Dynamic drag modeling of submerged aquatic vegetation canopy flows

Vegetation has a profound effect on flow and sediment transport processes in natural rivers, by increasing both skin friction and form drag. The increase in drag introduces a drag discontinuity between the in-canopy flow and the flow above, which leads to the development of an inflection point in the velocity profile, resembling a free shear layer. Therefore, drag acts as the primary driver for the entire canopy system. Most current numerical hydraulic models which incorporate vegetation rely either on simple, static plant forms, or canopy-scaled drag terms. However, it is suggested that these are insufficient as vegetation canopies represent complex, dynamic, porous blockages within the flow, which are subject to spatially and temporally dynamic drag forces. Here we present a dynamic drag methodology within a CFD framework. Preliminary results for a benchmark cylinder case highlight the accuracy of the method, and suggest its applicability to more complex cases

Physical complexity to model morphological changes at a natural channel bend

This study developed a two‐dimensional (2‐D) depth‐averaged model for morphological changes at natural bends by including a secondary flow correction. The model was tested in two laboratory‐scale events. A field study was further adopted to demonstrate the capability of the model in predicting bed deformation at natural bends. Further, a series of scenarios with different setups of sediment‐related parameters were tested to explore the possibility of a 2‐D model to simulate morphological changes at a natural bend, and to investigate how much physical complexity is needed for reliable modeling. The results suggest that a 2‐D depth‐averaged model can reconstruct the hydrodynamic and morphological features at a bend reasonably provided that the model addresses a secondary flow correction, and reasonably parameterize grain‐sizes within a channel in a pragmatic way. The factors, such as sediment transport formula and roughness height, have relatively less significance on the bed change pattern at a bend. The study reveals that the secondary flow effect and grain‐size parameterization should be given a first priority among other parameters when modeling bed deformation at a natural bend using a 2‐D model

Assessment of a numerical model to reproduce event-scale erosion and deposition distributions in a braided river

Becky Goodsell and Eric Scott are thanked for field assistance. Antony Smith assisted with figure production. The field campaign wa funded by NERC Grant NE/G005427/1 and NERC Geophysical Equipment Facility Loan 892. Richard Williams was funded by NERC Grant NE/G005427/1during fieldwork and by a British Hydrological Society Travel Grant whilst visiting NIW

Assessing river bed changes by morphological and numerical analysis

Die vorliegende Arbeit befasst sich mit der Beurteilung von Flussbettveränderungen durch Anwendung unterschiedlicher Analysemethoden. Hierfür werden regelmäßig durchgeführte Sohlgrundaufnahmen der Donau östlich von Wien bis zur österreichisch-slowakischen Staatsgrenze über einen Zeitraum von 10 Jahren ausgewertet. Zur Beschreibung und Quantifizierung relevanter morphologischer Strukturen und Prozesse werden Erosions-Anlandungs-Muster erstellt und analysiert, bestehende morphologische Parameter angewendet und neue Parameter vorgestellt.Numerische Methoden werden durchgeführt, um das Verständnis der den gemessenen Flussbettveränderungen zugrunde liegenden Prozesse zu erweitern. Für diesen Zweck wird das dreidimensionale numerische Modell SSIIM zur Berechnung von ungleichförmigen Sedimenttransportprozessen modifiziert. Die Sedimenttransportformeln von Wu et al. (2000), die einen Ansatz zur Beschreibung von Verbergens- und Expositionsvorgängen enthalten, werden in das bestehende Modell implementiert. Das modifizierte Modell wird an Laborexperimenten in einer Gerinnekrümmung mit einer Sedimentmischung unter instationären Verhältnissen validiert.Sowohl Sohlverformungen als auch Sortierungsprozesse werden berechnet.Der Nutzen des implementierten Ansatzes wird anhand eines Vergleichs von berechneten Ergebnissen des modifizierten und des ursprünglichen Modells mit gemessenen Daten ermittelt. Anschließend wird das validierte Modell an einem 6 km langen Donauabschnitt angewendet. Hierbei werden instationäre Berechnungen von Strömung und Sedimenttransport eines 100-jährlichen Hochwasserereignisses durchgeführt. Die berechneten Ergebnisse werden mit gemessenen Sohlhöhenveränderungen verglichen.This study deals with the assessment of river bed changes using different analysing methods. Regular bed level surveys of the Danube river between Vienna and the Austrian-Slovakian border are processed over a period of ten years. Erosion-deposition patterns are analysed, commonly applied morphological parameters are used, and new parameters are introduced to describe and quantify relevant morphological structures and processes. Numerical studies are performed to enhance the understanding of the processes leading to measured river bed changes.Therefore, the three-dimensional numerical model SSIIM is modified to compute nonuniform sediment transport processes. The sediment transport formulas of Wu et al. (2000) considering hiding-exposure algorithms are incorporated into the existing model. The modified numerical model is validated on laboratory experiments in a channel bend under unsteady flow conditions using graded bed material. Both bed deformation and sorting processes are computed. The benefit of the newly incorporated approach is assessed by comparing the computed results of the modified and the default model to measured data. Afterwards, the validated model is applied to compute unsteady flow and sediment transport processes of a 6 km long reach of the Danube river induced by a 100 year flood event.The computed results are compared to measured river bed changes.17