Processes and controls of meander development in the Allier, France

Abstract

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 equilibrium slope. When the bank became steeper, opposite of a resistant bank, scroll bars were stopped in their development and erosion could temporarily take place. Scroll bars are distinguishable through swales, that develop best in large bends, where there was more space than along the sharp inner bends. Pointbar morphology and scroll bar development were often influenced by the crossing of flow (riffles). Erosive channels often crossed long, narrow pointbars to a swale located on the inside of the pointbar, increasing the local waterlevel gradient. Riffles developed along long complex pointbars with average discharges, but were remoulded to scroll bars by high discharges. At the upstream end of a pointbar, riffles or riffle sections migrated downstream forming the newest phase of scroll bar development. The potential of meanders in the Allier to cause erosion and sedimentation is largely determined by bend radius and discharge. Actual meander development, form and direction, is related to the local factors vegetation, bend length, bank resistance and upstream meander development.