121 research outputs found
Past River Channel Alignments Reconstruction by Combining a Meander Migration Model and Vegetation Succession
No full textMeandering rivers dynamics has intrigued scientists since the nineteen fifties (Hooke, 2013). Meander migration models are regularly used to predict planimetric changes (Crosato, 2018), but they have not considered the figure, the reconstructed historical channel is shown. This alignment was obtained from the analysis of the existent water marks and channel trajectories that can be identified in satellite imagery. reconstruction of their historical alignments. This study deals with the reconstruction of a 9 km long reach of the Bodoquero River (see Figure 1), located in the Caquetá department, Colombia, combining the potential of meander migration models and vegetation succession to validate the modeled historical alignments.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.Environmental Fluid Mechanic
Bank erosion in regulated navigable rivers: Towards a process-based model of bank retreat
No full textAfter the European Water Framework Directive, riverbanks in several countries had the protections removed to improve the water quality and the river ecosystem. Particularly, the Meuse River currently has several kilometres of freely eroding banks, which may have consequences for other river functions such as navigation and flood conveyance. The understanding, quantification and prediction of the morphological evolution of restored banks is thus relevant to manage the integrity of all river functions and improve future restoration practices. This work analyses the results of a recently developed model to estimate bank retreat in regulated waterways and compares them with measured profiles. The model essentially accounts for the major drivers of erosion, i.e., primary and secondary ship waves, considers homogenous cohesive banks, and computes erosion rates through a Partheniades-type of formulation. The results show a good qualitative and quantitative agreement with measurements. Erosion rates are yet not accurate with the current approach, for which future work will focus on improving the temporal representation through the inclusion of other factors and processes affecting erosion rates. These are, for instance, statistically representative time series of ship waves, currents during floods, and elements affecting erosion processes such as mass failures, slump-block dynamics and vegetation.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.Environmental Fluid Mechanic
Improved floodplain vegetation roughness for 1D hydraulic models
No full text1D hydraulic models are largely used to simulate the propagation of flood waves and for flood mapping along river systems. The most common approach to account for the hydraulic effects of vegetated floodplains consists of imposing higher roughness coefficients. However, the flow resistance of vegetation is governed by plant submergence, which is water-depth dependent and varies with the discharge, and thus with time. An improved method properly incorporating floodplain vegetation roughness in 1D models is presented here. The Manning coefficient is derived from a simplification of Baptist’s formula assuming horizontal floodplains, i.e., with the same water depth everywhere. Considering the dependency of vegetation roughness on local water depth (in case of variable flow conditions), a predictor-corrector approach of the derived formula is proposed to be applied at every computational time-step. If different types of vegetation are present, the roughness coefficient, one for each floodplain, is derived as a weighted average. The method is tested on a recently restored stream located in the Netherlands, the Lunterse Beek, using the HEC-RAS code. The results support the implementation of the proposed method, but validation is needed for river floodplains with non-uniform vegetation cover.Accepted Author ManuscriptEnvironmental Fluid Mechanic
Morphological Response of Rivers to Withdrawal of Water
No full textIt is very convenient to make use of rivers as water supply for various purposes, i.e. irrigation, industrial and domestic uses. Even if it is a very common practice, the utilization of river waters must be carefully planned and its consequences analysed. For example, before starting on a new withdrawal of water the need of water supply more downstream must be considered. Furthermore, whenever those needs are satisfied, the morphological changes of the river must be taken into account. Characteristic consequences of withdrawal of water are: aggradation of the river bed; eventual rise of the water level and decrease of the water depth downstream of the extraction point. All these morphological changes can affect the existing structures along the river and the navigability of the channel. For this reason the morphological response of the river must be predicted. In this way it is possible to take measures in advance, when necessary. Here the river response to water withdrawal is analyzed by means of mathematical models. After a short description of the underlying theory, the morphological changes are investigated computationally by using the computer program "ODIRMO" (Vermeer [1985]) available at the Delft University of Technology. In order to give a more general insight into the involved processes the computations are carried out for several conditions. Furthermore, the input data and the output layout are comparable with those adopted in the study made by Hendrickx [1988]. In this way the morphological changes due to water withdrawal can be compared with the response generated by sediment mining. The results of