The relationship between water depth and multiple longitudinal grooves formed in a mix bedrock - alluvial channel
Abstract
When the sediment supply in a bedrock river is sufficiently smaller than the transport capacity, observations show that multiple longitudinal grooves can form on the bedrock bed surface. As documented by various investigations, these grooves are formed by impact wear of the bed caused by the movement of bed load particles. However, the lateral spacing of these features or, equivalently, the transverse number of grooves formed in a river of a given width, is not well understood, as the controls on the transverse number of incisional grooves are unclear. In general terms, there are two possibilities. First, the spacing could be determined through an interaction between the flow and the sediment that gives rise to a preferred lateral wavelength, much as the interaction of flow and sediment movement gives rise to the initial wavelength of alluvial bars or bedforms. The second possibility is that the lateral groove spacing could be forced entirely by flow structure and not be truly self-organized. To explore the question of which of these is true, we experimentally investigated the dependence of the groove spacing and number on the water depth using a soft mortar bed that imitates bedrock and can be eroded by moving sediment in similar manner. Using a suite of experiments for different conditions, the experimental results showed that the groove number increases with increasing water depth. We also made detailed multidimensional measurements of the flow structure without grooves (before groove formation) and with grooves. The results of the flow velocity measurements revealed that multiple longitudinal vortices are formed in the flat-bedded channel flow, as documented in other field and laboratory experiments. The longitudinal bed grooves appear at the spacing of those longitudinal vortices, suggesting that the spacing of the grooves is simply forced by flow structure, at least initially. The flow vortices tend to concentrate moving sediment into multiple pathways leading to the formation of bedrock incisional grooves. Notably, as the groove increase in amplitude (depth), the spacing changes slightly in response to interactions between the grooves and the vortices.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP53I2248I
- Keywords:
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- 1625 Geomorphology and weathering;
- GLOBAL CHANGE;
- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1856 River channels;
- HYDROLOGY