Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Earth Surface
Predicting how rivers evolve in response to natural or human-induced changes in water flow, sediment supply, sediment type (e.g., gravel vs. sand) and reference elevation (e.g. sea level) remains challenging. Notwithstanding recent advances in the study of river dynamics, physical processes can be so complex that empirical relations remain critical to formulate models. Typical examples of such relations are those to model interactions between riverbed irregularities due to bumps and hollows on the bed surface and fluid flow. Significant progress was recently done in modeling the evolution of rivers with a bed fully covered by loose sediment grains (alluvial rivers), but much less attention has been given to the study of the transport, erosion, and deposition of sediment grains in rivers with bed and banks that are not fully covered by loose sediment (bedrock rivers).
Models describing sediment transport, erosion, and deposition in bedrock rivers have failed to capture some important dynamics. In particular, these models do not predict the formation of sediment patches (that is, the accumulation of gravel in a particular location on the bare bedrock surface) or the sudden transition from a bedrock to an alluvial reach in presence of steep slopes, both of which had been observed in nature and experiments. Modeling such behaviors matters a lot, as these behaviors can have a strong impact on the quality of river habitats and on water and sediment fluxes during storms.
Previous observations in experiments suggested that some of these behaviors can be related to changes in riverbed roughness. In their first paper, Cho and Nelson [2024a] introduce a formulation to compute bed roughness that is novel in two ways. Firstly, it considers the presence of moving grains and geometric irregularities (bumps and hollows) on the bedrock surface, which can lead to the formation of large sediment patches resembling gravel bars. Secondly, it accounts for the fact that these large sediment patches will affect the ability for the river to transport sediment.
In their second paper, Cho and Nelson [2024b] then describe, test, and validate their new model. They demonstrate that when their new formulation is implemented, the model can capture the formation of gravel bars in alluvial channels and of sediment patches in bedrock channels. The model also shows additional complex behaviors that previous models could not capture. This work has important implications for those who aim to understand and predict how river habitats (e.g., the gravel patches that are crucial for fish spawning) and sediment fluxes will respond to future natural or human-induced changes in rainfall intensity, water flow or land use.
Citations:
Cho, J., & Nelson, P. A. (2024a). Patterns of alluviation in mixed bedrock-alluvial channels: 1. Numerical model. Journal of Geophysical Research: Earth Surface, 129, e2023JF007292. https://doi.org/10.1029/2023JF007292
Cho, J., & Nelson, P. A. (2024b). Patterns of alluviation in mixed bedrock-alluvial channels: 2. controls on the formation of alluvial patches. Journal of Geophysical Research: Earth Surface, 129, e2023JF007293. https://doi.org/10.1029/2023JF007293
—Enrica Viparelli, Associate Editor, and Mikaël Attal, Editor, JGR: Earth Surface
