Gemeente Amsterdam al sinds 1979 plannen aan het maken voor een mogelijke uitbreiding in , oostelijke richting. Onderdeel van deze uitbreiding is de wijk IJburg, die zich uit zal strekken in het IJmeer. Hiertoe zal zo'n 600 hectare land gewonnen (opgehoogd) moeten worden, waarop dan circa 18.000 woningen een plaats zullen vinden. Om een beter inzicht te verkrijgen in de stabiliteit, werkmethode en kosten is gekozen voor de aanleg van een proefeiland van 360 bij 160 m op bodemniveau. Onderdeel van de uitvoering van dit proefeiland is het sproeien van zand onder water om zo een ophoging te creëren van de bodem tot aan de waterlijn. Het sproeien werd uitgevoerd met behulp van een speciaal voor dit project ontwikkeld sproeiponton met het doel: \u95 de mengselstroom over een grote breedte (12 m) te verspreiden; \u95 de mengselstroomsnelheid dermate te verlagen dat het sproeiproces nauwkeurig beheerst kon worden en het ontstaan van erosiekuilen op de bodem werd tegengegaan. Tijdens de voorbereiding was het verschijnsel van het ontstaan van een sedimentatiestroom over de bodem al gesignaleerd, maar in de overgang van de voorbereiding naar de uitvoering is de informatie hierover verloren gegaan. Tijdens de uitvoering heeft men aan de hand van dieptepeilingen het verhaaltraject van het ponton toch weten te bepalen en te sturen.Hydraulic EngineeringCivil Engineering and Geoscience
High and variable drag in a sinuous estuary with intermittent stratification
No full text© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bo, T., Ralston, D. K., Kranenburg, W. M., Geyer, W. R., & Traykovski, P. High and variable drag in a sinuous estuary with intermittent stratification. Journal of Geophysical Research: Oceans, 126(10), (2021): e2021JC017327, https://doi.org/10.1029/2021JC017327In field observations from a sinuous estuary, the drag coefficient C based on the momentum balance was in the range of 5-20 X10-3, much greater than expected from bottom friction alone. C also varied at tidal and seasonal timescales. CD was greater during flood tides than ebbs, most notably during spring tides. The ebb tide CD was negatively correlated with river discharge, while the flood tide CD showed no dependence on discharge. The large values of CD are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of CD, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with CD during flood tides. While CD generally increased with water depth, CD decreased for the highest water levels that corresponded with overbank flow. The decrease in CD may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of CD during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature-induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.The research leading to these results was funded by NSF awards OCE-1634480, OCE-1634481, and OCE-2123002.2022-03-2
Leakage of Oil Following Rupture of Pipeline P/15-4 to Hook of Holland
No full textThis report presents computational results concerning the buoyancy-driven leakage of oil resulting from a possible rupture of the planned pipeline P/15-4 to Hook of Holland. The computations were commissioned by R.J. Brown and Associates (Netherlands) b.v. (RJBA Job No. 7004.02). The specifications of the pipeline and oil as made available by RJBA are listed in Table 1. Assumed properties of seawater are also listed. Leak volumes up to 400 m were calculated as functions of time after rupture for two oil temperatures (5 degrees C and 15 degrees C) and four pipe inclinations, namely 0 (horizontal pipe), 0.001, 0.002 and 0.01.Hydraulic EngineeringCivil Engineering and Geoscience
Salt intrusion in the river Maputo, Mozambique; field survey and one-dimensional modelling
No full textThe aim of the study was to give an indication of the accuracy and applicability of one-dimensional salt intrusion models in estuaries in Mozambique. Attention is mainly centered on tidally averaged modeis, although some preliminary testing of a real- time model is included as weIl. Testing of the models is done with data, obtained from a field study in the river Maputo in Mozambique. The results show that, notwithstanding the many limitations, tidally averaged, one-dimensional models can be an indicatory tooI for description of saltintrusion in the estuary considered. Furthermore the study reveals that description of dispersion mechanisms, solely ?n the basis of theoretical considerations, is still not feasible but always needs some sort of empirical relationship. It was found that during preliminary investigation of estuaries more emphasis has to be laid on measuring salinity distributions under varying circumstances in the estuary than on detailed measuring of the geometry.Hydraulic EngineeringCivil Engineering and Geoscience
Undular bottom topography as a salt intrusion mitigation measure: A study on the potential of trapped internal waves to enhance vertical mixing
