COMPARING MODELED AND MEASURED CHANNEL GEOMETRY: IMPLICATIONS FOR THE ROLE OF BEDROCK PROPERTIES

Rock properties (e.g., strength, fracture spacing) influence the cross-section shape of bedrock rivers. But models for channel cross-section evolution rely on a concept of erodibility that is difficult to quantify in the field. Comparing modeled and measured cross-sections across different rock types may reveal patterns in model-data misfit, indicating which properties lead to the greatest deviation from expected cross-section form. We might, for example, expect plucking-dominated reaches—those with closely spaced fractures—to exhibit greater divergence from model expectations than abrasion-dominated reaches due to the spatially and temporally stochastic nature of plucking processes. Here we compare modeled and measured channel geometry in streams draining sandstones and shales in the central Appalachians to assess the role of rock properties in causing natural channels to deviate from model expectations.

We use a combination of field and laboratory methods to investigate this relationship. LiDAR and Total-Station surveys are used to measure channel geometry throughout the study area. We then compare the measured and modeled geometries for a given slope and discharge.

Preliminary, qualitative observations suggest that areas with plucking-dominated erosive regimes diverge from the modeled geometry more than reaches dominated by abrasive erosion. This highlights the influence of discontinuity spacing in the development of channel geometry. Further analysis of additional reaches in other lithologic contexts will help to develop the relationship between rock properties and channel geometry and further our ability to model landscape evolution in lithologically heterogeneous landscapes.