Publication Date:
2023-01-25
Description:
Large, rarely mobile boulders are observed globally in mountainous bedrock channels. Recent studies suggest that high concentrations of boulders could be associated with channel morphological adjustment. However, a process‐based understanding of large boulder effects on channel morphology is limited, and data are scarce and ambiguous. Here, we develop a theory of steady‐state channel width and slope as a function of boulder concentration. Our theory assumes that channel morphology adjusts to maintain two fundamental mass balances: (a) grade, in which the channel transports the same sediment flux downstream despite boulders acting as roughness elements and (b) bedrock erosion, by which the channel erodes at the background tectonic uplift rate. Model predictions are normalized by a reference, boulder‐free channel width and slope, accounting for variations due to sediment supply, discharge, and lithology. Models are tested against a new data set from the Liwu River, Taiwan, showing steepening and widening with increasing boulder concentration. Whereas one of the explored mechanisms successfully explains the observed steepening trend, none of the models accuratly account for the observed width variability. We propose that this contrast arises from different adjustment timescales: while sediment bed slope adjusts within a few floods, width adjustment takes a much longer time. Overall, we find that boulders represent a significant perturbation to fluvial landscapes. Channels tend to respond by forming a new morphology that differs from boulder‐free channels. The general approach presented here can be further expanded to explore the role of other hydrodynamic effects associated with large, rarely mobile boulders.
Description:
Plain Language Summary:
Large boulders are a significant feature in mountainous landscapes. Recent studies suggested that boulders residing in rivers interfere with the flow and sediment transport, forcing their geometry, specifically width and slope, to change. Our ability to understand and predict such changes is challenged by scarce field data and a general lack of models capable of explaining the processes underlying channel geometry adjustment in the presence of boulders. Here, we develop a theory and several models for the variation of channel width and slope as with channel boulder coverage. Our theory builds on the assumption that the geometry of boulder‐bed channels evolves to a new configuration to maintain steadiness of erosion rate and sediment transport. Predictions from the various models are tested against data from the steep Liwu River in Taiwan. These data show that width and slope increase with more boulders. We find that channel slope increases to overcome the greater resistance to sediment transport due to the boulders. In contrast, the scattered nature of the width data and the overall models inability to explain width variability likely reflect a longer adjustment period for width than for slope. This study demonstrates the important role of boulders in shaping landscapes.
Description:
Key Points:
We develop a theory for steady‐state reach‐scale channel morphology responding to large, rarely mobile boulders in bedrock rivers.
Predictions of boulder‐bed channel width and slope are derived based on grade equilibrium and bedrock erosional balance.
Theory is tested against new data from the Liwu River, Taiwan, showing steepening and widening with increasing boulder concentration.
Description:
Israel Science Foundation
http://dx.doi.org/10.13039/501100003977
Description:
NSF‐BSF
Description:
https://zenodo.org/record/6371224#.YjdBkOpByUk
Keywords:
ddc:551.3
;
boulders
;
slope
;
width
;
bedrock erosion
;
sediment transport
;
grade
Language:
English
Type:
doc-type:article
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