No full textDue to climate change and human interventions, saltwater intrusion is becoming a topic of increasing concern worldwide. Salt water intrudes into the Rotterdam Waterway (RWW) by an exchange flow, where the denser sea water propagates landwards at the bottom. The main competing mechanism for this stratified exchange flow is vertical mixing, which can be realised by internal wave induced shear instabilities or wave breaking. The goal of this study is to investigate whether internal waves generated over undular bottom topography in the RWW can generate additional vertical mixing. The underlying assumption is that a decrease in stratification decreases salt intrusion. The approach to answer the main research question is a combination of an analytical and a numerical analysis. The analytical study is based on frictionless linear theory. Internal wave behaviour is further analysed with FinLab, a finite element model which includes the non-hydrostatic processes and effects of density differences. FinLab is evaluated for the application of this study by means of a validation case. In the analytical study, linear theory is applied to obtain a relation between the bed wave parameters and average internal wave energy density E for internal waves generated over sinusoidal bottom topography in a linearly stratified fluid. The derived expression describes that the bottom topography amplitude h0 and bed wave number kT both have a positive quadratic relation with the energy. Additionally, kTkinfluences the resonance conditions. To validate FinLab for internal wave breaking and mixing an experiment in a wave tank, according to an example from literature, is simulated. The validation case reveals a shortcoming in the turbulent mixing parameterization. However, on scales relevant for the RWW the effect of this will not have the same significance. The validation case offers a suggestion for a subgrid closure of diffusion, where density effects are taken into account. Numerical simulations of a 2D channel stretch with sinusoidal bottom topography, a linearly stratified fluid and a linearly varying background velocity, show generation of resonant trapped internal waves for the first two resonant modes. These occurrences correspond to the highest values of kinetic energy as function of vertical velocity averaged over the bed wave domain. The vertical buoyancy flux b is downward directed during occurrences of internal waves and becomes upward directed for increasing background flow. Vertical mixing is associated with an increase in average potential energy Ep, which is 17% higher for the base case (containing bed waves) than for a similar case without bed waves. This increase is larger when bottom shear stress increases. Richardson numbers below 0.25, associated with shear instabilities and mixing, are only observed near the bed, mainly when internal waves are present. The effect of variations in bottom topography wavelength LT and amplitude h0 on internal wave energy can be explained by the analytical formulation. The effect of bed wave parameter changes on b and relative increase in Ep can be related to the effect of the changed amount of bed friction rather than the difference in wave energy. The first resonant mode is the most energetic, however, the average energy density found for these waves is only 0.4% to 6.7% of the potential energy anomaly (PEA); the energy required to fully mix a stratified water column. In the simulations the only mechanism that could transfer internal wave energy to turbulent kinetic energy are shear instabilities near the bed. Over the full simulation, the net vertical buoyancy transport is of negligible magnitude, where Ep shows significant increases between 6% and 99% compared to similar cases without bed waves and is enhanced during the presence of internal waves. The main discussion point is that the quantification of vertical mixing requires improvement, particularly to determine the importance of mixing by internal wave-induced shear instabilities and by bed shear. Mixing by local shear instabilities (of which the relevant scales cannot be resolved with the current grid resolution) does not have an adequate parameterization, because density effects are not included in the turbulence closure. The bed friction parameter, which greatly influences the behaviour of the system, has to be validated. Furthermore, cases where internal waves might break in practice (e.g. at banks) were not considered. Finally, the observed internal wave energy is of small magnitude, however field measurements by Pietrzak(1991) shows that turbulence production by internal waves was significant. Civil Engineering | Hydraulic Engineerin
Kwaliteitsaspecten van het retourwater bij de berging van baggerspecie in een grootschalige locatie op de Maasvlakte
No full textDeze studie heeft betrekking op de kwaliteit van lozingswater (retourwater) bij de berging van baggerspecie in een grootschalige locatie op de Maasvlakte, ook wel het "Slufterplan" genoemd, in het kustgebied bij Voorne. Wat betreft de locatie van de berging is er sprake van een viertal varianten, te weten: varianten I, Ir en III t.p.v. de Maasvlakte en variant IV in de monding van het Haringvliet (zie figuren 1 tot en met 9). De oppervlakte van de verschillende varianten bedraagt ongeveer 300 ha. De berging is bedoeld om gedurende 15 jaar het aanbod van specie uit de oostelijke havens en middenhavens (klasse II en III specie) te kunnen verwerken. De benodigde bergingscapaciteit voor deze periode bedraagt ca. 150 miljoen m3. De diepte in het bassin zal bij aanvang van de berging = 18 m -N.A.P. bedragen en de bedoeling is dat tot =15 m +N.A.P. het storten van baggerspecie door zal gaan. Ten gevolge van verdringing zal in het oorspronkelijk zoute bekken verzoeting optreden door het ontsnappen van poriënwater uit de baggerspecie. Bij het omhoogkomen van het bodemniveau zal de waterdiepte van het bassin in de tijd afnemen. Externe factoren zoals wind, stroming en golven zullen een steeds grotere rol gaan spelen in het suspensiegedrag van het slib.Hydraulic EngineeringCivil Engineering and Geoscience
Applicability of dilution discharge measurements during flood wave conditions
