Controls of Dune Growth and Morphology in Rivers
Abstract
Dunes in sand-bedded rivers are often thought to be controlled by flow depth (h) with height (H) scaling as 1/6h and length (L) as 5h. However, some authors have argued that flow strength is an important control on dune dimensions. A recent compilation of dune dimension data shows up to two orders of magnitude of scatter about depth scaling relations, providing further evidence that depth is not a fundamental control on dune dimensions. Furthermore, steep asymmetric dunes in shallow flows (h < 2.5m) grow relatively higher than predicted by 1/6h while low-angle dunes in deeper flows, where suspension dominates, are relatively smaller than 1/6h. Here, we use a series of flume experiments to systematically test controls on dune growth and morphology. A mobile bed of 550 micron sand was flattened before each run and the bed morphology was scanned and mapped as the bed evolved to equilibrium features using 32 Seatek sensors. Three series of experiments were undertaken at flow depths of 15, 20 and 25 cm, respectively. For each series, different flow strengths were imposed and held constant with transport stages that ranged from the threshold of sediment motion to upper stage plane bed. The results show that, under constant h, the time to reach equilibrium decreases with transport stage and different equilibrium H and L emerge for different transport stages. Dune height increases with flow strength until a point where increasing flow strength further leads to smaller H. Length continues to increase with flow strength until dunes wash out to a flat bed. In general, dunes grow higher in the flow when significant bedload transport occurs but when suspended sediment dominates at higher transport stages, dunes are flatter and longer. The results also show significant variability in flow conditions and bedform dimensions between scans, especially at higher transport stages. Ultimately, dunes scales with transport stage, which is a function of slope, grain size and h. Since dunes do not fundamentally scale with h alone, relying on simple depth scaling relations and not accounting for the natural variability in dunes, will lead to poor predictions of dune dimensions in rivers.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMEP33G..02B
- Keywords:
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- 1824 Geomorphology: general;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY