24 research outputs found
Numerical modeling of sediment transport over hydraulic structures
Hydraulic structures are present in the designs of different Room for the River projects in the Netherlands. Examples are longitudinal weirs, groins, summer dikes and weirs in the inlet of a side channel. Morphological simulations with Delft3D are frequently carried out to investigate the effect of such projects on for example hindrance for shipping and dredging costs. It is important that also the physical processes around hydraulic structures are correctly modeled in these situations. At the upstream slope of a hydraulic structure, the larger depth-averaged velocity causes an increased sediment transport capacity and increased actual bed shear stresses. The latter is reinforced by a change of the velocity distribution over the vertical with respect to uniform flow. Opposite, the gravity component along the slope results in a higher critical bed shear stress than in flat bottom conditions. At steep slopes, (partial) bed-load transport blockage could occur. Delft3D is meant to model flow phenomena of which the horizontal length and time scales are significantly larger than the vertical scales. Near hydraulic structures, this is generally not the case. These structures are parameterized as weirs in a depth-averaged Delft3D model in engineering practice. The only effect of these weirs is an additional energy loss in the momentum equation. The parameterization aims at representing the influence of the weirs on the flow at larger scales. The local flow around the structures (including turbulence, vertical velocity components and actual shear stresses) is not correctly modeled. Moreover, there is no direct influence of the weir on sediment transport (like increased critical shear stresses and bed-load transport blockage). This inaccurate way of modeling could result in errors in the prediction of the morphological effects of hydraulic structures. The objectives of this study are: (1) Assessing the performance of the current way of Delft3D modeling of sediment transport around hydraulic structures in three-dimensional flows. (2) Making recommendations on the modeling of sediment transport around hydraulic structures in hydraulic engineering practice. The performance of Delft3D has been judged by comparing the results with the results of the numerical model FLUENT. FLUENT is an advanced flow modeling system, in which sediment transport can be studied by analyzing the trajectories of discrete particles. Firstly, some laboratory experiments describing flow and transport over structures have been modeled. In this way, the performance of both models has been investigated and mutually compared. The results of FLUENT gave confidence to use FLUENT as an instrument to judge the performance of Delft3D in modeling three-dimensional flow and transport over hydraulic structures. A three-dimensional flow situation has been designed, which resembles the flow over a longitudinal weir. In Delft3D, all bed-load transport and suspended-load transport that reaches the weir also passes the weir. In FLUENT, this is not the case. Suspended-load transport is distributed between the main channel and the zone behind the weir in the same ratio as the discharge. The distribution of bed-load transport strongly depends on the particle diameter. This difference shows that the parameterization of weirs in depth-averaged Delft3D models gives significant errors in the prediction of sediment transport over hydraulic structures, especially when bed-load transport is dominant. The transport magnitude can be reduced by increasing the bed level points near the weir to crest level. In this schematization, nearly all bed-load transport is blocked and suspended-load transport is reduced. A weir without increased bed level points overestimates the sediment transport over the structure. When the bed level points are increased until crest level of the weir, the sediment transport over the weir is underestimated. The sediment transport over the weir can be tuned by an increased bed level somewhere between zero and crest level. The distribution of sediment between the main channel (index 1) and the area behind the weir (index 2) can be described with a relation S2/S1 = C*Q2/Q1: The value of C as given by Delft3D can be judged with the following rules of thumb: (1) Suspended-load transport is distributed between the main channel and the zone behind the weir in the same ratio as the discharge, so C = 1. (2) For bed-load transport in three-dimensional situations with clearly oblique flow over the weir, the coefficient C can be related to the excess shear stress at the upstream slope, in which the actual and critical Shields parameter are adjusted for slope effects. (3) In situations where the flow is directed almost perpendicular to the crest of the structure, the conclusions of Lauchlan (2001) are recommended. Nearly all mobile sediment is transported over the structure in these situations. The coefficient C in Delft3D can be influenced by giving the bed level points near the weir the right height.Hydraulic EngineeringCivil Engineering and Geoscience
Building safety with nature: Salt marshes for flood risk reduction
Flood risk reduction in coastal areas is traditionally approached from a conventional engineering perspective, where dikes and dams are built to withstand the forces of tides, surges and waves. Recently, a nature-based approach to flood risk reduction is increasingly promoted, in which the benefits of coastal ecosystems for reducing the impact of extreme weather events are utilized. Ecosystems such as salt marshes, mangrove forests, coral reefs and sand dunes are preserved, enhanced or even created, in order to reduce flood risk in coastal areas. Nature-based flood defenses can work stand-alone, like sand dunes, but can also function in combination with engineered defenses, for example when vegetated foreshores reduce wave loads on dikes or dams.Coastal Engineerin
Overstromingskansen voor de Nederlandse kust
In opdracht van het Deltaprogramma Kust is in kaart gebracht hoe het gesteld is met de huidige veiligheid van de Nederlandse kust. Als maat voor de veiligheid is de overstromingskans van de kustverdediging genomen. Deze overstromingskans kan opgevat worden als de kans op falen van een willekeurige locatie langs de kering, zodanig dat een overstroming optreedt in binnendijks gebied. Voor elke JarKus-raai langs de Nederlandse kust zijn voor het jaar 2011 faalkansen berekend, zowel voor duinen als voor dijken. Hiermee is als het ware een foto met hoge resolutie beschikbaar gekomen die de kustveiligheid anno nu toont.Voor de berekeningen voor elke JarKus-raai is het programma PC-Ring gebruikt. Hiermee sluit dit onderzoek aan bij de werkwijze van VNK, waarbinnen dit programma standaard wordt gebruikt. Dit programma berekent een faalkans voor een bepaald faalmechanisme. Voor duinen is het faalmechanisme duinafslag gehanteerd. Hierbij is met DUROS+ de kans berekend dat de eerste duinenrij van een duinmassief wegslaat bij een storm. De basis - DUROS+ voor de eerste duinenrij - is dus vergelijkbaar met de methode van de toetsing. Indien achter de eerste duinenrij meerdere duinenrijen aanwezig zijn, is de faalkans in waarde gecorrigeerd aan de hand van de omvang van het gehele duinmassief. Voor dijken zijn de faalmechanismen overloop en golfoverslag beschouwd. Hierbij is de kans berekend dat een dijk bezwijkt door ofwel een hoge waterstand, ofwel sterke golfslag. Zoals gebruikelijk bij zeedijken, zijn de mechanismen piping en stabiliteit niet beschouwd, omdat extreme belasting bij zeedijken van relatief korte tijdsduur zijn.Uit het onderzoek zijn de volgende conclusies naar voren gekomen: 1. De voor 2011 berekende faalkansen langs de Noordzeekust zijn in het algemeen laag tot zeer laag. Ongeveer 85% van de zeewering heeft een verwaarloosbare faalkans. Minder dan 1% van de waterkeringen heeft een faalkans tussen 1/10.000 per jaar en 1/1.000 per jaar. Deze trajecten zijn veelal opgenomen in de lopende of geplande verbeterprogramma\u92s. 2. In Nederland zijn de dijken over het algemeen zwakker dan de duinen. De berekende faalkansen voor dijken zijn sterk afhankelijk van het gehanteerde kritieke overslagdebiet. De faalkans neemt vooral sterk af bij een toenemend kritiek overslagdebiet tussen 0.1 en 10 l/s/m. De waarde van 10 l/s/m is in deze studie als uitgangspunt gehanteerd. 3. De WV21-rapporten gaan ervan uit dat de zandige zeeweringen een kleine bijdrage leveren aan het overstromingsrisico vergeleken met de harde keringen. De kansen op falen van de zachte weringen zijn ook in dit onderzoek aanzienlijk kleiner gebleken dan van de harde keringen. De aanname van de WV21-rapporten is dus juist, mits de gevolgen van het falen van dijken en duinen vergelijkbaar van omvang zijn (omdat risico = kans gevolg). 4. Het beeld ten aanzien van zwakke plekken langs de kust komt grotendeels overeen met de huidige, veronderstelde inzichten.Deltaprogramm
Wave attenuation by salt marsh vegetation
Salt marshes are a characteristic feature of estuaries and coastal seas. They are found in the upper coastal intertidal zones between land and water, which are regularly flooded by tides and surges. They are covered with salt-tolerant vegetation types, such as herbs and grasses. Sheltering from continuous intense hydrodynamic forcing by waves and currents and sufficient supply of (fine) sediment are the two main requirements for salt marsh development. The presence of vegetation accelerates the sediment settlement by reducing the wave forces on the bed material. Addition-ally, the roots of the plants stabilize the accumulated sediments and amplify the process of subsoil drainage, consoli-dation and compaction. Salt marshes and the intertidal flats in front form a coherent system with many mutual dependencies.Coastal flood risk reduction by creating and restoring ecosystems is increasingly seen as a promising supplement to conventional coastal engineering methods. Salt marshes, mangrove forests and reed fields can act as a vegetated foreshore in front of a coastal dike. In such a combined dike-foreshore system, the foreshore plays a role in attenuating storm waves, whereas the dike retains the surge and the remaining wave energy. The current study focuses on the process of wave attenuation by vegetation and, vice versa, the process of breakage of vegetation due to wave action.Hydraulic Structures and Flood Ris
