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Processes and Controls of Meander Development in the Allier, France

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Processes and Controls of Meander Development in the Allier, France Processes and controls of meander development in the Allier, France a case study on meander change in the period 1960 – 2003 final version Supervisors: Dr. H. Middelkoop Dr. J.H. van den Berg November, 2005 Maarten Bakker 9907610 Department Physical Geography Faculty of Geosciences Contents Preface ……………………………………………………………………………... 1 Summary ………………………………………………………………………….. 2 1. Introduction ……………………………………………………………………. 3 1.1 Research background ……………………………………………………… 3 1.2 Research objectives ………………………………………………………... 4 2. Description of research area ………………………………………………….. 6 2.1 The Allier river ……………………………………………………………. 6 2.2 Study area near Moulins ……………………………………………………. 7 2.3 Study sites (meanders) ……………………………………….……………... 7 3. Meander flow and morphological development ……………………………. 9 3.1 Secondary flow in meanders ………………………………………………... 9 3.2 Meander development ……………………………………………………… 9 3.2.1 Progressive meander development ………………………………….. 9 3.2.2 Compound bend formation ………………………………………….. 10 3.2.3 Meander cutoff ………………………………………………………... 10 3.3 Erosion and sedimentation ………………………………………………….. 11 3.3.1 Beginning of motion ………………………………………………….. 11 3.3.2 Discharge ……………………………………………………………. 11 3.3.3 Bend radius ………………………………………………………….. 12 3.3.3 Resistance to flow ……………………………………………………. 13 3.4 Flow – morphology equilibrium ……………………………………………. 13 3.4.1 Upstream influences on meander development; adaptation length ….. 13 3.4.2 Channel cross section ………………………………………………... 14 3.5 Bar characteristics and development ……………………………………… 15 3.5.1 Bar morphology and structure ………………………………………... 15 3.5.2 Scroll bar development ……………………………………………… 16 3.5.3 Counter pointbar development ……………………………………… 17 4. Methods …………………………………………………………………….….. 18 4.1 Aerial photographs ……………………………………………………….…. 18 4.1.1 Determining bend radius ………………………………………..….…. 18 4.1.2 Channel migration: downstream and lateral sedimentation/erosion ….. 19 4.1.3 Morphological description ……………………………………..….….. 21 4.2 Levelling, GPS and dGPS …………………………………………….……. 22 4.3 Field observations ………………………………………………………….. 22 4.4 Discharge …………………………………………………………….……. 23 5. Allier discharge ………………………………………………………………... 24 6. Meander and pointbar development case 1: St. Loup ……………………… 26 6.1 Meander development 1960 - 1993 ………………………………………... 26 6.2 Meander development 1993 - 2003 ………………………………………... 28 6.2.1 Outer bend erosion ……………………………………………………. 28 ii 6.2.2 Pointbar sedimentation ……………………………………………… 28 6.3 Overall meander change ……………………………………………………. 33 7. Meander and pointbar development case 2: Chatel de Neuvre ……………. 34 7.1 Long term meander history ………………………………………………... 34 7.2 Meander development between 1992-2003 ………………………………... 36 7.2.1 Outer bend erosion ……………………………………………………. 36 7.2.2 Pointbar sedimentation ……………………………………………… 38 7.3 Overall meander change ……………………………………………………. 40 8. Meander and pointbar development case 3: Chemilly ……………………… 42 8.1 Long term meander history ………………………………………………... 42 8.2 Bend development between 1992 and 2003 ………………………………... 44 8.2.1 Outer bend erosion ……………………………………………………. 44 8.2.2 Sedimentation ………………………………………………………... 44 8.3 Overall meander change ……………………………………………………. 47 9. Meander and pointbar development case 4: Chateau de Lis ……………… 48 9.1 Long term meander history ………………………………………………... 48 9.2 Bend development between 1992 and 2003 ……………………………… 50 9.2.1 Outer bend erosion ………………………………………………….. 50 9.2.2 Sedimentation ………………………………………………………... 50 9.3 Overall meander change ……………………………………………………. 54 10. Meander and pointbar development case 5: Beauregard ………………... 55 10.1 Long term meander history ………………………………………………... 55 10.2 Meander development between 1993-2003 ……………………………… 57 10.2.1 Outer bend erosion ………………………………………………….. 57 10.2.2 Point bar sedimentation …………………………………………….. 58 10.3 Overall meander change ………………………………………………….. 60 11. Influence of discharge on meander development ………………………….. 62 11.1 Relation discharge – meander development of individual bends ………... 62 11.2 General relationship discharge – meander development (all bends combined) ………………………………………………………………... 63 11.2.1 Direct correlation and regression ……………………………………. 63 11.2.2 Correlation strength with varying discharge measures and corrections ………………………………………………………….. 64 11.2.3 Lateral extent of erosion and sedimentation ……………………...… 64 11.2.4 Amounts of erosion and transport ………………………………….. 65 11.2.5 Downstream component of sedimentation and erosion along meanders ……………………………………………………………. 67 12. Influence of bend radius on meander development ………………………... 69 12.1 The development of bend radius ………………………………………….. 