Extended Gravel-Bedrock Eroder 1.0: a Landlab component for sediment and bedrock dynamics across a range of river systems

Earth's rivers, especially in mountainous settings, carry sediment with a wide variety of properties, among which are size and lithology. Despite their critical influence on bedrock incision, transport/deposition patterns, and topographic forms, sediment properties are often overlooked in landscape evolution models. Here we present a new set of Landlab components centered on the Enhanced Gravel Bedrock Eroder (EGBE), which simultaneously describes the evolution of a gravel-sized alluvium layer and the underlying bedrock in a network of rivers. The component implements numerical solutions to fluvial sediment transport, deposition, attrition, and bedrock incision, taking into account sediment load heterogeneity in size or toughness. Additionally, EGBE allows the user to select between two assumptions regarding channel geometry: a fixed-width model (in which channel width scales with water discharge) and a dynamic-width model (in which channel width adjusts such that the bed shear stress is slightly above the transport threshold for the median-size sediment grain). EGBE relies on other Landlab components that handle flow routing and mass exchange among different sediment classes, and it can be coupled with hillslope sediment transport components. These components are integrated in a code called EGBE-LEM.

A set of 1D EGBE numerical experiments highlights the importance of sediment size for channel steepness. These experiments illustrate how an upstream source of coarse, resistant gravel leads to a steeper overall profile. Conversely, a downstream source steepens only the lower portion of the profile, leading to a break-in-slope that coincides with the lithologic transition. Several additional EGM-LEM experiments explored the impact of sediment toughness heterogeneity on landscape evolution. These demonstrate how upstream variations in toughness can influence the form and dynamics of channel networks downstream. For example, upstream variations in the toughness of source material can lead to asymmetric drainage divides and contrasts in the steepness index across adjacent drainage basins. In addition, we demonstrate how lithologic heterogeneity can influence river network concavity, planform geometry and steepness.

EGBE improves on previous modeling efforts by explicitly representing sediment attrition and size-dependent transport of heterogeneous sediment load: a useful addition given that size and lithology are two of the most commonly used and accessible measures of river systems. In addition, the EGBE-LEM code provides an advanced integrated modeling platform for understanding mechanisms and dynamics across a range of river types and geological settings.