Grain-size controls on sand storage in rivers
Abstract
Temporal and spatial non-uniformity in supplies of water and sand in rivers lead to sand transport that is in local disequilibrium with the sand supply. In such rivers, sand is transported downstream as elongating waves in which coupled changes in grain size and transport occur. Depending on the magnitude of each sand-supplying event and the interval between such events, changes in bed-sand grain size associated with sand-wave passage may be the dominant regulator of sand transport. In such cases, bed-sand grain size will exert a substantial control on the amount of sand that can be stored in a river reach. We demonstrate that temporal and spatial changes in bed-sand grain size regulate sand storage in two very different river systems: the dam-altered Colorado River in Grand Canyon National Park and the quasi-natural Little Snake - Yampa river network near and in Dinosaur National Monument.
In the Colorado River, sand supplied by tributaries is finer than the typical antecedent bed-sand grain-size distribution. Thus, changes in sand storage are associated with changes in bed-sand grain size. As sand accumulates upon arrival of the leading edge of a sand wave, the bed and suspended sand fine and sand concentrations increase independently of water discharge; as sand erodes in the tail of a sand wave, the bed and suspended sand coarsen and sand concentrations decrease. Thus, owing to the lower flux boundary condition, the advection-diffusion equation for suspended sand, and mass conservation of sand between the bed and the flow, the addition of sand causes bed-sand fining and increased downstream transport, thereby limiting the amount of sand that can be stored. This sand-storage limitation is evident in the discharges at which sand is observed to accumulate in long reaches, with accumulation occurring at higher discharges over initially coarser beds. In the Little Snake - Yampa case, natural changes in bed-sand grain size associated with the passage of tributary-generated sand wave(s) 50 years ago still exert a strong control on sand storage. In the tail of a sand wave, sand transport increases in the downstream direction because the bed sand fines in this direction. This process is evident in the modern Little Snake - Yampa network, where sand is eroding and the bed-sand grain size has been progressively coarsening over many decades.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP31C2358T
- Keywords:
-
- 1825 Geomorphology: fluvial;
- HYDROLOGYDE: 1856 River channels;
- HYDROLOGYDE: 1860 Streamflow;
- HYDROLOGYDE: 1862 Sediment transport;
- HYDROLOGY