Mechanisms linking sediment supply and bar morphology: Results from a straight flume with alternate bars
Abstract
One of the largest uncertainties in routing sediment through drainage networks is predicting the degree to which changes in sediment supply are mitigated by changes in sediment storage in bars. We hypothesize that changes in topographic steering link sediment supply and bar morphology. This hypothesis posits that increased sediment supply would enhance topographic steering over the bar, causing the bar to advance laterally into the pool. Contrarily, decreased supply would diminish topographic steering, causing bars to shrink laterally and pools to expand. We examined this hypothesis in a 28-m long, 0.86-m wide flume with a constant discharge of 5.4 l/s and sand with a median grain size of 0.8 mm. The conditions were sufficient to support alternate bars downstream of the upper 8-10 m, and the flume was run for approximately 65 hours at a constant discharge and sediment feed. The equilibrium slope was 0.0033. Once the channel reached equilibrium, the sediment feed was turned off while the discharge was held constant. During the first 25 hours after the feed was stopped, the sediment transport rate at the downstream end of the flume increased from 9.8 to 10.5 kg/hr as sediment was provided by incision at the upstream end of the flume. Because the bed was the only sediment source, sediment supply increased with distance down the flume. Once the feed was shut off, bar response differed between the upstream and downstream portions of the flume. Bars at the downstream end of the flume, where sediment supply increased relative to equilibrium conditions, swelled into the pool. These changes in bar morphology were accompanied by a decrease in surface grain size. Further upstream, where sediment supply was lower, the bars shrunk. These results suggest that there may be a linkage between topographic steering, bar morphology, and sediment supply, but further numerical tests and experimental manipulations are necessary to verify this linkage and to account for the effect of the decreased surface grain size of the bars.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H53C1075B
- Keywords:
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- 1824 Geomorphology: general (1625);
- 1856 River channels (0483;
- 0744);
- 1862 Sediment transport (4558)