Long Term Morphodynamics of Alternate Bars in Straightened Rivers: a Multiple Perspective

i “Adami” — 2016/4/12 — 13:13 — page i — #1 i i i University of Trento Department of Civil, Environmental and Mechanical Engineering Luca Adami Long term morphodynamics of alternate bars in straightened rivers: a multiple perspective PhD Thesis April 2016 i i i i i “Adami” — 2016/4/12 — 13:13 — page ii — #2 i i i (this page has intentionally been left blank) i i i i i “Adami” — 2016/4/12 — 13:13 — page iii — #3 i i i Luca Adami Academic year 2014/2015 28th cycle PhD Student at Department of Civil, Environmental and Me- chanical Engineering, University of Trento, Italy. Academic Guest at Laboratory of Hydraulics, Hydrology and Glaciology, Zurich, Switzerland. Home Institution: Department of Civil, Environmental and Mechanical Engineer- ing, University of Trento, Trento, Italy Host Institution: Swiss Federal Institute of Technology Zurich, Laboratory of Hy- draulics, Hydrology and Glaciology, Zurich, Switzerland Supervisors: Guido Zolezzi, Department of Civil, Environmental and Me- chanical Engineering, University of Trento, Italy. Walter Bertoldi, Department of Civil, Environmental and Mechanical Engineering, University of Trento, Italy. i i i i i “Adami” — 2016/4/12 — 13:13 — page iv — #4 i i i (this page has intentionally been left blank) i i i i i “Adami” — 2016/4/12 — 13:13 — page v — #5 i i i Abstract Alpine rivers have been regulated to claim productive land in valley bot- toms since the last two centuries. Width reduction and rectification of- ten 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 Land- v i i i i i “Adami” — 2016/4/12 — 13:13 — page vi — #6 i i i sat 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 con- sidering 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 reso- i i i i i “Adami” — 2016/4/12 — 13:13 — page vii — #7 i i i lution 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. i i i i i “Adami” — 2016/4/12 — 13:13 — page viii — #8 i i i i i i i i “Adami” — 2016/4/12 — 13:13 — page ix — #9 i i i Contents Abstract v Contents xi List of Figures xv List of Tables xvii 1 Introduction 1 1.1 Context . .1 1.2 Research questions . .5 1.3 Outline of the thesis . .6 2 Alternate bars in straight channels: theoretical background 9 2.1 Introduction: types of alternate bars . .9 2.2 Governing dimensionless parameters . 12 2.3 Governing equations . 13 2.4 Free bars in straight channels . 16 2.4.1 Free bars formation and linear stability . 16 2.4.2 Free bars amplitude and non-linear growth . 24 2.5 Forced bars in straight channels . 26 2.5.1 Linear theory of forced bars in straight channels . 29 2.5.2 Weakly non-linear theory of forced bars in straight channels . 34 2.5.3 Illustration of steady bar patterns in straight channel reaches with local planform perturbations . 38 ix i i i i i “Adami” — 2016/4/12 — 13:13 — page x — #10 i i i 3 Multi-decadal alternate bar dynamics of the Alpine Rhine river 43 3.1 Introduction . 44 3.2 Study site and methods . 47 3.2.1 The Alpine Rhine River . 47 3.2.2 Image database . 49 3.2.3 Bed topography database . 50 3.2.4 Monitoring of bar properties . 52 3.3 Results . 54 3.3.1 Bar wavelength . 55 3.3.2 Bar migration . 59 3.3.3 Bar amplitude . 65 3.4 Discussion . 66 3.4.1 The Alpine Rhine alternate bar dataset . 66 3.4.2 Observed bar morphodynamics: wavelength and migration . 68 3.5 Conclusions . 69 3.6 Acknowledgements . 70 4 Application of analytical morphodynamic theories to observations on alternate bars in the Alpine Rhine 71 4.1 An approach for the application of analytical bar theories to field cases . 73 4.1.1 Theoretical setting . 74 4.1.2 Corresponding setting in a field case . 77 4.1.3 The proposed approach . 81 4.2 The field case: Alpine Rhine river . 86 4.2.1 Alpine Rhine river . 86 4.2.2 Data and methods . 86 4.2.3 Summary of the observed properties of alternate bars and of the emerging research questions . 87 4.3 Results: application of bar theories . 92 4.3.1 Computation of theoretical parameters . 92 4.3.2 Application of bar theories . 94 4.3.3 Insights emerging from the application of the bar theories . 99 i i i i i “Adami” — 2016/4/12 — 13:13 — page xi — #11 i i i 4.4 Steady and migrating bars in the Alpine Rhine: why a spatially selective behavior? . 102 4.4.1 Synthesis of observations . 102 4.4.2 Development of possible Interpretations . 103 4.5 Discussion and conclusions . 112 4.6 Conclusions . 115 4.6.1 From chapter 3 . 115 5 Reproducing free and forced alternate bars with numerical models 117 5.1 Introduction . 117 5.2 Shallow-water-Exner model: numerical vs analytical solutions119 5.2.1 The numerical model Basement . 119 5.2.2 Numerical vs analytical solution: link between the two sets of governing equations and closure relation- ships . 120 5.2.3 Correction of local longitudinal slope . 122 5.3 Benchmark simulations on alternate bar stability . 125 5.4 The role of the coefficient of lateral transport r ...... 129 5.5 Numerical simulations of the Alpine Rhine river . 133 5.5.1 Numerical simulations . 133 5.6 Discussion and conclusions . 140 6 Conclusions 145 6.1 Summary of chapter conclusions . 145 6.2 Open issues .

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