Longshore sediment transport by large-scale lake circulations at low-energy, non-tidal beaches: A field and model study
Low-energy, non-tidal lake beaches are known to be subject to longshore morphodynamics, but little is known about how they are driven by wind and wave-driven currents. Lake Markermeer is a shallow (∼4 m deep), wind-dominated lake, of approximately 700 km2. A gradient in wind-induced water level set-up at the leeward shore induces a flow from the shallower to the deeper parts of the lake, thereby generating a large-scale, horizontal circulation. Flow measurements and results from a numerical Delft3D model of the lake show that these circulations impact the nearshore currents greatly, even more than wave-driven longshore currents for most wind conditions. From nearshore measurements at the first study site in lake Markermeer, we found a clear relation between longshore sediment transport capacity and the measured longshore volume flux. The model numerical can predict flow direction and magnitude for any wind condition. Using wind statistics, the net transport capacity for a short period or a long term mean can be predicted. The relation is confirmed for a second study site, which shows a distinct net transport capacity that could not be explained from wave-driven longshore flow alone. Concluding, large-scale lake circulations are of great significance for the morphological development of low-energy, non-tidal beaches in shallow, wind-driven water bodies. Knowledge of these circulations and their dependence on wind characteristics is a crucial factor to better understand and predict sediment losses of lake beaches.Coastal EngineeringHydraulic Engineerin
Sandy beaches in low-energy, non-tidal environments: Linking morphological development to hydrodynamic forcing
The morphodynamic behaviour of low-energy beaches is poorly understood, compared to that of exposed coasts. This study analyses the morphological development of sandy, low-energy beaches and the steering hydrodynamic processes. Four densely-monitored study sites in the non-tidal lake Markermeer in the Netherlands offered a unique opportunity to examine the relation between their hydraulic boundary conditions and morphodynamics. Regular bathymetric surveys were executed at all locations. Furthermore, the wave climate was monitored at one of these four sites. All four sites exhibit a commonly found low-energy beach morphology, with a narrow beach face and a low-gradient, subaqueous platform. This platform reaches an equilibrium depth quickly and then stays relatively stable. The stable elevation of the platform is located near Hallermeier's depth of closure. A sediment budget analysis over time demonstrates that the beach faces at all study sites have eroded during more energetic periods, and sediment accumulated offshore. During the monitoring periods of 2 to 4 years, the elevation of the platforms reached an equilibrium, but other morphological dimensions are still developing. The new insights gained from this study enable the prediction of platform elevations along sandy beaches in low-energy, non-tidal environments, and have contributed to our insight in the underlying processes driving the morphological evolution.Coastal EngineeringHydraulic Engineerin
The Termination Principle of Multiple Limit Cycles for the Kukles Cubic System
We carry out the global bifurcation analysis of the Kukles system representing a planar polynomial dynamical system with arbitrary linear and cubic right-hand sides and having an anti-saddle at the origin. Using the Wintner–Perko termination principle of multiple limit cycles, we solve the problem on the maximum number and distribution of limit cycles in this system.Numerical experiments are done to illustrate the obtained results.Numerical Analysi
Bifurcation Analysis of a Multi-Parameter Liénard Polynomial System
In this paper, we study a multi-parameter Liénard polynomial system carrying out its global bifurcation analysis. To control the global bifurcations of limit cycle in this systems, it is necessary to know the properties and combine the effects of all its field rotation parameters. It can be done by means of the development of our bifurcational geometric method based on the application of a canonical system with field rotation parameters. Using this method, we present a solution of Hilbert's Sixteenth Problem on the maximum number of limit cycles and their distribution for the Liénard polynomial system. We also conduct some numerical experiments to illustrate the obtained results.Numerical Analysi