the morphological changes due to water withdrawal show the importance of investigating the transitional period and not only the final situation. During the transitional period the water level initially drops and then rises. The decrease of the waterlevel can temporarily affect, for example, the intake of irrigation canals further downstream. Consequently its intensity and duration must be taken into account when planning a new water withdrawal. The performed computations show how effecting the schematization of the computational model can be on the results. The decoupling of flow and bed computations might yield to undesired oscillations of the bed when the celerity of the bed rise is two high. Here a criterion for a restriction of validity of that schematization via a dimensionless parameter is suggested. The lack of computational examples, together with the uncertainty added by (i) the problems related to the accuracy of the computations, (ii) the non-optimization of the flow computation routine, lead to the impossibility of estimating a critical value of the dimensionless parameter by means of the computational results of this study only.Hydraulic EngineeringCivil Engineering and Geoscience
Effects of smoothing and regridding in numerical meander migration models
No full textMeander migration models include an as yet poorly investigated source of numerical errors related to the computation of the channel curvature, which are amplified by the procedure of adding and deleting grid points as the river planform evolves. The methods adopted to reduce these errors may influence size, form, and migration rate of the developing meanders, which creates uncertainties in the analysis of the results, limits the model applicability, and makes it necessary to treat the bank erodibility coefficients as calibration parameters. This becomes evident from a series of computational tests performed in order to compare two different methods of error reduction in the computed local channel curvature: cubic spline interpolations versus different levels of curvature smoothing. Since the problems discussed are common to most meander migration models, the tests performed were carried out for three models of different complexity. These were derived by applying different degrees of simplification to the basic equations for water flow and sediment motion of shallow curved channels.Hydraulic EngineeringCivil Engineering and Geoscience
Analysis and modelling of river meandering
No full textThis thesis examines the morphological changes of non-tidal meandering rivers at the spatial scale of several meanders. With this purpose, a physics-based mathematical model, MIANDRAS, has been developed for the simulation of the medium-term to long-term evolution of meandering rivers. Application to several real rivers shows that MIANDRAS can properly simulate both equilibrium river bed topography and planimetric changes. Three models of different complexity can be obtained by applying different degrees of simplification to the equations. These models, along with experimental tests and field data, constitute the tools for several analyses. At conditions of initiation of meandering, it is found that river bends can migrate upstream and downstream. This depends on meander wave length and width-to-depth ratio, irrespective of whether the parameters are in the subresonant or the superresonant range. Varying lag distances between flow velocity and bed topography are found to offer an explanation why local channel migration rates reach a maximum at a certain bend sharpness, in addition to previous explanations based on flow separation. A new method has been developed to calculate the number of bars in a river channel with given width. It predicts successfully whether reducing or enlarging the river width would lead to meandering or braiding. Channel migration coefficients are demonstrated to depend not only on physical properties of the eroding bank, but also on physical properties of the accreting bank and the numerical scheme. Moreover, they need to account for overbank flows. Model results and a re-examination of experimental observations suggest that intrinsic initiation of meandering is not necessarily related to steady bars due to a permanent upstream disturbance. They may also be related to a steady bed deformation due to small quickly-varying periodic or random disturbances, for instance due to the presence of migrating alternate bars. The latter finding still requires further confirmation.Civil Engineering and Geoscience
Simulation of meandering river processes
No full textA simple mathematical model for the simulation of river meandering processes is presented and analysed. The model is schematized as follows: computation of steady-state flow field and riverbed topography; and computation of bank erosion rate as a function of the near-bank hydraulic and morphological properties. The model is linear for the prediction of flow field and river bed topography, while geometrical non-linearities arise from the bank erosion equation. The flow and river bed model is derived by fully coupling flow field, bed topography and sediment transport and it (roughly) accounts for the secondary flow momentum convection.Civil Engineering and Geoscience
Comparison between anatomical and integrated approaches to atrial fibrillation ablation: Adjunctive role of electrical pulmonary vein disconnection
No full textOn the relationship between flow-field and bank erosion in rivers: insights from large-eddy simulations
No full textGreen 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.Environmental Fluid Mechanic
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