Conditions Necessary to Create and Maintain Meandering Channels: Inferences From Flume Experiments
Abstract
Stream restoration projects often involve constructing single-thread meandering channels, and their success requires understanding the conditions necessary to develop and maintain a meandering pattern. Empirical studies indicate that meandering channels occur under a specific range of width-depth ratio, slope and Froude number. We hypothesize, however, that meandering rivers also require: 1) bank strength from either cohesive material or vegetation, 2) overbank flows to attach bars to their floodplains, and 3) fine sediment to fill the downstream end of bars and chutes. These latter conditions place significant additional constraints in gravel bedded channels, especially ones designed to be self-maintaining while laterally shifting. We tested whether these conditions were sufficient in a sand-bedded laboratory flume using alfalfa as model vegetation and a non- cohesive lightweight plastic as fine sediment. We conducted two experiments in a 6.1 m wide and 17 m long basin with a valley slope of 0.0046. The first experiment was conducted with a two-stage hydrograph with a bankfull flow of 1.8 l/s and an overbank flow of 2.7 l/s. Prior to the first experiment, we carved a channel with an initial geometry of 1.9 cm deep and of 40 cm wide. The second experiment began with the final morphology from the first experiment, and had a steady 1.8 l/s flow. We were able to create and maintain a meandering channel during both experiments. The channel maintained a meandering morphology for over 71 hours during the first experiment and an additional 35 hours to date during the steady experiment without the channel braiding. The alfalfa sprouts slowed down bank erosion enough to allow bars to grow to the elevation of the floodplain, and the fine sediment plugged chutes at the upstream end of bars. During the first experiment, the width initially increased, but then stabilized as the bars grew vertically. The width depth ratio had returned to its original value (21) by 51 hours, just prior to a cutoff that forced more water overbank and increased the width-depth ratio to 25. During the steady flow experiment, the channel reduced its flow capacity so that the discharge resulted in a steady overbank flow, and the sinuosity increased relative to the first experiment. This prevented us from testing whether overbank flows were necessary, but did show that a meandering morphology can be maintained with a steady flow. These experiments indicate that bank strength (here derived from vegetation) and fine sediment are essential components of meandering channels in the laboratory and should likely be incorporated into channel design.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.H34A..03B
- Keywords:
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- 1825 Geomorphology: fluvial (1625)