69 12.2 Relation bend radius – meander development of individual bends ……… 69 12.3 General relationship bend radius – meander development (all bends combined) ………………………………………………………………... 70 12.3.1 Correlation and regression ………………………………………….. 70 iii 12.3.2 Area of pointbar accretion ………………………………………….. 72 13. Planform pointbar development ………………………………………...….. 73 13.1 Lateral and downstream displacement ……………………………………. 73 13.2 Complex and compound bends ……………………………………………. 73 13.3 Bend development along a resistant bank ………………………………... 74 13.4 Cutoffs ……………………………………………………………………. 74 13.4.1 Meander cutoff ……………………………………………………… 74 13.4.2 Chute cutoff ……………………………………………………….. 75 14. Equilibrium between erosion and sedimentation ……………………….…. 76 14.1 Relation sedimentation / erosion ……………………………………….…. 76 14.2 Equilibrium pointbar slope ………………………………………………... 77 15. Scroll bar development ……………………………………………………… 78 15.1 Ridge and swale topography ………………….………………….……….. 78 15.1.1 Scroll bar characteristics ……………………………………………. 78 15.1.2 Swale development ………………………………………………….. 78 15.2 Circumstances facilitating scroll bar development ………………………... 79 15.2.1 Pointbar slope ……………………………………………………… 79 15.2.2 Sediment composition ……………………………………………… 80 15.2.3 Flow obstruction ……………………………………………………. 80 15.3 Interaction between riffle and scroll bar development ……………………. 81 15.3.1 Riffle development ………………………………………………….. 81 15.3.2 Interaction erosion and sedimentation ……………………………… 81 15.3.3 Overlap downstream riffle with scroll bars ………………………... 82 15.3.4 Riffle presence along a pointbar ……………………………………. 82 15.3.5 Overlap upstream riffle with scroll bars ……………………………. 82 15.3.6 Scroll bars originating from the riffle ……………………………… 83 15.4 Bars in the outer bend ……………………………………………………. 83 16. Conclusions ………………………………………………………………….. 84 16.1 Meander development ……………………………………………………. 84 16.2 Sedimentation processes ………………………………………………….. 85 References ………………………………………………………………………... 87 iv Preface This report was written as part of Master’s research in Physical Geography at Utrecht University. It contains the results and conclusions from graduate fieldwork done along the Allier river in the summer of 2003. I would like to thank everyone that was involved in the fieldwork. Supervisors Dr. H. Middelkoop and Dr. J.H. van den Berg for their guidance and constructive discussions. Fieldwork partners Sander van Rooy and Eisse Wijma for the good cooperation in the field and behind the computer, but also for the good times we had. The staff of the Moscow State University, TU Delft and Seva, Maxim, Misha, Emilie and Mathys for their help in the field. 1 Summary Morphological development was researched along five meanders in the Allier river, France. This includes large scale development of meander form and direction and guiding factors and processes. On a smaller scale channel processes that form morphology, including bars and (secondary) channels. Five meanders in the Allier river, France, demonstrated various types of development in time and space; lateral, downstream, rotational, compound bends and cutoffs. Bend radii decreased with lateral meander development, that occurred where bends were long enough to develop helicoidal flow (adjustment length) and where pointbar vegetation forced flow outwards. Downstream meander propagation and increasing bend radius occurred along resistant banks and recent cutoffs, that directed flow in downstream direction. The development of rotational and compound bends result from lateral (in the first case) and downstream (second case) erosion where flow meanders within bends. Meander migration increased with discharge. Lateral erosion and sedimentation are (significantly) related to discharges above 550 m3/s, that accounted for 20-30 % of their variation. Downstream sedimentation showed the strongest relationship with average discharge, accounting for 10-15 % of variation. Helicoidal flow was fully developed halfway most bends, causing lateral development, while lower discharges “meandered” within a bend causing bend expansion near the downstream end (and upstream for compound bends). A curve that related bend radius to erosion amounts, accounted for about 40 % of variation, while no correlation was found with sedimentation. Relative Bend radius (greater than 2.5) was related to maximum lateral and downstream sedimentation amounts (together with erosion). The steepest and clearest trend was found with erosion. Bend radius and discharge provided conditions that controlled potential erosion and sedimentation, while local factors (vegetation, bank resistance, upstream meander development etc.) limited actual amounts. Average pointbar slopes near the apex increased from 0.025 to 0.035 with decreasing bend radius (till 2.5) and strengthening helicoidal flow. Sedimentation intensity, visible through the presence and amount of scroll bars, was greatest where slopes deviated most from the steeper
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