No full textA severe restriction to dilution discharge measurements is that they are to be applied to steady flow conditions only. Usinga computational model, dilution discharge measurenents by constant-rate injection are simulated during unsteady flow conditions caused by Gaussian flood waves propagating down a rectangular uniform channel. The computational results are evaluated in order to provide a measured a discharge measurement's accuracy. The main source causing discrepancies between the actual and measured discharges is the difference in propagation velocities of the tracer cloud and the flood wave. Correction procedures are recommended, since maximum inaccuracies can easily be larger by an order of magnitude than inaccuracies during steady-state conditionsHydraulic EngineeringCivil Engineering and Geoscience
The influence of basin geometry on equilibrium and stability of double inlet systems
No full textThis study investigates the influence of basin geometry on the cross-sectional stability of double inlet systems. The inlet is in equilibrium when the amplitude of the inlet velocities equals the equilibrium velocity (~1 m s-1). This equilibrium is stable when after a perturbation the cross-sections of both inlets return to their original equilibrium value. The necessary amplitudes of the inlet velocities are obtained using an idealized 2DH hydrodynamic that calculates tidal elevation and flow in a geometry consisting of several adjacent rectangular compartments. Model results suggest that regardless of the inclusion or exclusion of bottom friction in the basin, stable equilibrium states exist. Qualitatively, the influence of basin geometry does not change the presence of stable equilibrium. Quantitatively, however, taking a basin surface area of 1200 km2, equilibrium values can differ up to a factor 2 depending on the geometry of the basin.Hydraulic EngineeringCivil Engineering and Geoscience
Reversed lateral circulation in a sharp estuarine bend with weak stratification
No full textAuthor Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(6), (2019):1619-1637, doi:10.1175/JPO-D-18-0175.1.Although the hydrodynamics of river meanders are well studied, the influence of curvature on flow in estuaries, with alternating tidal flow and varying water levels and salinity gradients, is less well understood. This paper describes a field study on curvature effects in a narrow salt-marsh creek with sharp bends. The key observations, obtained during times of negligible stratification, are 1) distinct differences between secondary flow during ebb and flood, with helical circulation as in rivers during ebb and a reversed circulation during flood, and 2) maximum (ebb and flood) streamwise velocities near the inside of the bend, unlike typical river bend flow. The streamwise velocity structure is explained by the lack of a distinct point bar and the relatively deep cross section in the estuary, which means that curvature-induced inward momentum redistribution is not overcome by outward redistribution by frictional and topographic effects. Through differential advection of the along-estuary salinity gradient, the laterally sheared streamwise velocity generates lateral salinity differences, with the saltiest water near the inside during flood. The resulting lateral baroclinic pressure gradient force enhances the standard helical circulation during ebb but counteracts it during flood. This first leads to a reversed secondary circulation during flood in the outer part of the cross section, which triggers a positive feedback mechanism by bringing slower-moving water from the outside inward along the surface. This leads to a reversal of the vertical shear in the streamwise flow, and therefore in the centrifugal force, which further enhances the reversed secondary circulation.This project was funded by NSF Grant OCE-1634490. During this work W.M. Kranenburg was supported as USGS Postdoctoral Scholar at Woods Hole Oceanographic Institution. A.M.P. Garcia was supported by the Michael J. Kowalski Fellowship in Ocean Science and Engineering (AMPG), and the Diversity Fellowship of the MIT Office of the Dean of Graduate Education (AMPG). The authors thank Jay Sisson for the technical support and Peter Traykovski for providing the bathymetric data. Also, the suggestions for improvement by Dr. K. Blanckaert and an anonymous reviewer are thankfully acknowledged
Gedrag van vloeibare sliblagen op een hellende bodem
No full textOm de stroom van slib in water te bestuderen, werden verschillende proeven In het Laboratorium voor Vloeistof mechanica van de TU Delft uitgevoerd. De gebruikte vloeistof (een mengsel van kaoliniet en water) gekenmerkt door een rheologische non-Newtoniaan gedrag, zoals een Binghams vloeistof, stroomt op een bodem die een kleine helling vertoont. De gebruikte dichtheden voor de vloeistof variëren tussen 1050 en 1230 kg/m'. Bij elke opstelling werd In drie secties opgemeten: twee secties om het gedrag van het lichaam van de stroom te bestuderen, en de laatste, om het gedrag van de kop te analyseren. De resultaten werden vergeleken met bestaande theorieën die een gelijkaardig verschijnsel behandelen. We hebben gemerkt dat de waarde van de 'entrainment' coëfficiënt (Ew) overeenstemt met de theorie van Garcia. Voor de 'drag' coëfficiënt, werden, voor turbulente stromen, interessante resultaten gevonden, maar niet voor laminaire stromen. In verband met de kop van de stroom konden twee theoretische relaties geverifieerd worden.vloeistofmechanicaHydraulic EngineeringCivil Engineering and Geoscience
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