Adami, Luca (2016) Long term morphodynamics of alternate bars in straightened rivers: a multiple perspective. PhD thesis, University of Trento.
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Alpine rivers have been regulated to claim productive land in valley bottoms since the last two centuries. Width reduction and rectiﬁcation often induced the development of regular scour-deposition sequences, called alternate bars, with implications for ﬂood protection, river navigation, environmental integrity. Understanding how alternate bars evolves in rivers and deﬁning the key aspects that inﬂuence the development of these regular deposits of sediments represents a challenge that is not fully described. Most studies on alternate bars are in fact based on mathematical theories, laboratory experiments and recently numerical simulations, but only few studies on ﬁeld cases have been performed so far. The goals of this work are: i) to quantify the morphodynamics of alternate bars in the Alpine Rhine River, with a particular emphasis on bar migration; ii) to assess to what extent the predictions of analytical bar theories are consistent with ﬁeld observations and to explore how theories may help interpret observed alternate bars dynamics; iii) to determine the ability of a numerical model to simulate correctly the formation and the length scale of alternate bars and inﬂuence of diﬀerent multi-decadal inﬂow conditions. The 42-km chosen reach is located along the border between Austria and Switzerland, between the conﬂuences of Landquart and Ill rivers. The whole reach has been completely embanked starting from the 19th century, so alternate bars are present for more than a century. Moreover the simpliﬁcation of the cross section, together with the presence of only few bends, puts the Alpine Rhine in the ideal position to be compared with analytical theories of alternate bars in straight channels. The goals are achieved by analyzing a dataset of freely available Landsat imagery, which combine unprecedented temporal length (3 decades), spatial length (more than 400 channel widths) and temporal resolution (around 3 images per year). Bars show a spatially selective behavior, with short, bars occurring in distinct straight reaches with respect to longer bars. The same evidence is found in terms of bar migration, so that short bars are shown to migrate more than longer bars, in agreement with theoretical predictions. A full range of bar wavelengths and more complex patterns occur in reaches with bends and ramps. Bar height, obtained from cross section monitoring, was found to be much more uniform. The temporally long dataset, including approximately 30 ﬂoods with diﬀerent magnitude and duration, allowed the investigation of bar migration as a function of discharge, showing that bars migrate faster for intermediate foods. Predicted values of linear theories for free and forced bars in straight channels are in good general agreement with ﬁeld observations, when considering conditions of bar formation and bar wavelength. Comparing theories and observations suggests that theoretical outcomes may represent the boundaries of the actual, wide range of bars’ behaviour, which likely reﬂects non-linear interactions, ﬂow unsteadiness, sediment size hetero- geneity and ﬁnite length of straight reaches, which are not retained in linear theories. Non-linear interactions are investigated through the 2D numerical morphodynamic model Basement, developed at the Swiss Federal Institute of Technology of Zurich. Preliminary investigations focus on the role of the transversal sediment transport, that behaves as a diﬀusive term. The numerical diﬀusion can be indirectly evaluated starting from the calibration of the coeﬃcient of the diﬀusive term and a benchmark methodology to evaluate the lateral and numerical diﬀusion is deﬁned. The results are used in the morphological calibration of the model. The spatial trend of wavelengths is in general agreement with the ﬁeld data, and the migration take place mainly in correspondence of short bars, while long bars tend to elongate with time. The choice of a constant discharge or a real hydrograph inﬂuences the time scale of bar evolution. The present analysis results in the longest spatial and temporal ﬁeld case study of river bars in channelized rivers with a temporal survey resolution that allows the investigation of the eﬀect of individual ﬂood events, and provides new quantitative data on bar wavelength and migration. The dataset provides useful information to assess the applicability of analytical bar theories, so far mainly tested against ﬂume experiments, and following recent attempts in French and Dutch streams. Moreover, a novel two-dimensional morphological benchmark to access the role of numerical diﬀusion is proposed. The new insights are crucial to design future management scenarios accounting for hydraulic safety and environmental quality.
|Item Type:||Doctoral Thesis (PhD)|
|Doctoral School:||Environmental Engineering|
|Subjects:||Area 08 - Ingegneria civile e Architettura > ICAR/01 IDRAULICA|
|Repository Staff approval on:||22 Apr 2016 11:47|
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