Zeespiegelmonitor
Voor het Nederlandse beleid van kustveiligheid en kustlijnhandhaving is een betrouwbare en robuuste schatting van de zeespiegelstijging belangrijk. Het voorliggende rapport beschrijft verschillende exercities die zijn gedaan om de zeespiegelstijging langs de Nederlandse kust met een hogere betrouwbaarheid vast te stellen ten opzichte van eerdere studies, zoals bijvoorbeeld Deltares (2010). Hiervoor zijn zowel daggemiddelde als jaargemiddelde waterstanden gebruikt. Er is onderzocht of de trend in de tijdreeks met waterstanden en de variantie van dit signaal kunnen worden verklaard aan de hand van diverse verklarende variabelen. De variabelen harmonisch getij, 18,6-jarige nodale getijcyclus, windsnelheid, windrichting, luchtdruk, en rivierafvoer zijn bij het onderzoek aan bod gekomen. Als de zeespiegel op een tijdschaal van dagen wordt bekeken, zijn het harmonisch getij en de windsnelheid verantwoordelijk voor de grootste variatie van de waterstand. De variatie in de jaargemiddelde waterstanden wordt vooral veroorzaakt door zeespiegelstijging, gevolgd door wind en de 18,6-jarige nodale getijcyclus. Het toevoegen van verklarende variabelen is zinvol gebleken, het reduceert de variantie, en vergemakkelijkt daardoor het herkennen van een eventuele versnelling van de zeespiegelstijging. Verschillende resultaten duiden op die versnelling van de zeespiegelstijging, maar op dit moment is dit nog niet met voldoende zekerheid vast te stellen. Met behulp van satellietdata is geen betrouwbaardere maat voor de zeespiegelstijging langs de Nederlandse kust te verkrijgen dan met behulp van de getijdestations. De satellietdata kunnen wel als bevestiging gelden van hetgeen met de getijdestations gevonden is. Het is belangrijk om de langst mogelijke reeks te gebruiken. Voor de Nederlandse kust is dit de periode 1890-2012. Om te voorkomen dat de berekende actuele zeespiegelstijging sterk afhankelijk is van de trends van vele decennia terug, kan gebruik worden gemaakt van lokale regressie met de LOESS methode. Deze methode geeft goed de ontwikkelingen weer in de recente periode, terwijl wel een consistente weergave (een vloeiende lijn zonder sprongen of knikken) van de gehele tijdsperiode verkregen wordt. Dit rapport bevat een voorstel om de zeespiegelschattingen op basis van klimaatmodellen (KNMI) en op basis van analyse van meetgegevens (dit rapport) te combineren. Om de resultaten makkelijk beschikbaar te maken is een web-applicatie ontwikkeld (sealevel in openearth) waar de in deze studie ontwikkelde methoden op een simpele manier toegepast kunnen worden op de gemeten zeespiegelstanden van de 6 verschillende hoofdpeilmeetstations langs de Nederlandse kust en op alle andere getijdestations over de wereld. Voor de Nederlandse stations kan gekozen worden met welke variabelen en met welke polynoom de zeespiegelstijging berekend en getoond wordt. De zeespiegel is slechts een van de vele kust-indicatoren die goed geborgd moet zijn. Voorgesteld wordt om de gemaakte web-applicatie te combineren met een van de andere websites waar indicatoren beschikbaar worden gemaakt, bijvoorbeeld de Kustviewer. Zo kan een centraal en consistent portfolio van kustkengetallen gevormd worden
Taylor least squares reconstruction technique for material point methods
The material point method (MPM) is an effective computational tool for simulating problems involving large deformations. However, its direct mapping of the material-point data to the background grid frequently leads to severe inaccuracies. The standard function reconstruction techniques can considerably decrease these errors, but do not always guarantee the conservation of the total mass and linear momentum as the MPM algorithm does. In this paper, we introduce a novel technique, called Taylor Least Squares (TLS), which combines the Least Squares approximation with Taylor basis functions to reconstruct functions from scattered data. Within each element, the TLS technique approximates quantities of interest, such as stress and density, and when used with a suitable quadrature rule, it conserves the total mass and linear momentum after mapping the material-point information to the grid. The numerical and physical properties of the reconstruction technique are first illustrated on one- and two-dimensional functions. Then the TLS technique is tested as part of MPM, Dual Domain Material Point Method (DDMPM), and B-spline MPM (BSMPM) on a one-dimensional problem experiencing small and large deformations. The obtained results show that applying the TLS approximation significantly improves the accuracy of the considered versions of the material point method, while preserving the physical properties of the standard MPM.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Numerical Analysi