Chute Channels in Large, Sand-Bed Meandering Rivers
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Dynamics and Morphodynamic Implications of Chute Channels in Large, Sand-Bed Meandering Rivers Michael C. Grenfell Doctor of Philosophy in Geography College of Life and Environmental Sciences July 2012 Dynamics and Morphodynamic Implications of Chute Channels in Large, Sand-Bed Meandering Rivers Submitted by Michael Cyril Grenfell to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Geography in the College of Life and Environmental Sciences July 2012 This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified, and that no material has previously been submitted and approved for the award of a degree by this or any other University. Michael Cyril Grenfell 2 ABSTRACT Chute channel formation is a key process in the transition from a single-thread meandering to a braided channel pattern, but the physical mechanisms driving the process remain unclear. This research combines GIS and spatial statistical analyses, field survey, Delft3D hydrodynamic and morphodynamic modelling, and Pb-210 alpha-geochronology, to investigate controls on chute initiation and stability, and the role of chute channels in the planform dynamics of large, sand-bed meandering rivers. Sand-bed reaches of four large, tropical rivers form the focus of detailed investigations; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Binary logistic regression analysis identifies bend migration style as a key control on chute channel initiation, with most chute channels forming at bends that are subject to a rapid rate of extension (elongation in a direction perpendicular to the valley axis). Bend extension rates are shown to track variation in potential specific stream power, such that reaches and sub-reaches of the rivers studied fit within a planform continuum expressed though increasing bend extension rates and chute initiation frequency, and driven by increasing stream power relative to bedload calibre. Field observations of point bar geomorphology and vegetation dynamics illustrate the importance of rapid bend extension in forming wide sloughs between scroll bars that are aligned with the direction of over-bar flow, and in breaking the continuity of vegetation encroachment on point bars. Bathymetric surveys and Delft3D simulations for the Strickland River provide insight into flow and sediment division at bifurcate meander bends. Coupled with GIS analyses, these simulations show that stable chute channels have higher gradient advantages than chute channels subject to infill, but that upstream and downstream changes in bend orientation can also influence chute stability. The process of bend extension is typically associated with an increase in the chute gradient advantage, further elucidating the role of bend migration style in chute 3 stability. At the reach scale, rivers with higher sediment loads (Qs/Q) are characterised by higher rates of chute infill. Strickland River floodplain sedimentation rates derived through Pb-210 alpha- geochronology are substantially higher adjacent to single-thread bends than adjacent to bifurcate bends, potentially due to an observed increase in channel capacity (and reduction in floodplain inundation frequency) associated with bend bifurcation. Further research is needed to determine whether this observation is significant in light of high uncertainty in the spatial variability of sedimentation rate estimates, but the data presented highlight a need for carefully considered stratified sampling approaches in floodplain coring campaigns, and illustrate the complexity of possible sediment dispersal mechanisms, and associated ecological responses. GIS analysis of the response of the Ok Tedi in Papua New Guinea to direct addition of mine tailings elucidates interplay between channel steepening due to the propagation of a tailings sediment slug, and mid-channel bar formation induced by the increased sediment load, with associated topographic forcing of bend and chute development. A temporal pattern of increased chute initiation frequency on the Ok Tedi mirrors the inter- and intra-reach spatial pattern of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power is associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold value of bend extension for chute initiation is shown to scale with reach-scale stream power, reminiscent of slope-ratio thresholds in river avulsion. However, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation is also rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy, selectively targeting sites of greatest change, and thereby mediating the river response. 4 LIST OF CONTENTS Abstract 2 List of Contents 4 List of Tables 7 List of Figures 8 Acknowledgements 13 1. Introduction 15 1.1. Large Rivers Research 15 1.2. Planform Dynamics of Alluvial Rivers 19 Channel Planform Classification 19 Anabranching in Alluvial Rivers 22 The Meandering-Braided Channel Pattern Continuum 24 1.3. Chute Channels in Meandering Rivers 28 1.4. Thesis Aim and Primary Research Questions 30 1.5. Thesis Structure 31 2. Chute Channel Dynamics in Large, Sand-Bed Meandering Rivers 32 2.1. Introduction 32 2.2. Mechanisms of Chute Formation in Meandering Rivers 36 2.3. River Locations and Physiographic Settings 40 Strickland River, Papua New Guinea 40 Paraguay River, Paraguay/Argentina 41 Beni River, Bolivia 44 2.4. Methods 45 Image Acquisition and Preparation 45 Measurement of Channel Planform Characteristics and Dynamics 46 Analysis of Error 49 Binary Logistic Regression Analysis 51 Assumption Testing 52 2.5. Results 53 5 Bend-Scale Controls on Chute Channel Initiation 53 Reach-Scale Controls on Chute Channel Initiation and Stability 55 2.6. Discussion 62 2.7. Conclusion 72 3. Morphology and Stability of Bifurcate Meander Bends in the 73 Strickland River, Papua New Guinea 3.1. Introduction 73 Shields Stress and Transverse Bed Slope Effects at the Nodal Point 75 Bifurcation Planform Geometry (Bifurcation Angle, Bifurcate Gradient 77 Advantage) Alignment of Upstream Flow (e.g. Due to an Upstream Bend) 79 3.2. Morphology of Bifurcate Meander Bends 80 3.3. GIS Analysis of Chute Stability 80 3.4. Channel Bathymetry and Bed Sediment Survey 83 3.5. Modelling Approach and Results 88 Discharge and Sediment Transport Ratios at Hydrodynamic 94 Equilibrium Morphodynamic Sensitivity to Alpha-bn 100 3.6. Conclusion 105 4. Effects of Meander Bend Bifurcation on Floodplain Sediment 107 Dispersal: Strickland River, Papua New Guinea 4.1. Introduction 107 4.2. Spatial Variation in Floodplain Sedimentation 108 4.3. Methods 110 2003 Field Campaign 110 Pb-210 Alpha-Geochronology 112 4.4. Results 115 Synthesis of Overbank Sedimentation Rates 115 Local Floodplain Sedimentation, Topography, and Channel Planform 116 Dynamics 6 4.5. Discussion 121 4.6 Further Implications 123 5. Mediative Adjustment of River Dynamics: Interplay between 126 Sediment Supply, Channel Migration, and Chute Formation in Large, Sand-Bed Meandering Rivers 5.1. Introduction 126 Process Sedimentology in Large River Systems 126 Planform Dynamics of Meandering Rivers 128 5.2. Chute Channel Dynamics and Channel Planform Transitions 129 Spatial Statistical Analysis of Chute Channel Dynamics 129 Spatial Variation in Chute Initiation 130 Temporal Variation in Chute Initiation: Insights from the Ok Tedi, 132 Papua New Guinea Rate-Frequency Distributions as a Function of River Energy 137 5.3. Mediative Adjustment of River Dynamics 142 5.4. Implications 147 6. Conclusion 150 6.1. Project Summary 150 6.2. Research Forecast: Large River Processes 152 References 154 7 LIST OF TABLES 2.1. Summary of image sources used in the analysis. 46 2.2. Estimated error in migration rate measurements due to image 50 overlay offsets and digitising inconsistencies. 2.3. Attributes of bend planform characteristics and dynamics 52 used in the binary logistic regression analysis. 2.4. Results of the binary logistic regression analysis. 54 2.5. Comparison of descriptive statistics for Curvature (R:w), 56 entrance angle and sinuosity, for cases in which bends are single-thread, and cases in which bends are partitioned by chute channels. 2.6. Reach-scale and within-reach variation in slope, average 57 annual water (Q) and suspended sediment (Qs) discharge, and average annual sediment load (Qs/Q) data for the rivers studied. 2.7. Summary statistics describing trends in migration style and 58 rates of bend migration for the rivers studied. 2.8. Summary statistics describing chute channel initiation and 59 stability in the rivers studied. 3.1. Parameter values used in Delft3D hydrodynamic simulations 91 for the Strickland River bifurcate meander bends studied in this chapter. 4.1. Summary of sedimentation rates for cores located within 100 116 m of the active Strickland River channel. 4.2. Local curvature at transect locations, and bend average 118 migration rates prior to the survey. 8 LIST OF FIGURES 1.1. A general, simplified qualitative classification of river channel 20 planform, based on planimetric pattern and temporal dynamics (modified from Brice, 1982). 1.2. ‘Landsatlook’ Landsat 7 ETM+ false-colour composites showing 21 single-thread channel patterns. 1.3. ‘Landsatlook’ Landsat 7 ETM+ false-colour composites showing 24 multiple-thread channel patterns. 1.4. The meandering-braided channel pattern continuum illustrated 25 in Papua New Guinea rivers. 2.1. Bifurcate meander bends in large, sand-bed meandering rivers. 33 2.2. Mechanisms of chute formation (and chute cutoff) in meandering 38 rivers. 2.3. Study-reach SRTM digital elevation models for the Strickland, 42 Paraguay, and Beni Rivers. 2.4. Bend migration style classification scheme (after Hooke, 1977; 48 1984). 2.5.