Mechanisms of point bar growth and accretion in experimental bedload-dominated streams
Abstract
As channels migrate across their floodplains they erode their outer banks and create new floodplain on their inner bank via deposition of a point bar. While there has been considerable work on flow through bends and on erosion of the outer bank, there has been comparatively little study of the mechanisms of lateral point bar growth and accretion. We hypothesize that if slope and discharge are sufficient for meandering channels, meandering requires three additional conditions: (1) increased bank strength so that bar growth can keep pace with bank erosion; (2) overbank flows so the point bar can grow to the elevation of the floodplain; and (3) finer (suspendable) particles to allow the point bar to attach to the floodplain downstream of the bar apex. The first condition stems from experiments conducted with the same bed and bank material without sprouts that resulted in a braided morphology and observations of characteristics of meandering channels in the field. The second condition arises because sediment deposition to the elevation of the floodplain requires flow depth that exceeds the elevation of floodplain. The last condition emerges from the strong tendency for the downstream tail of point bars to form a ridge away from the inner bank and leave a trough where bedload sediment doesn't enter. These hypotheses are based on initial single bend experiments in a 3.7 m by 6.1 m flume with a slope of 0.005 that uses alfalfa sprouts to provide bank strength. New experiments are now underway using a 6.1 m wide and 16.5 m long flume with a valley slope of 0.004. The initial channel is set to 1.9 cm deep and 40 cm wide. As before, we have used alfalfa sprouts to increase the bank strength and reduce the bank erosion rate. The previous experiments consisted of two runs with 4 discharges ranging from the critical flow for the initiation of sediment transport to a flow double bankfull. In the experiments in the larger basin, we will use a two-stage hydrograph with a long-duration bankfull flow and a shorter-duration overbank flow. Our goal is to model a meandering gravel bedded stream, which puts particular emphasis on the availability of finer sediment to bridge the gap between the bar tail and the floodplain. Here we use two sediment types to represent gravel and sand. The model gravel was sand with a median size of 0.85 mm and a specific gravity of 2.66. We used plastic sediment with a median size of 370 microns and specific gravity of 1.5 as the model sand. The lower specific gravity allows the plastic to be re-entrained more easily than denser particles with the same settling velocity. We anticipate being able to develop four bends, with the lower ones evolving to cutoff. Initial experiments without plastic were able to connect the bar to the floodplain upstream of the bar apex but the trough remained downstream of the bar apex. Subsequent test runs with both plastic and sand showed that the plastic was able to deposit where the trough formed downstream of the bar apex.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.H14D..02B
- Keywords:
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- 1820 Floodplain dynamics;
- 1825 Geomorphology: fluvial (1625);
- 1862 Sediment transport (4558)