A surface roughness model for predicting alluvial cover and bed load transport rate in bedrock channels. Issue 10 (9th October 2014)

• Record Type: Journal Article
• Authors: Johnson, Joel P. L.
• Is Part Of: Journal of geophysical research. Volume 119:Issue 10(2014)
• Page Start: 2147
• Page End: 2173
• Publication Date: 2014-10-09
• Subjects: Geomorphology -- Periodicals; 551.3
• DOI: 10.1002/2013JF003000
• Languages: English
• ISSNs: 2169-9003

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>Partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics. Flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity. Existing cover models impose different unique relationships between cover fraction and reach‐scale transport variables (e.g., slope, discharge, sediment supply, and grain size). Individually, these models cannot predict the range of cover behaviors observed experimentally, suggesting that additional variables may be important. I propose a 1‐D model in which bedrock surface roughness is a control on partial alluvial cover. In the model, surface roughness affects both shear stresses and thresholds of grain motion and therefore sediment transport capacity. The roughness of the combined bedrock‐alluvial bed surface varies with the cover fraction. Model trends are reasonably consistent with previous experiments over ranges of roughness, slope, and shear stress. When bedrock roughness is much greater than grain size, cover increases approximately linearly with the ratio of sediment supply to transport capacity. When bedrock roughness is much less than the sediment diameter, the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover. This runaway alluviation occurs for model solutions that are unstable to perturbations in supply, which is the case when transport capacity decreases as alluvial cover increases. In addition, a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux‐dependent erosion at landscape scales.</p> </abstract>